Visceral innervation by the sympathetic autonomic system
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Shaheer Akhtar and Prof. Hyung-Shik Shin",publishedDate:null,coverURL:"https://cdn.intechopen.com/books/images_new/10582.jpg",keywords:"Ion Implantation, Photomask Fabrication, Photovoltaic Materials, Solar Thermal, Mass Spectrometric, Electrochemical, Molecular Thermodynamics, Sustainable Energy Conversion, Energy Production and Storage, Green Technologies, Bioenergy and Biofuels to the Storage, Bioinspired Materials and Systems",numberOfDownloads:null,numberOfWosCitations:0,numberOfCrossrefCitations:null,numberOfDimensionsCitations:null,numberOfTotalCitations:null,isAvailableForWebshopOrdering:!0,dateEndFirstStepPublish:"November 17th 2020",dateEndSecondStepPublish:"February 17th 2021",dateEndThirdStepPublish:"April 18th 2021",dateEndFourthStepPublish:"July 7th 2021",dateEndFifthStepPublish:"September 5th 2021",remainingDaysToSecondStep:"20 days",secondStepPassed:!0,currentStepOfPublishingProcess:3,editedByType:null,kuFlag:!1,biosketch:"Professor Sadia Ameen is a Gold Medalist in academics and recipient of the Best Researcher Award. She has more than 130 peer-reviewed papers in the field of solar cells, catalysts, sensors, contributed to book chapters, edited books, and is inventor/co-inventor of patents.",coeditorOneBiosketch:"Associate professor at Jeonbuk National University, Korea. He is an expert in the synthesis of semiconductor nanomaterials, composite materials, polymer-based solid-state films, solid polymer electrolytes, and electrode materials, solar cells, small molecules based organic solar cells, and photocatalytic reactions.",coeditorTwoBiosketch:"Professor in School of Chemical Engineering, Jeonbuk National University, and also President of Korea Basic Science Institute (KBSI), Republic of Korea. The high impact of his work has been recognized by invitations to speak at international/national conferences and scientific meetings.",coeditorThreeBiosketch:null,coeditorFourBiosketch:null,coeditorFiveBiosketch:null,editors:[{id:"52613",title:"Dr.",name:"Sadia",middleName:null,surname:"Ameen",slug:"sadia-ameen",fullName:"Sadia Ameen",profilePictureURL:"https://mts.intechopen.com/storage/users/52613/images/system/52613.jpeg",biography:"Professor Sadia Ameen obtained her Ph.D. in Chemistry (2008) and then moved to Jeonbuk National University. Presently she is working as an Assistant Professor in the Department of Bio-Convergence Science, Jeongeup Campus, Jeonbuk National University. Her current research focuses on dye-sensitized solar cells, perovskite solar cells, organic solar cells, sensors, catalyst, and optoelectronic devices. She specializes in manufacturing advanced energy materials and nanocomposites. She has achieved a gold medal in academics and is the holder of a merit scholarship for the best academic performance. She is the recipient of the Best Researcher Award. She has published more than 130 peer-reviewed papers in the field of solar cells, catalysts and sensors, contributed to book chapters, edited books, and is an inventor/co-inventor of patents.",institutionString:"Jeonbuk National University",position:null,outsideEditionCount:0,totalCites:0,totalAuthoredChapters:"5",totalChapterViews:"0",totalEditedBooks:"3",institution:{name:"Jeonbuk National University",institutionURL:null,country:{name:"Korea, South"}}}],coeditorOne:{id:"218191",title:"Dr.",name:"M. Shaheer",middleName:null,surname:"Akhtar",slug:"m.-shaheer-akhtar",fullName:"M. Shaheer Akhtar",profilePictureURL:"https://mts.intechopen.com/storage/users/218191/images/system/218191.jpg",biography:"Professor M. Shaheer Akhtar completed his Ph.D. in Chemical Engineering, 2008, from Jeonbuk National University, Republic of Korea. Presently, he is working as Associate Professor at Jeonbuk National University, the Republic of Korea. His research interest constitutes the photo-electrochemical characterizations of thin-film semiconductor nanomaterials, composite materials, polymer-based solid-state films, solid polymer electrolytes and electrode materials for dye-sensitized solar cells (DSSCs), hybrid organic-inorganic solar cells, small molecules based organic solar cells, and photocatalytic reactions.",institutionString:"Jeonbuk National University",position:null,outsideEditionCount:0,totalCites:0,totalAuthoredChapters:"1",totalChapterViews:"0",totalEditedBooks:"0",institution:{name:"Jeonbuk National University",institutionURL:null,country:{name:"Korea, South"}}},coeditorTwo:{id:"36666",title:"Prof.",name:"Hyung-Shik",middleName:null,surname:"Shin",slug:"hyung-shik-shin",fullName:"Hyung-Shik Shin",profilePictureURL:"https://mts.intechopen.com/storage/users/36666/images/system/36666.jpeg",biography:"Professor Hyung-Shik Shin received a Ph.D. in the kinetics of the initial oxidation Al (111) surface from Cornell University, USA, in 1984. He is a Professor in the School of Chemical Engineering, Jeonbuk National University, and also President of Korea Basic Science Institute (KBSI), Gwahak-ro, Yuseong-gu, Daejon, Republic of Korea. He has been a promising researcher and visited several universities as a visiting professor/invited speaker worldwide. He is an active executive member of various renowned scientific committees such as KiChE, copyright protection, KAERI, etc. He has extensive experience in electrochemistry, renewable energy sources, solar cells, organic solar cells, charge transport properties of organic semiconductors, inorganic-organic solar cells, biosensors, chemical sensors, nano-patterning of thin film materials, and photocatalytic degradation.",institutionString:"Jeonbuk National University",position:null,outsideEditionCount:0,totalCites:0,totalAuthoredChapters:"3",totalChapterViews:"0",totalEditedBooks:"0",institution:{name:"Jeonbuk National University",institutionURL:null,country:{name:"Korea, South"}}},coeditorThree:null,coeditorFour:null,coeditorFive:null,topics:[{id:"8",title:"Chemistry",slug:"chemistry"}],chapters:null,productType:{id:"1",title:"Edited Volume",chapterContentType:"chapter",authoredCaption:"Edited by"},personalPublishingAssistant:{id:"194667",firstName:"Marijana",lastName:"Francetic",middleName:null,title:"Ms.",imageUrl:"https://mts.intechopen.com/storage/users/194667/images/4752_n.jpg",email:"marijana@intechopen.com",biography:"As an Author Service Manager my responsibilities include monitoring and facilitating all publishing activities for authors and editors. 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Venkateswarlu",coverURL:"https://cdn.intechopen.com/books/images_new/371.jpg",editedByType:"Edited by",editors:[{id:"58592",title:"Dr.",name:"Arun",surname:"Shanker",slug:"arun-shanker",fullName:"Arun Shanker"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}}]},chapter:{item:{type:"chapter",id:"38231",title:"Abdominal Surgery: Advances in the Use of Ultrasound-Guided Truncal Blocks for Perioperative Pain Management",doi:"10.5772/48255",slug:"abdominal-surgery-advances-in-the-use-of-ultrasound-guided-truncal-blocks-for-perioperative-pain-man",body:'In the order to get an overview of the mechanisms for the conduction of pain stimulus from the abdominal area it is clearly important to refresh the memory concerning some of the basic anatomical considerations. This knowledge constitutes an important factor with the on-going quest of providing efficient and safe post- and perioperative pain management to patients undergoing abdominal surgery.
The viscera are innervated by the vagal nerve (parasympathetic innervation) and by the splanchnic nerves (sympathetic innervation). The splanchnic nerves carry both visceral efferent and afferent nerve fibers. The sensory (or afferent) part of the splanchnic nerves reach the spinal column at certain spinal segments. Table 1 attempts to give a brief overview of the visceral innervation by the sympathetic autonomic system.
If post- or perioperative pain sensations are predominantly transmitted via the autonomic nervous system, then the choice of analgesic management would today primarily rely on continuous intrathecal and especially epidural infusions of local anaesthetic. It is also possible to block central visceral pain conduction with thoracic paravertebral blockade or maybe even with the novel quadratus lumborum block (Carney 2011). We will deal with these blocks later in this chapter. Finally, opioids administered either orally or intravenously will also reduce visceral pain significantly.
Greater splanchnic nerve | Th5-Th9(10) | Celiac ganglia |
Lesser splanchnic nerve | Th10-Th11 | Superior mesenteric & Aorticorenal ganglia |
Least splanchnic nerve | Th10 | Renal plexus |
Lumbar splanchnic nerves | L1-L2 | Inferior mesenteric ganglia & ganglia of intermesenteric and hypogastric plexuses |
Sacral splanchnic nervers | Sacral part of sympathetic trunk | Inferior hypogastric plexus and ganglia to the pelvic vicera |
Visceral innervation by the sympathetic autonomic system
The innervation of the anterolateral abdominal wall by the somatic nervous system arises from the anterior rami of the thoracolumbar spinal nerves (Th6-L1) (Børglum 2011). Branches from the anterior rami include the intercostal nerves (Th6-T11), the subcostal nerve (Th12), and the iliohypogastric and ilioinguinal nerves (L1). Furthermore, Th6-Th12 nerves provide motor innervation to the pyramidalis and rectus muscles in the anterior abdomen, and Th6-L1 nerves innervate the intercostal muscles, the external and internal oblique muscles, the transversus abdominis muscles and also provide sensory innervation to the parietal peritoneum (Børglum 2011). However, many previous descriptions of the thoracolumbar spinal nerves innervating the abdominal wall have been inconsistent leading to misunderstanding and faulty attempts to provide sufficient anaesthesia (Rozen 2008). Conducting a thorough cadaveric study including comprehensive tracing of nerves and their branches Rozen et al. were able to describe the pattern and course of all thoracolumbar nerves innervating the anterior abdominal wall. The thoracolumbar nerves were found to travel as multiple mixed segmental nerves (running with their accompanying blood vessels), which branch and communicate widely within the neurovascular plane called the transversus abdominis plane (TAP) (Rozen 2008). Such large branch communications were found antero-laterally (the intercostal plexus – Th6-Th9), and in plexuses that run with the deep circumflex iliac artery (DCIA) (the classical TAP plexus – Th10-L1) and the deep inferior epigastric artery (DIEA) (rectus sheath plexus – Th6-L1) (Børglum 2012, Rozen 2008). Segmental nerves Th6 to Th9 emerged from the costal margin to enter the TAP between the midline and the anterior axillary line. Th6 entered the TAP just lateral to the linea alba, while Th7-Th9 emerged from the costal margin at increasingly lateral positions. It was also found that Th9 emerged from the costal margin either medial (predominantly) or lateral to the anterior axillary line (Rozen 2008).
Dr. Louis Gaston Labat was a pioneer in the world of regional anaesthesia. In the early 20th century, he brought to the United States knowledge he had acquired from his mentor, the French surgery professor Victor Pauchet. Thus, the spread of regional anaesthesia in the United States was greatly facilitated by the work of Dr. Gaston Labat. Recruited to work at the Mayo Clinic, Dr. Labat there published his original textbook, Regional Anesthesia, in which he laid out his techniques to the next generation of physician specialists (Labat 1922, Bacon 2002). Regional anaesthesia in the United States was popularized by Dr. Labat’s book, and many physician anaesthesiologists in the 1920s and 1930s learned regional techniques this way. Most interestingly concerning the current chapter, in relation to abdominal operations, regional anaesthesia had already then been found to provide superior muscle relaxation with fewer complications than deep ether anaesthesia. Dr. Labat has also been given credit for the posterior approach to splanchnic nerve block,use of intercostal block instead of paravertebral block for breast surgery, the use of abdominal field blocks, level of dural puncture and many other regional anaesthetic techniques (Côté 2003). Further, in the early 1920s Dr. Labat wrote about the combined caudal, trans-sacral and paravertebral (lower lumbar) block for resection of the rectum. He also elaborated extensively on other regional techniques: “With the abdominal field block procedure, colostomy is performed painlessly, provided the patient is not too obese and the mesocolon is not too short. Exploration is possible in the majority of cases, if gentleness is used. The sacral block, consisting of the caudal or epidural and transsacral block, added to the paravertebral block of the last three lumbar nerves on both sides, constitutes the method of choice for the posterior resection of the carcinomatous rectum and rectosigmoid”. Thus, this very brief historical entry highlights the fact that the use of truncal blocks certainly is not a new phenomenon in anaesthesia practice. The old masters relied on landmark-based techniques as well as a thorough understanding of anatomy and anatomical variations.
In 2010 Abrahams et al. performed a systematic search of the medical literature in the quest to describe evidence-based medicine in relation to ultrasound (US) guidance for truncal block administration (Abrahams 2010). In this review article it is mentioned that anaesthesia and analgesia of the trunk can be achieved with perineural injections, which could have several advantages compared with neuraxial blockade; i.e. reduced sympathectomy, less severe consequences of infection or bleeding at the injection site, minimal interference with bladder and bowel function, and less incidence of lower extremity motor weakness (Abrahams 2010). It is also clearly stated that Thoracic Paravertebral blocks (TPVB) from Th6-L1, TAP blocks, Rectus Sheath (RS) blocks, and Ilioinguinal and Iliohypogastric nerve (IIN and IHN) blocks can provide anaesthesia and analgesia of the abdominal wall. Abraham et al. did not compare the efficacy of various truncal blocks against each other or against the golden standard of the continuous epidural blockade. In the following we will go through the most common truncal blocks suitable for the purpose of alleviating the patients from pain following abdominal surgery. Table 2 provides an overview of the recommended ultrasound-guided (USG) truncal nerve blocks specifically in relation to abdominal surgical procedures.
Truncal nerve blockade | Indications |
Bilateral Dual – Transversus Abdominal Plane (BD-TAP) block | Fig. 1: Intercostal TAP block (IC-TAP) block providing anaesthesia to the upper abdomen (Th6-Th9) (PPM) Fig. 2: Classical TAP block (CL-TAP) providing anaesthesia to the lower abdomen (Th10-Th12) (PPM) |
Ilioinguinal/iliohypogastric nerve (IIN/IHN) block (L1) | Fig. 3: Open and laparoscopic inguinal hernia repair (PPM) |
Rectus sheath (RS) block (Th6-L1) | Fig. 4: Midline incisions and trochar holes (PPM) |
Intercostal nerve (ICN) block (single or multiple injection technique) | Fig.5: Cholecystectomy, trochar holes high in the epigastric area (PPM) |
Thoracic paravertebral block (TPVB) | Fig. 6: Providing anaesthesia both to the upper (Th6-Th9) and lower (Th10-L1) abdomen depending on the site of administration and the volume of local anaesthetic. Has the potential to block the visceral pain in addition to the somatic sensory pain. |
Quadratus lumborum (QL) block | Fig. 7: Seems to be able to provide anaesthesia from the Th5 to the L1. Is currently still rather inadequately described. Seems to have the potential to block the visceral pain in addition to the somatic sensory pain. |
Overview of recommended truncal nerve blocks specifically in relation to abdominal surgical procedures. Indications only include postoperative pain management (PPM).
In order to enhance the clinical implementation process, to support further education and to advance improvements in clinical practice, the American Society of Regional Anesthesia and Pain Medicine (ASRA) and the European Society of Regional Anaesthesia and Pain Therapy (ESRA) encouraged all institutions that conduct USG PNB to support a quality improvement process (Sites 2010). The joint committee of ASRA and ESRA advocated a focus on the following issues: (i) ten common tasks used when performing an ultrasound-guided nerve block, (ii) the core competencies and skills associated with UGS PNB, and (iii) a training practice pathway for postgraduate anaesthesiologists and a residency-based training pathway (Sites 2010). Table 3 lists the first proposal from the joint committee of ASRA and ESRA.
High block expertise requires both anatomical knowledge and extensive hands-on experience (Jensen 2011, Orebaugh 2009). In this pursuit it is probably wise to adhere to the principles or EFSUMB (www.efsumb.org) when dividing practitioners into various catagories (levels of expertice) when formulating a strategy for enhancing the continuous education of physicians. As to the other proposals regarding core competencies and skills associated with USG peripheral nerve blocks and the proposed training practice pathway at any institution, this chapter refers to the original publication (Sites 2010). Our primary aim with this chapter is to provide the reader with an easy pathway to perform USG truncal nerve blocks in daily clinical practise. The USG truncal nerve blocks can in reality be performed by any trained physician qualified in the field of emergency medicine, acute pain management and trauma as well as anaesthesiologists providing for surgical anaesthesia and postoperative pain management. In the following we will provide recommendations on how to perform the various USG truncal blocks and show relevant clinical photographs together with ultrasound recordings to advice the reader accordingly. However, it is necessary to mention early on, that the special field of USG truncal blocks differs from the performance of USG peripheral nerve blocks on the upper and lower extremities; i.e. with USG truncal blocks you very rarely actually see the nerves ultrasonographically. Rather, the focus for the physician must be on the surrounding perineural structures (muscle layer, fascia, neurovascular plane, bone etc.) and a thorough knowledge of anatomy.
1 | Visualize key landmark structures including blood vessels, muscles, fascia, and bone |
2 | Identify the nerves or plexus on short-axis imaging |
3 | Confirm normal anatomy and recognize anatomic variation(s) |
4 | Plan for a needle approach that avoids unnecessary tissue trauma |
5 | Maintain an aseptic technique with respect to the ultrasound equipment |
6 | Follow the needle under real-time visualization as it advances toward the target |
7 | Consider a secondary confirmation technique, such as nerve stimulation |
8 | When the needle tip is presumed to be in the correct position, inject a small volume of a test solution. If solution is not visualized during this test injection, presume that the needle tip is intravascular or out of the imaging plane. |
9 | Make necessary needle adjustments if an undesired pattern of local anaesthetic spread is visualized. The visualization of local anaesthetic should occur through the entirety of the injection to avoid an intravascular injection |
10 | Maintain traditional safety guidelines including the presence of resuscitation equipment, frequent aspiration, intravascular test dosing, standard monitoring, patient response, and assessment of injection characteristics |
Ten tasks helpful in performing USG peripheral nerve blocks. (Sites 2012)
The idea of the TAP block is to anaesthetize part of - or the entire - abdominal wall instead of using intrathecal or epidural techniques, that may or may not elicit more negative side effects by the application. By adhering to this principle one would block the nerves as peripheral as possible but only as centrally a necessary (to quote Professor Peter Marhofer, Austria). The technique builds on anaesthetizing the peripheral nerves to the abdomen using a direct approach. Since the first description of the TAP block technique by Rafi (or something very similar to what it is conceived as today), this block has been increasingly used to provide somatic anaesthesia of the antero-lateral abdominal wall (Rafi 2001, Abrahams 2010, Petersen 2010, Børglum 2011, Koscielniak-Nielsen 2011, Børglum 2012). This landmark-based blind approach to deposit local anaesthetic at the neurovascular plane was since thoroughly described by McDonnell et al. and further documented using computerized tomography (McDonnell 2004, 2007). As of today, it would seem that four different TAP block approaches are in common use. First, let us begin with the landmark-based blind approach at the triangle of Petit. McDonnell et al. and Carney et al. have provided ample evidence with many scientific publications for the huge success when using their TAP block approach where ultrasound guidance is not used (McDonnell 2007, Carney 2009, 2011), but where extensive dermatomal anaesthesia is achived and postoperatively pain management is improved significantly for a great many surgical procedures. Second, we find the USG approach to the TAP very well described by El-Dawlatly et al. and Shibata et al. Both use two separate injections deposited at the lateral classical TAP plexus; i.e. one injection on each hemi-abdomen (above the iliac crest and below the thoracic cage) (El-Dawlatly 2009, Shibata 2007). This simple and very efficient technique is probably the method most commonly used today. Thirdly, we find the brilliant classification by Hebbard et al. where the USG continuous oblique subcostal transversus abdominis plane blockade technique is described (Hebbard 2010). Dr. Hebbard must be thankfully accredited to address the issue of providing consistent anaesthesia to both the lower (Th10-L1) and upper (Th6-Th9) abdominal wall on a continuous basis. In addition, Dr. Hebbard´s research has showed that it possible to use more peripheral approaches to continuous block the entire abdominal wall. Dr. Hebbard has however clearly expressed that his technique for providing safe and continuous anaesthesia to the entire abdomen requires considerable skills and serious anatomical knowledge (Hebbard 2010). Having said that, one must not rule out that McDonnell et al. have also found their landmarked-based and blind approach to be able to anaesthetize the entire abdominal wall for an extended period. Finally – and fourth – is our own approach called the bilateral dual TAP (BD-TAP) block based on four single shot injections with the aim to provide anaesthesia to the entire abdominal wall in a fast and safe sequence (Børglum 2011, Børglum 2012). Our method does not rely on relatively lengthy or sophisticated methods for the insertion of catheters; rather our technique relies on simple anatomical knowledge and structured ultrasonographic recognition (Børglum 2011, Børglum 2012). The BD-TAP block will normally take the well-trained anaesthesiologist approximately 5-6 minutes to perform. Probably even shorter time if the block is administered prior to surgery. The BD-TAP block will anaesthetize the dermatomes Th6-Th12, the antero-medial muscles of the abdominal wall and the underlying parietal peritoneum. It would also be fair to say that the BD-TAP block builds to some extent on a “mixture” of previously described techniques by El-Dawlatly et al. and Shibata et al. as well as Dr. Hebbards research (El-Dawlatly 2009, Shibata 2007, Hebbard 2010). In addition, the BD-TAP technique has been proved not to result in inhibition of the accessory respiratory function attributed to the abdominal wall muscles – mainly forced expiration (Petersen 2011). The efficacy of the BD-TAP block technique has been ascertained by magnetic resonance imaging (MRI) (Børglum 2012), and it seems obvious that it is not possible to anaesthetize the entire abdominal wall (TH6-L1) with the so-called lateral classical TAP block technique alone, but that the intercostal TAP plexus in the upper abdomen (epigastric area) must also be anaesthetized by a direct approach in addition.
Generally speaking when considering outcome measures of the various techniques, TAP blocks have been described as an effective component of multimodal postoperative analgesic protocols for a wide variety of abdominal surgical procedures including laparotomy for colorectal surgery, open and laparoscopic appendectomy, caesarean section, abdominal hysterectomy, laparoscopic cholecystectomy, open prostatectomy and renal transplant surgery. In an uncontrolled study, patients undergoing lower abdominal gynaecological surgery received bilateral TAP block catheters, and the authors found an average pain at rest and on movement below 2 on a 10-point VAS scale for up to 48 hours postoperatively, with no occurrences of nausea or side effects (Fujita 2012). Compared to systemic opioids, patients receiving TAP blocks after major abdominal surgery had less pain up to 24 hours postoperatively than non-TAP block groups, but in that study no statistical differences were found with respect to nausea (Siddiqui 2011). The benefits of TAP blocks are so far measured in relation to reduced postoperative opioid requirements, lower pain scores or a reduction in opioid-related side effects (Shin 2011). As an example, a meta-analysis of 7 studies demonstrated an average reduction in 24-hour morphine consumption of 22 mg compared with systemic opioids, and TAP blocks were associated with reduced early postoperative pain VAS in 4 of the 7 studies (Petersen 2010). Postoperative sedation, as well as PONV, was marginally reduced in patients having TAP blocks administered. Newer studies confirm these findings, and also observe a higher patient satisfaction in the TAP block groups (Hivelin 2011). Despite the numerous descriptive studies on TAP blocks, however, results of comparative studies have been inconsistent. The current scientific evidence is lacking to definitively identify the surgical procedures, dosing, techniques, and timing that provide optimal analgesia following TAP block (Abdallah 2012).
With the aim to render the entire abdominal wall pain-free after surgery (or during the surgical procedure) one must anaesthetize all the antero-lateral rami of the thoracoabdominal nerves (Th6-Th12). In doing so, one must anaesthetize both the intercostal TAP plexus (Th6-Th9) situated in the epigastric area just below the xiphoid process medially to the costal curvature (Fig. 1), and one must also target the lateral classical TAP plexus (Th10-Th12) situated in the lower abdomen. This must be done on both hemi-abdomens. When administering local anaesthetic to the uppermost branches of the intercostal TAP plexus the physician must use the USG intercostal TAP (IC-TAP) block, where the IC-TAP plexus lies in the fascial plane between the rectus abdominis (RA) muscle (or rather deep to the posterior rectus sheath) and the transversus abdominis (TA) muscle (Fig. 1). When blocking the lateral classical TAP (CL-TAP) plexus the point of skin penetration must be in the anterior axillary line above the iliac crest and below the thoracic cage (Fig. 2). The needle is then advanced posterior and increasingly lateral, and the point of injection will be in the middle axillary line between the internal oblique (IO) and TA muscles. Thus, the BD-TAP block technique can best be described as a fast and simple four-point USG single-shot TAP block approach. For all TAP blocks the patient is placed in a supine position, and a linear transducer (6-15 MHz) is placed with its medial end pointing medially (Fig. 1-2). The needle is inserted in-plane to the transducer in a medial to lateral direction with the endpoint in the fascial neurovascular plane between the RA and TA (IC-TAP block) or between the IO and TA (CL-TAP block). The spread of the injectate should be observed to be distributed within the neurovascular plane.
USG IC-TAP block (Th6-Th9). PC: peritoneal cavity.
USG CL-TAP block (Th10-Th12). EO: external oblique muscle, PC: peritoneal cavity.
Very recently a thorough systematic review concerning the various TAP blocks was published, and the review centres on postoperative analgesia following abdominal surgery (Abdallah et al. 2012). According to this systematic review improved analgesia was found in patients subjected to laparotomy for colorectal surgery, laparoscopic cholecystectomy, and open and laparoscopic appendectomy (Abdallah et al. 2012). Superior analgesic outcomes was also found when 15 mL of local anaesthetic or more was used per side, compared with lesser volumes, and TAP blocks performed in the triangle of Petit and along the midaxillary line both demonstrated some analgesic advantages (Abdallah et al. 2012). Finally, this systematic review also found that although the majority of trials reviewed suggested superior early pain control, they were unable to definitively identify the surgical procedures, local anaesthetic doses, techniques, and timing (pre- or post-incisional) that would ensure optimal analgesia following the TAP blocks. Thus, there is still much work to be done.
This is a selective block of the ventral ramus of the L1. The IIN provides sensation to the upper medial part of the thigh and the upper part of the genitalia. The IHN provides sensation to the buttock and abdominal wall above the pubis (Abrahams 2010). In our own previous studies, we have been unable to register effective dermatomal anaesthesia of the L1 branch with our BD-TAP block technique, but other studies have shown the L1 branches to be blocked by other versions of the TAP block technique (Børglum 2012, Carney 2011). For the selective USG IIN/IHN block the patient is placed in a supine position, and the anterior superior iliac spine (ASIS) is localized by palpation first and since ultrasonographically (Fig. 3). A linear transducer (6-15 MHz) is placed with its lateral end at or just superior to the ASIS. The needle is inserted in-plane to the transducer in a lateral to medial direction, and the neurovascular plane between the IO and the TA is located. The IIN and IHN can often be seen together with the deep circumflex iliac artery in the neurovascular plane. This is rather specific for this particular truncal block, because trunk nerves are not as easily discovered by US as are the peripheral nerves of the upper and lower extremities. The tip of the needle is placed in this plane, and the spread of the injectate should be observed to expand in the fascial neurovascular plane (3).
USG IIN/IHN block. EO: external oblique muscle.
Traditional techniques were landmark-based and relied on one or two facial “clicks”, but the old techniques are largely abandoned now, since US imaging subsequent to the so-called blind blocks has demonstrated incorrect placement of the local anaesthetic administered (Weintraud 2008). The US technique is very well established already (Willschke 2005, Willschke 2006, Eichenberger 2007). Most IIN/IHN blocks are placed for analgesia after inguinal hernia repairs in children, but have also been shown to provide similar analgesia to caudal blocks during orchidopexy and hydrocele repair (Abrahams 2010). In his review article Dr. Abrahams gave the use of US guidance for the IIN/IHN block a Grade A recommendation (Abrahams 2010). Our own very recent study concerning primary open inguinal hernia repair (ad modum Lichtenstein) in adult males showed significant reduction in pain scores at mobilization and at rest in the group of patients having active bupivacaine USG IIN/IHN blocks administered prior to surgery (Bærentzen 2012). The pain scores were recorded when the patients arrived at the post anaesthesia care unit (PACU) and after 30 minutes stay. Pain at rest was similarly reduced in the active group at the time of discharge. Most importantly, patients with severe (NRS>5) and moderate (NRS>3) pain at mobilization and rest, respectively, were significantly reduced in the group of patients having the block (Bærentzen 2012). Thus, it would seem that the USG IIN/IHN block also has a place in the post- and perioperative pain management in adult patients.
The central portion of the anterior abdominal wall is innervated by the ventral branches of the thoracolumbar nerves (Th6-L1), and in the beginning of this chapter we have already mentioned the rectus sheath plexus and its anatomical position. The ventral branches lie deep to the RA muscle but ventral to the posterior rectus sheath. Since the tendinous inscriptions of the rectus muscle are not attached to the posterior RS the local anaesthetic administered into the spatial space can in theory spread both in the cranial and caudal direction. However, the RS block may have been over-shadowed by the various TAP block techniques, but the evidence base for its use is very good; i.e. a grade A recommendation for the use of US guidance for the RS block has been granted (Abrahams 2010). The RS block has been utilized to provide analgesia for midline incisions and laparoscopic procedures (Ferguson 1996, Abrahams 2010). RS blocks may also be effective in reducing postoperative pain in upper abdominal surgery as an alternative method to epidural analgesia in anti-coagulated patients (Osaka 2010).
However, we find that the RS block has a potential drawback, since the risk for inadvertent injections deep to the posterior RS (intra-peritoneal) seems to be higher that for the USG TAP blocks where the TA muscle lies deep to the point of injection (Dolan 2009). To our knowledge, no comparison between TAP blocks and RS blocks has yet been done. For the selective USG RS block, the patient is placed in a supine position, and a linear transducer (6-15 MHz) is placed with its medial end just above the linea alba (LA) (Fig. 4). The needle is inserted in-plane to the transducer in a medial to lateral direction, and the division between the belly of the rectus abdominis muscle and the posterior rectus sheath is visualized. The tip of the needle is placed in this space. The spread of the injectate should be observed to advance in a lateral direction.
The thoracoabdominal nerves Th6-Th11 are all intercostal nerves per se, before they become abdominal nerves when they leave the thoracic cage and contribute to the formation of the IC-TAP, CL-TAP and RS-TAP plexuses in the anterolateral abdominal wall (Rozen 2008). Thus the potential to provide efficient abdominal analgesia employing US guidance to block the intercostal nerves are obviously there if a multiple injection technique is used. In the past, the landmark-based (blind) technique has been employed to provide analgesia for various surgical procedures in the abdominal area; i.e. following renal transplantation, cholecystectomy and appendectomy (Knowles 1998, Vieira 2003, Bunting 1988). It would seem to be obvious that the USG ICN block (multiple injection technique) could very well be used postoperatively; i.e. either as an effective rescue block or because TAP blocks were not possible due to surgical incisions, tissue swelling etc. For the selective USG ICN block the patient is placed in the lateral decubitus position, and a linear transducer (6-15 MHz) is placed in a sagittal paravertebral plane (Fig. 5). The needle is inserted in-plane to the transducer in a cranial to caudal direction, and the three intercostal muscles (external, internal and innermost) are visualized between two costae. The tip of the needle is placed in the fascial plane between the internal and innermost intercostal muscles. The spread of the injectate should be observed to occur in this fascial plane. It is very important to visualize the tip of the needle at all times and its close proximity to the parietal pleura.
USG RS block. PC: peritoneal cavity, LA: linia alba.
USG ICN block: parasagittal plane. CO: costa, EX,IN,INM: external, internal, innermost intercostal muscles, PL: pleura.
The conventional technique of a TPVB involves inserting the needle perpendicular to all planes, making contact with the transverse process, and then walking off the bone with the needle until the physician feels the loss of resistance when penetrating deep to the internal intercostal membrane and entering the thoracic paravertebral space (TPVS). The TPVB has been used to provide pain relief for many surgical procedures in the abdominal area (Naja 2002, Moussa 2008, Ho 2004, Berta 2008). Conducting an USG TPVB or using the traditional landmark-based methods is the technique of injecting local anaesthetic adjacent to the thoracic vertebra close to the actual site where spinal nerves emerge from the intervertebral foramina. This results in ipsi-lateral somatic and sympathetic nerve blockade in multiple contiguous thoracic dermatomes above and below the site of injection (Karmakar 2001). How much dermatomal anaesthesia in the abdominal area results from specific volumes of local anaesthetic is to our knowledge still not fully elucidated, at least when it concerns the USG TPVBs. There is bound to be considerable individual variations as well. From previous studies it would seem that the point of injection within the TPVS must influence the distribution pattern of a paravertebral blockade. Apparently, injections made in the more ventral part of the TPVS, supposedly anterior to the endothoracic fascia, will result in a multisegmental longitudinal spreading pattern (evaluated by radiographic spreading patterns), whereas injections dorsal to the endothoracic fascia will result in a cloud-like spreading pattern, with only limited distribution over adjacent segments (Naja 2004). Whether US guidance can make the administration of local anaesthetic more beneficial remains to be evaluated in future studies. Even when using the USG technique, the endothoracic fascia is very difficult to visualize if at all possible with the current ultrasound machines. The TPVB is effective in treating pain of resulting from surgery in the chest and abdomen (Karmakar 2001). The potential advantage over the various TAP blocks could be that the visceral pain is more reliably blocked with the TPVB. Further, the potential failure rates and complications using the traditional techniques have already been brilliantly described previously (Lönnqvist 1995). Finally, insertion of catheters using the USG technique in the TPVS is indeed possible with at high success rate, thus making this technique a potential replacement of the epidural continuous infusion catheters (Renes 2010).
USG TPVB block. IIM: Internal intercostal membrane (continuous with the superior costotransverse ligament medially), PL: Pleura, TP: Transverse process
For the selective USG TPVB block the patient is placed in the lateral decubitus position, and a linear transducer (6-15 MHz) is placed parallel to and in-between two costae in an axial transverse plane (Fig. 6). It is important to visualize the pleura very clearly at all times. The transducer is then gradually aligned in a medial direction until the acoustic shadow of the transverse process is clearly visualized in the medial part of the sonographic image. The needle is inserted in-plane to the transducer in a lateral to medial direction until the tip of the needle is seen to penetrate the internal intercostal membrane. We do not recommend that the needle tip should be advanced under the acoustic shadow of the transverse process. Rather, we recommend that the spread of the injectate should be observed to occur above the pleura in the triangular space (which is the TPVS) and thus depressing the pleura and filling up the TPVS. It is very important to visualize the tip of the needle at all times and its close proximity to the parietal pleura.
As we are approaching the end of the description of the various USG truncal blocks we would like to introduce the reader to the “new kid on the block”. The so-called Blanco block (as it is known by some anaesthetists in the United Kingdom) is an USG block administered to the quadratus lumborum space first described by Professor R. Blanco in May 2007 during his presentation at ESRA 2007 at the XXVI Annual ESRA Congress in Valencia, Spain. Professor Blanco describes a potential space posterior to the abdominal wall muscles and lateral to the quadratus lumborum muscle. Thus, this new block has also been called the Quadratus lumborum (QL) block. It has been used in abdominoplasties, caesarean sections and lower abdominal operations since 2006 providing complete pain relief in the distribution area from Th6 to L1 dermatomes. Apparently, in operations with peritoneal involvement the morphine consumption was significantly reduced to less than 30% of the control groups. It is hard however, to find any solid scientific evidence to support these findings in the literature, and much of the knowledge of the QL block relies regrettably to this day on personal communication, which is certainly not the best of documentation. Much research effort at many centres is currently directed towards the description and qualification of this new block, and we have found it highly relevant to include the block in this chapter, since the block holds some very positive potential benefits. If may well be seen as a lumbar approach to the TPVS. The block apparently produces distribution of the local anaesthetic extending proximally and over both sides of the surface of the QL muscle, in between the anterior and intermediate layers of the thoracolumbar fascias. It also pushes the fascia transversalis and the perinephric fat towards the peritoneum without the risk of intrabdominal puncture. The block does not rely on the feeling of any pops or fascial clicks because depending of the angle of the needle several pops can be felt without reaching the target zone, which is lateral to the quadratus lumborum muscle. Actually, the block has never been intended to be conducted without the use of US guidance, and the block is thus a purely USG block. In an absolutely brilliant paper by Carney et al. the block is compared to other TAP block techniques using volunteers rather than patients (Carney 2011). Dr. Carney found that there was a non-contiguous paravertebral, epidural and lymphatic contrast enhancement Th5-Th10 in one subject, and similarly contrast at Th6-Th10 in two other subjects (Carney 2011). Carney et al. concluded that the posterior USG approach (as they have named the QL block in their recent publication) produced a more extensive, predictable and posterior spread of contrast, similar to that seen with their own landmark-based and blind approach at the triangle of Petit. The contrast extended postero-medially to the paravertebral region from the 5th thoracic vertebral level rostrally, to the first lumbar vertebral level caudally, indicating that this US guidance approach is the optimal site for injection to reproduce the analgesia of the blind landmark TAP block favoured by Dr. McDonnell and Dr. Carney. Fig. 7 depicts one method to administer the USG guided QL block as we have found it most easy to perform in our daily clinical practise (Jensen 2012). Again, the patient rests in a supine position. We have found it easier to conduct the block using a low frequency transducer (2-6 MHz) as compared to a linear transducer. The low frequency transducer is placed on the lateral abdomen above the iliac crest and below the thoracic cage. The transducer is then gradually aligned in a more posterior and lateral direction parallel to the inter-crista line. It is always possible to observe, that the TA muscle becomes aponeurotic, and this aponeurosis is followed until the QL muscle is clearly visualized. Thus, it is indeed possible to visualize the QL muscle lateral and posterior to the abdominal wall muscles. It is also clearly possible to visualize the thoracolumbar fascia at the lateral edge of the QL muscle. We have set this to be the point of injection of local anaesthetic. Following the injection we could observe the local anaesthetic spread along the ventral side of the QL muscle. Apparently this block results in a block that is longer lasting and more extensive than what we have previously observed with the BD-TAP block, but it remains to be further elucidated in RCT trials.
USG QL block. Arrows in upper left corner indicating needle shaft approaching in a medial to lateral-posterior direction towards the injection point (IP). PC: Peritoneal cavity, IP: Injection point, QL: Quadratus lumborum muscle, PNF: Perinephric fat.
Epidural analgesia is an effective method of anaesthetizing the sensory nerves to the abdominal wall. The impulse conduction of sensory roots protruding from the spinal cord is considerably reduced in a dermatomal fashion, optimally extending from the 4th to the 12th thoracic dermatome. Not all spinothalamic nerve transmission is reduced however, and some sensory input is perceived by the brain (Lund 1991). An element of habituation (so-called tachyphylaxis) is also present, in which the nerve roots require increasing amounts of local anaesthetic to maintain a sufficient nerve block over days. This inconvenient effect may be reduced by the addition of opioids, administered epidurally or systemically. Common side effects include a subsidiary block of the sympathetic trunk in the dermatomes anaesthetized, causing arteriole relaxation, reduced peripheral vascular resistance with hypotension and reflex tachycardia; motor block of the lower extremities when the epidural extends below thoracic dermatomes; and central nervous system side effects such as drowsiness, nausea or pruritus when opioids or other drugs are added to the local anaesthetic. The placement of an epidural catheter requires some experience, and the handling of an epidural catheter is particularly resource heavy for infusion maintenance and monitoring of potential side effects.
The benefits and risks of epidural analgesia have been extensively documented ever since its introduction almost 100 years ago (Thompson 1917). In the past ten years alone, more than two publications have appeared every single day on the subject, equally distributed between analgesic effects in major surgery, obstetric anaesthesia, side effects and technical aspects (Jensen 2012). Apart from subarachnoidal analgesia, which was first introduced in 1898 (Bier 1899), it is hard to imagine a method of analgesia having been similarly subjected to scientific study. In the case of spinal anaesthesia, the foundation for its use is still overwhelming by its significant reduction in perioperative morbidity and mortality compared to general anaesthesia (Rodgers 2000). Epidural analgesia is a well-established technique that is often regarded as the gold standard in postoperative pain management. However, newer and evidence-based outcome data show that its benefits are not as significant as previously believed, and that these benefits are probably limited to high-risk patients undergoing major abdominal surgery receiving epidural analgesia with local anaesthetic drugs only. There is increasing evidence that less invasive regional analgesic techniques are as effective as epidural analgesia, and while pain relief associated with epidurals can be outstanding, clinicians expect more from this invasive, high-cost, labour-intensive technique (Niraj 2009; Rawal 2012). A plethora of cardiovascular, neurologic and infectious side effects of epidural analgesia have been published, and given its modest success rate (at 70-80%) and the potential of motor block of the lower extremities, the scale is beginning to tip in the direction of USG truncal nerve blocks, in particular when the need for short-term analgesia of the abdominal wall is anticipated or when patients are not high-risk.
Invasive surgery induces a combination of local response to tissue injury and generalized activation of systemic metabolic and hormonal pathways via afferent nerve pathways and the central nervous system. The local inflammatory responses and the parallel neurohumoral pathways are linked through complex signalling networks. The magnitude of the response is related to the site of injury (greater in abdomen or thorax) and the extent of the trauma. The changes include alterations in metabolic, hormonal, inflammatory, and immune systems that collectively are termed the stress response. Integral to the stress response are the effects of nociceptive afferent stimuli on systemic and pulmonary vascular resistance, heart rate and blood pressure. Opioid doses required to provide analgesia are less than those required for haemodynamic stability in response to surgery, and these are in turn less than those required to suppress most aspects of the stress response. In contrast to this considerable dose dependency, neuraxial nerve blocks allow blockade of the afferent and efferent sympathetic pathways at relatively low doses resulting in profound suppression of hemodynamic and stress responses to surgery (Wolf 2012). Intraoperative stress may therefore suppress the adaptive immune system. Abolished pro-inflammatory lymphocyte function is associated with higher risk of infection and postoperative complications. During major abdominal surgery, plasma concentrations of epinephrine and cortisol are significantly lower in the epidural group compared to the non-epidural group. Lymphocyte numbers and T-helper cells are significantly higher in the epidural group on day one, whereas no significant differences may be detected among IL-2, HLA-DR, or the postoperative clinical course. Intraoperative use of a thoracic epidural catheter may therefore reduce stress response and prevent stress-induced perioperative impairment of pro-inflammatory lymphocyte function (Ahlers 2008). However, other studies find that epidural analgesia cannot suppress postoperative lymphocyte apoptosis, increases in cortisol, CRP or ESR compared with general anaesthesia, so the evidence is equivocal (Papadima 2009). Epidurals modify the electrical activity of the heart in addition to ventricular function and wall motion. Improvements in regional blood flow and a reduction of the major determinants of cardiac oxygen consumption lead to less severity of the ischemic injury. Although epidural analgesia negatively affects the performance of intercostal muscles, is spares diaphragmatic function and, when limited to the first five thoracic segments, affects pulmonary volumes to a lesser extent. Improved gastrointestinal blood flow and motility are clear in animals, and in clinical studies epidurals have been shown to improve recovery after major abdominal surgery. Liver perfusion increases with thoracic but not lumbar epidural analgesia after major abdominal surgery in most patients (Kortgen 2009). However, its use alone cannot prevent postoperative morbidity and mortality (Clemente 2008). Overall though, the evidence is strong that epidural analgesia is superior to systemic opioids after major abdominal surgery (www.postoppain.org). Postoperative pulmonary function, mobilization, food intake and general well-being are all increased (Catro-Alves 2011). Its benefit on postoperative analgesia is most evident in surgery involving high-risk surgery or high-risk patients (Siriussawakul 2010; van Lier 2011; Panaretou 2012).
Data generally indicate that the perioperative use of regional anaesthesia and analgesia may be associated with improvement in both major outcomes and rehabilitation. The majority of evidence favours an ability of epidural analgesia to reduce postoperative cardiovascular and pulmonary complications, and there is also consistent evidence that epidural analgesia with LA is associated with faster resolution of postoperative ileus after major abdominal surgery, compared to systemic opioids (Hanna 2009). But while there is evidence favouring epidural analgesia following major surgery in high-risk patients, controversy exists as to whether epidural analgesia also reduces the intensive care resources following major surgery. In a study where patients were followed after thoraco-abdominal oesophagectomy, higher calculated costs of epidural versus systemic pain treatment were outweighed by lower postoperative costs of intensive care, and the overall costs of postoperative care were in fact the same in the two groups (Bartha 2008). In a prospective but non-randomized study on pancreato-duodenectomy, patients receiving epidurals had, surprisingly, significantly higher rates of major complications (pancreatic fistulae, postoperative ileus), and more often required discharge to rehabilitation facilities. Also, 31% of epidural infusions were aborted before anticipated because of haemodynamic compromise or inadequate analgesia (Pratt 2008). Similar adverse events were observed in a study on liver resections; 20% epidurals failed, and patients with epidurals required more intravenous colloid than patients on systemic opioids (Revie 2011). Few individual clinical trials have had sufficient subject numbers to definitively determine the effects of postoperative analgesia on major outcomes. In two comprehensive systematic reviews, the majority of evidence favours an ability of epidural analgesia to reduce postoperative cardiovascular and pulmonary complications only after major vascular surgery or in high-risk patients. However, this finding may become irrelevant because of rapid conversion of major surgery to minimally invasive techniques that carry less risk of complications. There is also consistent evidence that epidural analgesia with local anaesthetics is associated with faster resolution of postoperative ileus after major abdominal surgery, but this finding may also become irrelevant with increasing use of laparoscopic and multimodal fast-track protocols (Liu 2007). No differences were found in mortality, length of stay in hospital, or other morbidity variables (Seller 2008). In yet another exhaustive meta-analysis comparing epidural versus systemic analgesia, the authors found that epidurals carried a reduced risk of pneumonia, independent of site of surgery, catheter insertion, duration of analgesia, or regimen. Epidural analgesia reduced the need for prolonged ventilation or re-intubation, improved lung function and blood oxygenation, but also increased the risk of hypotension, urinary retention, and pruritus. In addition, the beneficial effect on pulmonary function has in fact lessened considerably over the last 35 years because of an overall decrease in the baseline risk (Pöpping 2008). As for the risk of bladder paresis, urinary retention requiring catheterization carries the risk of infection and is generally a problem after abdominal surgery. In a recent meta-analysis, the authors found that the duration of detrusor dysfunction following neuraxial anaesthesia was correlated with LA dose and potency, and the incidence of urinary retention was increased by the presence of neuraxial opioids (Choi 2012).
There can be no doubt that the administration of continuous epidural analgesia following abdominal surgery has remained - to this day - the golden standard for the provision of post- and perioperative pain management following major abdominal surgery. Only few studies have compared the efficacy of TAP blocks to epidural analgesia. In a matched-control study comparing continuous TAP block catheters to thoracic epidurals, no differences in pain scores were seen over a 3-day follow-up period. Therapeutic failure rate was higher in the epidural group, and the incidence of hypotension was also greater (Kadam 2011). Niraj et al. are also amongst the few having compared the new techniques versus the older and more established techniques (Niraj 2011). Dr. Niraj compared the analgesic efficacy of the subcostal TAP block catheter technique (very much resembling the technique described by Dr. Hebbard) with the epidural analgesia for patients undergoing elective open hepatobiliary or renal surgery. The primary outcome measure was visual analogue pain scores during coughing at 8, 24, 48 and 72 hours after surgery, and they found no significant differences in median VAS during coughing. Tramadol consumption was, however, significantly greater in the TAP group. Very recently, one of the major pioneers of modern regional anaesthesiological practices has published a rather controversial special article (Rawal 2012). The conclusion of the paper is very direct: “It is therefore no exaggeration to suggest that the diminishing role of epidural analgesia can be expected to diminish further. Epidural analgesia remains the gold standard for pain relief in labour because there are currently no good alternatives.This can no longer be said of the use of the epidural analgesia after surgery, and it can therefore no longer be described as the gold standard in postoperative analgesia. The continued use of epidural techniques in your institution should be based on a careful evaluation of its risks and benefits drawn from local audit data, rather than on a tradition that is increasingly being viewed as outdated”.
Continuing along this venue of argumentation, a recent study on continuous wound installation after laparotomy found that this method was in fact the most cost-effective compared to epidural or systemic therapy (Tilleul 2012). A pre-peritoneal catheter reduced the demand for epidural analgesia after colonic surgery (Ozturk 2011). Continuous paravertebral nerve blocks provided excellent analgesia after major abdominal or retroperitoneal procedures (Burns 2008). Finally, continuous paravertebral nerve blocks provided better pain at rest and during coughing, less opioid consumption, superior pulmonary function, and were associated with less nausea and hypotension than epidural analgesia in patients undergoing thoracotomy (Davis 2006). Similar benefits of the paravertebral nerve blocks were observed in abdominal, pelvic and urological surgery (Bigler 1989, Burns 2008; Ben-Ari 2009). Although still tentative, these studies suggest that a diverse group of truncal blocks may at the very least be as effective as the epidural block. Some of the physiological effects and potential analgesic and side effects of these techniques are outlined in Table 4. Future research will no doubt be able to enhance our knowledge concerning direct comparison between the various techniques.
Parameter | Epidural block | Truncal block |
Physiology | ||
Lung volumes | Reduced | No change |
Postoperative pulmonary dysfunction | Reduced | ? |
Postoperative pneumonia | Probably reduced | ? |
Heart frequency | Increased | No change |
Blood pressure | Reduced | No change |
Coronary blood flow | Increased | ? |
Myocardial infarction risk | Slightly reduced | ? |
Splanchnic venous pooling | Increased | Probably no change |
Gastrointestinal circulation | Probably increased | ? |
Postoperative bowel function | Increased if LA alone | ? |
Urinary bladder paralysis | Yes | No |
Perioperative immune suppression | Probably reduced | ? |
Postoperative rehabilitation | ||
Postoperative pain | Reduced | Reduced |
Continuous analgesia | Yes | Yes, if intermittent boluses or catheter |
Prevention of chronic pain | No | ? |
Opioid demands | Reduced | Reduced |
Out-of-bed mobilization | Increased | Probably increased |
Side effects and logistics | ||
Block failure rate | 20-30% | 10-20% |
Lower extremity motor weakness | 5-15% | 0% |
Risk of systemic LA toxicity | Yes | Yes |
Risk of neurological damage | Yes | Probably not |
Usable during anticoagulation | No | Yes |
Pruritus | Yes, if opioids | No |
Comparison of the epidural and truncal blocks
In 1869, the first synthetic polymer was invented in response to a commercial $10,000 prize to provide a suitable replacement to ivory. A continuous string of discoveries and inventions contributed new polymers to meet the various requirements of society. Polymers are constructed of long chains of atoms, organized in repeating components or units often exceeding those found in nature. Plastic can refer to matter that is pliable and easily shaped. Recent usage finds it to be a name for materials called polymers. High molecular weight organic polymers derived from various hydrocarbon and petroleum materials are now referred to as plastics [1].
\nSynthetic polymers are constructed of long chains of smaller molecules connected by strong chemical bonds and arranged in repeating units which provide desirable properties. The chain length of the polymers and patterns of polymeric assembly provide properties such as strength, flexibility, and a lightweight feature that identify them as plastics. The properties have demonstrated the general utility of polymers and their manipulation for construction of a multitude of widely useful items leading to a world saturation and recognition of their unattractive properties too. A major trend of ever increasing consumption of plastics has been seen in the areas of industrial and domestic applications. Much of this polymer production is composed of plastic materials that are generally non-biodegradable. This widespread use of plastics raises a significant threat to the environment due to the lack of proper waste management and a until recently cavalier community behavior to maintain proper control of this waste stream. Response to these conditions has elicited an effort to devise innovative strategies for plastic waste management, invention of biodegradable polymers, and education to promote proper disposal. Technologies available for current polymer degradation strategies are chemical, thermal, photo, and biological techniques [2, 3, 4, 5, 6]. The physical properties displayed in Table 1 show little differences in density but remarkable differences in crystallinity and lifespan. Crystallinity has been shown to play a very directing role in certain biodegradation processes on select polymers.
\nPolymer | \nAbbreviation | \nDensity (23/4°C) | \nCrystallinity (%) | \nLifespan (year) | \n
---|---|---|---|---|
Polyethylene | \nPE | \n0.91–0.925 | \n50 | \n10–600 | \n
Polypropylene | \nPP | \n0.94–0.97 | \n50 | \n10–600 | \n
Polystyrene | \nPS | \n0.902–0.909 | \n0 | \n50–80 | \n
Polyethylene glycol terephthalate | \nPET | \n1.03–1.09 | \n0–50 | \n450 | \n
Polyvinyl chloride | \nPVC | \n1.35–1.45 | \n0 | \n50–100+ | \n
Selected features of major commercial thermoplastic polymers [7].
Polymers are generally carbon-based commercialized polymeric materials that have been found to have desirable physical and chemical properties in a wide range of applications. A recent assessment attests to the broad range of commercial materials that entered to global economy since 1950 as plastics. The mass production of virgin polymers has been assessed to be 8300 million metric tons for the period of 1950 through 2015 [8]. Globally consumed at a pace of some 311 million tons per year with 90% having a petroleum origin, plastic materials have become a major worldwide solid waste problem. Plastic composition of solid waste has increased for less than 1% in 1960 to greater than 10% in 2005 which was attributed largely to packaging. Packaging plastics are recycled in remarkably low quantities. Should current production and waste management trends continue, landfill plastic waste and that in the natural environment could exceed 12,000 Mt of plastic waste by 2050 [9].
\nA polymer is easily recognized as a valuable chemical made of many repeating units [10]. The basic repeating unit of a polymer is referred to as the “-mer” with “poly-mer” denoting a chemical composed of many repeating units. Polymers can be chemically synthesized in a variety of ways depending on the chemical characteristics of the monomers thus forming a desired product. Nature affords many examples of polymers which can be used directly or transformed to form materials required by society serving specific needs. The polymers of concern are generally composed of carbon and hydrogen with extension to oxygen, nitrogen and chlorine functionalities (see Figure 1 for examples). Chemical resistance, thermal and electrical insulation, strong and light-weight, and myriad applications where no alternative exists are polymer characteristics that continue to make polymers attractive. Significant polymer application can be found in the automotive, building and construction, and packaging industries [12].
\nStructures of major commercial thermoplastic polymers [11].
The environmental behavior of polymers can be only discerned through an understanding of the interaction between polymers and environment under ambient conditions. This interaction can be observed from surface properties changes that lead to new chemical functionality formation in the polymer matrix. New functional groups contribute to continued deterioration of the polymeric structure in conditions such as weathering. Discoloration and mechanical stiffness of the polymeric mass are often hallmarks of the degradative cycle in which heat, mechanical energy, radiation, and ozone are contributing factors [13].
\nPolyolefins (PO) are the front-runners of the global industrial polymer market where a broad range of commercial products contribute to our daily lives in the form o packaging, bottles, automobile parts and piping. The PO class family is comprised of saturated hydrocarbon polymers such as high-density polyethylene (HDPE), low-density polyethylene (LDPE) and linear low-density polyethylene (LLDPE), propylene and higher terminal olefins or monomer combinations as copolymers. The sources of these polymers are low-cost petrochemicals and natural gas with monomers production dependent on cracking or refining of petroleum. This class of polymers has a unique advantage derived from their basic composition of carbon and hydrogen in contrast to other available polymers such as polyurethanes, poly(vinyl chloride) and polyamides [14].
\nThe copolymers of ethylene and propylene are produced in quantities that exceed 40% of plastics produced per annum with no production leveling in sight. This continuous increase suggests that as material use broadens yearly, the amount of waste will also increase and present waste disposal problems. Polyolefin biological and chemical inertness continues to be recognized as an advantage. However, this remarkable stability found at many environmental conditions and the degradation resistance leads to environmental accumulation and an obvious increase to visible pollution and ancillary contributing problems. Desired environmental properties impact the polyolefin market on the production side as well as product recyclability [15].
\nBiodegradation utilizes the functions of microbial species to convert organic substrates (polymers) to small molecular weight fragments that can be further degraded to carbon dioxide and water [16, 17, 18, 19, 20, 21]. The physical and chemical properties of a polymer are important to biodegradation. Biodegradation efficiency achieved by the microorganisms is directly related to the key properties such as molecular weight and crystallinity of the polymers. Enzymes engaged in polymer degradation initially are outside the cell and are referred to as exo-enzymes having a wide reactivity ranging from oxidative to hydrolytic functionality. Their action on the polymer can be generally described as depolymerization. The exo-enzymes generally degrade complex polymer structure to smaller, simple units that can take in the microbial cell to complete the process of degradation.
\nPolymer degradation proceeds to form new products during the degradation path leading to mineralization which results in the formation of process end-products such as, e.g., CO2, H2O or CH4 [22]. Oxygen is the required terminal electron acceptor for the aerobic degradation process. Aerobic conditions lead to the formation of CO2 and H2O in addition to the cellular biomass of microorganisms during the degradation of the plastic forms. Where sulfidogenic conditions are found, polymer biodegradation leads to the formation of CO2 and H2O. Polymer degradation accomplished under anaerobic conditions produces organic acids, H2O, CO2, and CH4. Contrasting aerobic degradation with anaerobic conditions, the aerobic process is found to be more efficient. When considering energy production the anaerobic process produces less energy due to the absence of O2, serving the electron acceptor which is more efficient in comparison to CO2 and SO4−2 [23].
\nAs solid materials, plastics encounter the effects of biodegradation at the exposed surface. In the unweathered polymeric structure, the surface is affected by biodegradation whereas the inner part is generally unavailable to the effects of biodegradation. Weathering may mechanically affect the structural integrity of the plastic to permit intrusion of bacteria or fungal hyphae to initiate biodegradation at inner loci of the plastic. The rate of biodegradation is functionally dependent on the surface area of the plastic. As the microbial-colonized surface area increases, a faster biodegradation rate will be observed assuming all other environmental conditions to be equal [24].
\nMicroorganisms can break organic chemicals into simpler chemical forms through biochemical transformation. Polymer biodegradation is a process in which any change in the polymer structure occurs as a result of polymer properties alteration resulting from the transformative action of microbial enzymes, molecular weight reduction, and changes to mechanical strength and surface properties attributable to microbial action. The biodegradation reaction for a carbon-based polymer under aerobic conditions can be formulated as follows:
\n\n
Assimilation of the carbon comprising the polymer (Cpolymer) by microorganisms results in conversion to CO2 and H2O with production of more microbial biomass (Cbiomass). In turn, Cbiomass is mineralized across time by the microbial community or held in reserve as storage polymers [25].
\nThe following set of equations is a more complete description of the aerobic plastic biodegradation process:
\n\n
where Cpolymer and newly formed oligomers are converted into Cbiomass but Cbiomass converts to CO2 under a different kinetics scheme. The conversion to CO2 is referred to as microbial mineralization. Each oligomeric fragment is expected to proceed through of sequential steps in which the chemical and physical properties are altered leading to the desired benign result. A technology for monitoring aerobic biodegradation has been developed and optimized for small organic pollutants using oxygen respirometry where the pollutant degrades at a sufficiently rapid rate for respirometry to provide expected rates of biodegradation. When polymers are considered, a variety of analytical approaches relating to physical and chemical changes are employed such as differential scanning calorimetry, scanning electron microscopy, thermal gravimetric analysis, Fourier transform infrared spectrometry, gas chromatograph-mass spectrometry, and atomic force microscopy [26].
\nSince most polymer disposal occurs in our oxygen atmosphere, it is important to recognize that aerobic biodegradation will be our focus but environmental anaerobic conditions do exist that may be useful to polymer degradation. The distinction between aerobic and anaerobic degradation is quite important since it has been observed that anaerobic conditions support slower biodegradation kinetics. Anaerobic biodegradation can occur in the environment in a variety of situations. Burial of polymeric materials initiates a complex series of chemical and biological reactions. Oxygen entrained in the buried materials is initially depleted by aerobic bacteria. The following oxygen depleted conditions provide conditions for the initiation of anaerobic biodegradation. The buried strata are generally covered by 3-m-thick layers which prevent oxygen replenishment. The alternate electron acceptors such as nitrate, sulfate, or methanogenic conditions enable the initiation of anaerobic biodegradation. Any introduction of oxygen will halt an established anaerobic degradation process.
\nThis formulation for the aerobic biodegradation of polymers can be improved due to the complexity of the processes involved in polymer biodegradation [27]. Biodegradation, defined as a decomposition of substances by the action of microorganisms, leading to mineralization and the formation of new biomass is not conveniently summarized. A new analysis is necessary to assist the formulation of comparative protocols to estimate biodegradability. In this context, polymer biodegradation is defined as a complex process composed of the stages of biodeterioration, biofragmentation, and assimilation [28].
\nThe biological activity inferred in the term biodegradation is predominantly composed of, biological effects but within nature biotic and abiotic features act synergistically in the organic matter degradation process. Degradation modifying mechanical, physical and chemical properties of a material is generally referred to as deterioration. Abiotic and biotic effects combine to exert changes to these properties. This biological action occurs from the growth of microorganisms on the polymer surface or inside polymer material. Mechanical, chemical, and enzymatic means are exerted by microorganisms, thereby modifying the gross polymer material properties. Environmental conditions such as atmospheric pollutants, humidity, and weather strongly contribute to the overall process. The adsorbed pollutants can assist the material colonization by microbial species. A diverse collection of bacteria, protozoa, algae, and fungi are expected participants involved in biodeterioration. The development of different biota can increase biodeterioration by facilitating the production of simple molecules.
\nFragmentation is a material breaking phenomenon required to meet the constraints for the subsequent event called assimilation. Polymeric material has a high molecular weight which is restricted by its size in its transit across the cell wall or cytoplasmic membrane. Reduction of polymeric molecule size is indispensable to this process. Changes to molecular size can occur through the involvement of abiotic and biotic processes which are expected to reduce molecular weight and size. The utility of enzymes derived from the microbial biomass could provide the required molecular weight reductions. Mixtures of oligomers and/or monomers are the expected products of the biological fragmentation.
\nAssimilation describes the integration of atoms from fragments of polymeric materials inside microbial cells. The microorganisms benefit from the input of energy, electrons and elements (i.e., carbon, nitrogen, oxygen, phosphorus, sulfur and so forth) required for the cell growth. Assimilated substrates are expected to be derived from biodeterioration and biofragmentation effects. Non-assimilated materials, impermeable to cellular membranes, are subject to biotransformation reactions yielding products that may be assimilated. Molecules transported across the cell membrane can be oxidized through catabolic pathways for energy storage and structural cell elements. Assimilation supports microbial growth and reproduction as nutrient substrates (e.g., polymeric materials) are consumed from the environment.
\nThe polymer substrate properties are highly important to any colonization of the surface by either bacteria or fungi [29]. The topology of the surface may also be important to the colonization process. The polymer properties of molecular weight, shape, size and additives are each unique features which can limit biodegradability. The molecular weight of a polymer can be very limiting since the microbial colonization depends on surface features that enable the microorganisms to establish a locus from which to expand growth. Polymer crystallinity can play a strong role since it has been observed that microbial attachment to the polymer surface occurs and utilizes polymer material in amorphous sections of the polymer surface. Polymer additives are generally low molecular weight organic chemicals that can provide a starting point for microbial colonization due to their ease of biodegradation (Figure 2).
\nFactors controlling polymer biodegradation [30].
Weather is responsible for the deterioration of most exposed materials. Abiotic contributors to these conditions are moisture in its variety of forms, non-ionizing radiation, and atmospheric temperature. When combined with wind effects, pollution, and atmospheric gases, the overall process of deterioration can be quite formable. The ultraviolet (UV) component of the solar spectrum contributes ionizing radiation which plays a significant role in initiating weathering effects. Visible and near-infrared radiation can also contribute to the weathering process. Other factors couple with solar radiation synergistically to significantly influence the weathering processes. The quality and quantity of solar radiation, geographic location changes, time of day and year, and climatological conditions contribute to the overall effects. Effects of ozone and atmospheric pollutants are also important since each can interact with atmospheric radiation to result in mechanical stress such as stiffening and cracking. Moisture when combined with temperature effects can assist microbial colonization. The biotic contributors can strongly assist the colonization by providing the necessary nutrients for microbial growth. Hydrophilic surfaces may provide a more suitable place for colonization to ensue. Readily available exoenzymes from the colonized area can initiate the degradation process.
\nCommunities of microorganisms attached to a surface are referred to as biofilms [31]. The microorganisms forming a biofilm undergo remarkable changes during the transition from planktonic (free-swimming) biota to components of a complex, surface-attached community (Figure 3). The process is quite simple with planktonic microorganism encountering a surface where some adsorb followed by surface release to final attachment by the secretion of exopolysaccharides which act as an adhesive for the growing biofilm [33]. New phenotypic characteristics are exhibited by the bacteria of a biofilm in response to environmental signals. Initial cell-polymer surface interactions, biofilm maturation, and the return to planktonic mode of growth have regulatory circuits and genetic elements controlling these diverse functions. Studies have been conducted to explore the genetic basis of biofilm development with the development of new insights. Compositionally, these films have been found to be a single microbial species or multiple microbial species with attachment to a range of biotic and abiotic surfaces [34, 35]. Mixed-species biofilms are generally encountered in most environments. Under the proper nutrient and carbon substrate supply, biofilms can grow to massive sizes. With growth, the biofilm can achieve large film structures that may be sensitive to physical forces such as agitation. Under such energy regimes, the biofilm can detach. An example of biofilm attachment and utility can be found in the waste water treatment sector where large polypropylene disks are rotated through industrial or agriculture waste water and then exposed to the atmosphere to treat pollutants through the intermediacy of cultured biofilms attached to the rotating polypropylene disk.
\nMicrobial attachment processes to a polymer surface [32].
Biofilm formation and activity to polymer biodegradation are complex and dynamic [36]. The physical attachment offers a unique scenario for the attached microorganism and its participation in the biodegradation. After attachment as a biofilm component, individual microorganisms can excrete exoenzymes which can provide a range of functions. Due to the mixed-species composition found in most environments, a broad spectrum of enzymatic activity is generally possible with wide functionalities. Biofilm formation can be assisted by the presence of pollutant chemical available at the polymer surface. The converse is also possible where surfaces contaminated with certain chemicals can prohibit biofilm formation. Biofilms continue to grow with the input of fresh nutrients, but when nutrients are deprived, the films will detach from the surface and return to a planktonic mode of growth. Overall hydrophobicity of the polymer surface and the surface charge of a bacterium may provide a reasonable prediction of surfaces to which a microorganism might colonize [37]. These initial cell-surface and cell-cell interactions are very useful to biofilm formation but incomplete (Figure 4). Microbial surfaces are heterogeneous, and can change widely in response to environmental changes. Five stages of biofilm development: have been identified as (1) initial attachment, (2) irreversible attachment, (3) maturation I, (4) maturation II, and (5) dispersion. Further research is required to provide the understanding of microbial components involved in biofilm development and regulation of their production to assemble to various facets of this complex microbial phenomenon [38].
\nBiofilm formation and processes [34].
The activities envisioned in this scenario (depicted in Figure 4) are the reversible adsorption of bacteria occurring at the later time scale, irreversible attachment of bacteria occurring at the second-minute time scale, growth and division of bacteria in hours-days, exopolymer production and biofilm formation in hours-days, and attachment and other organisms to biofilm in days-months.
\nThe evaluation of the extent of polymer biodegradation is made difficult by the dependence on polymer surface and the departure of degradation kinetics from the techniques available for small pollutant molecule techniques [39]. For applications for polymer biodegradation a variety of techniques have been applied. Visual observations, weight loss measurements, molar mass and mechanical properties, carbon dioxide evolution and/or oxygen consumption, radiolabeling, clear-zone formation, enzymatic degradation, and compost test under controlled conditions have been cited for their utility [27]. The testing regime must be explicitly described within a protocol of steps that can be collected for various polymers and compared on an equal basis. National and international efforts have developed such protocols to enable the desired comparisons using rigorous data collecting techniques and interpretation [40].
\nThe conventional polymers such as (PE), (PP), (PS), (PUR), and (PET) are recognized for their persistence in the environment [41]. Each of these polymers is subject to very slow fragmentation to form small particles in a process expected to require centuries of exposure to photo-, physical, and biological degradation processes. Until recently, the commercial polymers were not expected to biodegrade. The current perspective supports polymer biodegradation with hopeful expectation that these newly encountered biodegradation processes can be transformed into technologies capable of providing major assistance to the ongoing task of waste polymer management.
\nThe polyolefins such as polyethylene (PE) have been recognized as a polymer remarkably resistant to degradation [42]. Products made with PE are very diverse and a testament to its chemical and biological inertness. The biodegradation of the polyolefins is complex and incompletely understood. Pure strains elicited from the environment have been used to investigate metabolic pathways or to gain a better understanding of the effect that environmental conditions have on polyolefin degradation. This strategy ignores the importance of different microbial species that could participate in a cooperative process. Treatment of the complex environments associated with polymeric solid waste could be difficult with information based on pure strain analysis. Mixed and complex microbial communities have been used and encountered in different bioremediation environments [43].
\nA variety of common PE types, low-density PE (LDPE), high-density PE (HDPE), linear low-density PE (LLDPE) and cross-linked PE (XLPE), differ in their density, degree of branching and availability of functional groups at the surface. The type of polymer used as the substrate can strongly influence the microbial community structure colonizing PE surface. A significant number of microbial strains have been identified for the deterioration caused by their interaction with the polymer surface [44]. Microorganisms have been categorized for their involvement in PE colonization and biodegradation or the combination. Some research studies did not conduct all the tests required to verify PE biodegradation. A more inclusive approach to assessing community composition, including the non-culturable fraction of microorganisms invisible by traditional microbiology methods is required in future assessments. The diversity of microorganisms capable of degrading PE extends beyond 17 genera of bacteria and nine genera of fungi [45]. These numbers are expected to increase with the use of more sensitive isolation and characterization techniques using rDNA sequencing. Polymer additives can affect the kinds of microorganisms colonizing the surfaces of these polymers. The ability of microorganisms to colonize the PE surfaces exhibits a variety of effects on polymer properties. Seven different characteristics have been identified and are used to monitor the extent of polymer surface change resulting from biodegradation of the polymer. The characteristics are hydrophobicity/hydrophilicity, crystallinity, surface topography, functional groups on the surface, mechanical properties, and molecular weight distribution. The use of surfactants has become important to PE biodegradation. Complete solubilization of PE in water by a Pseudomonas fluorescens treated for a month followed by biosurfactant treatment for a subsequent month in the second month and finally a 10% sodium dodecyl sulfate treatment at 60°C for a third month led to complete polymer degradation. A combination of P. fluorescens, surfactant and biosurfactant treatments as a single treatment significantly exhibited polymer oxidation and biodegradation [46]. The metabolically diverse genus Pseudomonas has been investigated for its capabilities to degrade and metabolize synthetic plastics. Pseudomonas species found in environmental matrices have been identified to degrade a variety of polymers including PE, and PP [47]. The unique capabilities of Pseudomonas species related to degradation and metabolism of synthetic polymers requires a focus on: the interactions controlling cell surface attachment of biofilms to polymer surfaces, extracellular polymer oxidation and/or hydrolytic enzyme activity, metabolic pathways mediating polymer uptake and degradation of polymer fragments within the microbial cell through catabolism, and the importance of development of the implementation of enhancing factors such as pretreatments, microbial consortia and nutrient availability while minimizing the effects of constraining factors such as alternative carbon sources and inhibitory by-products. In an ancillary study, thermophilic consortia of Brevibacillus sps. and Aneurinibacillus sp. from waste management landfills and sewage treatment plants exhibited enhanced PE and PP degradation [48].
\nThe larval stage of two waxworm species, Galleria mellonella and Plodia interpunctella, has been observed to degrade LDPE without pretreatment [49, 50]. The worms could macerate PE as thin film shopping bags and metabolize the film to ethylene glycol which in turn biodegrades rapidly. The remarkable ability to digest a polymer considered non-edible may parallel the worm’s ability utilize beeswax as a food source. From the guts of Plodia interpunctella waxworms two strains of bacteria, Enterobacter asburiae YP1 and Bacillus sp. YP1, were isolated and found to degrade PE in laboratory conditions. The two strains of bacteria were shown to reduce the polymer film hydrophobicity during a 28-day incubation. Changes to the film surface as cavities and pits were observed using scanning electron microscopy and atomic-force microscopy. Simple contact of ~100 Galleria mellonella worms with a commercial PE shopping bag for 12 hours resulted in a mass loss of 92 mg. The waxworm research has been scrutinized and found to be lacking the necessary information to support the claims of the original Galleria mellonella report [51].
\nPolypropylene (PP) is very similar to PE, in solution behavior and electrical properties. Mechanical properties and thermal resistance are improved with the addition of the methyl group but chemical resistance decreases. There are three forms of propylene selectively formed from the monomer isotactic, syndiotactic, and atactic due to the different geometric relationships achievable through polymerization technology. PP properties are strongly directed by tacticity or the methyl group orientation as related the methyl groups in neighboring monomer units. Isotactic PP has a greater degree of crystallinity than atactic and syndiotactic PP and therefore more difficult to biodegrade. The high molar mass of PP prohibits permeation through the microbial cell membrane which thwarts metabolism by living organisms. It is generally recognized that abiotic degradation provides a foothold for microorganisms to form a biofilm. With partial destruction of the polymer surface by abiotic effects the microbes can then start breaking the damaged polymer chains [52].
\nPS is a sturdy thermoplastic commonly used in short-lifetime items that contribute broadly to the mass of poorly controlled polymers [53]. Various forms of PS such as general purpose (GPPS)/oriented polystyrene (OPS), polystyrene foam, and expanded polystyrene (EPS) foam are available for different commercial leading to a broad solid waste composition. PS has been thought to be non-biodegradable. The rate of biodegradation encountered in the environment is very slow leading to prolonged persistence as solid waste. In the past, PS was recycled through mechanical, chemical, and thermal technologies yielding gaseous and liquid daughter products [54]. A rather large collection of studies has shown that PS is subject to biodegradation but at a very slow rate in the environment. A sheet of PS buried for 32 years. in soil showed no indication of biotic or abiotic degradation [55]. The hydrophobicity of the polymer surface, a function of molecular structure and composition, detracts from the effectiveness of microbial attachment [56, 57]. The general lack of water solubility of PS prohibits the transport into microbial cells for metabolism.
\nA narrow range of microorganisms have been elicited for the environment and found to degrade PS [53]. Bacillus and Pseudomonas strains isolated from soil samples have been shown to degrade brominated high impact PS. The activity was seen in weight loss and surface changes to the PS film. Soil invertebrates such as the larvae of the mealworm (Tenebrio molitor Linnaeus) have been shown to chew and eat Styrofoam [57]. Samples of the larvae were fed Styrofoam as the sole diet for 30 days and compared with worms fed a conventional diet. The worms feeding Styrofoam survived for 1 month after which they stopped eating as they entered the pupae stage and emerged as adults after a subsequent 2 weeks. It appears that Styrofoam feeding did not lead to any lethality for the mealworms. The ingested PS mass was efficiently depolymerized within the larval gut during the retention time of 24 hours and converted to CO2 [51]. This remarkable behavior by the mealworm can be considered the action of an efficient bioreactor. The mealworm can provide all the necessary components for PS treatment starting with chewing, ingesting, mixing, reacting with gut contents, and microbial degradation by gut microbial consortia. A PS-degrading bacterial strain Exiguobacterium sp. strain YT2 was isolated from the gut of mealworms and found to degrade PS films outside the mealworm gut. Superworms (Zophobas morio) were found to exhibit similar activity toward Styrofoam. Brominated high impact polystyrene (blend of polystyrene and polybutadiene) has been found to be degraded by Pseudomonas and Bacillus strains [58]. In a complementary study, four non-pathogenic cultures (Enterobacter sp., Citrobacter sedlakii, Alcaligenes sp. and Brevundimonas diminuta) were isolated from partially degraded polymer samples from a rural market setting and each were found to degrade high impact polystyrene [59].
\nPVC is manufactured in two forms rigid and flexible. The rigid form can be found in the construction industry as pipe or in structural applications. The soft and flexible form can be made through the incorporation of plasticizers such as phthalates. Credit cards, bottles, and non-food packaging are notable products with a PVC composition. PVC has been known from its inception as a polymer with remarkable resistance to degradation [60]. Thermal and photodegradation processes are widely recognized for their role in the weathering processes found with PVC [61, 62]. The recalcitrant feature of polyvinyl chloride resistance to biodegradation becomes a matter of environmental concern across the all processes extending from manufacturing to waste disposal. Few reports are available relating the extent of PVC biodegradation. Early studies investigated the biodegradation of low-molecular weight PVC by white rot fungi [63]. Plasticized PVC was found to be degraded by fungi such as As. fumigatus, Phanerochaete chrysosporium, Lentinus tigrinus, As. niger, and Aspergillus sydowii [64].
\nModifying the PVC film composition with adjuvants such as cellulose and starch provided a substrate that fungi could also degrade [65]. Several investigations of soil bacteria for the ability to degrade PVC from enrichment cultures were conducted on different locations [66]. Mixed cultures containing bacteria and fungi were isolated and found to grow on plasticized PVC [67]. Significant differences were observed for the colonization by the various components of the mixed isolates during very long exposure times [68]. Significant drift in isolate activity was averted through the use of talc. Consortia composed of a combination of different bacterial strains of Pseudomonas otitidis, Bacillus cereus, and Acanthopleurobacter pedis have the ability to degrade PVC in the environment [64]. These results offer the opportunity to optimization conditions for consortia growth in PVC and use as a treatment technology to degrade large collections of PVC. PVC film blends were shown to degrade by partnering biodegradable polymers with PVC [69].
\nPUR encompass a broad field of polymer synthesis where a di- or polyisocyanate is chemically linked through carbamate (urethane) formation. These thermosetting and thermoplastic polymers have been utilized to form microcellular foams, high performance adhesives, synthetic fibers, surface coatings, and automobile parts along with a myriad of other applications. The carbamate linkage can be severed by chemical and biological processes [70].
\nAromatic esters and the extent of the crystalline fraction of the polymer have been identified as important factors affecting the biodegradation of PUR [71, 72]. Acid and base hydrolysis strategies can sever the carbamate bond of the polymer. Microbial ureases, esterases and proteases can enable the hydrolysis the carbamate and ester bonds of a PUR polymer [71, 73, 74]. Bacteria have been found to be good sources for enzymes capable of degrading PUR polymers [75, 76, 77, 78, 79, 80, 81, 82]. Fungi are also quite capable of degrading PUR polymers [83, 84, 85]. Each of the enzyme systems has their preferential targets: ureases attack the urea linkages [86, 87, 88] with esterases and proteases hydrolyzing the ester bonds of the polyester PUR as a major mechanism for its enzymatic depolymerization [89, 90, 91, 92]. PUR polymers appear to be more amenable to enzymatic depolymerization or degradation but further searches and inquiry into hitherto unrecognized microbial PUR degrading activities is expected to offer significant PUR degrading activities.
\nPET is a polyester commonly marketed as a thermoplastic polymer resin finding use as synthetic fibers in clothing and carpeting, food and liquid containers, manufactured objects made through thermoforming, and engineering resins with glass fiber. Composed of terephthalic acid and ethylene glycol through the formation of ester bonds, PET has found a substantial role in packaging materials, beverage bottles and the textile industry. Characterized as a recalcitrant polymer of remarkable durability, the polymer’s properties are reflective of its aromatic units in its backbone and a limited polymer chain mobility [91]. In many of its commercial forms, PET is semicrystalline having crystalline and amorphous phases which has a major effect on PET biodegradability. The environmental accumulation of PET is a testament of its versatility and the apparent lack of chemical/physical mechanisms capable of attacking its structural integrity show it to be a major environmental pollution problem.
\nThe durability and the resulting low biodegradability of PET are due to the presence of repeating aromatic terephthalate units in its backbone and the corresponding limited mobility of the polymer chains [92]. The semicrystalline PET polymer also contains both amorphous and crystalline fractions with a strong effect on its biodegradability. Crystallinity exceeding 30% in PET beverage bottles and fibers having even higher crystalline compositions presents major hurdles to enzyme-induced degradation [93, 94]. At higher temperatures, the amorphous fraction of PET becomes more flexible and available to enzymatic degradation [95, 96]. The hydrolysis of PET by enzymes has been identified as a surface erosion process [97, 98, 99, 100]. The hydrophobic surface significantly limits biodegradation due to the limited ability for microbial attachment. The hydrophobic nature of PET poses a significant barrier to microbial colonization of the polymer surface thus attenuating effective adsorption and access by hydrolytic enzymes to accomplish the polymer degradation [101].
\nA wide array of hydrolytic enzymes including hydrolases, lipases, esterases, and cutinases has been shown to have the ability to hydrolyze amorphous PET polymers and modify PET film surfaces. Microbes from a vast collection of waste sites and dumping situations have been studied for their ability to degrade PET. A subunit of PET, diethylene glycol phthalate has been found to be a source of carbon and energy necessary to the sustenance of microbial life. Enzyme modification may be effectively employed to improve the efficiency and specificity of the polyester degrading enzymes acknowledged to be active degraders of PET [102]. Significant efforts have been extended to developing an understanding of the enzymatic activity of high-performing candidate enzymes through selection processes, mechanistic probes, and enzyme engineering. In addition to hydrolytic enzymes already identified, enzymes found in thermophilic anaerobic sludge were found to degrade PET copolymers formed into beverage bottles [103].
\nRecently, the discovery of microbial activity capable of complete degradation of widely used beverage bottle plastic expands the range of technology options available for PET treatment. A microorganism isolated from the area adjacent to a plastic bottle-recycling facility was shown to aerobically degrade PET to small molecular daughter products and eventually to CO2 and H2O. This new research shows that a newly isolated microbial species, Ideonella sakaiensis 201-F6, degrades PET through hydrolytic transformations by the action of two enzymes, which are extracellular and intracellular hydrolases. A primary hydrolysis reaction intermediate, mono (hydroxy-2-ethyl) terephthalate is formed and can be subsequently degraded to ethylene glycol and terephthalic acid which can be utilized by the microorganism for growth [104, 105, 106, 107, 108, 109].
\nThis discovery could be a candidate as a single vessel system that could competently accomplish PET hydrolysis as an enzyme reactor. This may be the beginning of viable technology development applicable to the solution of the global plastic problem recognized for its terrestrial component as well as the water contamination problem found in the sea. These remarkable discoveries offer a new perspective on the recalcitrant nature of PET and how future environmental management of PET waste may be conducted using the power of enzymes. The recognition of current limiting steps in the biological depolymerization of PET are expected to enable the design of a enzymes-based process to reutilized the natural assets contained in scrap PET [110] (Figure 5).
\nMicrobial depolymerization of poly(ethylene terephthalate).
The major commercial polymers have been shown to be biodegradable in a variety of circumstances despite a strong predisposition suggesting that many of these polymers were recalcitrant to the effects of biodegradation. The question of whether bioremediation can play a significant role in the necessary management of polymer waste remains to be determined. Treatment technology for massive waste polymer treatment must be sufficiently robust to be reliable at large scale use and adaptable to conditions throughout the environment where this treatment is required. The status of information relating to the application of biodegradation treatment to existing and future polymer solid waste is at early stages of development for several waste polymers. The discovery of that invertebrate species (insect larvae) can reduce the size of the waste polymer by ingesting and degradation in the gut via enzymes which aid or complete degradation is rather amazing and requires additional scrutiny. There is an outside change that a polymer recycling technology based on these findings is a future possibility.
\nThe views expressed in this book chapter are those of the author and do not necessarily represent the views or policies of the U.S. Environmental Protection Agency.
\nNo “conflict of interest” is known or expected.
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