Open access peer-reviewed chapter
Abstract
The epidural space is present above the dura also called as extradural space. This space contains spinal nerve roots and other contents with Batson’s venous plexus. The lumbar epidural space is more than atmospheric pressure. Hence, one of the hypothesis for loss of resistance (LOR) during epidural is the loss of pressure exerted by dense ligamentum flavum. There are many methods to find the loss of resistance (LOR) technique. Two most common methods followed are loss of air technique and loss of saline technique. The recent advances speak about epidural waveform analysis for correct position of epidural catheter which is helpful in labor analgesia.
Keywords
- epidural
- loss of resistance
- saline
- air
- methods
- pressure
1. Introduction
The epidural needle (also known as extradural space or peridural space) pierces the skin, subcutaneous tissue, supraspinous ligament, interspinous ligament, ligamentum flavum, and finally epidural space (EDS). The failure rate of lumbar epidural is 27% and thoracic epidural is 32% [1].
2. History of origin of loss of resistance to epidural space
In 1921, Sicard and Forestier described loss of resistance (LOR) using lipiodol. Lipiodol is a chemical compound with 40% iodine with vegetable oil or poppy seed oil [2].
In 1928, Heldt and Moloney attempted to check the epidural pressure with spinal puncture needle with stopcock. They attached the manometer to the stopcock to measure the pressure. A negative pressure of −1 to −18 mm of mercury was recorded by the manometer once the needle was pushed deeper to the ligamentum flavum [3].
3. Different methods available for the loss of resistance
Various methods are described for epidural loss of resistance.
3.1 Classification based on type of sensation appreciated
Three categories of LOR have been described as:
tactile end point (loss of resistance),
visual end point (negative pressure recognition), and
auditory end point (acoustic fall in tonal pitch) [4].
The other classification of various methods to identify EDS (epidural space).
3.2 Based on level or position of needle and epidural space
Three categories are described:
Needle before entering the epidural space or guiding the needle to EDS,
Needle during entry into epidural space or identifying entry into the EDS, and
Post needle entry into epidural space or confirming needle/catheter location in the EDS [5].
Guiding the needle to EDS.
Ultrasound-guided techniques
Needle tracking methods
Guidance positioning system for regional anesthesia (SonixGPS)
Real-time 3D/4D ultrasonography
Ultrasound imaging with pre-acquired three-dimensional images of spine
Ultrasound through needle
Needle through ultrasound
Machine vision
Acoustic radiation force impulse (ARFI) imaging
Fluoroscopy
Identifying entry into epidural space (modifications of the loss of resistance technique).
Membrane in syringe technique
Epidural balloon
Epidrum
Episure Autodetect
Auditory and visual display of pressure wave
Bioimpedance
Optical reflectance spectroscopy
Optical coherence tomography (OCT)
Confirm catheter location in epidural space.
Epidural stimulation test
Electrocardiography (ECG)-guided system
Epidurography
Epidural pressure waveform analysis
Near-infrared tracking system
Ultrasound
3.3 The detection of epidural space can also be classified as two types: Subjective and objective types
The subjective type can be subdivided into two types—human-based subjective type and equipment-based subjective type.
The objective type can be subdivided into two types—equipment-based objective type and ultrasound-based objective type.
3.3.1 Human-based subjective type epidural space detection
Loss of air
Loss of fluid
Saline with air bubble
Lidocaine
KSMM (the combined plunger pressure—manometer method)
BiP (Bidigital Pressure) test
Use of hanging drop
Cerebral spinal fluid (CSF) ceased to drip
Sterile water injection
Loss of guidewire resistance (LOGR, epidural space finder)
Membrane in syringe technique
Epidural Queckenstedt test
Cold test
Hissing sound
Second needle method
3.3.2 Equipment-based subjective-type epidural space detection
Acoustic puncture assist device (APAD)
LOR syringe with fluoroscopy
EpiFaith syringe
Episure autodetect syringe
Epidrum
Epidural balloon
Electrocardiography (ECG)-guided system
Drip infusion method
3.3.3 Equipment-based objective-type epidural space detection
Epidural waveform analysis
CompuFlo
Epidural stimulation test
Epidurography
Near-infrared tracking system
Epiduroscope
3.3.4 Ultrasound-based objective-type epidural space detection
Ultrasonography.
Preprocedural ultrasound scanning
Real-time ultrasound guidance
Guidance positioning system for regional anesthesia (SonixGPS)
Three- and four-dimensional real-time ultrasound
Preprocedure 3D high-resolution images
Machine vision
Acoustic radiation force impulse (ARFI)
Doppler method.
4. Human-based subjective-type epidural space detection
4.1 Loss of air
It is the oldest and most common method followed for the loss of resistance technique to detect the epidural space [6]. This technique is done by filling air into syringe and connected to epidural needle to detect the loss of resistance.
Some complications are observed by the use of loss of resistance to air. They are pneumocephalus, air embolism, insufficient analgesia, delayed onset, higher incidence of dural puncture, nerve root compression, and subcutaneous emphysema [7].
4.2 Loss of fluid
Dogliotti described the loss of fluid technique in epidural space. He described that while advancing the needle by exerting the pressure on the piston of the syringe. Once this needle enters the epidural space, there is the sensation of needle slipping, simultaneous disappearance of resistance to injection, and the pressure felt on the piston of the syringe will disappear. He with his team also described about polyethylene catheter insertion to epidural space [8].
4.3 Saline with air bubble
Odom devised this method by attaching the capillary glass tube with air bubble and fluid. He modified the capillary method of saline with air bubble of 2 ml. He said that the movement of this air bubble to the needle indicates epidural space, and the movement of the air bubble away from the needle indicates subarachnoid space (dural tap) [9].
The fluid LOR does not provide the same feel as air for appreciating the LOR.
4.4 Lidocaine
2–3 ml test dose of lignocaine 1–2% confirms the epidural space. If the lidocaine is administered to subarachnoid space, there is be sudden onset of weakness in bilateral lower limbs, and it does not occur when it is administered into epidural space. This property is used as test dose to confirm the epidural space.
4.5 KSMM (The combined plunger pressure-manometer method)
It is a satisfactory alternative for the loss of resistance technique [10].
4.6 BiP test (Bidigital pressure test)
It is a simple procedure where pressures of two fingers are used to feel the LOR for epidural [11]. It is described that pressures of two fingers are adequate to feel the epidural space.
4.7 Use of hanging drop
Alberto Gutierrez discovered the hanging drop technique in February 1933. He was searching for epidural space with LOR technique with fluid. Due to stiff resistance, he removed LOR syringe and noted a small drop of liquid hanging at the tip of the needle. As he advanced the needle, the drop suddenly disappeared or moved inside the epidural space due to negative pressure of the epidural space [12].
4.8 CSF (Cerebral spinal fluid) ceased to drip
A method described by Sebrechts describes to do a dural tap in the beginning by inserting the needle into the subarachnoid space. Later, the needle is withdrawn until CSF ceased to drip to locate the epidural space [6].
4.9 Sterile water injection
Lund used 5 cc syringe filled with distilled water for epidural space identification. The distilled water causes burning pain if the patient is awake, and in asleep patient it causes slight movement [13]. It is due to irritation caused by sterile water in epidural space.
4.10 Loss of guidewire resistance (LOGR, epidural space finder)
The guidewire is used instead of air in the LOR technique. When the needle goes through the ligamentum flavum, there was constant rotation of dials on both sides leading to resistance of guidewire. Once the needle entered the epidural space, the dial lost the resistance and the guidewire moved a few centimeters into the epidural space [14].
4.11 Membrane in syringe technique
In this technique, a plastic membrane is placed in the middle of the syringe, dividing it into two compartments. The distal compartment of syringe has nozzle and is filled with saline. The proximal part of the syringe has plunger and is filled with air. Once this enters, the epidural space wrinkling of the plastic membrane is seen which can be appreciated by the loss of air. This technique also avoids injection of air into the epidural space. Even sometimes due to no appreciation of loss of resistance, the epidural space is identified by visible wrinkling of the membrane [15].
4.12 Epidural Queckenstedt test
Compression of bilateral internal jugular veins lead to increased subarachnoid pressures and increased epidural pressure which is used to confirm epidural puncture in this test [16].
4.13 Cold test
The injection of local anesthetic into epidural space gives a cold sensation in the back, and it is called as the cold test [17].
4.14 Hissing sound
In loss of resistance to air technique, once the needle enters the epidural space the air rushes into the epidural space. This is detectable on a stethoscope attached to epidural needle and audible as hissing sound [18].
4.15 Second needle method
After first needle LOR is obtained, the second needle LOR is obtained in the adjacent intervertebral space. If both these needles are in epidural space, 5 ml of rapid injection of normal saline from one needle will lead to fluid leakage from the other needle called the second needle method [19].
Current scientific evidence:
False-positive LORs can happen in 17% of the patients. Saline with air bubble loss of resistance technique is preferred by most users.
Studies are done to compare other syringes with epidural loss of resistance syringe, and it has been found to be 66–81% similar [20]. Hence, it is always better to use epidural loss of resistance syringe.
Three different techniques were used for LOR assessment: incremental needle advancement with intermittent LOR assessment, continuous needle advancement with high-frequency intermittent LOR assessment, and continuous needle advancement with continuous LOR assessment. Postdural puncture headache is due to increased needle tip overshoot distance after LOR. Needle tip overshoot is defined as the distance traveled by the needle tip beyond the point where LOR can be first obtained. Of all the three techniques, incremental needle advancement with intermittent LOR assessment resulted in deepest needle tip overshoot [21]. The needle tip overshoot was less with continuous needle advancement with continuous loss of resistance assessment.
Any drug or air pushed through the epidural space has a risk to enter the circulation and cause unwanted systemic effects. Hence, anesthesiologists should be always cautious of the quantity of air pushed in epidural space and its placement [22].
Limitations and disadvantages of some popular techniques
There has been some explanation by MRI studies for false loss of resistance. Supraspinous ligament and interspinous ligament in lumbar region are biconcave axially. These gaps lead to deposition of anterior fat giving false loss of resistance [23].
The other causes of false-positive loss of resistance are as follows:
Age-related cysts in interspinous ligaments seen in higher number of patients above 60 years;
Paravertebral muscles give a false LOR;
Frequent midline gaps hamper the gap between ligamentum flavum and epidural space;
Intermuscular planes between the muscles.
5. Equipment-based subjective-type epidural space detection
These methods use equipments to detect the loss of resistance. It is not definitive that the loss of resistance is in epidural space. It is therefore highly subjective in the detection of epidural space.
5.1 Acoustic puncture assist device (APAD)
It documents pressure throughout the procedure of epidural. It provides real-time auditory and visual displays of pressure waveforms. It simultaneously uses three senses: hear (auditory signal), see (pressure and auditory graph on the screen), and touch (LOR from the needle) [24].
5.2 LOR technique with fluoroscopy
It is done in the prone position. Once the needle reaches L5-T1 interlaminar foramen, fluoroscopy of AP (anteroposterior) is done to confirm the placement. When LOR is done, lateral view of the fluoroscopy is done [25]. The disadvantage of this technique is patient needs to be put in prone position to undergo fluoroscopy.
5.3 EpiFaith syringe
The EpiFaith syringe has a spring-loaded plunger with the syringe. On the loss of resistance, the syringe moves by itself thus showing the epidural space [26].
5.4 Episure autodetect syringe
It is a spring-loaded loss of resistance syringe with a coaxial compression spring within a LOR syringe. This syringe gives constant pressure when it is being advanced. Hence, the operator can use both his hands in advancing the needle [12]. To minimize the false loss of resistance, the episure syringe is attached to the needle once its tip is in the interspinous ligament as this does not detect epidural space, but it detects only the loss of resistance [27].
5.5 Epidrum
It contains a small drum with a diaphragm on one of its sides. The device is placed between Tuohy needle and syringe for epidural placement. On penetration of epidural space, there is sudden collapse of the diaphragm giving visual evidence confirming the loss of resistance. It takes less attempts and shorter time in comparison with standard LOR [28].
5.6 Epidural balloon
A small inflated balloon is attached to the hub of the epidural needle, while the epidural needle is being inserted. On obtaining the epidural space, the balloon collapses due to negative pressure [29]. A Y-shaped connector is attached to the epidural needle where the one end is having the balloon and the other end is attached to the syringe for charging the balloon. The balloon collapse on entering the epidural space is faster than traditional LOR giving a visual evidence of loss of resistance [30].
5.7 Electrocardiography (ECG)-guided system
In this system, the epidural tip is having or contains one of the ECG lead. Another surface electrode is positioned at desired dermatomal level. Once this ECG enabled epidural catheter reaches the desired segment, it matches with the reading of the surface electrode [31]. It confirms the needle entry into respective dermatomal level.
5.8 Drip infusion method
It combines method of LOR with drip infusion for confirmation of the epidural space. It identifies the true epidural space by LOR and later with fluid dripping present. In pseudo or false LOR, there is no fluid dripping [32].
Current scientific evidence:
Less commonly used modern methods (epidrum, lidocaine, acoustic device, and Macintosh balloon) are better compared to air, saline, and both with relation to loss of resistance for the identification of epidural space [33].
The risk of epidural vein cannulation is higher in sitting position (15.7%) in comparison with lateral position (3.7%) of the parturients. This study shows the correlation between posture and epidural catheter insertion [34]. Hence, in parturients, insertion of epidural needle insertion with catheter in lateral position is always preferred.
Limitations and disadvantages of some popular techniques
The advantage of ECG-guided epidural catheter is that it can be used in the presence of local anesthetic and neuromuscular blocking agents. This technique helps to match the vertebral level of particular dermatome but does not confirm its location in epidural space.
6. Equipment-based objective-type epidural space detection
These are the techniques or methods which take the help of equipments and identify the epidural space accurately most of the times.
6.1 Epidural waveform analysis
In the epidural pressure waveform, there is a drop in pressure once the catheter enters the epidural space. Epidural space pressure waveform and computed tomography cathetergram are helpful in identifying the epidural space and location of epidural catheters. It is obtained by transducing an epidural catheter. The pressure changes observed on epidural waveform when the needle went through ligamentum flavum is around 82 ± 25 mmHg. This pressure dropped to 6.5 ± 11.6 mmHg once the needle entered the epidural space [35]. The quantity of saline required for epidural waveform analysis is 5 ml.
The epidural pressure waveform analysis has reported 81% sensitivity, 100% specificity, 100% positive predictive value, and 17% negative predictive value [36].
6.2 CompuFlo
The CompuFlo epidural instrument is a computer controlled drug delivery system that can precisely measure the pressure of human tissues in real time at the tip of the needle. It gives real time exit-pressure data at the needle tip. Both visual and audio graphic of exit pressure is provided. With CompuFlo, the needle entry into the ligamentum flavum increase pressure and audible tone. Once it enters the epidural space there is a big drop in pressure and audiotone. A drop in pressure for more than 5 seconds confirms entry into epidural space [37, 38].
CompuFlo helps in identification of true loss of resistance by sudden and sustained drop (more than 5 seconds) in pressure (greater than 50% of maximum pressure) and pitch of audible tone with formation of low and stable plateau pressure. In case of false LOR any one of the above is not achieved. If the pressure increases after drop in pressure it is a false LOR [39].
6.3 Epidural stimulation test
It involves electrical stimulation of nerves passing through the epidural space. Motor or sensory response to stimulation of 1–10 mA indicates epidural location of catheter. Stimulation <1 mA happens when the catheter is in subarachnoid position, subdural space, or close to nerve root [40].
6.4 Epidurography
It is obtaining fluoroscopy of contrast dye administered through epidural catheter. Typical epidural spread can be appreciated on fluoroscopic image. Equipment required and radiation risks are the disadvantages of this technique [41].
6.5 Near-infrared tracking system
A fiberoptic wire is placed in an epidural catheter which emits infrared signal picked up by infrared camera. This infrared tracking system helps in the identification of the epidural space. In this technique, the signal is poor in obese patients, and when catheter passes under the lamina making it is difficult to track the epidural space [42].
6.6 Epiduroscope
It is a fiberscope or a camera which is advanced through the 18-guage Tuohy needle into the epidural space. The outer diameter of this scope is 0.8 mm [43]. This epiduroscope gives the visualization of the epidural space.
Current scientific evidence
Attempts were made to check the epidural pressure. It was found that at T3–T5 level, the median epidural pressure was 1 mmHg (−1 to 4.5 mmHg). The epidural pressure at T7–T10 level is 4 mmHg (2 to 7.8 mmHg). The subatmospheric epidural pressure is higher at mid-thoracic area in comparison with lower thoracic area [44]. The lumbar epidural pressure is greater than atmospheric pressure when referenced to zero at the dorsal spine level [45].
Limitations and disadvantages of some popular techniques
The CompuFlo can be used to localize the epidural space, but it does not decrease the overall catheter failure rate or accidental dural puncture rate. It does not help in the identification of midline or guide the trajectory of the epidural needle [46].
The epidural pressure waveform will not aid in the detection of subarachnoid, intravascular, or subdural catheter misplacement [47]. Hence, it is not cent percent effective in the confirmation of epidural space.
The disadvantages of epidural stimulation are that it becomes ineffective on the administration of local anesthetic, neuromuscular blocking agents, and in preexisting neuromuscular disease. It becomes ineffective in those conditions where the nervous system is affected.
7. Ultrasound-based objective-type epidural space detection
7.1 Ultrasonography
Two methods are used.
7.1.1 Preprocedural ultrasound scanning
A handheld ultrasound device for epidural identification—A wireless ultrasound device is used to limit the storage space for the ultrasound equipment. The software is programmed to calculate the depth of the epidural space. Horizontal and vertical lines are drawn from the midpoint to the probe at each inter spinous space. Later, the ultrasound device is kept aside. Using the marked lines, the epidural needle is inserted to the depth achieved by the ultrasound earlier to get the epidural space. The success of up to 87% has been achieved for the first pass of epidural needle to identify the epidural space by the ultrasound method [48].
7.1.2 Real-time ultrasound guidance
Though it is real-time placement of epidural catheter, two operators are considered necessary for real-time ultrasound guidance [49]. Some have reported that single person technique is possible by in-plane method, and it requires a lot of expertise in doing the same. There is also risk of ultrasound gel entering the epidural space [50].
Different types of ultrasound modalities are used to help localize the epidural space.
Guidance positioning system for regional anesthesia (SonixGPS).
This uses an electromagnetic motion tracking system which helps in determining the position of the needle. It is more useful in out-of-plane approach [51].
Three- and four-dimensional real-time ultrasound.
Three- and four-dimensional real-time ultrasound has shown to help in regional anesthesia [52]. Certain difficulties are present in 3D/4D ultrasound images. They are varying bony shadows and artifacts due to complex anatomy of the spine. There are also problems with poor resolution, poor needle visibility, and reduced frame rate. Hence, it requires a high skill to perform the needle insertion into the epidural space.
Preprocedure 3D high-resolution images.
The preprocedure three-dimensionsl high-resolution images are constructed and later used in real-time 2D ultrasound image. This helps in the point of insertion of needle, trajectory of the needle, and depth of epidural space, making it more easier in getting the epidural space.
Machine vision.
The machine vision is a form of artificial intelligence where computer is helping to recognize images. It compares with previous stored data, and once familiar structures are recognized, they are pointed out by ultrasound image [53]. This artificial intelligence helps and gives clues to direct the epidural needle in the right direction and right depth. It helps by correlating the present ultrasound image with previous stored images.
Acoustic Radiation Force Impulse (ARFI).
In the acoustic radiation force impulse (ARFI), the images are derived from differences in the mechanical properties of tissue rather than acoustic properties. It is used to know the tissue structural and mechanical properties. In ARFI imaging plane, if imaging needles are out of required plane, the local stiffening effect of the needle can be visualized within ARFI imaging plane, thus helping in needle visualization [54].
7.2 Doppler method
The epidural needle is connected to a Doppler probe which is connected to a speaker and a paper recorder. On entry to epidural space through loss of resistance, “whoosh” sound is heard followed by spontaneous drop in Doppler flow tracing [55]. The appreciation of this sound helps in the identification of the epidural space.
Limitations and disadvantages of some popular techniques
The difficulty with real-time two-dimensional ultrasound is tracking of needle. From entry to epidural space is visualization of needle, or needle tip is difficult. This can be taken care with the help of needle tracking and navigation tools.
Needle visibility is a problem with ultrasound methods. Usually 2D B-mode ultrasound guidance is used for needle entry guidance to epidural space. The needle visibility is difficult when insertion angle is too steep, difference between imaging plane and needle plane and small needle gauge.
8. Future of epidural space detection
Raman spectroscopy has shown unique spectrum for all paravertebral and neuraxial tissue layers in porcine tissues [56]. Further studies of Raman spectroscopy is required on human subjects. If proved or invented, unique spectrum of Raman spectroscopy can be used to identify the epidural space.
Color flow Doppler function is being used to visualize the flow in epidural space on the injection of normal saline or air (1 ml over 1 second) [57]. Higher skills training is required, and it might require a long learning curve for expertise.
CompuFlo or technology built on the same with real ultrasound guidance could be the future instrument or technique of choice for difficult epidural.
Fiber Bragg grating (FBG) sensor for loss of resistance—This uses a novel soft system (SS) based on one fiber Bragg grating sensor (FBG) which is present in a soft polymeric matrix for LOR detection [58].
Ultrasound—Embedded needle is used for the identification of epidural space, and placement of catheter in real time with axial resolution of 0.15 mm has been tried on porcine models [59]. Further human trials are required to evaluate the effectiveness on humans.
Bioimpedance—Different tissues have different electrical impedance. This property helps to distinguish different types of tissues. Epidural space has higher fat content compared to nearby structures. This property could be utilized by bioimpedance in future to detect epidural space [60].
Optical spectroscopy—This method is tested on animal models. The needle is customized with optical fibers integrated into the cannula. The optical spectra (visible and near-infrared wavelength) are obtained at different depths. The estimates of blood and lipid volume fractions are determined. The lipid fractions obtained from epidural space were in the range of 1.9- to 20-fold higher than muscle, and epidural vein has high blood volume fraction [61].
Optical coherence tomography (OCT)—This is a type of B-mode ultrasonography using light reflection. This technology uses the light reflected to determine the depth of penetration to 2–7 mm to create the images of the tissue. This technology has helped to avoid neural damage in animal studies [62].
Due to reduced opportunities and to mitigate medical errors, simulation training for technical skills will help the trainees. To bridge the requirements and resources, multiple simulators from different manufactures are available in the market.
9. Conclusion
It has been more than 100 years since the discovery of LOR in epidural space, and we have not been able to find an ideal method to detect LOR space which has 100% specificity and 100% sensitivity and is safe and user-friendly. Research for better methods are still required to discover an ideal LOR instrument for the epidural space.
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