Summary of the studies assessed regarding LCA for waste tire rubber.
\r\n\tWith this goal in mind, together with the US Prof. John M. Ballato and the InechOpen publishing house since 2011 we have published in 2011, 2013, 2015 and 2017 4 books of our serial “Optoelectronics” and the book “Excitons”, edited in 2018 by Prof. Sergei L. Pyshkin. Publishing the new book “Luminescence” we are pleased to note the growing number of countries participating in this undertaking as well as for a long time fruitfully cooperating scientists from the United States and the Republic of Moldova.
\r\n\tSpecialists from all over the world have published in edited by us books their works in the field of research of the luminescent properties of various materials suitable for use in optoelectronic devices, the development of new structures and the results of their application in practice.
Approximately 1 billion unserviceable tires are discarded annually. The largest contributors are from the United States and the European Union, producing about 300 and 260 million, respectively [1, 2, 3]. Tires are a complex system containing 41% synthetic and natural rubber; up to 30 wt.% of additives such as silica and carbon black; 15 wt.% of reinforcing materials such as steel, polyester, and nylon; 6 wt.% of plasticizers and vulcanizing agents; and up to 2 wt.% of antiaging agents and other chemicals [4]. Figure 1 shows the main components of a tire.
Materials present in a tire.
Selecting the final destination of tires requires significant knowledge and responsibility, since inappropriate disposal can result in a range of negative effects, including fires and the proliferation of mosquitoes. According to the waste hierarchy, there are several ways of disposing waste tires to mitigate environmental impacts, the most common being reuse, retreading, regeneration, co-processing, pyrolysis, and landfills [5, 6].
Reuse involves using the whole tire or pieces of it to manufacture different rubber products for application in traffic and roadside barriers, the construction of parks and playgrounds, marine defense structures (dykes, wharfs, dams, and for coastal containment), channeling rainwater, artificial reefs, and biogas drainage [7, 8].
Tire reforming can be achieved through three different processes, namely recapping, retreading, and remolding. All involve replacing one or more worn regions with crude rubber and submitting them to revulcanization to acquire the properties of a new tire. Recapping consists of replacing the tread, retreading replaces both the tread and its shoulder, and remolding, also known as bead-to-bead retreading, involves replacing the tread, shoulder, and entire sidewall surface [9, 10].
Reforming is an interesting strategy for used tire recovery, since it promotes savings in iron, rubber, and petroliferous resources and minimizes the problems associated with the disposal of used tires [11, 12]. Reforming is used primarily in the truck tire market, which can be retreaded three or four times [13, 14]. Retreading also provides energy savings because the energy required to manufacture a new tire is around 2.3 times greater than that needed for retreading [14, 15].
The presence of rubber and steel makes tire grinding a complex process. Rubber is an elastomeric material that requires special care, and steel has excellent mechanical properties, which hampers the molding process. Grinding can be carried out at ambient temperature, by ultrasound or cryogenically to produce small pieces of rubber for a variety of applications, including as a base for artificial grass pitches and playgrounds or an additive to asphalt [16, 17].
In grinding, vulcanized rubber is initially reduced to 7–10 cm particles that are placed into another grinder and processed at ambient temperature into smaller granules, removing steel (by magnetism) and fibers (using vibratory sieves and screens). Depending on the required product, additional processing (tertiary grinding) may be necessary to obtain even smaller particle sizes [17, 18, 19].
In grinding by ultrasound, whole tires are fed into a rotary grinder where ultrasound is generated, and the material is ground into 2 μm particles. The metal is removed by magnetic separators, and the final mixture consists of rubber and fabric [20, 21].
In cryogenic grinding, vulcanized rubber is first reduced to 50 mm particles in a mechanical pulverizer and then frozen at temperatures below −120°C in a cryogenic tunnel. The resulting rubber is fragile and can therefore be broken into small pieces in a mill. Metal and fibers are also removed, as occurs in mechanical grinding [18, 22, 23, 24].
In the case of regeneration, waste tires undergo chemical modification (degradation) in order to become more plastic, malleable, less viscous, and processable, that is, with properties similar to those of virgin rubber. Regeneration prompts the breaking of covalent carbon-carbon (C-C), carbon-sulfur (C-S), and sulfur-sulfur (S-S) bonds. If a number of C-C bonds are broken during the process, the main rubber chain may rupture, leading to serious structural disintegration [12].
The quality of products regenerated from waste tires varies according to their composition and the selectivity of the methods used in terms of the type and number of bonds to be broken. For regenerated waste to be deemed good quality, at least 70% of cross-linking must be carried out. It must also remain stable for at least 6 months and still be capable of being revulcanized at temperatures close to 170°C. Rubber regeneration can be carried out in the presence of a specific catalyst, which attacks the cross-linking points, or by applying enough energy to break these bonds. This process generally requires heat, chemical products, and mechanical energy. In principle, regeneration is used to obtain a product to replace virgin rubber with fewer technical requirements than the original product. Rubber is considered regenerated when it recovers its flow capacity and the characteristics of the original compound. Regenerated rubber can be used in carpets, furniture, asphalt mixtures, glues, and adhesives [25].
Co-processing is defined as the use of waste materials to replace fuels and/or primary raw materials. Whole or ground tires are burned in a cement kiln to produce clinker, an intermediate product in cement manufacturing. The ash generated is not problematic because it is incorporated into the clinker, preventing the need for subsequent collection and treatment [16]. Silica and iron (contained in the tire) are used as secondary raw materials to replace sand and iron oxide in cement. The high temperatures (1500–1600°C) and oxidizing atmosphere in the cement production kiln allow complete combustion of the tire and almost total combustion of the volatile material produced during burning [7, 11].
The tires can be fed into the kiln whole or ground. Whole tires must be fed into the calcination zone of the kiln, while ground tires can be introduced into the burner zone [7, 20].
The use of fossil fuels (conventional power plants) in the form of coal, oil, and gas accounts for about 80% of the global energy demand [26, 27]. Nitrogen compounds and sulfur oxides produced by coal combustion have a significant effect on the environment and are responsible for acidification (acid formation) (HNO3, H2SO4), increased ozone concentration at low altitudes, and high levels of particulate material [28, 29]. According to Singh et al. [30], using tires as a source material to generate energy in coal-fired power plants reduces NOx emissions and recovers the energy contained in the material. In this process, ground tires are combined with coal in the combustion unit to generate electrical energy. An important advantage of this process is that it lowers fossil fuel consumption [16]. Nevertheless, the energy conversion efficiency of power stations that use tires as raw material is 25–30% but far higher in conventional power stations. However, CO2 emissions are around 23% lower when tires are used for energy generation [16].
Pyrolysis is a high-temperature chemical process that generates oil, gas, and carbon black. First, the tire is ground into 20 mm particles, fed into the pyrolytic reactor, and submitted to temperature (400–700°C) and pressure (0.01–0.04 MPa) conditions under which elastomers degrade. The products of the process consist of the following fractions: gaseous (hydrogen, methane, and carbonic oxides), liquid (water and oils), and residual solids (metals and dust) [16, 19].
An interesting process for the degradation of waste tires is thermolysis under pressure, which involves applying superheated steam and high pressure to obtain oligomers, gas, and liquid fuel. Used tires are placed into a preheating chamber (60–100°C), then fed into the reactor, and submitted to temperatures of 300–500°C and pressures of 1–1.2 atm. The resulting volatile hydrocarbons are removed and condensed, and the carbon residue is separated from the remaining metal [20, 31].
Another recycling technique for degrading tire rubber to obtain commercial products of interest is barodestruction, which is based on the pseudo-liquefaction of rubber at high pressure. Whole or ground tires are fed into the chamber at high pressure. The pseudo-liquefied rubber flows through the holes, and the nylon and metals are separated from the rubber. The metal is removed in the first step, and the rubber and nylon mixture is then passed through a grinder to separate the nylon. The gaseous emissions are treated using filters [20, 32].
This type of disposal consists of simply discarding tires in landfills, which is prohibited in Europe, according to Directive 2000/53/EU [33], and in countries such as Brazil [34]. In addition to shortening the useful life of the landfill, this practice impoverishes the soil, favors the proliferation of mosquitos, and makes the site prone to fires [7, 9, 15]. Fires caused by tires are difficult to extinguish. A tire has around 75% of hollow space in relation to its entire volume, preventing these fires from being extinguished with water because the oxygen in this space feeds the fire. Additionally, the pyrolysis oil generated is a significant atmospheric, soil, and water pollutant [1, 2].
The most sustainable final destination for end-of-life tires is difficult to determine among the different possibilities available. The LCA tool has contributed to the decision-making process, requiring different technologies for each situation, region, and condition. As such, the aim of this chapter is to present studies that used LCA to investigate tire disposal options. Studies were reviewed by continent, and the environmental impact of each technology was evaluated.
The methodology used was divided into two stages. The first was to understand the different technologies applied for end-of-life tire disposal, and the second was to analyze life cycle studies that assessed these technologies in different parts of the world, including Europe, Asia, and America. To that end, a bibliographic review was conducted in different databases, such as ScienceDirect, Scopus, and Web of Science. The study selection criteria were directly related to the subject of the chapter, that is, end-of-life tire disposal based on life cycle assessment. The data from the selected articles are presented and summarized in Table 1.
Authors and reference | Country | Impact method | Technology studied and/or process for end-of-life tires |
---|---|---|---|
Corti and Lombardi [18] | Italy | Ecopoint |
|
Ferrão, Ribeiro, Silva [15] | Portugal | Ecopoint |
|
Li et al. [19] | China | Eco-indicator 99 |
|
Clauzade et al. [7] | France | Not declared |
|
Fiksel et al. [16] | USA | Traci |
|
Feraldi et al. [27] | USA | Traci |
|
Li et al. [35] | China | Eco-indicator 99 |
|
Sun et al. [36] | China | CML |
|
Ortíz-Rodriguez et al. [8] | Colombia | CML |
|
Summary of the studies assessed regarding LCA for waste tire rubber.
Life cycle assessment (LCA) can be used to quantify the impact of waste tire disposal and determine the most environmentally beneficial alternative for product manufacture and managing used products. LCA has also been applied to identify the most environmentally appropriate final destination for waste tires [3, 4, 5, 6, 7].
LCA can be applied to quantify the potential environmental impacts of a product and the resources used during its life cycle, including the acquisition of raw materials, production and use, and waste management. It can also be used to determine the best alternative for managing used products, encompassing their disposal, recycling, and reuse [37]. It is a broad assessment that considers all of the attributes or aspects of the natural environment, from human health to natural resources [38].
In order to standardize environmental management methodology, the International Organization for Standardization (ISO) developed the ISO 14.040 global standard [39], which defines the method for LCA application. An LCA study is divided into four phases: goal and scope definition, life cycle inventory (LCI) analysis, life cycle impact assessment (LCIA), and interpretation [40].
Defining the goal and scope includes establishing the motives for the study, the intended application, and target audience. The limits of the system under study are also described in this phase, in addition to defining the functional unit [40], which is a quantitative measure of the functions that the products (or services) perform. The results of the LCI provide information on the inputs (resources) and outputs (emissions) of the product during its life cycle in relation to the functional unit. The aim of the LCIA is to determine and evaluate the magnitude and significance of the potential environmental impacts of the system studied. In this stage, the functional units allow the relevant data to be compared. Inventory data are separated into midpoint [41] and endpoint (human health, ecosystem quality, and resource consumption) and converted into units via weighting factors for comparison [42]. Since the functional units have yet to be standardized, several names have been proposed, including Ecopoint unit. In this case, the values for each impact category are summed to produce a single value known as the Ecopoint, which corresponds to the environmental load of 1000 Europeans over a 1-year period [42, 43]. In the interpretation phase, the results of the previous stages are compared with the goal and scope in order to draw conclusions and provide recommendations [39].
In order to understand the state of the art, the papers developed in relation to the end-of-life tire destination that used the life cycle assessment were grouped by continents.
Ferrão et al. [15] carried out an LCA of a new tire, whose life cycle phases were production, distribution, use, disposal, collection of the used tire, and recycling. The aim was to assess the impacts of a new tire during its life cycle as well as of four forms of recycling (recycling, retreading, fuel replacement, and energy generation) and disposal in a landfill. The Ecopoint approach was adopted, and the functional unit was a metric ton of used tires.
The results indicated that the most relevant phase in terms of environmental impacts was tire use. This was expected, since fossil fuels are the main fuel consumed during tire use and have a significant effect on the environment. Despite its impact, this phase is important in guaranteeing the safety of the vehicle, since the greater friction between the tire and the ground, the more secure the vehicle, but the more fuel it will consume [15].
Impacts resulting from landfill disposal are mainly related to the leaching of metals, stabilizers, flame retardants, and plasticizers, which are mixed with the rubber during tire manufacturing. Retreading is the most cost-efficient alternative in terms of the recovery of material and energy [11]. Although energy is consumed during retreading, consumption is 2.3 times greater when manufacturing a new tire. An important benefit of recycling is that it prevents the use of virgin material [15].
Burning whole tires to generate energy means they do not require grinding. However, a sophisticated burning system is needed to allow the use of high temperatures at specific points, and emissions must be kept within admissible limits [9]. Tire pyrolysis generates three products, namely, gas, oil, and carbon black. The energy potential of gas and oil (used to replace fuel) is similar to that of conventional products [44]. According to Van Beukering and Janssen [45], an important advantage of energy generation in cement kilns is that it does not produce solid residues and the sulfur emissions are not a significant problem because the sulfur generated is incorporated into the gypsum, which is added to the final product.
The results obtained in studies that applied LCA to analyze rubber recycling processes are detailed below. Corti and Lombardi [18] evaluated the following processes using LCA: mechanical pulverization, cryogenic pulverization, energy generation, and fuel replacement, the last applied in cement kilns. The functional unit was a metric ton of tires. The emissions generated were obtained via observations by the authors at different power plants, and average values were calculated. The only exception was the energy generation process, whose values were obtained from a thermodynamic model. The Ecopoint approach was adopted for the emission values.
Of the processes studied, cryogenic pulverization generated the most negative impacts due to its high water consumption when compared to the other processes. Other negative aspects include the greenhouse effect, eutrophication, and carcinogenic emissions (which were higher in cryogenic pulverization) [18].
The greenhouse effect, water consumption, and energy consumption were analyzed in greater detail. The impact on the greenhouse effect is assessed based on the equivalent CO2 emissions into the atmosphere. According to Corti and Lombardi [18], cryogenic pulverization produces the poorest results, emitting around 450 kg of CO2 equiv. per ton of tire processed. The energy generation process was the most beneficial because it consumes conventional materials, whereas fuel replacement showed no notable positive or negative influence on the greenhouse effect.
As previously mentioned, cryogenic pulverization displayed the most negative impact on water consumption, since it is high in the cryogenic step of this process. The energy recycling processes produced the best results, since water is not consumed to obtain energy.
In regard to energy consumption, cryogenic pulverization once again produced the worst results. As expected, much higher values were obtained in the two energy processes, given that they generate energy as opposed to consuming it.
Silvestraviciute and Karaliunaite [20] studied fuel replacement, mechanical pulverization, mechanical pulverization with ultrasound, thermolysis, and barodestruction recycling in terms of energy, atmospheric emissions, solid waste, and water consumption. The authors did not adopt a specific methodology for life cycle impact assessment, and only the values for each emission category were reported.
It can be concluded that the use of tires in the fuel replacement process is of significant interest in terms of energy; however, the emissions are similar to those produced using carbon as fuel [20]. In a study carried out by Corti and Lombardi [18], emission values were lower and negative, that is, the process did not result in new emissions. In the process studied by Corti and Lombardi [18], ground tires were added to the burner zone of the furnace, whereas Silvestraviciute and Karaliunaite [20] used whole tires added to the calcination zone. The advantage of the latter is the absence of the grinding step, since the whole tire is used; however, the drawback is that the metal is not recovered (during grinding, iron can be separated out and reused in another process).
In the process studied by Silvestraviciute and Karaliunaite [20], water consumption and solid waste generation were very low and not limiting factors. Gas and dust emissions are associated with fuel replacement and are zero or insignificant in the other processes.
Clauzade et al. [7] used LCA to assess used tire rubber as a substitute for different materials in a range of applications, including as a replacement for filler in retention dykes (concrete and polyethylene blocks) and infiltration (gravel substitute); as a filler at steelworks and foundries (to complement steel), in synthetic grass (instead of ethylene propylene diene copolymer–EPDM), at sports grounds (to replace sand), and in molded objects (instead of polyurethane); and as fuel for heating (coal substitute) and in cement plants (to replace fuel and raw materials). The study considered the transport of material from the generation center to the processing location, the impacts of the processes, and those prevented by the replacement. The authors concluded that reusing rubber as a filler for molded objects and synthetic grass provides the greatest environmental benefits. Additionally, the logistics of collection and transport is an important stage of the process.
Li et al. [19] analyzed four processes for use in LCA: mechanical pulverization, regeneration, pyrolysis, and oil extraction. As in the studies mentioned above, the functional unit was 1 metric ton of tires. In accordance with Eco-indicator 99, disability-adjusted life years (DALY) were used to evaluate human health-associated impacts. The impact of one unit on this scale corresponds to the loss of 1 year of life. The unit used for ecosystem quality was the potentially disappeared fraction of species (PDF), in the form of PDF*m2*yr (where m2 is an area in square meters and yr, a year). An impact value of 1 for this unit indicates that all species within one square meter disappear over a year. For the resources category, the unit used was MJ of surplus energy, where an impact value of 1 indicates that an area previously used to extract resources requires 1 MJ of additional energy in order to be used again due to the decline in the natural resources available [46].
The following impacts were considered in the present study: ecotoxicity, acidification and nitrification, emission of carcinogenic materials, global warming potential, emissions of inorganic and organic materials harmful to human health, and the consumption of fossil fuels.
Global warming is caused primarily by the emission of CO2, CO, N2O, and CH4. This study [19] found that only the oil extraction process caused negative effects. The processes that obtained the best environmental performance were mechanical pulverization and pyrolysis. The effects of the first three processes (mechanical pulverization, regeneration, and pyrolysis) are negligible when compared to oil extraction, which uses carbon as an energy source and generates large amounts of heavy metals.
The impacts assessed in the ecotoxicity category were those related to heavy metal and aromatic compound levels in the soil or air. Once again, oil extraction had the most negative impact because carbon is burned as an energy source.
In relation to fossil fuel consumption, all the processes obtained negative values because virgin material was not required, precluding the need for energy consumption during extraction. Even in oil extraction, fuel consumption is avoided, since the oil generated is an energy source [35].
The predominant management option in the Chinese end-of-life tire market is the production of ground rubber [35] for regeneration. In order to improve the environmental performance of ground rubber production, Li et al. [35] made a series of technical recommendations based on the Eco-indicator 99 method. The process consists of three main stages: ground rubber preparation, regeneration, and refining.
According to the authors [35], respiratory inorganics obtained the most severe results, that is, the highest relative contribution among the other impact categories assessed. With respect to regeneration, devulcanization was responsible for most of the environmental loads, corresponding to 66.2% of the total impact. Moreover, improvements in the flue gas treatment contributed to better performance. The use of renewable and clean energy can improve environmental performance by approximately 22%. These results could be used as a guide to reduce the environmental load when producing ground rubber from scrap tires. Moreover, increasing energy efficiency, improving environmental protection equipment, and using clean energy are effective measures to achieve this goal [35].
Still in regard to the Chinese tire market, Sun et al. [36] assessed the environmental impacts of radial tires for passenger vehicles. The authors used the CML method to analyze raw material extraction, tire production, use, and end of life. However, they considered only five out of eight impact categories, namely global warming potential (GWP), acidification potential (AP), photochemical oxidant creation potential (POCP), eutrophication potential (EP), and human toxicity potential (HTP), since these are easier to explain and based on direct emissions that are easy to correlate, in addition to being more important to tire production.
It was assumed that all end-of-life tires were collected and recycled and that, after separating the different tire components, the rubber was completely regenerated to replace synthetic rubber. This recovery and recycling process only showed negative impacts for GWP, EP, and HTP, meaning it prevents emissions as opposed to causing them. However, the main environmental impacts observed during the production of reclaimed rubber and waste treatment were for AP and POCP [36].
Fiksel et al. [16] studied fuel replacement, energy generation, retreading, and mechanical grinding. The grinding process analyzed was aimed at the application of rubber in civil construction (as asphalt and a base for synthetic grass) and as a filler in new products. The authors found that using waste tires as raw material for synthetic grass is the most promising alternative, followed by energy recovery (co-processing in cement kilns and energy generation). However, the study was conducted in the United States, where the market for artificial grass is saturated, and, as such, they concluded that energy recovery is currently the most viable alternative.
Feraldi et al. [27] evaluated two final destinations for tires in the United States: grinding and energy recovery. The authors used the TRACI method and analyzed the future prospects for tire disposal considering changes in US energy matrix. The results identified grinding as the ideal final destination, given that energy recovery involves burning and emission of harmful compounds. With regard to future prospects, the authors concluded that the reduction in the impacts of each process would be negligible.
In Colombia, Ortíz-Rodríguez, Ocampo-Duque, and Duque-Salazar [8] used LCA to estimate the environmental impacts of three different alternatives for tires at the end of their useful lives in a case study at the Valle del Cauca Department. The first option was reuse and retreading, the second incineration, and the third grinding to obtain new products. CML-2001 was used to calculate the environmental impact indicators.
Grinding to manufacture flooring and rubber incineration in cement plants exhibited the best environmental results, largely because they prevent harmful effects by recovering the material. Comparison of the different waste tire recovery and disposal processes indicated that retreading and the production of multipart asphalt displayed the worst environmental performance. The performance categories used were global warming potential, ozone layer depletion, acidification, abiotic resource depletion, and photochemical ozone formation. The phases that most contributed to the recovery process were fuel consumption, initial synthetic rubber production, and conversion into liquid asphalt [8].
A comparison of the papers presented in Table 1 shows that the studies are concentrated in Europe [7, 15, 18], the United States [16, 27], and China [19, 35, 36]. With respect to different forms of disposal, it is noteworthy that earlier studies describe a larger number of options, while current research focuses on comparing alternatives to recycling, as well as exploring different applications and recycling techniques [8, 35].
There is no consensus regarding the best impact method for tire recovery studies, although regional preferences are observed. European studies showed a preference for Ecopoint [15, 18], while American papers used only the TRACI method [16, 27], Chinese authors applied both Eco-indicator 99 and CML, and Colombian studies the CML [8, 19, 35, 36].
It is important to underscore that more LCA studies are needed to better understand the impacts of alternatives to traditional tire management, particularly when tires are submitted to new industrial processes, such as recycling [21, 47, 48].
End-of-life tire disposal was shown to be of great interest in Europe, Asia, and America, as a means of contributing to the decision-making process in selecting the best technological alternative from an environmental standpoint. Studies demonstrated that the best environmental performance, in general, was mechanical recycling for use in synthetic grass. The worst environmental performance was observed in co-processed and retreaded tires. There is no consensus regarding the best tire recovery method, although regional preferences are observed. European studies showed a preference for Ecopoint, while their American counterparts prefer Traci methodology for life cycle assessment.
The authors would like to thank the National Council for Scientific and Technological Development (CNPq) and the National Council for the Improvement of Higher Education (CAPES).
Aortic root dilation (AoD) is frequently an incidentally discovered, asymptomatic finding in that is seen on various imaging modalities [1]. The anatomy of the aortic root includes the annulus, sinuses of Valsalva, sinotubular junction and ascending aorta [1], with the size being a function of a patient’s biologic variables such as height, age, BSA, and gender [1, 2]. However, while natural variations in the size of the aortic root are well known, the identification of progression from normal to pathologic AoD is a key clinical diagnosis that carries significant cardiovascular risk including aortic dissection, rupture, valvular regurgitation and cardiac tamponade [1, 3, 4, 5]. The etiology of pathological AoD is varied, ranging from congenital, infectious, autoimmune, and idiopathic conditions; and influences the medical and surgical management [1, 5]. Due to the variety of clinical conditions that can result in AoD, and the risks associated with worsening AoD, a thorough understanding of the pathophysiology of AoD, noninvasive imaging modalities and pharmacologic therapies is critical. The aim of this chapter is to review the most common conditions associated with AoD, appropriate imaging modalities, and treatment strategies to manage AoD.
\nMultiple etiologies of AoD exist such as Marfan syndrome, bicuspid aortic valve, Loeys-Dietz and Ehler-Danlos syndromes, idiopathic conditions, hypertension, infections, and inflammatory disorders. In this chapter, we will discuss the various etiologies categorized into two standardized groups—genetically-mediated and nongenetically mediated AoD.
\nGenetically-mediated aortic root dilation or enlargement is the leading cause of thoracic aortic aneurysms. Marfan syndrome (MFS), the prototype condition for AoD, and bicuspid aortic valve has led to a greater understanding of AoD pathophysiology, pharmacologic treatment, timing of surgical intervention and optimal surveillance strategies with noninvasive imaging [6].
\nMFS is one of the most common hereditary disorders of connective tissues and is characterized by manifestations in cardiovascular, skeletal, and ocular systems [7]. MFS is the most common genetic cause of thoracic aortic aneurysms (TAAs). Its inheritance is almost exclusively autosomal dominant and mostly involves a mutation of the fibrillin-1 (FBN1) gene encoding the connective tissue structural protein fibrillin-1 [8]. The widely accepted incidence of Marfan syndrome is ~1 in 5000 individuals [9].
\nAlthough the inheritance pattern is predominantly autosomal dominant, rare cases of autosomal recessive FBN1 gene mutations has been described [10]. While patients with Marfan phenotypes usually have an affected family member, 25% of the cases are sporadic due to de novo mutations [9]. In addition, in <10% of Marfan cases, no mutation of FBN1 was determined [11]. Since it was first identified as the main cause of Marfan syndrome, FBN1 mutations, depending on how it is mutated, were linked to a variety of syndromes and phenotypes [12]. Animal studies investigating the pathophysiology of the disease demonstrated over-expression of TGF-β in the mitral valve preceding prolapse, the aorta associated with dilatation, skeletal muscle associated with myopathy, and the dura leading to ectasia [12]. Later, mutations in TGF-beta receptor 2 (TGFBR2) and TGFBR1 genes were identified in some patients with Marfan phenotypes and subsequently implicated in the disease process in FBN1 mutation negative individuals [13, 14, 15]. These genes were also linked to another condition later, namely Loeys-Dietz syndrome (LDS) [14].
\nThe diagnosis of Marfan syndrome is established by using a combination of clinical manifestations, family history, and the presence of FBN1 mutation. In order to facilitate accurate recognition of the syndrome and improve patient management and counseling, a set of defined clinical criteria, called the Ghent nosology was developed [16] and later revised [17] (Table 1). Apart from the genetic testing for FBN1 mutation, Ghent nosology uses a systemic score calculation using clinical manifestations of Marfan and an aortic root dilatation Z-score (see noninvasive imaging below).
\nPatients with family history of Marfan disease | \n
\n
| \n
\n
| \n
\n
| \n
Patients without family history of Marfan disease | \n
\n
| \n
\n
| \n
\n
| \n
\n
| \n
Revised Ghent nosology.
One of the major causes of mortality and clinical hallmark of Marfan syndrome is aortic root dilation and related complications such as dissection, rupture and/or aortic valvular regurgitation. Aortic root dilation is typically first identified on echocardiography in 60–80% of Marfan patients [18]. Therefore, surveillance echocardiography has been routinely used to serially monitor aortic dimensions. If the aortic root diameter is above 4.5 cm in adults, aortic dilation rates are above 0.5 cm/year, and/or significant aortic insufficiency is already present, more frequent monitoring is recommended [6] (see Diagnosis and Surveillance of Aortic Root Dilation below for more detailed guidelines).
\nBicuspid aortic valve is one of the most frequent congenital heart anomalies in adults, affecting 0.9–2% of the population [19]. Most cases of bicuspid aortic valve are familial and studies show that heritability of the disease is ~90% making it an autosomal dominant pattern with incomplete penetrance [20]. Bicuspid aortic valve can occur alone or with other congenital cardiovascular disorders such as coarctation of the aorta, supravalvular or subvalvular aortic stenosis, and ventricular septal defect [21].
\nThe diagnosis is often established by transthoracic echocardiogram (TTE), which has high sensitivity (~92%) and specificity (~96%) [22]. TTE is also useful for surveillance of potential complications of bicuspid aorta such as aortic stenosis, aortic dilation, aortic regurgitation, and infective endocarditis [23]. Given the risk of inheritance, first degree relatives are also recommended to be screened with TTE [21].
\nPrevalence of aortic dilation in patients with bicuspid aortic valve disease ranges from 20 to 84% depending on the criteria used in different studies [24]. The risk of aortic dilation increases with age and the risk of dissection increases as the aortic diameter increases [25, 26]. When the aortic root diameter is above 4.5 cm, there is a family history of aortic dissection, or aortic diameter change is rapid it is recommended to perform echocardiogram annually [21]. More frequent surveillance is recommended for patients with aortic root diameters approaching surgical thresholds (see Surgical Interventions section below).
\nLoeys-Dietz syndrome (LDS) is a rare congenital syndrome characterized by hypertelorism (widely spaced eyes), a split uvula or cleft palate, tortuous arteries and aortic aneurysms. LDS shares many features with Marfan syndrome [14]. Most of the LDS cases are sporadic or show an autosomal dominant pattern of inheritance [14].
\nThe incidence and prevalence of the disease is still not well established.
\nLoeys-Dietz syndrome was initially classified into two subtypes based on the severity of the cutaneous and craniofacial features but later was divided into six subtypes stratified by genotypes [27]. These subtypes are labeled 1–6 and associated with mutations in TGFBR1, TGFBR2, SMAD3, TGFB2, TGFB3, SMAD2, respectively [27]. Type 1 and type 2 are the most commonly seen subtypes with frequencies of 20 and 55% among all subtypes, respectively [28].
\nAortic root dilation is a hallmark feature of this disease entity and is frequently seen in patients (~80%) [29]. Another vascular manifestation is aneurysms throughout the arterial tree. This is a concerning clinical manifestations of the disease and can cause aggressive arteriopathy; therefore, early operative intervention at ascending aortic diameters of ≥42 mm is recommended [30].
\nEhlers-Danlos syndromes (EDS) are a heterogeneous and relatively rare group of connective tissue disorders characterized by skin hyperextensibility, joint hypermobility, and tissue fragility [31]. Ehler-Danlos syndrome can present with a variety of clinical manifestations and can be caused by different kinds of genetic mutations. Overall prevalence of EDS is ~1 in 5000 and EDS hypermobile (hEDS) is the most common type [31].
\nVascular complications can be seen with different types of EDS; however, it is most commonly seen in type IV (vascular or arterial ecchymotic type; vESD), characterized by an autosomal dominant mutation in COL3A1 (collagen, type III, α-1 gene) encoding type III procollagen [32]. Up to 80% with vESD patients suffer from vascular complications by the age 40 years [32]. Therefore EDS patients, especially vEDS, patients should be routinely evaluated for aortic root disease. These patients are recommended to undergo elective operation at smaller diameters (4.0–5.0 cm) to avoid acute dissection or rupture. Patients with a growth rate of more than 0.5 cm/year in an aorta that is <5.5 cm in diameter are recommended to be considered for operation [33].
\nAortic root dilation is an established phenomenon that has shown strong correlations to key pathobiological factors such as age, body surface area (BSA), height and gender. The correlation of aortic root size with age and BSA were initially described in the development of screening nomograms using M-mode echocardiograms [34]. Follow up studies with 2D echocardiography further validated these correlations, allowing for the development of nomograms for normal patient populations or adjusted for patients with underlying congenital disorders (i.e., Marfan syndrome) [2, 35]. These studies evaluating AoD by echocardiograms are further supported by reviews of autopsy data that show clear correlations to key pathobiological factors such as increased age and height with AoD [36].
\nDespite the validation of age as being correlated strongly with AoD, the mechanism of age on the development of AoD still remains an area of active research. One of the predominant hypotheses is based on the idea of cyclic stress, and how the aorta degrades through gradual mechanical decline of elastin proteins [37]. Elastic arteries, namely the aorta, are estimated to dilate by 10% with each beat [38]. It is hypothesized that the shear stress over a normal lifetime results in the degradation of elastic lamella, resulting in arterial dilation and stiffening [38]. This is corroborated by histologic data demonstrating damage to medial elastin of the proximal aorta [38]. Furthermore, there is evidence to suggest that in the absence of risk factors such as hypertension or atherosclerosis, the aortic wall undergoes age-associated reprograming that is proinflammatory promotes progression of arterial disease [39]. Wang et al. demonstrated in pathologic samples of aortas that age correlated with increased smooth muscle cell invasion, and increased production of downstream angiotensin II mediators [39].
\nIn addition to age and BSA, gender is another key factor which can increase the risk and progression of AoD [40]. In the Framingham study of 1849 men and 2152 women, not currently diagnosed with cardiac disease or having a cardiac history, aortic root was 2.4 mm smaller in women than men with m-mode echocardiography [40]. A systematic review in 2014 of 10,741 patients with hypertension revealed men had a significantly higher incidence of AoD relative to women [41].
\nIn conclusion, a series of biological variables are correlated with AoD, and it is important to take these into account as they are potential confounders or contributors in the evaluation of patients with pathologic AoD. Even exercise capacity has been correlated with AoD, with a recent meta-analysis showing that athletes defined by participation in National Collegiate Athletic Association (NCAA) or international equivalent had an aortic root diameter that was larger than nonathletic controls [42], and a statistically significant increase by measurement of sinuses of Valsalva and aortic root annulus [42]. It is important to understand the significance of biological variables such as age, height, BSA, or gender to fully evaluate pathologic AoD without the influence of known confounders.
\nHypertension is a well-known risk factor for aortic dissection, and in some studies, it is estimated to factor into roughly half of the overall risk for aortic dissection [43]. A recent prospective cohort study of 30,447 patients, 86% of patients who developed aortic dissection had hypertension [4]. However the relationship between hypertension and AoD is not as clearly established. In a Framingham study of 3195 patients, there was no relationship between the development of AoD with hypertension [44]. A subsequent follow up study of Framingham participants evaluating aortic root diameter was positively correlated with mean arterial pressure, but negatively associated with pulse pressure, indicating that the mechanism behind AoD is more complicated [45]. Moreover, investigations have shown that in patients with other comorbidities for AoD, such as, Turner syndrome, hypertension is significantly associated with increased prevalence of AoD [45]. This has led to interesting insights into the cyclic stress hypothesis of the development of AoD [43]. If AoD develops due to chronic shear stress, then it would be expected that AoD is correlated with higher pulse pressure (PP), which would presumably lead to greater stress and aortic dilation [43]. However, studies have reported an inverse relationship between AoD and PP [43]. Additionally a systematic review in 2014 showed that in a population of 10,791 hypertensive patients, 9.1% had AoD with a significant gender skew toward men [41]. However there was no significant correlation of mean arterial pressure or pulse pressure values and AoD [41]. While hypertensive patients have a higher incidence of AoD, the mechanism remains to be further investigated. Moreover, these unclear correlations between MAP, PP, and AoD suggest that the aorta is not static, but a dynamic structure whose response to stress, such as hypertension, is still being elucidated [43].
\nSince the first mass production of penicillin in 1945, the modern era of antibiotics has resulted in a decrease in the prevalence of mycotic aneurysms due to bacterial infections in developed countries [46, 47]. However they can still be found in developing countries, and are rare but well described causes of mycotic aneurysms [46]. Most common pathogens include Salmonella, Staphylococcus and Streptococcus pneumonia, and while rare have been in the pathogenesis of mycotic aneurysms of the aortic root [46, 48, 49]. Other common bacteria include Mycobacterium tuberculosis and Treponema pallidum, which will be discussed below, and more rare causes include Listeria, Bacteroides, Clostridium septicum, and Campylobacter jejuni [46]. With the majority of bacterial aortitis, aneurysm development is generally saccular, and Salmonella has been reported in case studies to predominantly affect the abdominal aorta than the thoracic [46, 48]. Infections with Staphylococcal species generally are related to underlying aortic valve infections, but have been reported to progress into aneurysms of the aortic root [46, 49].
\n\nTreponema pallidum, a sexually transmitted spirochete which is the causative organism of syphilis, is a well characterized cause of aortitis [46, 50, 51]. Cardiovascular involvement is limited to late stage, or tertiary syphilis, and generally occurs 5 to upwards of 40 years after primary infection [50, 51]. Aortitis, and aneurysm development is due to invasion of the vasa vasorum, resulting in obliterative endarteritis that leads to degradation of the aortic media [50, 51]. The chronic inflammation results in fibrosis of the intima, a phenomenon known as “tree-barking” that ultimately progresses to aneurysm development. In an autopsy study in 1960 of 51 aortic aneurysms secondary to syphilitic aortitis, 7.8% were found at the sinuses of Valsalva and 29.4% involved the ascending aorta, representing a majority of the sample [52]. This predominance to the ascending thoracic aorta have been further corroborated in later studies, however the detailed echocardiographic analysis of syphilitic aortitis, specifically in relation to AoD is limited due to the rarity of the disease presentation [46, 50].
\nTuberculosis is a relatively common infection especially in developing countries [53]. Mycobacterium tuberculosis, the causative pathogen, is a known cause of mycotic aortic aneurysms [46, 50]. Pathogenesis of tuberculous mycotic aneurysm is believed to be due to lymphatic spread or hematogenous seeding, and mortality rates are reported as high as 60% in patients who develop this complication [50]. While more commonly affecting the distal aortic arch and descending aorta, there are case reports detailing aortic root aneurysms due to tuberculosis [50, 54].
\nThere have been case reports proposing an association between aortic aneurysms and HIV [50]. In a variety of these cases the causes are generally multifactorial, as the majority of cases have reported coinfections (Q fever and leishmaniasis) or comorbid autoimmune conditions (giant cell arteritis) [55, 56]. It is still an area of investigation as to whether there is a true association, and there is sparse data showing a relationship with AoD.
\nAnkylosing spondylitis, a seronegative spondyloarthropathy, is a chronic, progressive rheumatologic disorder, and was one of the first found to be associated with aortitis [50, 57]. The proposed mechanism of AoD in ankylosing spondylitis is fibrous growth development along the intima, which leads to subsequent weakening [57]. Prior TEE studies further evaluated the prevalence of AoD in ankylosing spondylitis, and 82% of patients with ankylosing spondylitis had aortic root abnormalities [58]. Specifically, 61% of patients had aortic root thickening and 25% of patients had AoD [58]. AoD in these populations is a relatively common phenomenon and is associated with significant cardiac morbidity [45, 57].
\nRelapsing polychondritis is another autoimmune disorder, which is a multisystem inflammatory disorder that primarily affects the cartilaginous structures of the body [59]. Cardiovascular involvement is common, estimated to be the second most frequent cause of death and can result in aneurysm development in 5% of cases of both thoracic and abdominal aorta [50, 59]. AoD has been known to occur, albeit rare, with cases of requiring surgical revision after the development of aortic regurgitation [60, 61].
\nTakayasu arteritis is a chronic granulomatous large vessel vasculitis, predominantly found in pediatric populations [50, 62]. A rare disorder, the pathogenesis is characterized by granulomatous panarteritis that can affect the entirety of the aorta and major branches, however predominantly affects the common carotid and subclavian artery [62]. While rare, there are reports of AoD from Takayasu arteritis resulting in aortic regurgitation [63, 64].
\nGiant cell arteritis is a large vessel vasculitis that is characterized by chronic granulomatous inflammation [50]. While commonly affecting carotid, temporal and vertebral arteries, it has been known to affect the ascending aorta, at times resulting in dissection or aortic valve insufficiency [50]. The development of AoD from GCA may be influenced by other comorbid conditions such as HIV; however, this association is currently only supported with case reports [55].
\nAdditionally left ventricular hypertrophy is reported to be positively correlated with AoD. Early retrospective reviews of echocardiographic studies have shown a positive relationship between LVH and AoD, and this has been further supported in subsequent systematic reviews [41, 65]. Patients with AoD with concomitant left ventricular hypertrophy are shown to have an increased risk of adjusted cardiovascular events [66]. However as with previous studies, the exact mechanism between LVH and AoD is still being determined.
\nAortic root dilation is typically a silent disease, with most cases being diagnosed incidentally on imaging. AoD can become symptomatic as the aneurysm enlarges. Aortic root aneurysms grow at an average of 1–4 mm/year [5], with a faster rate of growth noted in patients with bicuspid aortic valves, Marfan syndrome, ESRD, male gender, and smokers [5, 67]. When the aneurysm enlarges to the point of compressing surrounding structures the patient may begin to observe symptoms—the most common of which is chest pain, seen in up to 75% of patients [5, 68]. Other nonspecific symptoms can include back pain, abdominal pain and fatigue (though only present in 5% of patients).
\nAdditionally, patients may present with symptoms secondary to complications of a dilated aortic root such as aortic insufficiency and congestive heart failure. Thus, patients can develop dyspnea as the presenting symptom of aortic root dilation up to 40% of the time [68]. Other presenting symptoms may be related to the complications noted in Table 2 [69, 70, 71, 72, 73, 74].
\nComplication of aortic root aneurysm | \nPresenting symptom | \n
---|---|
Aortic insufficiency, aortic regurgitation | \nDyspnea, diastolic murmur, congestive heart failure symptoms | \n
Aortic dissection | \nSharp chest pain, may radiate to the back | \n
Thromboembolism | \nSymptoms of stroke | \n
Compression of tracheal or bronchus | \nHemoptysis, cough, recurrent pneumonitis | \n
Compression of left recurrent laryngeal nerve | \nHoarseness | \n
Compression of superior vena cava | \nSigns of superior vena cava syndrome | \n
Compression of esophagus | \nDysphagia | \n
Complications and presenting symptoms of aortic root dilation.
Acute aortic emergencies that occur secondary to aortic root dilation include dissection, rupture, and aortic insufficiency. As the aortic root diameter increases, the risk for aortic dissection and rupture rises [75]. Aortic dissections are the most common acute aortic emergencies [76], and can be classified depending on the segment of the aorta affected: type A dissections involve the ascending aorta (seen in aortic root dilation), while type B dissections are those that occur distal to the left subclavian artery.
\nAortic dissection most commonly presents with acute onset chest pain that may radiate to the back. The character of the pain has traditionally been described as ripping or tearing in nature, however over half of patients may instead complain of sharp pain [77]. In addition, geriatric populations are less likely to have an acute onset of pain [78]. Physical exam findings that may be present include unequal blood pressures in the upper extremities, a new diastolic murmur indicative of acute aortic regurgitation, or muffled heart sounds secondary to tamponade (with proximal extension of the dissection). Imaging may be notable for widening of the mediastinum on CXR [77]. In order to aid in the diagnosis of a dissection, an aortic dissection detection risk score (ADD-RS) has been developed. The score is comprised of three categories: the presence of high risk conditions such as Marfan syndrome, the presence of typical symptoms (such as abrupt onset chest pain), and the presence of physical exam findings such as unequal blood pressure readings in the upper extremities. Each group is given a score of 1 if a feature is present, and the total score helps pave the next steps of workup—a score of 0 can be followed by diagnostic workup of other pathologies, while scores of 2–3 should be followed by expedited workup and immediate surgical consultation for possible aortic dissection [79].
\nAortic rupture is also an acute and life-threatening complication of aortic root dilation. It can present similarly to aortic dissection with regards to chest pain, however rupture can lead to severe and abrupt hypotension. Moreover, contingent with the site of rupture the patient may have symptoms such as hemoptysis [80] (if there is rupture into the lung), hematemesis [81] (if there is rupture into the esophagus), or cardiogenic shock [82] (if there is rupture into the pericardial cavity with resultant tamponade physiology).
\nAortic root dilation may also lead to aortic insufficiency. Roughly 30% of aortic insufficiency is now recognized as being caused by aortic root dilation, surpassing the incidence of any valvular cause [83]. The pathophysiology is related to stretching of the aortic valve annulus secondary to aortic root dilation, which results in incomplete closure of the aortic leaflets during diastole. Unfortunately, at the onset of aortic regurgitation, patients may be asymptomatic; therefore, congestive heart failure can develop when the regurgitant valve goes unnoticed.
\nWhile aortic root aneurysms are known to grow at an average of 1–4 mm/year [5], it is difficult to ascertain how fast an individual’s aortic root aneurysm will grow, therefore necessitating surveillance imaging. The frequency of surveillance imaging recommended is dependent on the etiology of the aortic root dilatation as well as its size, with genetically mediated aortic disease having a lower threshold for more frequent (biannual) imaging [84]. At the very least however patients are recommended to have annual imaging for aortic root dilation to closely monitor the aortic diameter. The impact that frequent imaging (CT, MR angiography or echocardiography) has on public health is likely significant, with cumulative costs. In addition, any patient with a bicuspid aortic valve should be screened for a thoracic aortic aneurysm, as well as screening all first-degree family members of a patient with genetic conditions such as Marfan syndrome [85].
\nThe aortic root is the most proximal segment of the aorta. It extends from the annulus of the aortic valve to the sinotubular junction (STJ). It is composed of the left, right, and non coronary sinuses of Valsalva. The diameter of the aorta decreases as it moves distally. The aortic root is assessed using multimodality imaging techniques. These include transthoracic echocardiogram (TTE), cardiac magnetic resonance imaging (cMRI), and cardiac computed tomography angiography (cCTA).
\nTTE is widely used for the detection and monitoring of aortic root pathology. Early studies established age- and sex-specific nomograms for aortic root measurements [86]. These studies used the motion mode (M-mode) of TTE, in which the amplitude of the ultrasound pulses amplitudes is converted to corresponding level on gray-scale imaging [86]. Using the M-mode, the American Society of Echocardiography (ASE) has recommended using the leading-edge to leading-edge approach for measuring the aortic root [87]. Later studies used 2D TTE and obtained reference measurements of the aortic root. This is now preferred over M-mode images, which may be off-axis and are subject to aortic motion that may produce erroneous measurements.
\nOn echocardiogram, the aortic root diameter is typically measured in the parasternal long-axis view from the right coronary sinus to the opposite sinus of Valsalva. When unable to obtain the long axis view, the parasternal short axis view may provide more accurate measurements. However, universal landmarks to measure the root in this view have not been established. Some suggest measuring the diameter from the right coronary sinus to the opposite commissure. These measurements are typically performed at end diastole, as this represents the resting aortic diameter [88]. In adults, these measurements correlate with age and body size. In addition, the aorta is about 2 mm larger in men compared to women due to differences in body size [89]. Normal values stratified by body surface area and age have been published by the ASE [87].
\nImportantly, TTE is limited by its two-dimensional images and thus does not give a complete depiction of the aortic root. It is also limited by patient factors that limit the visualization windows and thus aortic root measurement. Since the aorta is not a straight tube, it can be imaged obliquely leading to over-estimation of its true diameter. Newer modalities, such cMRI and cCTA, can provide three-dimensional images.
\nDespite ECG-gated CT being the most accurate modality for evaluating the thoracic aorta, it is limited by the radiation and contrast exposure. This is particularly important in younger patients with connective tissue disorders that require serial follow up imaging. Cardiac MRI provides an alternative approach for imaging the thoracic aorta including the aortic root and is considered the preferred modality in select groups. It can be performed with ECG gating to provide motion-free evaluation of the aorta. In addition, young patients, in whom this is more commonly used, can hold their breath for longer periods, allowing acquisition of images with high spatial resolution.
\nCardiac MRI evaluation of the aorta does not require contrast use. MRI sequences used include balanced steady-state free precession (SSFP) sequences, fast imaging employing steady-state acquisition (FIESTA), true fast imaging with steady-state precession (FISP), and balanced fast-field echo (FFE) sequences. These sequences provide a high signal-to-noise ratio and adequate contrast between vessel wall and blood pool [90]. When used with ECG gating and contrast enhanced MRA, images tend to have less artifact, higher resolution, and overall short imaging time. Another approach is to use ECG gating 2D balanced SSFP sequence that is oriented perpendicular to the aortic root in two planes to assess the aortic valve and root throughout the cardiac cycle. In addition, prospective ECG gating and respiratory navigation with three-dimensional balanced SSFP sequences can provide 3D aortic imaging without contrast administration [91, 92].
\nIt is important to note that different methods of aortic measurement have been described and guidelines are less well defined. Aortic root measurements can be challenging given different approaches. Burman et al. found in the Framingham Heart Study that cusp-commissure dimensions better corresponded with reference echocardiographic aortic root measurements [89, 93]. This best correlated with study measurements after averaging the three end-diastolic cusp-commissure measurements [93]. In addition, there is a lack of consensus with regard to measurements used (inner lumen only versus lumen and wall) and whether measurements should be adjusted to body surface area, sex, and age.
\nAlthough TTE is widely used for the imaging and surveillance of aortic root, cardiac computed tomography angiography (cCTA) is currently the most commonly used technique for the study of the thoracic aorta. Main advantages of cCTA are fast scanning times, low artifact sensibility, and wide availability including emergency rooms operating 24 h [94].
\nThe new generation CT scanners acquire high-resolution 3D datasets of the thoracic aorta, showing sensitivities up to 100% and specificities of 98–99% [95]. ECG synchronization is vital for detailed assessment of the aortic root anatomy since it allows suppression of pulsation artifacts [96]. ECG gating also allows viewing images in a particular phase of the cardiac cycle. Unfortunately, the ECG-gated technique can increase the acquisition time and required breath-hold time. In order to minimize the increased acquisition times, employment of a 64 or wider ECG-gated row detector system is suggested [95, 97].
\nModern CT scanners can be used to employ several different cardiac synchronization methods such as prospective ECG triggering where images are only acquired during a specified portion of the cardiac cycle, starting at a predetermined delay from the R wave; retrospective ECG gating where the desired cardiac phase is selected retrospectively from the raw data [95, 97]. The details of each technique will not be discussed in this chapter; however, it is important to determine the advantages and disadvantages of different techniques. The main limitations of CT are related to the radiation exposure and the use of iodinated contrast media and different techniques come at a higher cost of each limitation.
\nFor the surveillance of aortic root, any technique can be used and be useful; therefore, the technique with the least amount of radiation exposure should be selected such as prospective sequential triggering without padding, retrospective gating with tube-current modulation optimized for diastolic-phase datasets only, or a prospectively triggered high-pitch helical acquisition [95, 97]. Retrospective ECG gating acquires redundant helical CT data which allows the reconstruction of images at different cardiac phases and providing detailed images which can be useful in complicated cases and pre-/post-operative imaging since pseudoaneurysm or small leaks which are some of the most common complications of aortic root surgery can only be detected during a specific phase of the cardiac cycle. Iodinated contrast-media is another risk related to CT imaging given the risk of contrast induced nephropathy and allergic reactions of various severity. Surveillance CT data for the dimensions of aortic root can be acquired without contrast injection; however, a complete endoluminal evaluation can only be achieved by the injection of contrast-medium [97].
\nIt is standard of care to monitor the size of aortic aneurysms that are below surgical threshold, <5.5 cm for nongenetic aneurysms and <5.0 cm for genetically-mediated aneurysms [98]. In general, physicians should be conscientious about patient cumulative radiation exposure as there is evidence that it can increase cancer incidence and cancer mortality [99]. One study estimated that ionizing radiation exposure results in 0.7% of total expected baseline cancer incidence and 1% of total cancer mortality. These rates increase with greater cumulative exposure [99]. Therefore, physicians should opt to perform serial CT imaging with longer intervals in the most appropriate patients. A study investigating patients with moderate-risk thoracic aortic aneurysms (defined as size <5.0 cm) showed that patients with aneurysms below 4.3 cm had overall lower risk of significant aneurysm growth or size reaching surgical threshold. Thus, the authors suggested that these subset of patients undergo surveillance CT scans less frequently.
\nManagement focuses on slowing the rate of growth and the complications of aortic root dilation. The line of management that is chosen for a patient depends on symptoms and size of the aneurysm. For patients who are asymptomatic and have root dilation <55 mm, medical management is advised. In patients with Marfan syndrome or a bicuspid aortic valve, the cut off of ≤50 mm is used for medical management [1, 100].
\nThe use of beta blockers has shown a survival benefit in patients with aortic root dilation secondary to Marfan syndrome [101]. While data on survival benefits for patients with bicuspid aortic valves is sparse, the common practice is to also prescribe beta blockers given that both conditions share a similar pathology and therefore both are likely to benefit from beta blockade. The mechanism by which beta blockers slow the progression of aortic root dilation is through their negative inotropic and chronotropic effects, reducing the peak left ventricular ejection rate and therefore decreasing shear stress and the rate of aortic dilation [102].
\nThe goal blood pressure for patients with thoracic aortic aneurysms is <130/80 mmHg. In patients who cannot tolerate beta blockers, calcium channel blockers (CCB) are an alternative group of medications available. While less studied as compared to beta blockers, CCB have also been found to reduce the rate of aortic root dilation [103]. Other agents that can be used for additional blood pressure control include ACE-inhibitors and ARBs.
\nIn order to reduce the risk for complications such as aortic dissection, patients should be counseled on smoking cessation, and cessation of drugs that increase aortic wall stress such as cocaine or other stimulants. In addition patients should have dyslipidemia well controlled, which can be achieved through the use of atorvastatin 40–80 mg daily in individuals with aortic root aneurysms [104, 105].
\nPatients should be counseled on avoiding high intensity and collision sports, such as boxing and cycling. Instead patients should take part in low dynamic sports, such as, golf [5, 106]. Pregnancy should be avoided in patients with Marfan syndrome with an aortic diameter >40 mm, if a patient does chose to become pregnant however there must be close follow up with surveillance imaging of the aortic diameter [5, 101].
\nEmergent surgical interventions are indicated for management of an aortic dissection or rupture, or a symptomatic aneurysm. In addition, surgical repair can be performed electively in high risk patients to prevent propagation of an aneurysm (Table 3). Indications for elective surgical intervention include the absolute size of the aneurysm—if the diameter is over 55 mm, or over 50 mm in patients with Marfan syndrome or bicuspid valves. Other indications for elective surgery include if the rate of growth of an aneurysm surpasses 10 mm/year, and if there is concurrent aortic insufficiency [1, 100]. In addition, patients who undergo aortic insufficiency repair who have concurrent aortic root dilation should be considered for aortic replacement at the time of their surgery—that is since 25% of patients with aortic root diameters >40 mm will eventually also require intervention for their aortic aneurysm [107].
\nEmergent surgical repair | \nElective surgical repair | \n
---|---|
\n
| \n\n
| \n
Indications for emergent and elective surgical repair of aortic root dilation.
As opposed to supravalvular aortic aneurysms, aortic root aneurysms involve the coronary ostia as well as the aortic valve, which have implications on the type of surgical procedure available for patients. There are two approaches for a surgical intervention: radical and conservative. In a radial surgical intervention the patient’s aortic valve and root are replaced (commonly referred to as the Bentall procedure), whereas in conservative interventions only the aortic root is replaced [108].
\nThe Bentall procedure involves replacing the aortic valve with a prosthetic valve, and thus has the caveat of requiring indefinite anticoagulation [5]. If patients have a high bleeding risk it may therefore be worthwhile investigating replacement of the aortic root while preserving the valve. In addition, it is important to note that a large number of patients with aortic root dilation are young (secondary to its association with Marfan syndrome), and therefore lifelong anticoagulation in cases such as these confers a cumulative bleeding risk. Preserving the aortic valve while surgically treating the aortic root dilatation is made possible by the development of two surgical procedures: the first is removing the aortic root while keeping the valve intact. The second method is through re-implantation of the aortic valve [5]. Both the Bentall procedure as well as aortic valve-preserving procedures have been shown to have comparable short and long-term outcomes with regards to the risk of death and valve associated complications. The main difference however is that patients undergoing valve sparing operations were significantly more likely to develop moderate to severe aortic regurgitation later [108].
\nIn patients with both severe aortic stenosis and ascending aortic aneurysm, undergoing surgical aortic valve replacement (sAVR) and concomitant surgical intervention for aortic aneurysms above 4.5 cm is recommended by the American College of Cardiology (ACC) foundation guidelines [84]. However, in high-risk surgical patients, undergoing a transcatheter aortic valve replacement (TAVR) has become an alternative approach that obviates the need for parallel surgical aortic aneurysm intervention. This raises the concern for the safety of TAVR catheter-based delivery system in patients with aortic aneurysms since intraoperative rupture or dissection risk potentially increases. However, a clinical study showed that TAVR does not increase intraoperative aortic dissection/rupture risk or mortality with a median follow-up of 14 months [109]. Therefore, there are no recommendations against performing TAVR in patients with ascending aortic aneurysms.
\nNone.
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