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/lp/ou_press/wall-stress-correlates-with-intimal-entry-tear-localization-in-type-a-M5cECFX9f8
- Publisher
- Oxford University Press
- Copyright
- © The Author(s) 2018. Published by Oxford University Press on behalf of the European Association for Cardio-Thoracic Surgery. All rights reserved.
- ISSN
- 1569-9293
- eISSN
- 1569-9285
- DOI
- 10.1093/icvts/ivy158
- Publisher site
- See Article on Publisher Site
Abstract
Abstract OBJECTIVES The risk of aortic dissection should be assessed based not only on the aortic diameter, but also on other biomechanical parameters that have an impact on the stress in the aortic wall. This study evaluates very rare clinical scenarios of patients with both pre- and post-dissection computed tomography (CT) images and evaluates whether an increased wall stress correlates with the localization of an intimal entry tear in Type A aortic dissection. METHODS CT-angiography images of 4 patients performed shortly prior to and after developing Type A aortic dissection were evaluated. The stress distribution in the pre-dissection aortas was evaluated using the finite elements method. Then, the areas of high stress in the pre-dissection aortas were compared to the localization of the intimal entry tears in the dissected vessels. RESULTS In all the patients, the pre-dissection areas of high wall stress correlated with the location of the intimal tears. The highest stress was not observed in the most dilated segments of the aorta but was predominantly found in the areas of an abrupt change in the geometry of the aorta. CONCLUSIONS Wall stress can indicate the areas susceptible to the formation of an intimal tear and subsequent aortic dissection. Stress analysis may be a valuable tool to predict the risk of aortic dissection in patients with aortic dilatation. Aorta , Dissection , Wall stress , Aneurysm INTRODUCTION Aortic dissection is a phenomenon that is still not well understood. Patients who are considered to be at risk of experiencing aortic dissection are qualified for surgery based mainly on the diameter of the aorta. However, recent findings suggest that the assessment of the diameter is not sufficient and that aortic dissection usually occurs in patients with moderately dilated aortas that have a mean diameter of 40 mm [1]. This value is much smaller than the diameter thresholds provided in the current guidelines [2, 3]. Those findings indicate that the evaluation of the risk of aortic dissection should be based not only on the diameter, but also on other biomechanical parameters that have an impact on the stress in the aortic wall: the geometry of the vessel, including its diameter and shape, the arterial blood pressure, the longitudinal stretching of the aorta during systole and the aortic wall elasticity [4]. An estimation of the aortic wall stress that includes all these parameters can be performed non-invasively using the finite elements analysis. This method has been used in aortic research for many years [5–9]. However, to date, there has been no direct evidence of a correlation between the finite elements simulation results and ‘real life’ clinical scenarios. No studies have objectively confirmed the existence of a direct relationship between the aortic wall stress and the localization of the intimal entry tear. The biomechanical studies, which analysed the wall stress in the aortic dissection, were based on the analysis of the already dissected vessels. An association between the stress in the aortic wall and the localization of the entry tear can be assessed by comparing the stress distribution in the non-dissected aorta with the localization of the entry tear after the dissection in the same patient. However, such clinical scenarios are very rare, as there are very few patients who have computed tomography (CT) angiography images of the aorta performed shortly before and after Type A aortic dissection. The aim of the study was to analyse whether the areas of high wall stress in the aorta prior to dissection correlate with the localization of the intimal entry tear in the dissected vessel in the same patient. PATIENTS AND METHODS Patients Two databases from 2 cardiac surgery centres (University Hospital in Wroclaw and Heart Centre Freiburg University) were evaluated to identify patients who suffered from aortic dissection (N = 710) between 2006 and 2016. Eleven patients with Type A aortic dissection who underwent CT scans performed shortly before (less than 6 months before the dissection) and after the dissection (less than 24 h after the onset of the symptoms of dissection) were identified and selected for initial assessment. Patients with poor quality pre- and/or post-dissection CT-images [no contrast, non-electrocardiography (ECG) gated, non-identifiable intimal entry tear localization or the presence of artefacts that made the good-quality 3-dimensional (3D) reconstruction impossible] were excluded. Finally, the CT images of 4 patients were suitable for analysis. Pre-dissection stress analysis Pre-dissection CT-scans (slice thickness ≤2 mm) were used to create computational 3D models of the thoracic aorta from the level of the ventriculo-aortic junction to the mid-descending aorta including proximal segments of the aortic arch branches. Then, the surface models were covered with a net of 2 mm thick SHELL181 type elements during the discretization process. Each element consisted of 3 layers of different material properties to simulate the 3-layer aortic wall (Table 1). Then, the simulations using the finite elements method were performed for each pre-dissection model (ANSYS Software v0.16, USA). They were then subjected to a luminal pressure of 120 mmHg and 10 mm up-and-down stretching of the ascending aorta caused by the systolic heart motion. Based on the results of these simulations, stress distribution and areas of peak wall stress were assessed in the models of pre-dissection aortas. Table 1: Mechanical parameters of the simulated aortic wall Aortic wall layers Thickness (mm) Young’s modulus (MPa) Poisson’s ratio Intimal 0.2 2.98 0.49 Medial 1.2 8.95 0.49 External 0.6 2.98 0.49 Aortic wall layers Thickness (mm) Young’s modulus (MPa) Poisson’s ratio Intimal 0.2 2.98 0.49 Medial 1.2 8.95 0.49 External 0.6 2.98 0.49 Table 1: Mechanical parameters of the simulated aortic wall Aortic wall layers Thickness (mm) Young’s modulus (MPa) Poisson’s ratio Intimal 0.2 2.98 0.49 Medial 1.2 8.95 0.49 External 0.6 2.98 0.49 Aortic wall layers Thickness (mm) Young’s modulus (MPa) Poisson’s ratio Intimal 0.2 2.98 0.49 Medial 1.2 8.95 0.49 External 0.6 2.98 0.49 Post-dissection images Post-dissection CT-angiography scans (slice thickness ≤2 mm) were evaluated. The maximum aortic dimensions were measured and the localization of the intimal entry tears was identified. Finally, the areas of high wall stress in the pre-dissection models and the localization of the intimal entry tear were compared. The study was approved by the local ethics committees of the Wroclaw Medical University and the University of Freiburg. No informed patient consent was required for this retrospective analysis. RESULTS All patients were diagnosed with an acute Type A dissection. The pre-dissection CT-scans were performed due to a suspicion of pulmonary embolism (2 patients) and as the next step in the diagnostic process after the identification of aortic dilatation in transthoracic echocardiography (2 patients). None of the patients had undergone cardiac or aortic surgery before the dissection. The maximum aortic diameter prior to the dissection was on average 43.8 mm (range 35–50 mm). The most dilated segments before the dissection were: the tubular ascending aortas in 2 patients, the aortic root in 1 patient and the sinotubular junction in 1 patient. The morphology of the ascending aorta before the dissection differed between patients (tubular type moderate dilatation in 2 patients, root type moderate dilatation in 1 patient and normal non-dilated aorta in 1 patient). The time interval between pre- and post-dissection CT scans was on average 92 days (range 75–122 days). The maximum aortic diameter after the dissection measured on average 51.5 mm (range 40–62 mm). The aortic root (n = 2, 50%) and the tubular ascending aorta (n = 2, 50%) were the most dilated segments following the dissection. The length of the ascending aorta before the dissection measured from the ventriculo-aortic junction to the aortic arch was on average 110.3 mm (range 97–121 mm). All patients had tricuspid aortic valves. The detailed patient data are presented in Table 2. Table 2: Patient characteristics Patient Gender Age (years) Interval between CT-scans (days) Maximum pre-dissection diameter (mm) Maximum post-dissection diameter (mm) Type of aortic valve Morphology of the aorta Length of the aorta (from VAJ to aortic arch) (mm) Location of the entry tear 1 Male 85 122 50 (ascending aorta) 62 (ascending aorta) Tricuspid Tubular type 114 Mid-ascending aorta 2 Male 71 75 47 (ascending aorta) 58 (ascending aorta) Tricuspid Tubular type 121 Sinotubular junction 3 Female 51 92 35 (aortic root) 40 (sinotubular junction) Tricuspid Normal 97 Sinotubular junction 4 Female 78 88 43 (aortic root) 46 (aortic root) Tricuspid Root type 109 Mid-ascending aorta Patient Gender Age (years) Interval between CT-scans (days) Maximum pre-dissection diameter (mm) Maximum post-dissection diameter (mm) Type of aortic valve Morphology of the aorta Length of the aorta (from VAJ to aortic arch) (mm) Location of the entry tear 1 Male 85 122 50 (ascending aorta) 62 (ascending aorta) Tricuspid Tubular type 114 Mid-ascending aorta 2 Male 71 75 47 (ascending aorta) 58 (ascending aorta) Tricuspid Tubular type 121 Sinotubular junction 3 Female 51 92 35 (aortic root) 40 (sinotubular junction) Tricuspid Normal 97 Sinotubular junction 4 Female 78 88 43 (aortic root) 46 (aortic root) Tricuspid Root type 109 Mid-ascending aorta CT: computed tomography; VAJ: ventriculo-aortic junction. Table 2: Patient characteristics Patient Gender Age (years) Interval between CT-scans (days) Maximum pre-dissection diameter (mm) Maximum post-dissection diameter (mm) Type of aortic valve Morphology of the aorta Length of the aorta (from VAJ to aortic arch) (mm) Location of the entry tear 1 Male 85 122 50 (ascending aorta) 62 (ascending aorta) Tricuspid Tubular type 114 Mid-ascending aorta 2 Male 71 75 47 (ascending aorta) 58 (ascending aorta) Tricuspid Tubular type 121 Sinotubular junction 3 Female 51 92 35 (aortic root) 40 (sinotubular junction) Tricuspid Normal 97 Sinotubular junction 4 Female 78 88 43 (aortic root) 46 (aortic root) Tricuspid Root type 109 Mid-ascending aorta Patient Gender Age (years) Interval between CT-scans (days) Maximum pre-dissection diameter (mm) Maximum post-dissection diameter (mm) Type of aortic valve Morphology of the aorta Length of the aorta (from VAJ to aortic arch) (mm) Location of the entry tear 1 Male 85 122 50 (ascending aorta) 62 (ascending aorta) Tricuspid Tubular type 114 Mid-ascending aorta 2 Male 71 75 47 (ascending aorta) 58 (ascending aorta) Tricuspid Tubular type 121 Sinotubular junction 3 Female 51 92 35 (aortic root) 40 (sinotubular junction) Tricuspid Normal 97 Sinotubular junction 4 Female 78 88 43 (aortic root) 46 (aortic root) Tricuspid Root type 109 Mid-ascending aorta CT: computed tomography; VAJ: ventriculo-aortic junction. The distribution of stress differed between patients. In the most dilated segments of the aorta a moderately elevated wall stress was observed (0.2–0.5 MPa). The highest stress was predominantly present in the areas of an abrupt change in the geometry of the aorta and it ranged from 0.4 MPa to 0.6 MPa. The peak wall stress was present at the junction of the ascending aorta and the innominate artery (in all patients), in the mid-ascending aorta (2 patients) and at the level of the sinotubular junction (2 patients) (Fig. 1). Figure 1: View largeDownload slide Correlation between aortic wall stress before dissection and localization of the entry tears in the dissected aortas. CT: computed tomography. Figure 1: View largeDownload slide Correlation between aortic wall stress before dissection and localization of the entry tears in the dissected aortas. CT: computed tomography. The entry tears were localized in the mid-ascending aorta in 2 patients and at the level of the sinotubular junction in the remaining 2 patients. In all of the patients, the areas of high wall stress correlated with the intimal entry tears localization in the dissected vessels. DISCUSSION This is the first study that compares the aorta before and after the dissection with respect to the pre-dissection wall stress and the localization of the entry tears. The findings of the study can be summarized as follows: (i) high wall stress correlated with the areas of intimal tears and (ii) peak wall stress was predominantly located in the areas of abrupt geometrical change rather than in the most dilated segments. The dissection of the thoracic aorta is an unpredictable phenomenon. The parameter that is commonly used to determine the risk of this complication is the maximum diameter of the aorta. However, recent studies suggest that this is not a good prognostic factor for evaluating the risk of aortic dissection [1, 9–11]. Despite the fact that patients with large aneurysms of the thoracic aorta are at a high risk of dissection, most cases of dissection occur in patients with slightly or moderately dilated aortas [1, 10, 11]. The findings of our study support the thesis that the diameter alone is not sufficient to assess the risk of an aortic dissection. In our study, the dimensions of the aorta did not reach the threshold value being the indication for surgical repair in any of the patients [2]. The aortic wall is a complex structure that differs significantly between patients. There are numerous direct and indirect biomechanical factors that impact the wall stress of the aorta, i.e. longitudinal stretching caused by the systolic motion of the heart, blood pressure, the geometry of the aorta and aortic wall elasticity [4]. Most of them can be measured based on standard imaging diagnostic tools and a physical examination. An accurate and non-invasive evaluation of aortic elasticity is currently very difficult. There are various indirect findings which may suggest that the vessel has impaired elasticity, i.e. the presence of connective tissue disorders, a family history of aortic dissection or the presence of a bicuspid aortic valve [12–17]. All of our patients had tricuspid aortic valves, hence the effect of a bicuspid aortic valve on the wall stress was not assessed in this study. One of the factors that may correlate with wall elasticity is aortic distensibility, but it remains unclear whether this parameter is a good indicator of the loss of aortic elasticity [18, 19]. Our findings support the thesis that high stress is localized in areas of an abrupt geometrical change and not in the most dilated segments of the aorta. In our opinion, the assessment of the shape of the vessel may be an additional parameter used to better predict the risk of aortic complications. The ‘malignant geometries’ of the aorta should be evaluated in detail in studies involving a much larger number of patients. One of the parameters that may indicate a ‘malignant geometry’ is aortic elongation [20, 21]. In our study, the pre-dissection ascending aorta (the ventriculo-aortic junction to the aortic arch distance) was 11–12 cm long in 3 (75%) patients. According to the tubingen aortic pathoanatomy (TAIPAN) study, such a distance was associated with a higher risk of dissection [20]. The results of this study suggest that stress analysis may indicate where the dissection is likely to occur in the examined aorta. However, it does not provide us with an answer which aorta is more likely to dissect. It remains unclear why in some patients areas of high stress undergo dissection, while in others they remain intact. Our initial results obtained from patients who did not develop aortic dissection indicate that the peak wall stress values do not differ significantly (0.3–0.6 MPa) from those observed in the 4 patients who suffered from dissection (0.4–0.6 MPa). Therefore, this study did not determine the definitive stress values indicative of a high risk of dissection. A much larger group of patients needs to be examined to define the features of the ‘hazardous stress’ (including peak stress value, stress distribution, or its correlation with other clinical or biomechanical factors, i.e. wall elasticity). The high wall stress was also observed at the junction between the innominate artery and the ascending aorta/aortic arch in all of the patients. However, none of the patients had an intimal entry tear in this area. A similar zone of high stress has been observed in our preliminary analyses in patients who did not suffer from aortic dissection. We assume that this high stress was primarily caused by the imperfection of current numeric models, which were unable to correctly reflect the biomechanics of the transition zone between the aorta and its branches. Based on the results of this study, the intimal entry tears are located in the areas of high wall stress. Thus, developing diagnostic tools to assess the risk of aortic dissection based on the estimated aortic wall stress is warranted. Such an estimation can be based on computational simulations, which incorporate various mechanical factors, such as the geometry of the aorta as well as the stretching of the aorta and wall elasticity. Patient-specific finite elements simulations may aid in this diagnostic process. However, such simulations are time-consuming and introducing them in daily clinical practice would be difficult. In our opinion, a non-invasive stress analysis based on standard diagnostic examinations, i.e. a risk models based on the geometric features of the aorta or the measurement of aortic strain assessed in echocardiography or magnetic resonance imaging, could be a clinically valid solution for the assessment of the risk of aortic dissection in patients with non-dilated or moderately dilated aortas. This may lead to a preventive treatment of patients with a high risk of developing aortic complications including pharmacotherapy or surgery using less invasive methods, i.e. external aortic support (PEARS) or new generation thoracic endovascular aortic repair [22, 23]. It is still impossible to indicate which patients would benefit from invasive treatment based solely on the stress analysis. Hopefully, future studies will improve the accuracy of risk stratification in aortic diseases. Limitations The study was performed on a small group of patients due to the fact that few patients had good quality CT-angiography images both, pre- and post-dissection. Several simplifications had to be made in the computational models and simulations: the aortic valve and the tissues surrounding the aorta were not simulated. Current diagnostic tools do not allow for a precise estimation of the aortic wall thickness and the aortic wall in the computational model had the same biomechanical properties (thickness, elasticity) along the course of the whole aorta. The study involved only those patients who suffered from Type A aortic dissection. Further studies involving patients diagnosed with Type B dissection could provide clinically relevant data that may be useful in creating more accurate risk prediction models for aortic dissection. CONCLUSIONS Peak wall stress correlated with the localization of the intimal entry tears of the dissected aortas and was primarily located in the areas of abrupt geometrical change and not in the most dilated aortic segments. These findings may explain why the aortic diameter is not a sufficient predictor of the risk of aortic dissection. Stress analysis has a potential to become a valuable tool to predict aortic dissection in patients with aortic dilatation. However, further prospective studies are necessary to fully validate stress analysis as a diagnostic method and to define normal values for stress and its distribution on the surface of the aortic wall. Funding This work was supported by a statutory grant from the Wroclaw Medical University [grant number STM.C050.16.008]. Calculations were carried out using resources provided by the Wroclaw Centre for Networking and Supercomputing (http://wcss.pl) [grant number 423]. Conflict of interest: none declared. 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Eur J Cardiothorac Surg 2017 ; doi:10.1093/ejcts/ezx308. © The Author(s) 2018. Published by Oxford University Press on behalf of the European Association for Cardio-Thoracic Surgery. All rights reserved. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices)
Journal
Interactive CardioVascular and Thoracic Surgery – Oxford University Press
Published: Jun 4, 2018