The combined impact of mechanical factors on the wall stress of the human ascending aorta - a finite elements study

Reproducibility of 3D thoracic aortic displacement from 3D cine balanced SSFP at 3 T without contrast enhancement

PURPOSE

Aortic motion has direct impact on the mechanical stresses acting on the aorta. In aortic disease, increased stiffness of the aorta may lead to decreased aortic motion over time, which could be a predictor for aortic dissection or rupture. This study investigates the reproducibility of obtaining 3D displacement and diameter maps quantified using accelerated 3D cine MRI at 3 T.

METHODS

A noncontrast-enhanced, free-breathing 3D cine sequence based on balanced SSFP and pseudo-spiral undersampling with high spatial isotropic resolution was developed (spatial/temporal resolution [1.6 mm]3 /67 ms). The thoracic aorta of 14 healthy volunteers was prospectively scanned three times at 3 T: twice on the same day and a third time 2 weeks later. Aortic displacement was calculated using iterative closest point nonrigid registration of manual segmentations of the 3D aorta at end-systole and mid-diastole. Interexamination and interobserver regional analysis of mean displacement for five regions of interest was performed using Bland-Altman analysis. Additionally, a complementary voxel-by-voxel analysis was done, allowing a more local inspection of the method.

RESULTS

No significant differences were found in mean and maximum displacement for any of the regions of interest for the interexamination and interobserver analysis. The maximum displacement measured in the lower half of the ascending aorta was 11.0 ± 3.4 mm (range: 3.0-17.5 mm) for the first scan. The smallest detectable change in mean displacement in the lower half of the ascending aorta was 3 mm.

CONCLUSION

Detailed 3D cine balanced SSFP at 3 T allows for reproducible quantification of systolic-diastolic mean aortic displacement within acceptable limits.

Mechanics of ascending aortic aneurysms based on a modulus of elasticity dependent on aneurysm diameter and pressure

The mechanical stresses and strains are examined, in ascending thoracic aortic aneurysm (aTAA) models, in a patient-specific aTAA as well as in healthy thoracic aortic models, via Finite Element Analysis. The aneurysms are assumed spherical, 1.5 mm thick, with diameters between 47 mm and 80 mm, eccentrically positioned. The geometry and wall thickness distribution of the aorta along its length are based on open literature data for an average patient age of 66.25 years, accounting for the Body Surface Area (BSA) parameter. The vessel wall material is assumed isotropic and incompressible, with its Young's modulus varying with the aneurysm diameter and the applied intraluminal pressure (120 mmHg to 240 mmHg). In the aTAAs, peak stresses were found to increase nonlinearly with aneurysm diameter (for a given pressure) tending to reach a plateau, appearing at the proximal area of the aneurysm, whereas lower stresses were found at its distal part and even smaller at the aneurysm maximum diameter. Regarding the patient-specific aTAA model, the peak stresses appeared at the distal part of the aneurysm where a tear of the intima layer was detected during surgical intervention. Peak strains exhibited for each pressure a maximum at a certain aneurysm diameter beyond which they dropped so that essentially the vessel wall's distensibility was thus reduced. Examining more than 100 geometry cases and employing a failure stress criterion, the rupture diameter thresholds were estimated to be 65, 52.5, 50 and 47.5 mm for a pressure of 120, 160, 200 and 240 mmHg respectively.

Structural numerical analysis of a branched modular stent-graft for aneurysms encompassing all zones of the aortic arch

The development of stent-grafts for the total repair of aneurysms in the aortic arch is still a technical challenge due mainly to the anatomical complexity of this region. Research performed here structurally evaluated a modular branched stent-graft for aneurysms encompassing all zones of the aortic arch by means of numerical simulations using fluid-structure interaction. The geometric domain obtained by means of computed tomography was subjected to physiological boundary conditions. The blood was modelled as non-Newtonian by the Carreau model, and the arterial wall was modelled as anisotropic hyperelastic by the Holzapfel model. The material adopted for the stents was Nitinol, and expanded polytetrafluoroethylene (ePTFE) was used for the graft. A comparison of the structural behaviour of the aneurysmal aortic arch before and after stent-graft implantation was performed. The numerical flow model was experimentally verified in vitro on a representative test bench of blood flow in the aortic arch. The stent-graft was shown to minimally modify arterial wall dynamics and was not susceptible to migration and endoleak. Peak stresses and strains were found in the stents and graft, respectively, while the stresses in the aneurysm sac were significantly reduced, of the order of 97.5%, due to the isolation of the arterial wall by the stent-graft.

Comparative study between 1-way and 2-way coupled fluid-structure interaction in numerical simulation of aortic arch aneurysms

Hemodynamic forces are related to pathological variations of the cardiovascular system, and numerical simulations for fluid-structure interaction have been systematically used to analyze the behavior of blood flow and the arterial wall in aortic aneurysms. This paper proposes a comparative analysis of 1-way and 2-way coupled fluid-structure interaction for aortic arch aneurysm. The coupling models of fluid-structure interaction were conducted using 3D geometry of the thoracic aorta from computed tomography. Hyperelastic anisotropic properties were estimated for the Holzapfel arterial wall model. The rheological behavior of the blood was modeled by the Carreau-Yasuda model. The results showed that the 1-way approach tends to underestimate von Mises stress, displacement, and strain over the entire cardiac cycle, compared to the 2-way approach. In contrast, the behavior of the variables of flow field, velocity, wall shear stress, and Reynolds number when coupled by the 1-way model was overestimated at the systolic moment and tends to be equal at the diastolic moment. The quantitative differences found, especially during the systole, suggest the use of 2-way coupling in numerical simulations of aortic arch aneurysms due to the hyperelastic nature of the arterial wall, which leads to a strong iteration between the fluid and the arterial wall.

Hemodynamic Effect of Pulsatile on Blood Flow Distribution with VA ECMO: A Numerical Study

The pulsatile properties of arterial flow and pressure have been thought to be important. Nevertheless, a gap still exists in the hemodynamic effect of pulsatile flow in improving blood flow distribution of veno-arterial extracorporeal membrane oxygenation (VA ECMO) supported by the circulatory system. The finite-element models, consisting of the aorta, VA ECMO, and intra-aortic balloon pump (IABP) are proposed for fluid-structure interaction calculation of the mechanical response. Group A is cardiogenic shock with 1.5 L/min of cardiac output. Group B is cardiogenic shock with VA ECMO. Group C is added to IABP based on Group B. The sum of the blood flow of cardiac output and VA ECMO remains constant at 4.5 L/min in Group B and Group C. With the recovery of the left ventricular, the flow of VA ECMO declines, and the effective blood of IABP increases. IABP plays the function of balancing blood flow between left arteria femoralis and right arteria femoralis compared with VA ECMO only. The difference of the equivalent energy pressure (dEEP) is crossed at 2.0 L/min to 1.5 L/min of VA ECMO. PPI’ (the revised pulse pressure index) with IABP is twice as much as without IABP. The intersection with two opposing blood generates the region of the aortic arch for the VA ECMO (Group B). In contrast to the VA ECMO, the blood intersection appears from the descending aorta to the renal artery with VA ECMO and IABP. The maximum time-averaged wall shear stress (TAWSS) of the renal artery is a significant difference with or not IABP (VA ECMO: 2.02 vs. 1.98 vs. 2.37 vs. 2.61 vs. 2.86 Pa; VA ECMO and IABP: 8.02 vs. 6.99 vs. 6.62 vs. 6.30 vs. 5.83 Pa). In conclusion, with the recovery of the left ventricle, the flow of VA ECMO declines and the effective blood of IABP increases. The difference between the equivalent energy pressure (EEP) and the surplus hemodynamic energy (SHE) indicates the loss of pulsation from the left ventricular to VA ECMO. 2.0 L/min to 1.5 L/min of VA ECMO showing a similar hemodynamic energy loss with the weak influence of IABP.

Patient-Specific Analysis of Ascending Thoracic Aortic Aneurysm with the Living Heart Human Model

In ascending thoracic aortic aneurysms (ATAAs), aneurysm kinematics are driven by ventricular traction occurring every heartbeat, increasing the stress level of dilated aortic wall. Aortic elongation due to heart motion and aortic length are emerging as potential indicators of adverse events in ATAAs; however, simulation of ATAA that takes into account the cardiac mechanics is technically challenging. The objective of this study was to adapt the realistic Living Heart Human Model (LHHM) to the anatomy and physiology of a patient with ATAA to assess the role of cardiac motion on aortic wall stress distribution. Patient-specific segmentation and material parameter estimation were done using preoperative computed tomography angiography (CTA) and ex vivo biaxial testing of the harvested tissue collected during surgery. The lumped-parameter model of systemic circulation implemented in the LHHM was refined using clinical and echocardiographic data. The results showed that the longitudinal stress was highest in the major curvature of the aneurysm, with specific aortic quadrants having stress levels change from tensile to compressive in a transmural direction. This study revealed the key role of heart motion that stretches the aortic root and increases ATAA wall tension. The ATAA LHHM is a realistic cardiovascular platform where patient-specific information can be easily integrated to assess the aneurysm biomechanics and potentially support the clinical management of patients with ATAAs.

Reducing Aortic Barotrauma and Vascular Extracellular Matrix Degradation by Pacemaker-Mediated QRS Widening

Background

The extent of pressure‐related damage might be related to acceleration rate of the applied pressure (peak dP/dt) in the vascular system. In this study, we sought to determine whether dP/dt applied to the aortic wall (aortic dP/dt) and in turn vascular extracellular matrix degradation can be mitigated via modulation of left ventricular (LV) contractility (LV dP/dt) by pacemaker‐mediated desynchronization.

Methods and Results

First, in 34 patients, changes in aortic dP/dt values in 3 aortic segments in response to pacemaker‐mediated stepwise QRS widening leading to gradual desynchronization of the LV contraction by means of steadily changed atrioventricular delay (AVD) with temporary dual‐chamber pacing was examined before and after beta‐blocker (15 mg IV metoprolol) administration. Second, serum matrix metalloproteinase‐9 levels were measured in the 20 patients with permanent pacemaker while they were on sinus rhythm with normal QRS width and 3 weeks after wide QRS rhythm ensured by dual pacing, dual sensing, and dual response to sensing with short AVD. LV dP/dt substantially correlated with dP/dt measured in ascending (r=0.83), descending (r=0.89), and abdominal aorta (r=0.96). QRS width strongly correlated with dP/dt measured in ascending (r=−0.95), descending (r=−0.92), and abdominal (r=−0.96) aortic segments as well. In patients with permanent pacemaker, wide QRS rhythm led to a significant reduction in serum matrix metalloproteinase‐9 levels (from 142.5±32.9 pg/mL to 87.5±32.4 pg/mL [P<0.001]) at the end of 3 weeks follow‐up.

Conclusions

QRS prolongation by short AVD dual pacing, dual sensing, and dual response to sensing results in concomitant decreases in peak dP/dt values in the LV and in all aortic segments with or without background beta‐blocker administration, which in turn led to a significant reduction in circulating matrix metalloproteinase‐9 levels.

Registration

URL: https://www.clini​caltr​ials.gov; Unique identifier: NCT03665558.

Mechanical and structural properties of different types of human aortic atherosclerotic plaques

Atherosclerotic plaques are characterized by structural heterogeneity affecting aortic behaviour under mechanical loading. There is evidence of direct connections between the structural plaque arrangement and the risk of plaque rupture. As a consequence of aortic plaque rupture, plaque components are transferred by the bloodstream to smaller vessels, resulting in acute cardiovascular events with a poor prognosis, such as heart attacks or strokes. Hence, evaluation of the composition, structure, and biochemical profile of atherosclerotic plaques seems to be of great importance to assess the properties of a mechanically induced failure, indicating the strength and rupture vulnerability of plaque. The main goal of the research was to determine experimentally under uniaxial loading the mechanical properties of different types of the human abdominal aorta and human aortic atherosclerotic plaques identified based on vibrational spectra (ATR-FTIR and FT-Raman spectroscopy) analysis and validated by histological staining. The potential of spectroscopic techniques as a useful histopathological tool was demonstrated. Three types of atherosclerotic plaques - predominantly calcified (APC), lipid (APL), and fibrotic (APF) - were distinguished and confirmed by histopathological examinations. Compared to the normal aorta, fibrotic plaques were stiffer (median of E_H for circumferential and axial directions, respectively: 8.15 MPa and 6.56 MPa) and stronger (median of σ_M for APLc = 1.57 MPa and APLa = 1.64 MPa), lipidic plaques were the weakest (median of σ_M for APLc = 0.76 MPa and APLa = 0.51 MPa), and calcified plaques were the stiffest (median of E_H for circumferential and axial directions, respectively: 13.23 MPa and 6.67 MPa). Therefore, plaques detected as predominantly lipid and calcified are most prone to rupture; however, the failure process reflected by the simplification of the stress-stretch characteristics seems to vary depending on the plaque composition.

Numerical analysis of aortic hemodynamics under the support of venoarterial extracorporeal membrane oxygenation and intra-aortic balloon pump

BACKGROUND AND OBJECTIVE

A gap still exists in the hemodynamic effect of intra-aortic balloon pump (IABP), venoarterial extracorporeal membrane oxygenation (VA-ECMO), and VA-ECMO plus IABP on the blood perfusion of the coronary artery, brain, and lower limb; the relation between heart flow and ECMO flow; and the wall stress of vessels.

METHODS

A finite-element model of the aorta, ECMO, and IABP was proposed to calculate the mechanical response via fluid-structure interaction. Heart failure (HF), IABP, ECMO, and ECMO plus IABP were utilized to study the effect of support models.

RESULTS

For the pressure curve, VA-ECMO weakened the dicrotic notch of pressure compared with HF and the pulsatile index (0.494 vs. 0.706 vs. 0.471 vs. 0.613). IABP, ECMO, and ECMO plus IABP increased the perfusion of the coronary, brain, and renal artery compared with HF. However, ECMO and ECMO plus IABP clearly reduced the blood flow of the left arteria femoralis compared to that of the right arteria femoralis (ECMO: 194.04 vs. 730.80 mL/min; ECMO plus IABP: 342.15 vs. 947.22 mL/min). In addition, the flow of ECMO accessed the renal artery more than the left ventricular flow. Greater ventricular flow perfused to the renal artery at a diastolic period for ECMO plus IABP, especially at the time points of 2.192 s and 2.304 s. Compared to the velocity distribution with ECMO, the flow of the right arteria femoralis was increased in the process of IABP-on. According to these four cases, the stress of the vascular wall was increased for ECMO support at the systolic period. The peak wall stress of ECMO is increased by 20% at 1.68 s.

CONCLUSIONS

ECMO plus IABP is more conducive to the blood supply than other cases from the result of numerical simulation. The location of blood intersection was generated in the region of the renal artery, which is estimated carefully.

A comparison of aortic root measurements by echocardiography and computed tomography

OBJECTIVES

The aim of the study is to evaluate an optimal way to assess the dimensions of the aortic root and each of the sinuses of Valsalva and examine how a single measurement in 1 plane (echocardiography or 2-dimensional computed tomography) can underestimate the maximum dimension of the aortic root.

METHODS

Computed tomography and transthoracic echocardiography images of the aortic root and ascending aorta of 112 patients were analyzed. The minimum and maximum aortic root dimensions, the root perimeter, and the total area of all 3 sinuses of Valsalva were measured on a plane perpendicular to the long axis of the aorta using 3-dimensional multiplanar reconstruction. Moreover, the maximum root dimension was compared with the measurements obtained from the echocardiography and 2-dimensional computed tomography angiography measurements.

RESULTS

The difference in the measurements of the minimum and maximum root dimension was 5.4 ± 3.2 mm (range, 0-21 mm, P < .0001) and was significantly larger in patients with bicuspid aortic valves compared with those with tricuspid valves (6.3 ± 4 mm, range, 0-21 mm vs 4.9 ± 2.6 mm, range, 0-15 mm, P = .036). The maximum root dimension measured in 3-dimensional multiplanar reconstruction (49.1 ± 9.0 mm) differed significantly from the root dimension measured in transthoracic echocardiography in the parasternal long-axis view (44.8 ± 8.4 mm) and 2-dimensional computed tomography (axial plane: 45.5 ± 9.0 mm, coronal plane: 46.1 ± 8.8 mm, sagittal plane: 45.1 ± 8.9 mm) (P < .001).

CONCLUSIONS

The difference in the measurements of the minimum and maximum aortic root dimensions is significant and may exceed 20 mm, especially in patients with bicuspid aortic valves. Therefore, aortic root dimensions can be significantly underestimated with the measurement (echocardiography, computed tomography angiography) performed in only 1 plane.

The evaluation of the aortic annulus displacement during cardiac cycle using magnetic resonance imaging

Background

The stress in the ascending aorta results from many biomechanical factors including the geometry of the vessel and its maximum dimensions, arterial blood pressure and longitudinal systolic stretching due to heart motion. The stretching of the ascending aorta resulting from the longitudinal displacement of the aortic annulus during the heart cycle has not been examined in the general population so far. The aim of the study is to evaluate this parameter using cardiovascular magnetic resonance (CMR) imaging in the general population in all age groups.

Methods

The cardiac magnetic resonance images of 73 patients were evaluated. The maximum distance to which the ventriculo-aortic junction was pulled by the contracting heart (LDAA – longitudinal displacement of the aortic annulus) was measured in the cine coronal sequences. Moreover, the maximum dimensions of the aortic root and the ascending aorta were assessed.

Results

The LDAA value was on average 11.6 ± 2.9 mm (range: 3-19 mm; 95% CI: 10.9–12.3 mm) and did not differ between males and females (11.8 ± 2.9 mm vs. 11.2 ± 2.9 mm, p = .408). The diameter of the ascending aorta was 32 ± 6.3 mm (range: 20-57 mm). The maximal dimension of the aortic root was 35 ± 5.1 mm (range: 18-42 mm). There was a statistically significant negative correlation between the LDAA and the age of patients (r = −.38, p = .001). There was no significant correlation between the LDAA and aortic root dimension (r = .1, p = .409) and between the LDAA and diameter of the ascending aorta (r = .16, p = .170).

Conclusions

Human aortic root and ascending aorta are significantly stretched during systole and the distance to which the aorta is stretched decreases with age. The measurement of the longitudinal displacement of the aortic annulus using the CMR is feasible and reproducible.