Old Myths, New Concerns: the Long-Term Effects of Ascending Aorta Replacement with Dacron Grafts. Not All That Glitters Is Gold

Synthetic grafts are widely used in cardiac and vascular surgery since the mid-1970s. Despite their general good performance, inability of mimicking the elastomechanical characteristics of the native arterial tissue, and the consequent lack of adequate compliance, leads to a cascade of hemodynamic and biological alterations deeply affecting cardiovascular homeostasis. Those concerns have been reconsidered in more contemporaneous surgical and experimental reports which also triggered some research efforts in the tissue engineering field towards the realization of biomimetic arterial surrogates. The present review focuses on the significance of the “compliance mismatch” phenomenon occurring after aortic root or ascending aorta replacement with prosthetic grafts and discusses the clinical reflexes of this state of tissue incompatibility, as the loss of the native elastomechanical properties of the aorta can translate into detrimental effects on the normal efficiency of the aortic root complex with impact in the long-term results of patients undergoing aortic replacement.

Regional and directional compliance of the healthy aorta: an ex vivo study in a porcine model

OBJECTIVES

To gain differential knowledge about the physiological compliance and wall strength of the different regions of the aorta, including the ascending aorta, arch and descending aorta in both the circumferential and longitudinal directions, and to generate a hypothesis on the pathophysiological mechanisms that lead to Type A aortic dissection.

METHODS

Fresh tissue specimens from 22 ex vivo porcine aortas were analysed on a tensile tester. Regional and directional compliance, failure stress and failure strain were recorded.

RESULTS

Aortic compliance appeared as a linear function of the natural logarithm (ln) of wall stress. Compliance significantly decreased along the length of the aorta. In the ascending aorta, longitudinal compliance significantly (P = 0.003) exceeded circumferential compliance, and the outer curvature was more compliant than the inner curvature (P = 0.03). In the descending aorta, this relationship is reversed: the circumferential compliance exceeded the longitudinal compliance, and the outer aspect was more compliant (P = 0.003). The median circumferential failure stress of all aortic segments was in the range of 2000-2750 kPa, whereas the longitudinal failure stress in the ascending aorta and the arch had values of 750-1000 kPa, which were significantly lower (P < 0.05). Surprisingly, the longitudinal failure stress of the inner aspect of the descending aorta was extraordinarily high (2000 kPa). Failure strain, similar to compliance, was highest in the ascending aorta and decreased along the aorta.

CONCLUSION

The aorta appears to be a complex organ with distinct regional and directional differences in compliance and wall strength that is designed to effectively absorb the kinetic energy of cardiac systole and to cushion the momentum of systolic impact. Under normotensive conditions and a preconditioned physiological morphology, the aortic wall works in the steep part of the logarithmic strain-stress function; under hypertensive conditions and pathological morphology, the wall reacts in an non-compliant manner. The high longitudinal compliance and low failure stress of the ascending aorta and subsequent pathological changes may be the main determinants of the recurrent patho-anatomy of Type A aortic dissection.

Development of an In Vitro PIV Setup for Preliminary Investigation of the Effects of Aortic Compliance on Flow Patterns and Hemodynamics

The aorta with its compliance plays a major role in hemodynamics as it saves a portion of ejected blood during systole which is then released in diastole. The aortic compliance decreases with increasing age, which is related to several cardiovascular imparities and diseases. Changes in flow patterns and pressure curves, due to varying aortic compliance, are difficult to investigate in vivo. As a result, the aim of the present work was to develop an in vitro setup enabling standardized investigations on the effect of compliance changes on flow patterns and pressure curves. Therefore an experimental setup with an anatomically correct silicone phantom of the aortic arch was developed, suitable for optical flow measurements under pulsatile inflow conditions. The setup was developed for precise adjustments of different compliances and optical flow measurements. Particle image velocimetry measurements were carried out downstream of the aortic valve in the center plane perpendicular to the valve with compliance adjusted between 0.62 × 10^-3 to 1.82 × 10^-3 mmHg^-1. Preliminary results of the in vitro investigations showed that decreases in compliance results in significant increases in pressure changes with respect to time (dp/dt) and altered pressure curves in the aortic arch. In terms of flow, an increased aortic stiffness lead to higher mean velocities and decreased vortex development in the aortic sinuses. As in vivo validation and translation remains difficult, the results have to be considered as preliminary in vitro insights into the mechanisms of (age-related) compliance changes.

Influence of shape-memory stent grafts on local aortic compliance

The effect of repair techniques on the biomechanics of the aorta is poorly understood, resulting in significant levels of postoperative complications for patients worldwide. This study presents a computational analysis of the influence of Nitinol-based devices on the biomechanical performance of a healthy patient-specific human aorta. Simulations reveal that Nitinol stent-grafts stretch the artery wall so that collagen is stretched to a straightened high-stiffness configuration. The high-compliance regime (HCR) associated with low diastolic lumen pressure is eliminated, and the artery operates in a low-compliance regime (LCR) throughout the entire cardiac cycle. The slope of the lumen pressure–area curve for the LCR post-implantation is almost identical to that of the native vessel during systole. This negligible change from the native LCR slope occurs because the stent-graft increases its diameter from the crimped configuration during deployment so that it reaches a low-stiffness unloading plateau. The effective radial stiffness of the implant along this unloading plateau is negligible compared to the stiffness of the artery wall. Provided the Nitinol device unloads sufficiently during deployment to the unloading plateau, the degree of oversizing has a negligible effect on the pressure–area response of the vessel, as each device exerts approximately the same radial force, the slope of which is negligible compared to the LCR slope of the native artery. We show that 10% oversizing based on the observed diastolic diameter in the mid descending thoracic aorta results in a complete loss of contact between the device and the wall during systole, which could lead to an endoleak and stent migration. 20% oversizing reaches the Dacron enforced area limit (DEAL) during the pulse pressure and results in an effective zero-compliance in the later portion of systole.

The combined effect of hypertension and type 2 diabetes mellitus on aortic stiffness and endothelial dysfunction: an integrated study with high-resolution MRI

PURPOSE

The purpose of this study was to investigate the combined effect of hypertension and type 2 diabetes mellitus (DM2) on aortic stiffness and endothelial dysfunction by using an integrated MRI approach.

MATERIALS AND METHODS

A total of 31 non-hypertensive DM2 patients and 31 hypertensive DM2 patients underwent 3.0-T MRI. Aortic distensibility (AD), pulse wave velocity (PWV) and brachial artery flow-mediated dilation (FMD) were assessed. Student's t-test, Mann-Whitney U test, chi-squared test, Pearson correlation analysis, and univariable and multiple linear regression analyses were used for statistical analyses.

RESULTS

The hypertensive patients showed lower AD at multiple levels (ascending aorta [AA]: 2.07±0.98×10(-3)mm Hg(-1) vs. 3.21±1.70×10(-3)mm Hg(-1), p<0.01; proximal thoracic descending aorta [PDA]: 2.58±0.72×10(-3)mm Hg(-1) vs. 3.58±1.47×10(-3)mm Hg(-1), p<0.01; distal descending aorta [DDA]: 3.11±1.84×10(-3)mm Hg(-1) vs. 4.27±1.75×10(-3)mm Hg(-1), p<0.01); faster PWV (7.46±2.28m/s vs. 5.82±1.12m/s, p<0.05) and lower FMD (12.67%±6.49% vs. 20.66%±9.7%; p<0.01). Systolic blood pressure was an independent predictor of PWV, AA-AD, DDA-AD and FMD. FMD was statistically significantly associated with PWV (r=-0.37, p<0.01) and AD (p<0.01).

CONCLUSIONS

Hypertension has a contributive effect on aortic stiffness and endothelial dysfunction in DM2 patients.

Disparate Information Provided by Pulse Wave Velocity versus Other Measures of Aortic Compliance in End-Stage Renal Disease

INTRODUCTION

Unfavorable changes in cardiac and arterial function are related to poor prognosis in chronic kidney disease (CKD). We compared hemodynamic profiles between subjects with end-stage renal disease and 2 control groups with corresponding pulse wave velocities (PWVs).

METHODS

Noninvasive hemodynamics were recorded during passive head-up tilt in CKD stage 5 patients (n = 35), patients with primary hypertension (n = 35, n = 30 with antihypertensive medications), and subjects without cardiovascular or renal diseases and cardiovascular medications (n = 70). The groups were selected to have corresponding age, sex, body mass index, and PWV. Hemodynamic data were captured using whole-body impedance cardiography and radial tonometric pulse wave analysis.

RESULTS

Supine blood pressure did not differ between the groups, but upright diastolic blood pressure was lower in CKD patients than in the 2 control groups (p ≤ 0.001 for both, RANOVA). Despite similar PWV, supine aortic pulse pressure was higher in CKD patients versus nonmedicated subjects (p = 0.029). Two additional measures indicated reduced aortic compliance in CKD patients versus both control groups: lower ratio of stroke index to aortic pulse pressure (p ≤ 0.023) and higher aortic characteristic impedance (p ≤ 0.003). The subendocardial viability ratio was lower in the CKD group than in both control groups (p ≤ 0.039).

CONCLUSION

In the absence of differences in PWV, higher aortic pulse pressure and characteristic impedance, and lower ratio of stroke index to aortic pulse pressure, suggest reduced aortic compliance and impaired left ventricular function in CKD patients. A lower subendocardial viability ratio predisposes the CKD patients to impaired cardiac oxygen supply versus hypertensive patients and nonmedicated controls.

Computational model of coarctation of the aorta in rabbits suggests persistent ascending aortic remodeling post-correction

Coarctation of the aorta (CoA) is a common congenital cardiovascular lesion that presents as a localized narrowing of the proximal descending aorta. While improvements in surgical and catheter-based techniques have increased short-term survival, there is a high long-term risk of hypertension and a reduced average lifespan despite correction. Computational models can be used to estimate aortic remodeling and peripheral vascular compensation, potentially serving as key tools in developing a mechanistic understanding of the interplay between pre-treatment dynamics, post-treatment recovery, and long-term hypertension risk. In this study, we developed a lumped-parameter model of the heart and circulation to simulate CoA. After fitting model parameters using imaging and catheterization data from healthy rabbits, we then used the model to estimate differences in ascending aortic compliance and peripheral resistance between the healthy group and rabbits with both untreated and corrected CoA using their imaging and catheterization data. CoA was defined by the current putative clinical treatment threshold (a pressure gradient > 20 mm Hg). Model inputs were fitted such that outputs matched reported stroke volume, ejection fraction, systolic and diastolic aortic pressure, peak aortic flow, mean and peak blood pressure gradients, and upper-to-lower body flow split, with all results falling within one standard deviation of the data for all groups. In the untreated CoA and corrected simulations, a decrease in ascending aortic compliance was necessary to match reported hemodynamics. This suggests exposure to a pressure gradient > 20 mm Hg results in vascular remodeling that persists after repair, a process strongly correlated with hypertension.

Evaluation of the acute effects of inhaled iloprost on aortic compliance in pulmonary arterial hypertension using invasive methods

BACKGROUND

Pulmonary arterial hypertension (PAH) is characterized by vascular remodeling and elevated pulmonary vascular resistance. Aortic compliance indicates vascular stiffness and may be increased in PAH. Inhaled iloprost, a prostacyclin analog, is commonly used for PAH treatment and vasoreactivity testing. Its acute effects on aortic compliance remain unclear. This study evaluated the immediate impact of inhaled iloprost on aortic compliance through aortic pulse wave velocity (aPWV) measurements and pressure parameters in patients with PAH undergoing right heart catheterization.

METHODS

This single-center, cross-sectional study enrolled patients with group 1 PAH who underwent right heart catheterization with pulmonary vasoreactivity testing between August 2022 and May 2023. Aortic compliance was measured via aPWV before and after inhaling 20 mcg/mL iloprost. On the basis of the post-iloprost changes, 32 patients were categorized into high-aPWV (n = 4) or low-aPWV (n = 28) groups. Multivariate regression analysis identified significant predictors of impaired aortic stiffness.

RESULTS

The median patient age was 54.5 years (42.2-60.5). No significant differences were found between groups regarding percentage reductions in proximal/distal aortic pressure and pulmonary artery pressure after iloprost administration. The QRS interval was a significant predictor of impaired aortic stiffness (odds ratio: 1.072, 95% confidence interval: 1.002-1.197, p = 0.045). The high-aPWV group demonstrated significantly lower QRS intervals compared to the low-aPWV group (79.0 ms [70.5-84.0] vs. 96.0 ms [85.5-102.0], p = 0.011).

CONCLUSIONS

Inhaled iloprost effectively reduced both aortic pressure and pulmonary artery pressure regardless of aortic compliance in patients with group 1 PAH. The QRS interval emerged as an independent predictor of impaired aortic stiffness, offering potential for risk stratification in clinical practice.