Wall thickness of coronary vessels varies transmurally in the LV but not the RV: implications for local stress distribution

Abstract

Since the right and left ventricles (RV and LV) function under different loading conditions, it is not surprising that they differ in their mechanics (intramyocardial pressure), structure, and metabolism; such differences may also contribute to differences in the coronary vessel wall. Our hypothesis is that intima-media thickness (IMT), IMT-to-radius (IMT-to-R) ratio, and vessel wall stress vary transmurally in the LV, much more than in the RV. Five normal Yorkshire swine were used in this study. The major coronary arteries were cannulated through the aorta and perfusion fixed with 6.25% glutaraldehyde and casted with a catalyzed silicone-elastomer solution. Arterial and venous vessels were obtained from different transmural locations of the RV and LV, processed for histological analysis, and measured with an imaging software. A larger transmural gradient was found for IMT, IMT-to-R ratio, and diastolic circumferential stress in vessels from the LV than the nearly zero transmural slope in the RV. The IMT of arterial vessels in the LV showed a slope of 0.7 +/- 0.5 compared with 0.3 +/- 0.3 of arterial vessels in the RV (P <or= 0.05). The slope for venous vessels in the LV was 0.14 +/- 0.14 vs. 0.06 +/- 0.05 in the RV. The present data reflect the local structure-function relation, where the significant gradient in intramyocardial pressure in the LV is associated with a significant gradient of IMT and IMT-to-R ratio, unlike the RV. This has important implications for local adaptation of transmural loading on the vessel wall and vascular remodeling when the loading is perturbed in cardiac hypertrophy or heart failure.

Why is the subendocardium more vulnerable to ischemia? A new paradigm

“…Even small compliance differences may affect flow distribution, however, since flow depends on the fourth power of diameters (Poiseuille's law). Hence, the simulated wall thickness of each vessel was modified in the vessel-in-myocardium micromechanical submodel to vary linearly with transmural vessel location, consistent with experimental data (10).…”

“…In addition to the longitudinal differences in vessel wall thickness (i.e., differences between vessels of different diameters and between arteries, capillaries, and veins), recent data have shown that endocardial vessel walls are ϳ50% thinner than the respective epicardial vessels (10). This observation was expected to account for the generally small transmural differences in in situ vessel compliance, since in situ compliance is due to the combined effects of vessel and myocardium intrinsic mechanical properties (Fig.…”

“…Structural differences affecting vessel in situ compliance were assumed to stem from transmural differences in vascular anatomy, i.e., diameters (24), vessel numbers (24), and wall thickness (10). Additional possible factors may include vessel and myocardium constitutive properties and residual strains.…”

“…Specifically, we hypothesized that 1) the extravascular loading exerted by the contracting myocardium on its vessels induces lower transvascular pressures (⌬P) in the deep myocardial layers; 2) this effect, when combined with vessel pressure-dependent compliance, induces a higher compliance of the subendocardial vasculature and consequent flow redistribution away from the subendocardium under reduced perfusion pressure; and 3) the thinner subendocardial vascular walls (10) have greater compliance, which further enhances flow redistribution.…”

A validated predictive model of coronary fractional flow reserve

“…Pulsatile pressure and flow were continuously recorded using a Biopac MP 150 data acquisition system (Biopac Systems, Inc., Goleta, CA). A cast was made at 100 mmHg after the stenotic vessel was fixed with 6.25 per cent glutaraldehyde solution in 0.1 sodium cacodylate buffer (osmolarity of fixative was 1100 mosM) [22]. Photographs of small rings sectioned from the vessel and stenosis casts were then taken (figure 1b).…”