Evaluation of different indices to measure elastic properties of the aorta in a pulsatile mock circulation loop.

Objectives: Aim of this study was to investigate how different indices to express aortic stiffness can be understood and what the impact of different loading conditions is.

Methods: Thoracic aortas from swine (n = 6) and sheep (n = 6) were tested in a pulsatile mock circulation loop. Aortas were loaded at constant heart rate and stroke volume at increasing diastolic pressure with constant pulse pressure of 40 mmHg from 60/20 to 180/140 mmHg in 20 mmHg steps. Data was analyzed using commonly used formulas to calculate absolute strain, dynamic strain, circumferential strain, compliance, distensibility, pulse wave velocity, elastic modulus, Young's modulus, Petersons's elastic modulus, non-logarithmic and logarithmic stiffness index.

Results: Ovine samples had a smaller diastolic diameter at 60/20 mmHg compared to porcine samples (15.9 ± 0.6 vs 17.9 ± 0.7 mm) but a larger systolic and diastolic diameter at pressure of 180/140 mmHg (27.8 ± 1.5 vs 24.6 ± 0.7 and 26.6 ± 1.6 vs 23.9 ± 0.7 mm) indicating a higher aortic elasticity of ovine samples. An increase in pressure resulted in a stiffening of all samples with a significant difference in all included formulas (all p < 0.001). In the normotensive pressure range (100/60 to 140/100 mmHg) only non-logarithmic stiffness index in porcine samples showed no significant difference (p = 0.251) all other formulas showed a significant difference (p < 0.05).

Conclusions: In this experimental setup we could show that aortic elasticity is highly dependent on blood pressure. None of the formulas currently used provides a consistent value over the tested pressure range. A formula that can give a consistent value for aortic stiffness would be highly desirable.