Distensibility of Deformable Aortic Replicas Assessed by an Integrated In-Vitro and In-Silico Approach

Abstract

The correct estimation of the distensibility of deformable aorta replicas is a challenging issue, in particular when its local characterization is necessary. We propose a combined in-vitro and in-silico approach to face this problem. First, we tested an aortic silicone arch in a pulse-duplicator analyzing its dynamics under physiological working conditions. The aortic flow rate and pressure were measured by a flow meter at the inlet and two probes placed along the arch, respectively. Video imaging analysis allowed us to estimate the outer diameter of the aorta in some sections in time. Second, we replicated the in-vitro experiment through a Fluid-Structure Interaction simulation. Observed and computed values of pressures and variations in aorta diameters, during the cardiac cycle, were compared. Results were considered satisfactory enough to suggest that the estimation of local distensibility from in-silico tests is reliable, thus overcoming intrinsic experimental limitations. The aortic distensibility (AD) is found to vary significantly along the phantom by ranging from 3.0 × 10^-3 mmHg^-1 in the ascending and descending tracts to 4.2 × 10^-3 mmHg^-1 in the middle of the aortic arch. Interestingly, the above values underestimate the AD obtained in preliminary tests carried out on straight cylindrical samples made with the same material of the present phantom. Hence, the current results suggest that AD should be directly evaluated on the replica rather than on the samples of the adopted material. Moreover, tests should be suitably designed to estimate the local rather than only the global distensibility.

Keywords: FSI simulation; aortic compliance; aortic distensibility; aortic phantom; in-vitro experiments; pulse-duplicator.

Figure A1

Determination of the edge displacement. (a) Acquisition of the phantom image from the camera. (b) Segmented image by the morphology function of LabView. (c) Detection of the edge marker starting from the manually crossed segment (red lines). Blue circles denote the pixels used by the algorithm to track the edge contour (green circles); gray pixels are pixel selected that do not contribute to the contour definition. (d) Example of an estimate of displacements for cardiac cycle for the points 1 and 2 of panel a and b.

Figure 1

The customized mold to realize the silicone aorta. The mold is divided into six modules.

Figure 2

The Pulse Duplicator at the HER Lab. It is composed by the motor with the bellows, the left ventricle, the flowmeter, the aortic valve, the phantom of the aorta, the pressure probes, the systemic resistances, the Windkessel tank, the left atrium, and the mitral valve.

Figure 3

A frame of the silicone aortic phantom tested in the pulse duplicator, (a) with fish-bone-shaped configuration marked with black nail polish, and (b) with the pixel sizes estimated in six regions of the phantom. The values reported are in mm.

Figure 4

Aorta’s replica of the study. (a) Real silicone phantom placed in the Pulse Duplicator, and (b) corresponding in-silico models of the aorta (in red and blue the solid and fluid mesh, respectively). The boundary conditions are reported in both panels, i.e., flow rate Q (uniform velocity) at the inlet and time-varying pressure recorded by the probe p₂ at the outlet. p₁ reported in (a) shows the additional probe placed at the brachiocephalic artery.

Figure 5

Measures collected in the hydrodynamics test: (a) Pressures collected by the probes p₁ (green line) and p₂ (red line), and (b) flowrate recorded by the flowsensor (blue line).

Figure 6

Time-variation of the external diameters during the heartbeat in five sections of interest.

Figure 7

The comparison between the p₁ pressure measured in the pulse duplicator (black dashed line) and the corresponding values computed by the numerical model (green solid line).

Figure 8

Comparison between the computed (solid lines) and measured (dashed black lines) external diameter in three sections of interest.

Figure 9

Pressure analysis of the Aorta. (a) Time-varying pressure in some sections of the domain, and (b) the maximum (red line) and minimum pressures (blue line) calculated along the path line, s/L.

Figure 10

Aortic distensibility, AD, estimated along the normalized path line, s/L, according to Equation (1).