Supraceliac and Infrarenal Aortic Flow in Patients with Abdominal Aortic Aneurysms: Mean Flows, Waveforms, and Allometric Scaling Relationships

Abstract

Purpose Hemodynamic forces are thought to play a critical role in abdominal aortic aneurysm (AAA) growth. In silico and in vitro simulations can be used to study these forces, but require accurate aortic geometries and boundary conditions. Many AAA simulations use patient-specific geometries, but utilize inlet boundary conditions taken from a single, unrelated, healthy young adult. Methods In this study, we imaged 43 AAA patients using a 1.5 T MR scanner. A 24-frame cardiac-gated one-component phase-contrast magnetic resonance imaging sequence was used to measure volumetric flow at the supraceliac (SC) and infrarenal (IR) aorta, where flow information is typically needed for simulation. For the first 36 patients, individual waveforms were interpolated to a 12-mode Fourier curve, peak-aligned, and averaged. Allometric scaling equations were derived from log–log plots of mean SC and IR flow vs. body mass, height, body surface area (BSA), and fat-free body mass. The data from the last seven patients were used to validate our model. Results Both the SC and IR averaged waveforms had the biphasic shapes characteristic of older adults, and mean SC and IR flows over the cardiac cycle were 51.2 ± 10.3 and 17.5 ± 5.44 mL/s, respectively. Linear regression of the log–log plots revealed that BSA was most strongly predictive of mean SC (R2 = 0.29) and IR flow (R2 = 0.19), with the highest combined R2. When averaged, the measured and predicted waveforms for the last seven patients agreed well. Conclusions We present a method to estimate SC and IR mean flows and waveforms for AAA simulation.

A haemodynamic predictor of intraluminal thrombus formation in abdominal aortic aneurysms

“…The average number of elements for these models was 5 949 585 ± 346 503. The boundary condition at the supraceliac inlet was prescribed by imposing the averaged volumetric flow rate and waveform obtained by Les et al [17] from PC-MRI measurements on 36 patients affected by AAA. Stress conditions were simulated using the waveform and flow rate reported by Suh et al [18] for mild exercise (table 2).…”

“…The average cycle at rest and exercise was 0.9 and 0.6 s corresponding to heart rates of approximately 67 and 100 beats per minute, respectively. A Womersley velocity profile was prescribed at the inlet [17][18][19] and Windkessel RCR boundary conditions were imposed at all outlets using values reported in the full-body three-dimensional model of Xiao et al [16] (table 2). To avoid instabilities in the numerical solution in situations of back flow, we weakly constrained the shape of the velocity profiles to be parabolic at the outlets of the iliac and superior mesenteric arteries using the augmented-Lagrangian technique developed by Kim et al [20].…”

A comparison of hemodynamic metrics and intraluminal thrombus burden in a common iliac artery aneurysm

“…For the inlet boundary condition (SC aorta), we applied a mass flow waveform derived from volumetric flow data by Les et al [57] (see Figure 4), where the waveform was scaled to suit the patient's age-estimated fat-free body mass using the allometric scaling relationships provided by Les et al [57]. We explicitly coupled the 3D CFD simulation with a Windkessel model (RCR circuit) at each outlet boundary (see Figure 4) in order to approximate the resistance and compliance of the downstream vascular beds.…”

“…We explicitly coupled the 3D CFD simulation with a Windkessel model (RCR circuit) at each outlet boundary (see Figure 4) in order to approximate the resistance and compliance of the downstream vascular beds. This improves the estimation of pressure throughout the domain and allows the pressure waveform at the SC inlet to comply with the patient's measured systolic and diastolic pressures [57]. Our Windkessel parameters are calibrated according to previous methodology [35,58]; with 30% of the common iliac flow passing through to the internal iliac artery.…”

“…These assumptions are necessary, as patient-specific flow and pressure data for each outlet were not collected. Figure 4 shows the calculated IR flow waveform and how it compares with the IR flow data from the study by Les et al (from which the inlet waveform was derived) [57].…”