Hemodynamic Changes Quantified in Abdominal Aortic Aneurysms with Increasing Exercise Intensity Using MR Exercise Imaging and Image-Based Computational Fluid Dynamics

Abstract

Abdominal aortic aneurysm (AAA) is a vascular disease resulting in a permanent, localized enlargement of the abdominal aorta. We previously hypothesized that the progression of AAA may be slowed by altering the hemodynamics in the abdominal aorta through exercise. To quantify the effect of exercise intensity on hemodynamic conditions in 10 AAA subjects at rest and during mild and moderate intensities of lower-limb exercise (defined as 33 ± 10% and 63 ± 18% increase above resting heart rate, respectively), we used magnetic resonance imaging and computational fluid dynamics techniques. Subject-specific models were constructed from magnetic resonance angiography data and physiologic boundary conditions were derived from measurements made during dynamic exercise. We measured the abdominal aortic blood flow at rest and during exercise, and quantified mean wall shear stress (MWSS), oscillatory shear index (OSI), and particle residence time (PRT). We observed that an increase in the level of activity correlated with an increase of MWSS and a decrease of OSI at three locations in the abdominal aorta, and these changes were most significant below the renal arteries. As the level of activity increased, PRT in the aneurysm was significantly decreased: 50% of particles were cleared out of AAAs within 1.36 ± 0.43, 0.34 ± 0.10, and 0.22 ± 0.06 s at rest, mild exercise, and moderate exercise levels, respectively. Most of the reduction of PRT occurred from rest to the mild exercise level, suggesting that mild exercise may be sufficient to reduce flow stasis in AAAs.

Dynamics of pulsatile flow through model abdominal aortic aneurysms

“…Moreover, during the progression of the pathology the flow rate waveform may change in response to the alterations of the blood vessel geometry. In our study, we have used data presented in a recent article by Suh et al (2011), who recorded blood flow rate waveforms of 10 subjects (nine male, one female) suffering from abdominal aortic aneurysms. The recordings were made at an infrarenal location just upstream of the aneurysm, and during both rest and exercise conditions.…”

“…Details of the two waveforms are given in table 1. In Suh et al (2011) blood is considered to be a Newtonian fluid with a density of 1.06 g cm −3 and a viscosity of 0.004 Pa s. To convert the recorded volume flow rates to non-dimensional quantities, one also needs to know the diameters of the abdominal aorta at the locations of the recording. These values are not given in Suh et al (2011).…”

“…In Suh et al (2011) blood is considered to be a Newtonian fluid with a density of 1.06 g cm −3 and a viscosity of 0.004 Pa s. To convert the recorded volume flow rates to non-dimensional quantities, one also needs to know the diameters of the abdominal aorta at the locations of the recording. These values are not given in Suh et al (2011). The values of the Reynolds number and Womersley number in table 1, for example Re = 256 and Wo = 12 for waveform Q1, are based on a vessel diameter of 1.7 cm, which seems to be a typical value of the inlet diameter of an abdominal aneurysm.…”

“…An introduction to this approach can be found in Taylor & Draney (2004) and Taylor & Figueroa (2009). Recent studies of the haemodynamics of abdominal aortic aneurysms using this approach include Les et al (2010), Sheidaei et al (2011), Stamatopoulos et al (2011) and Suh et al (2011). Such studies can provide a quantitative description of the flow in an aneurysm and the distribution of shear stress along the vessel wall, and may even incorporate the presence of atherosclerotic plaque.…”

“…Here, the articles by Egelhoff et al (1999), Taylor, Hughes & Zarins (1999), Deplano et al (2007), Les et al (2010) and Suh et al (2011) should be mentioned. It has been hypothesized that prolonged physical exercise may eventually slow the growth of aneurysms, but supporting arguments are indecisive.…”