Development of Velocity Profiles and Retrograde Flow in the Porcine Abdominal Aorta under Different Haemodynamic Conditions

Abstract

Low and/or oscillating wall shear stresses are related to the development of atherosclerosis and this oscillation is influenced by changes in basic haemodynamics (exercise). The objective of this study was to provide in vivo data on the development of velocity profiles and oscillating blood velocities in the abdominal aorta under varying haemodynamic conditions. Six anaesthetized, 90-kg pigs were used in the study. Abdominal aortic velocity profiles across the anterior-posterior diameter were acquired at different axial positions using 10 MHz pulsed Doppler ultrasound. Measurements were obtained under normal conditions and during cardiac pacing up to 170 beats/ min. Velocity profiles were obtained during heart rates ranging between 58 and 169 beats/min, and during flow rates ranging between 0.57 and 2.89 l/min. Main outcome measures included minimum velocities, frequency index, shape of velocity profiles (velocity distribution index), Reynolds' numbers, and Womersley's frequency parameter. Velocity profiles were blunted, with lowest velocities at the distal posterior vessel wall. Multiple regression analysis showed the development of velocity profiles to be inversely correlated with the pulsatility index, Womersley's frequency parameter and the mean Reynolds' number (r = 0.89, p < 0.0005). Minimum velocities were negatively correlated with the PI, Womersley's frequency parameter and positively with the mean Reynolds' number (r = 0.94, p < 10-8). Retrograde velocities (and hence oscillating wall shear stresses) were present at mean Reynolds' number < 1000. The oscillation of blood velocities at the wall in the porcine abdominal aorta was highly dependent on general haemodynamics (i.e. flow, heart rate and vessel diameter as expressed in the Reynolds' numbers and Womersley's frequency parameters). The velocity profiles in the abdominal aorta were found to be far from parabolic. These findings have important implications for the understanding and future modelling of the complex haemodynamics in the abdominal aorta and their relation to the development of atherosclerotic disease.