The impact of heart rate and cardiac output on the flow inthe human thoracic aorta

Purpose: The purpose of the study is to determine the effects of heart rate (HR) and cardiac output(CO), in the temporal variation of CO on flow structures and related biomechanical markers. Methods: The pulsatile flow in the thoracic aorta was simulated for 15 combinations of HR (60-150 beats per minutes, BPM), CO and cardiac temporal profiles. In all cases, the Quemada viscositymodel was used. The results were analyzed in terms of biomechanical markers such as extent ofretrograde flow in the lumen and close to the wall, helicity parameters, commonly used wall shearstress (WSS) indicators along with proposed Endothelial Activation Indices (EAIs). Results: The simulations demonstrated the presence of helical motion in all cases. The helicalmotion depends on the spatial distribution of the flow by the aortic valve. Time- and space-averagedhelicity indices were found to have smallest values in the aortic arch and largest in the descendingpart of the aorta. For all cases, retrograde flow was observed. The extent of separated flow close tothe aortic wall depended strongly on the rate of decelerating CO during late systole as well aspossible axial flow deceleration periods during diastole. At high HR and CO, small scale flowstructures developed, indicating transition to turbulence. Time averaged WSS-related indicatorswere less distinctive in assessing the spatial and temporal impact as compared to the EAI indicators(EAI_Nobili and EAI_Soares) accounting for both accumulated stress and the temporal behavior of thestress. Conclusions: The results underpin the importance of temporal variation of the cardiac flowrate andthe impact of the deceleration phase of systole on retrograde flow and formation of helical flowstructures. As retrograde and helical flow has been found to be related to atherosclerosis, thetemporal contribution of the flowrate must be maintained, since time averaged biomechanicalindicators filter out information of potential diagnostic importance. Temporal flow behavior, up tocell response frequency, needs to be reflected by the biomechanical indicators as in the proposed EAI_Soares indicator.QC 20200806