4.4 FORWARD AND BACKWARD WAVES AT THE AORTIC ROOT: STEADY-STATE AND WAVE RE-REFLECTION CONSIDERATIONS

Abstract

Background: The assumption of steady-state oscillation is often overlooked when arterial pressure and flow waveforms are decomposed into backward (Pb) and forward waves (Pf). This has led to various misinterpretations including a significant reflection-free time during early-systole and attribution of the Pf to solely a product of left ventricular contraction and proximal aortic properties. Methods: Aortic pressure and flow were measured in anesthetized, open-chest dogs (n = 5). Wave reflections were modified with i.v. infusion of methoxamine (MTX) to increase reflections and nitroprusside (NTP) to decrease reflections. In a human sample (n = 226), central pressure and flow were measured using carotid tonometry and phase-contrast MRI, respectively. Results: Under conditions of baseline and increased reflections in both dogs and humans, peak of the forward wave (FWA) consistently occurred after time of peak flow (P < 0.001). FWA was systematically greater than peak flow multiplied by aortic characteristic impedance (QmaxZc) in dogs (P < 0.01) and humans (P < 0.01). Only when wave reflections were abolished vasoactively (NTP) in dogs was time of FWA and peak flow the same, leading to insignificant differences in FWA and QmaxZc (P = 0.59). Conclusion: In steady-state, wave reflections set up in previous cardiac cycles, wave re-reflections at the aortic root, and proximal reflections contribute to both the Pf and Pb waves, even during early-systole. Most importantly, peak aortic flow is also determined by aortic input impedance, which includes effects from properties distal of the proximal aorta. Under steady-state conditions, forward wave amplitude and morphology cannot be attributed solely to the LV and proximal aorta.