Immediate post renal transplant period and hypertension: concordance between clinic and ambulatory blood pressure

Abstract

Background: Markedly increased pulse wave velocity (PWV) with aging has been reported in numerous studies. Arterial wave reflection (WR) timing based on of the Augmentation Index (AI) framework does not change appreciably, despite increased PWV. The cause for this apparent discordance has not been established. Although a distal shift of reflecting sites has been proposed, this proposition has been exclusively based on the timing of the inflection point, which is a very limited approach. We hypothesize that AI is not capable of detecting early-systolic effects of reflection, particularly when PWV is high, leading to an apparent reflection-free period in early ejection. Methods: Simultaneous aortic pressure and flow were measured in anesthetized, open-chest dogs (n1⁄45). Intravenous infusion of methoxamine (MTX) and nitroprusside (NTP) were used to increase and decrease WRs, respectively. A coupled left ventricle-arterial system model was used to alter reflection magnitude and timing. Effects of reflection in early-systole were assessed with a systolic pressure-flow ratio (SPFR). Results: SPFR in the case of NTP remained close to aortic characteristic impedance (Zc) [horizontal dashed line] up until peak flow (Fig.1A). In control and MTX, SPFR grew increasingly divergent from Zc before peak flow (Fig.1B-C). Even when an inflection point is observed on the pressure waveform (Fig.1D, inset), SPFR diverges from Zc before the inflection, indicative of early-systole WRs. Modeling studies confirmed the dog experiments in a noise-free environment; only when wave reflections are substantially delayed and/or minimal does SPFR equal Zc in earlysystole. Divergence from this relation is due to WRs. Conclusion:WRs can alter pressure-flow relations in early-systole, prior to time of peak flow and the inflection point on the pressure waveform. Since AI is blind to early-systolic WRs, it should not be used to assess WR timing, especially when PWV is high. This is a particularly important limitation of AI to recognize when studying older subjects as well as those with hypertension in which PWV is elevated. When an appropriate method is used to gauge WR magnitude and timing in earlysystole, elevated PWV is indeed associated with earlier effects of reflections.