Defects in Vascular Mechanics Due to Aging in Rats: Studies on Arterial Wave Properties from a Single Aortic Pressure Pulse

Abstract

Changes in vascular mechanics due to aging include elevated vascular impedance, diminished aorta distensibility, and an accelerated return of pulse wave reflection, which may increase the systolic workload on the heart. Classically, the accurate measurement of vascular mechanics requires the simultaneous recording of aortic pressure and flow signals. In practice, it is feasible to estimate arterial wave properties in terms of wave transit time (τw) and wave reflection index (RI) by using aortic pressure signal alone. In this study, we determined the τw and magnitudes of the forward (∣Pf∣) and backward (∣Pb∣) pressure waves in Long–Evans male rats aged 4 (n = 14), 6 (n = 17), 12 (n = 17), and 18 (n = 24) months, based on the measured aortic pressure and an assumed triangular flow (Qtri). The pulsatile pressure wave was the only signal recorded in the ascending aorta by using a high-fidelity pressure sensor. The base of the unknown Qtri was constructed using a duration, which equals to the ejection time. The timing at the peak of the triangle was derived using the fourth-order derivative of the aortic pressure waveform. In the 18-month-old rats, the ratio of τw to left ventricular ejection time (LVET) decreased, indicating a decline in the distensibility of the aorta. The increased ∣Pb∣ associated with unaltered ∣Pf∣ enhanced the RI in the older rats. The augmentation index (AI) also increased significantly with age. A significant negative correlation between the AI and τw/LVET was observed: AI = −0.7424 − 0.9026 × (τw/LVET) (r = 0.4901; P < 0.0001). By contrast, RI was positively linearly correlated with the AI as follows: AI = −0.4844 + 2.3634 × RI (r = 0.8423; P < 0.0001). Both the decreased τw/LVET and increased RI suggested that the aging process may increase the AI, thereby increasing the systolic hydraulic load on the heart. The novelty of the study is that Qtri is constructed using the measured aortic pressure wave to approximate its corresponding flow signal, and that calibration of Qtri is not essential in the analysis.

Extracorporeal Life Support Enhances the Forward Pressure Wave to Cause a Mismatch between Cardiac Oxygen Demand and Supply

“…The unknown Q tri was derived from the pressure waveform measured in the ascending aorta 13,14,17 . The start and end time points of LV ejection were identified as the intersection of two vertical lines near the foot of the pressure wave (the first vertical blue line in each of Fig.…”

“…1A–D), respectively 35 . The time at the peak of the triangle was derived from the fourth-order derivative of the aortic pressure wave 13,14,17 (the pink curve in each of Fig. 1A–D).…”

“…After ejection commenced, the first zero-crossing curve from above to below (the second vertical blue line in each of Fig. 1A–D) determined the peak of the triangle of blood flow, which was the inflection point of the pressure wave 13,14,36 . Thus, the uncalibrated Q tri was approximated by a triangular shape (the green curves in each of Fig.…”

“…Moreover, Chang et al . 14 discovered that the aortic impulse response is effective for estimating arterial τ w by a single pressure pulse with its corresponding Q tri . The novelty of the concept proposed by Westerhof et al .…”

“…In this study, we used the measured aortic pressure and an assumed Q tri to determine the effects of ECLS on arterial wave properties and ventricular/arterial coupling in male Wistar rats. The construction of Q tri was derived from the fourth-order derivative of the measured aortic pressure wave 13,14,17 . The aortic input impedance ( Z i ) was calculated from the ratio of the ascending aortic pressure harmonics to the corresponding Q tri harmonics.…”