Heart rate and blood pressure dependence of aortic distensibility in rats: comparison of measured and calculated pulse wave velocity

Objectives: When assessing arterial stiffness, heart rate (HR) and blood pressure (BP) are potential confounders. It appears that the HR/BP dependences of pulse wave velocity (PWV) and distensibility are different, even though both assess arterial stiffness. This study aims to compare aortic PWV as measured using pulse transit time (PWVTT) and as calculated from distensibility (PWVdist) at the same measurement site and propose a solution to the disparity in dependences of PWV_TT and PWV_dist.

Methods: Adult anaesthetized rats (n = 24) were randomly paced at HRs 300-500 bpm, at 50 bpm steps. At each step, aortic PWV_TT (two pressure-tip catheters) and PWV_dist (pressure-tip catheter and ultrasound wall-tracking; abdominal aorta) were measured simultaneously while BP was varied pharmacologically.

Results: HR dependence of PWV_dist paradoxically decreased at higher levels of BP. In addition, BP dependence of PWV_dist was much larger than that of PWV_TT. These discrepancies are explained in that standard PWV_dist uses an approximate derivative of pressure to diameter, which overestimates PWV with increasing pulse pressure (PP). In vivo, PP decreases as HR increases, potentially causing a PWV_dist decrease with HR. Estimating the full pressure-diameter curve for each HR corrected for this effect by enabling calculation of the true derivative at diastolic BP. This correction yielded a PWV_dist that shows HR and BP dependences similar to those of PWV_TT. As expected, BP dependence of all PWV metrics was much larger than HR dependence.

Conclusion: Measured and calculated PWV have different dependences on HR and BP. These differences are, at least in part, because of approximations made in using systolic and diastolic values to calculate distensibility.