Rate of change of arterial pressure and assessment of baroreceptor function

Background: Baroreceptor function can be measured acutely by actively driving a change in mean arterial pressure (MAP) via administration of a vasoactive agent and measuring the resultant heart rate (HR) change. The timing and rate of administration of the vasoactive agent varies between experiments and between types of investigations. This study aimed to quantify the difference of rate of pressure change on quantification of baroreceptor function. Methods: Spontaneously Hypertensive and Wistar-Kyoto rats (n = 5, male, 13–20 weeks old) were anaesthetised and instrumented to measure HR and aortic pressure. MAP was increased (bolus intravenous injection of phenylephrine, 15 μg/kg) and the resultant HR recorded. The MAP-HR relationships, determined for the period of fast blood pressure increase from baseline to a peak value and for the period of slow blood pressure decrease from peak to baseline, were individually fitted with a 5-parameter dose response function. The maximum baroreceptor gain (BRSmax, bpm/mm Hg) and the MAP at which it occurred were determined from the derivative of the fitted function for the fast and slow periods of MAP change. Comparison was made by paired t-test and Bland-Altman analysis. Results: The fast rate of change (increase) in pressure was 6.6±1.2 mm Hg/s and the slow rate (decrease) was 0.7±0.2 mm Hg/s (9-fold difference, p = 0.005). There was no difference in BRSmax between slow and fast blood pressure changes (2.5±0.7 and 1.4±0.7 bpm/mm Hg, p = 0.23) or in the MAP at which this occurred (153±12 and 144±7 mm Hg, p = 0.28). The non-statistical difference in BRSmax was 1.0±1.6 bpm/mm Hg and in the associated MAP was 8±15 mm Hg. Conclusions: The rate of change and direction of change of blood pressure had no significant effect on quantification of BRSmax within a range of 0.7–6.6 mm Hg/s. This indicates that in adult rats, baroreceptor function can be readily assessed by the HR response to fast or slow changes in MAP.