Cardiac baroreflex sensitivity is not correlated to sympathetic baroreflex sensitivity within healthy, young humans

Abstract

The purpose of this study was to evaluate the relationship between the cardiac and sympathetic baroreflex sensitivities within healthy, young humans. The sensitivities of the cardiac and sympathetic baroreflexes were compared in 53 normotensive individuals (28 men and 25 women; age: 24.0 ± 0.9 years; body mass index: 24.0 ± 0.3 cm/kg², mean ± SEM). Heart rate, arterial blood pressure, and peroneal muscle sympathetic nerve activity were recorded under resting conditions (heart rate: 58 ± 1 bpm; systolic blood pressure: 126 ± 2 mm Hg; diastolic blood pressure: 72 ± 1 mm Hg; mean arterial blood pressure: 89 ± 1 mm Hg; muscle sympathetic nerve activity: 18 ± 1 bursts per min) and during rapid changes in blood pressure induced by sequential boluses of nitroprusside and phenylephrine. Cardiac and sympathetic baroreflex sensitivities were analyzed using the slopes of the linear portions of the muscle sympathetic nerve activity-diastolic blood pressure and R-R interval-systolic blood pressure relationships, respectively. When individual cardiac baroreflex sensitivity was compared with sympathetic baroreflex sensitivity, no correlation (R-R interval: r = -0.13; heart rate: r = 0.21) was observed when studied as a group. Analysis by sex unveiled a correlation in women between the cardiac and sympathetic baroreflex sensitivities (R-R interval: r = -0.54; P = 0.01; no correlation with hazard ratio: r = 0.29). No relationship was found in men (R-R interval: r = 0.17; heart rate: r = 0.12). These results indicate that, although both cardiac and sympathetic efferents function in baroreflex control of arterial pressure, there is no correlation in their sensitivities within healthy normotensive humans. However, sex-stratified data indicate that sex-based differential correlations might exist.

Figure 1

Regression analysis shows no relationship between sympathetic (MSNA) and cardiac baroreflex sensitivity within individuals when studied as a group including men (open circle) and women (closed circle). This absence of a relationship is evident whether cardiac baroreflex sensitivity is represented by (a) R-R Interval slope or (b) heart rate (HR) slope. AU; arbitrary units.

Figure 2

Linear regression analysis of the relationship between individual baroreflex sensitivity when separated on the basis of sex. In women there was a positive relationship between sympathetic and cardiac BRS shown by (a) RRI: r = −0.54, P<0.01. This relationship was not significant when HR was used to estimate cardiac BRS in (c) HR: r = 0.29. In men, there was no correlation between sympathetic and cardiac baroreflex sensitivity whether RRI (b) or HR (d) was used. AU; arbitrary units.