Relationship between spontaneous sympathetic baroreflex sensitivity and cardiac baroreflex sensitivity in healthy young individuals.

Low baroreflex sensitivity (BRS) is associated with elevated cardiovascular risk. However, the evidence is based primarily on measurements of cardiac BRS. It cannot be assumed that cardiac or sympathetic BRS alone represent a true reflection of baroreflex control of blood pressure. The aim of this study was to examine the relationship between spontaneous sympathetic and cardiac BRS in healthy, young individuals. Continuous measurements of blood pressure, heart rate, and muscle sympathetic nerve activity (MSNA) were made under resting conditions in 50 healthy individuals (18-28 years). Sympathetic BRS was quantified by plotting MSNA burst incidence against diastolic pressure (sympathetic BRSinc), and by plotting total MSNA against diastolic pressure (sympathetic BRStotal). Cardiac BRS was quantified by plotting R-R interval against systolic pressure using the sequence method. Significant sympathetic BRSinc and cardiac BRS slopes were obtained for 42 participants. A significant positive correlation was found between sympathetic BRSinc and cardiac BRS (r = 0.31, P = 0.049). Among this group, significant sympathetic baroreflex slopes were obtained for 39 participants when plotting total MSNA against diastolic pressure. In this subset, a significant positive correlation was observed between sympathetic BRStotal and cardiac BRS (r = 0.40, P = 0.012). When males and females were assessed separately, these modest relationships only remained significant in females. Analysis by gender revealed correlations in the females between sympathetic BRSinc and cardiac BRS (r = 0.49, P = 0.062), and between sympathetic BRStotal and cardiac BRS (r = 0.57, P = 0.025). These findings suggest that gender interactions exist in baroreflex control of blood pressure, and that cardiac BRS is not appropriate for estimating overall baroreflex function in healthy, young populations. This relationship warrants investigation in aging and clinical populations.