RESUMO
A variety of methods has been used experimentally to increase baroreceptor activity and provoke transient, vagally mediated cardiac slowing in humans. We studied baroreceptor-cardiac reflex physiology in eight resting volunteers by measuring R-R interval changes during and after spontaneous brief elevations of arterial pressure, preceded by bursts of muscle sympathetic nerve activity. Arterial pressure was measured with a catheter in a brachial artery, and muscle sympathetic activity was measured with a microelectrode positioned transcutaneously in a peroneal nerve. R-R intervals were related to preceding systolic pressures with linear regression analysis. When systolic pressures were correlated with R-R intervals of the next cardiac cycle, slopes (+/- SE) averaged 12.4 +/- 2.2 ms/mmHg and correlation coefficients averaged 0.81 +/- 0.09. Baroreflex slopes were inversely related to base-line systolic pressures. These results are similar to those obtained when baroreceptors are stimulated experimentally; they suggest that elevations of arterial pressure, which are preceded by bursts of muscle sympathetic activity, trigger with one heart-beat latency highly linear increases of efferent vagal-cardiac nerve activity.
