Carotid baroreflex resetting during drug-induced arterial pressure changes in humans

We studied human baroreflex resetting during 25 min of drug-induced arterial pressure changes in 10 healthy volunteers. Average (+/- SE) base-line systolic pressure of 113 +/- 4 fell to 102 +/- 3 during nitroprusside infusions and rose to 135 +/- 6 mmHg during phenylephrine infusions. Average base-line R-R intervals of 932 +/- 37 shortened to 820 +/- 39 during nitroprusside infusions and lengthened to 1,251 +/- 61 ms during phenylephrine infusions. Carotid baroreceptor-cardiac reflex responses were evaluated with a complex series of neck chamber pressure changes, and R-R intervals were plotted as functions of carotid distending pressure. Baroreceptor-cardiac reflex relations shifted on both R-R interval and arterial pressure axes during drug infusions, but there was no significant change of the maximum slope or range of R-R interval responses. The position of baseline R-R intervals on the reflex relation (operational point) changed significantly. Resting R-R intervals were closer to threshold during pressure reductions and closer to saturation for baroreceptor-cardiac responses during pressure elevations. These results document short-term partial resetting of human baroreceptor-cardiac reflex responses as early as 25 min after the onset of arterial pressure changes.