Human sympathetic and vagal baroreflex responses to sequential nitroprusside and phenylephrine

We evaluated a method of baroreflex testing involving sequential intravenou s bolus injections of nitroprusside followed by phenylephrine and phenyleph rine followed by nitroprusside in 18 healthy men and women, and we drew inf erences regarding human sympathetic and vagal baroreflex mechanisms. We rec orded the electrocardiogram, photoplethysmographic finger arterial pressure , and peroneal nerve muscle sympathetic activity. We then contrasted least squares linear regression slopes derived from the depressor (nitroprusside) and presser (phenylephrine) phases with 1) slopes derived from spontaneous fluctuations of systolic arterial pressures and R-R intervals, and 2) baro reflex gain derived from cross-spectral analyses of systolic pressures and R-R intervals. We calculated sympathetic baroreflex gain from integrated mu scle sympathetic nerve activity and diastolic pressures. We found that vaga l baroreflex slopes are less when arterial pressures are falling than when they are rising and that this hysteresis exists over pressure ranges both b elow and above baseline levels. Although pharmacological and spontaneous va gal baroreflex responses correlate closely, pharmacological baroreflex slop es tend to be lower than those derived from spontaneous fluctuations. Sympa thetic baroreflex slopes are similar when arterial pressure is falling and rising; however, small pressure elevations above baseline silence sympathet ic motoneurons. Vagal, but not sympathetic baroreflex gains vary inversely with subjects' ages and their baseline arterial pressures. There is no corr elation between sympathetic and vagal baroreflex gains. We recommend repeat ed sequential nitroprusside followed by phenylephrine doses as a simple, ef ficient-means to provoke and characterize human vagal and sympathetic baror eflex responses.