Human sympathetic and vagal baroreflex responses to sequential nitroprusside and phenylephrine

Author:

Rudas László1,Crossman Alexandra A.1,Morillo Carlos A.2,Halliwill John R.3,Tahvanainen Kari U. O.4,Kuusela Tom A.5,Eckberg Dwain L.1

Affiliation:

1. Departments of Physiology and Medicine, Medical College of Virginia at Virginia Commonwealth University, and Hunter Holmes McGuire Department of Veteran Affairs Medical Center, Richmond, Virginia 23249;

2. Department of Cardiology and Cardiovascular Sciences, Fundación Cardiovascular del Oriente Columbiano, AA 1143 Bucaramanga, Santander, Columbia;

3. Department of Anesthesia, Mayo Clinic and Foundation, Rochester, Minnesota 55905;

4. Department of Clinical Physiology, Kuopio University Hospital, FIN-70211 Kuopio; and

5. Department of Applied Mathematics, University of Turku, FIN-20014 Turku, Finland

Abstract

We evaluated a method of baroreflex testing involving sequential intravenous bolus injections of nitroprusside followed by phenylephrine and phenylephrine followed by nitroprusside in 18 healthy men and women, and we drew inferences regarding human sympathetic and vagal baroreflex mechanisms. We recorded 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 pressor (phenylephrine) phases with 1) slopes derived from spontaneous fluctuations of systolic arterial pressures and R-R intervals, and 2) baroreflex gain derived from cross-spectral analyses of systolic pressures and R-R intervals. We calculated sympathetic baroreflex gain from integrated muscle sympathetic nerve activity and diastolic pressures. We found that vagal baroreflex slopes are less when arterial pressures are falling than when they are rising and that this hysteresis exists over pressure ranges both below and above baseline levels. Although pharmacological and spontaneous vagal baroreflex responses correlate closely, pharmacological baroreflex slopes tend to be lower than those derived from spontaneous fluctuations. Sympathetic baroreflex slopes are similar when arterial pressure is falling and rising; however, small pressure elevations above baseline silence sympathetic motoneurons. Vagal, but not sympathetic baroreflex gains vary inversely with subjects’ ages and their baseline arterial pressures. There is no correlation between sympathetic and vagal baroreflex gains. We recommend repeated sequential nitroprusside followed by phenylephrine doses as a simple, efficientmeans to provoke and characterize human vagal and sympathetic baroreflex responses.

Publisher

American Physiological Society

Subject

Physiology (medical),Cardiology and Cardiovascular Medicine,Physiology

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