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1 Department of Physiology and Biomedical Engineering, Mayo Clinic College of Medicine, Rochester, MN, USA; General Clinical Research Center, Mayo Clinic College of Medicine, Rochester, MN, USA
2 Department of Physiology and Biomedical Engineering, Mayo Clinic College of Medicine, Rochester, MN, USA
3 Department of Anesthesiology, Mayo Clinic College of Medicine, Rochester, MN, USA
* To whom correspondence should be addressed. E-mail: charkoudian.nisha{at}mayo.edu.
Volume expansion often ameliorates symptoms of orthostatic intolerance; however, the influence of this increased volume on integrated baroreflex control of vascular sympathetic activity is unknown. We tested whether acute increases in central venous pressure (CVP) diminished subsequent responsiveness of muscle sympathetic nerve activity to rapid changes in arterial pressure. We studied healthy humans under 3 separate conditions: control (C), acute 10-degree head-down tilt (HDT) and saline infusion (SAL). In each condition, heart rate, arterial pressure, CVP and peroneal muscle sympathetic nerve activity (MSNA) were measured during 5 minutes of rest and then during rapid changes in arterial pressure induced by sequential boluses of nitroprusside and phenylephrine (modified Oxford technique). Sensitivities of integrated baroreflex control of MSNA and heart rate were assessed as the slopes of the linear portions of the MSNA-diastolic pressure and R-R interval-systolic pressure relations, respectively. CVP increased ~2 mmHg in both SAL and HDT conditions. Resting heart rate and mean arterial pressure were not different among trials. Sensitivity of baroreflex control of MSNA was decreased in both SAL and HDT conditions: -3.1 ± 0.6 (SAL) and -3.3 ± 1.0 (HDT) vs. -5.0 ± 0.6 (C) units/beat/mmHg (P < 0.05 for SAL and HDT vs. C). Sensitivity of baroreflex control of the heart was not different among conditions. Our results indicate that small increases in CVP decrease the sensitivity of integrated baroreflex control of sympathetic nerve activity in healthy humans.
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