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Am J Physiol Heart Circ Physiol (September 27, 2001). doi:10.1152/ajpheart.00535.2001
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Articles in PresS, published online ahead of print September 27, 2001
Am J Physiol Heart Circ Physiol, 10.1152/ajpheart.00535.2001
Submitted on June 24, 2001
Accepted on September 14, 2001

Hypovolemia and Neurovascular Control During Orthostatic Stress

Derek S Kimmerly1 and Kevin Shoemaker1*

1 School of Kinesiology, University of Western Ontario, London, Ontario, Canada

* To whom correspondence should be addressed. E-mail: dkimmerl{at}julian.uwo.ca.

Humans exposed to real or simulated microgravity experience decrements in blood pressure regulation during orthostatic stress that may be related to autonomic dysregulation and/or hypovolemia. We examined the hypothesis that hypovolemia, without the deconditioning effects of bed rest or space flight, would augment the sympathoneural and vasomotor response to graded orthostatic stress. Radial artery blood pressure (tonometry), stroke volume and brachial blood flow (Doppler ultrasound), heart rate (ECG), peroneal muscle sympathetic nerve activity (MSNA; microneurography), and estimated central venous pressure (CVP) were recorded during five levels (-5,-10,-15, -20 and -40 mmHg) of randomly assigned lower body negative pressure (LBNP) (n=8). Forearm (FVR) and total peripheral (TPR) vascular resistance were calculated. The test was repeated under randomly assigned placebo (Normovolemia) or diuretic (spironolactone: 100mg/day, 3 days) (Hypovolemia) conditions. The diuretic produced an ~16% reduction in plasma volume. Compared with Normovolemia, SV and cardiac output were reduced by ~12% and ~10% at baseline and during LBNP following the diuretic. During Hypovolemia there was an upward shift in the %{Delta}MSNA/{Delta}CVP, the {Delta}FVR/{Delta}CVP and {Delta}TPR/{Delta}CVP relationships during 0 to -20 mmHg LBNP. In contrast to Normovolemia, blood pressure increased at -40 mmHg LBNP during Hypovolemia due to larger gains in the %{Delta}MSNA/{Delta}CVP and {Delta}TPR/{Delta}CVP relationships. It was concluded that acute hypovolemia augmented the neurovascular component of blood pressure control during moderate orthostasis effectively compensating for decrements in stroke volume and cardiac output.




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