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Articles in PresS, published online ahead of print December 7, 2001
J Appl Physiol, 10.1152/jap.00465.2000
Submitted on May 8, 2000
Accepted on November 10, 2001
1 Department of Physiology, Institute of Basic Medical Sciences, Universitity of Oslo, Oslo, Norway
* To whom correspondence should be addressed. E-mail: ktoska{at}basalmed.uio.no.
Mechanisms involved in the control of arterial pressure during postural changes were studied by analysis of the dynamic time course of cardiovascular changes during head-up tilt and tilt-back to supine position. Beat-to-beat values of cardiovascular variables were recorded continuously before, during and after passive head-up tilt to 30°in seven healthy humans. Left cardiac stroke volume (SV, Doppler ultrasound), mean arterial blood pressure (MAP, Finapres), heart rate (HR), cardiac output (CO) and total peripheral conductance (TPC) were recorded. Upon head-up tilt, MAP at the level of the carotid baroreceptors decreased by some 5 mmHg. There was a striking asymmetry between the time courses of cardiovascular changes on head-up tilt and on tilt-back. Adjustments generally took up to 30 seconds after head-up tilt, whereas most changes were completed during the first 10 seconds after tilt-back to a supine position. The cardiovascular reflex adjustments of HR and TPC were more symmetrical. After head-up tilt, SV was maintained during the first 4-6 s and then decreased steadily during the next 30 s to a stable level some 25 % below its pre-tilt value. However, after tilt-back SV increased rapidly to its pre-tilt value in less than 10 s. This asymmetry in SV dynamics may be explained in part by a more rapid change in left cardiac filling after tilt-back than after tilt-up. On tilt-back, there must be a rapid inflow of stagnant blood from the legs, whereas venous valves will impede the backward filling of veins in the lower body on tilt-up. In conclusion, we have revealed a characteristic asymmetry in the cardiovascular responses to inverse variations in gravity forces in humans. This asymmetry can be explained in part by non-linear, hydrodynamic factors, such as the one-way effect of venous valves in the lower part of the body
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