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Comparison of hemodynamic and volume responses to different levels of lower body suction and head-up tilt.

Orthostatic challenge such as lower body suction (LBNP) or head-up tilt (HUT) induces marked shifts of blood from the cardiopulmonary into lower extremity vascular compartments. As a reaction to central hypovolemia, counterregulating mechanisms take place immediately, including central/peripheral circulatory responses and neurohumoral responses. Compensatory answers are exerted via autonomic reflexes, ie, cardiopulmonary and arterial baroreceptor mechanisms. The sympathetic nervous system increases heart rate and peripheral resistance to adjust the cardiovascular system to the stress situation. The physiological response to head-up tilt and LBNP are similar but not identical. Numerous investigations on cardiovascular changes during tilt and LBNP have been performed, but the data vary greatly because of different experimental protocols. Musgrave et al. have done systematic comparisons of the overall hemodynamic responses to these two stress stimuli in the same subjects. To our knowledge, no systematic comparative study of effects of LBNP and HUT of different magnitude, using identical subjects, has yet been performed. This study investigates two different grades of lower body suction and two tilt angles in 12 subjects. Before, during and after cardiovascular loading we recorded heart rate (HR), systolic (SBP), diastolic (DSP) and mean arterial blood pressure (MAP). For calculation of blood volume shifts, plasma density (PD) and hematocrit (Ht) was measured. We addressed the questions if 1) heart rate and blood pressure react similarly to LBNP and HUT, 2) there is evidence for unloading of high pressure receptors with low levels of orthostatic / circulatory stress, and 3) a correlation between amount of fluid loss and grade of stimulus intensity can be established.