Standing after prolonged bed rest may lead to orthostatic intolerance as a result of cardiovascular system dysregulation. The aim of this study was to assess the cardiovascular response of subjects' reambulation after a 35-day head-down tilt (HDT, -6°) bed rest.

Methods: 

Of the 9 subjects tested, only 5 completed the test (tolerant, TOL), whereas the other 4 fainted (intolerant, INT) within 3 min of standing. Heart rate (fH) was recorded continuously by ECG. Arterial pulse pressure profiles were obtained at a fingertip on the left arm by a non-invasive cuff pressure recorder (Portapres, FMS, Amsterdam, The Netherlands). Beat-by-beat mean arterial pressure (MAP) was computed as the integral mean of each pressure profile. Stroke volume was determined beat-by-beat by applying the Modelflow method (Wesseling et al., JAP 74: 2566, 1993), after which cardiac output (Q') was computed.

Results: 

During standing, in INT, MAP decreased reaching a threshold (MAP = 70.8±12.9 mmHg) without corresponding fH increase. At threshold, fH was 151±19.5 min^-1 and Q' was 4.7±1.3 L min^-1. Before fainting, MAP, fH and Q' were 43.9±3.5 mmHg, 139±28.1 min^-1, 2.9±0.4 L min^-1, respectively.

Conclusion: 

The fH increase due to stand results from a baroreflex response, whose operational point, however, has been reset during HDT. The MAP threshold found represents the low-pressure threshold of the reset baroreflex response.