COMPARISON OF CARDIOVASCULAR FUNCTION DURING THE EARLY HOURS OF BED REST AND SPACE-FLIGHT

This paper reviews the cardiovascular responses of six healthy male su bjects to 6 hours in a 5 degrees head-down bed rest model of weightles sness, and compares these responses to those obtained when subjects we re positioned in head-up tilts of 10 degrees, 20 degrees, and 42 degre es, simulating 1/6, 1/3, and 2/3 G, respectively. Thoracic fluid index , cardiac output, stroke volume, and peak flow were measured using imp edance cardiography. Cardiac dimensions and volumes were determined fr om two-dimensional guided M-mode echocardiograms in the left lateral d ecubitus position at 0, 2, 4, and 6 hours. Cardiovascular responses to a stand test were compared before and after bed rest. The impedance v alues were related to tilt angle for the first 2 hours of tilt; howeve r, after 3 hours, at all four angles, values began to converge, indica ting that cardiovascular homeostatic mechanisms seek a common adapted state, regardless of effective gravity level (tilt angle) up to 2/3 G. Echocardiography revealed that left ventricular end-diastolic and end -systolic volume, stroke volume, ejection fraction, heart rate, and ca rdiac output had returned to control values by hour 6 for all tilt ang les. The lack of a significant immediate change in left ventricular en d-diastolic volume, despite decrements in stroke volume (P <.05) and h eart rate (not significant), indicates that multiple factors may play a role in the adaptation to simulated hypogravity. The echocardiograph y data indicated that no angle of tilt, whether head-down or head-up f or 4 to 6 hours, mimicked exactly the changes in cardiovascular functi on recorded after 4 to 6 hours of space flight. Changes in left ventri cular end-diastolic volume during space flight and tilt may be similar , but follow a different time course. Nevertheless, head-down tilt at 5 degrees for a hours mimics some (stroke volume, systolic and diastol ic blood pressure, mean arterial blood pressure, and total resistance) , but not all, of the changes occurring in an equivalent time of space flight. The magnitude of the change in the mean heart rate response t o standing was greater after six hours of tilt at -5 degrees or 10 deg rees. Thus, results from the stand test after 6 hours of bed rest at - 5 degrees and 10 degrees, but not at 20 degrees or 42 degrees, are sim ilar to those obtained after space flight.