Echocardiograms During Six Hours of Bedrest at Head‐Down and Head‐Up Tilt and During Space Flight

Abstract

Left ventricular end‐diastolic volume increased after 4 1/2 to 6 hours of space flight, but was significantly decreased after 5 to 6 days of space flight. To determine the role of acute gravitational effects in this phenomenon, responses to a 6‐hour bedrest model of 0 gravity (G; 5° head‐down tilt) were compared with those of fractional gravity loads of 1/6G, 1/3G, and 2/3G by using head‐up tilts of 10°, 20°, and 42°, respectively. On 4 different days, six healthy male subjects were tilted at one of the four angles for 6 hours. Cardiac dimensions and volumes were determined from two‐dimensional and M‐mode echocardiograms in the left lateral decubitus position at control (0), 2, 4, and 6 hours. Stroke volume decreased with time (P < .05) for all tilt angles when compared with control. Ejection fraction (EF) at −5° was greater than at +20° and +42° (not significant); EF at +10° was greater than at +42° (not significant). For the tilt angles of −5°, +10°, and +20°, mean heart rate decreased during the first 2 hours, and returned to control or was slightly elevated above control (+20°) by 6 hours (not significant). At the +42° angle of tilt, heart rate was increased above control at hours 2, 4, and 6. There were no significant differences in cardiac output at any time point for any tilt angle. Left ventricular end‐diastolic volume did not change significantly with time or tilt angle, but there was a trend to a decrease by hour 2. Left ventricular end‐systolic volume was increased at hour 2 (not significant) and at hour 4 (not significant) for subjects at −5°. Systolic blood pressure did not change significantly. Left ventricular end‐diastolic volume, left ventricular end‐systolic volume, stroke volume, ejection fraction, heart rate, and cardiac output were at control values by hour 6 for all tilt angles. The lack of a significant immediate change in left ventricular end‐diastolic volume despite decrements in stroke volume (P < .05) and heart rate (not significant) suggests that multiple factors may play a role in the adaptation to simulated hypogravity. The data indicate that no angle of tilt, whether head‐down or head‐up for 4 to 6 hours, mimicked exactly the changes in cardiovascular function recorded after 4 to 6 hours of flight. Thus space and bedrest changes in left ventricular end‐diastolic volume may be similar but possess a different time course. Nevertheless, head‐down tilt at 5° for 6 hours mimics some (stroke volume, systolic and diastolic blood pressure, mean arterial blood pressure, total peripheral vascular resistance), but not all, of the changes occurring in an equivalent time of space flight.