ECHOCARDIOGRAMS DURING 6 HOURS OF BEDREST AT HEAD-DOWN AND HEAD-UP TILT AND DURING SPACE-FLIGHT

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-degrees head-down tilt) were compared with those of fractional grav ity loads of 1/6G, 1/3G, and 2/3G by using head-up tilts of 10-degrees , 20-degrees, and 42-degrees, respectively. On 4 different days, six h ealthy 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 c ontrol (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-degrees was greater than at +20-degrees and +42-degrees (no t significant); EF at +100 was greater than at +420 (not significant). For the tilt angles of -5-degrees, +10-degrees, and +20-degrees, mean heart rate decreased during the first 2 hours, and returned to contro l or was slightly elevated above control (+20-degrees) by 6 hours (not significant). At the +42-degrees angle of tilt, heart rate was increa sed above control at hours 2, 4, and 6. There were no significant diff erences in cardiac output at any time point for any tilt angle. Left v entricular end-diastolic volume did not change significantly with time or tilt angle, but there was a trend to a decrease by hour 2. Left ve ntricular end-systolic volume was increased at hour 2 (not significant ) and at hour 4 (not significant) for subjects at -5-degrees. Systolic blood pressure did not change significantly. Left ventricular end-dia stolic volume, left ventricular end-systolic volume, stroke volume, ej ection fraction, heart rate, and cardiac output were at control values by hour 6 for all tilt angles. The lack of a significant immediate ch ange in left ventricular end-diastolic volume despite decrements in st roke volume (P < .05) and heart rate (not significant) suggests that m ultiple factors may play a role in the adaptation to simulated hypogra vity. The data indicate that no angle of tilt, whether head-down or he ad-up for 4 to 6 hours, mimicked exactly the changes in cardiovascular function recorded after 4 to 6 hours of flight. Thus space and bedres t changes in left ventricular end-diastolic volume may be similar but possess a different time course. Nevertheless, head-down tilt at 5-deg rees for 6 hours mimics some (stroke volume, systolic and diastolic bl ood pressure, mean arterial blood pressure, total peripheral vascular resistance), but not all, of the changes occurring in an equivalent ti me of space flight.