ACUTE AND INTERMEDIATE CARDIOVASCULAR-RESPONSES TO ZERO GRAVITY AND TO FRACTIONAL GRAVITY LEVELS INDUCED BY HEAD-DOWN OR HEAD-UP TILT

Determination of early cardiovascular responses to simulated gravity levels between 0 and 1 G will add knowledge of cardiovascular responses to space flight. Cardiovascular responses to 6 hours in a −5° head‐down bedrest model of weightlessness (0 G) were compared to those in head‐up tilts of +10°, +20°, and +42° (1/6, 1/3, and 2/3 G, respectively). Six healthy young adult males experienced the four angles on separate days. Impedance cardiography was used to measure thoracic fluid index, cardiac output, stroke volume, and peak flow. Although much intersubject variation occurred, the mean thoracic fluid content at −5° decreased during the first hour and remained decreased; 6‐hour values were similar to + 10° and +20°. Heart rate decreased the first 2 hours for all angles, then increased, converging at 3–4 hours, and reached control by hour 6. Stroke volume decreased for the first 3 hours at −5°, +10°, +20°; values at all four angles converged at hour 3 and increased in unison thereafter. Cardiac output and peak aortic flow reflected the angle at start of tilt; values at all angles converged by the second hour, decreased through the third hour, and increased thereafter. Pulse pressure decreased for the first 3 hours for angles −5°, +10°, and +20°, converged at the fourth hour, and returned to control. Peak flow at +42° was constant for the first 3 hours and increased thereafter. Blood pressure decreased for the first 2 hours, although the greatest decrease occurred at −5° and +42°; thereafter, values at all angles increased in unison and converged at the fourth hour. Total peripheral resistance increased during the first hour at −5° and +20° and decreased from hour 3 to hours 5–6 at the +42° angle. Cardiovascular values were related to tilt angle for the first 2 hours of tilt, but after hour 3 values at all four angles began to converge, suggesting that cardiovascular homeostatic mechanisms seek a common adapted state regardless of effective gravity level (tilt angle) up to 2/3 G. 1990 American College of Clinical Pharmacology