ACUTE HEMODYNAMIC-RESPONSES TO WEIGHTLESSNESS DURING PARABOLIC FLIGHT

Pilots and astronauts experience fluid shifts in variable gravity. Acute effects of fluid shifts on the cardiovascular system were monitored on NASA's KC-135 aircraft during parabolic flight. The variability of R-R intervals in the electrocardiogram was measured as an indication of vagal cardiac neural activity. R-R intervals were measured during the gravity transition from 2-G to 0-G produced by parabolic flight to assess the involvement of the autonomic nervous system in regulating the acute effects of fluid shifts. In seven subjects, a BoMed noninvasive continuous cardiac output monitor (NCCOM 3) monitored thoracic fluid index (TFI, ohms), heart rate (bpm), and cardiac output (1/min). Data were stored on a lap-top computer with the subject in one of four postures: sitting, standing, supine, and semi-supine, during one of four sets of eight to ten parabolas. Five seconds of data were averaged: before parabola onset (1.3-G); parabola entry (1.9-G); 0-G; and parabola exit (1.7-G). Three to eight parabolas were averaged for subjects in each posture; the mean for each posture was calculated. In each of five additional subjects, the coefficient of variation was calculated by dividing mean value by the standard deviation of 3 to 15 R-R intervals. Eight to ten parabolas were averaged for each postural set. Compared with values collected before 0-G, standing values during 0-G showed that the thoracic fluid index decreased 2.5 ohms, heart rate decreased 22 bpm, and cardiac output increased 1 L/min. During sitting, thoractic fluid index decreased 1.25 ohms, heart rate decreased 10 bpm, whereas cardiac output increased 0.5 L/min. In the supine position, thoracic fluid index and heart rate were constant whereas cardiac output decreased 0.55 L/min. In the semi-supine position, thoracic fluid index and heart rate were constant. Compared with values collected from 2-G and 0-G the coefficient of variation increased 66.4% in the standing position, 53.4% in the sitting position, and 43.3% in the semi-supine position and decreased 11.6% in the supine position. The data indicated that cardiovascular changes are dependent on posture and gravity. During the four sets of parabolas in the four different postures, the greatest and smallest changes were observed in the standing and supine positions, respectively, during 0-G. Fluid shifts from the legs to the thorax occurred during 0-G in the supine and standing positions. The high values of the coefficient of variation at the onset of 0-G suggest that vagal cardiac neural activity increases, but not significantly, in all positions except supine.