Orthostatic intolerance and motion sickness after parabolic flight

Because it is not clear that the induction of orthostatic intolerance in re turning astronauts always requires prolonged exposure to microgravity, we i nvestigated orthostatic tolerance and autonomic cardiovascular function in 16 healthy subjects before and after the brief micro- and hypergravity of p arabolic flight. Concomitantly, we investigated the effect of parabolic fli ght-induced vomiting on orthostatic tolerance, R-wave-R-wave interval and a rterial pressure power spectra, and carotid-cardiac baroreflex and Valsalva responses. After parabolic flight 1) 8 of 16 subjects could not tolerate 3 0 min of upright tilt (compared to 2 of 16 before flight); 2) 6 of 16 subje cts vomited; 3) new intolerance to upright tilt was associated with exagger ated falls in total peripheral resistance, whereas vomiting was associated with increased R-wave-R-wave interval variability and carotid-cardiac baror eflex responsiveness; and 4) the proximate mode of new orthostatic failure differed in subjects who did and did not vomit, with vomiters experiencing comparatively isolated upright hypocapnia and cerebral vasoconstriction and nonvomiters experiencing signs and symptoms reminiscent of the clinical po stural tachycardia syndrome. Results suggest, first, that syndromes of orth ostatic intolerance resembling those developing after space flight can deve lop after a brief (i.e., 2-h) parabolic flight and, second, that recent vom iting can influence the results of tests of autonomic cardiovascular functi on commonly utilized in returning astronauts.