SUBNORMAL NOREPINEPHRINE RELEASE RELATES TO PRESYNCOPE IN ASTRONAUTS AFTER SPACEFLIGHT

Postflight orthostatic intolerance is experienced by virtually all ast ronauts but differs greatly in degree of severity. We studied cardiova scular responses to upright posture in 40 astronauts before and after spaceflights lasting up to 16 days. We separated individuals according to their ability to remain standing without assistance for 10 min on landing day. Astronauts who could not remain standing on landing day h ad significantly smaller increases in plasma norepinephrine levels wit h standing than did those who could remain standing (105 +/- 41 vs. 34 0 +/- 62 pg/ml; P = 0.05). In addition, they had significantly lower s tanding peripheral vascular resistance (23 +/- 3 vs. 34 +/- 3 mmHg . l (-1). min; P = 0.02) and greater decreases in systolic (-28 +/- 4 vs. -11 +/- 3 mmHg; P = 0.002) and diastolic (-14 +/- 7 vs. 3 +/- 2 mmHg; P = 0.0003) pressures. The presyncopal group also had significantly lo wer supine (16 +/- 1 vs. 21 +/- 2 mmHg . l(-1). min; P = 0.04) and sta nding (23 +/- 2 vs. 32 +/- 2 mmHg . l(-1). min; P = 0.038) vascular re sistance, supine (66 +/- 2 vs. 73 +/- 2 mmHg; P = 0.008) and standing (69 +/- 4 vs. 77 +/- 2 mmHg; P = 0.007) diastolic pressure, and supine (109 +/- 3 vs. 114 +/- 2 mmHg; P = 0.05) and standing (99 +/- 4 vs. 1 08 +/- 3 mmHg; P = 0.006) systolic pressures before Eight. This is the first study to clearly document these differences among presyncopal a nd nonpresyncopal astronauts after spaceflight and also offer the poss ibility of preflight prediction of postflight susceptibility. These re sults clearly point to hypoadrenergic responsiveness, possibly central ly mediated, as a contributing factor in postflight orthostatic intole rance. They may provide insights into autonomic dysfunction in Earthbo und patients.