Orthostatic hypotension of prolonged weightlessness: clinical models

Subnormal norepinephrine release relates to presyncope in astronauts after spaceflight

Postflight orthostatic intolerance is experienced by virtually all astronauts but differs greatly in degree of severity. We studied cardiovascular 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 had significantly smaller increases in plasma norepinephrine levels with standing than did those who could remain standing (105 +/- 41 vs. 340 +/- 62 pg/ml; P = 0.05). In addition, they had significantly lower standing peripheral vascular resistance (23 +/- 3 vs. 34 +/- 3 mmHg.1l-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 lower supine (16 +/- 1 vs. 21 +/- 2 mmHg.1l-1).min; P = 0.04) and standing (23 +/- 2 vs. 32 +/- 2 mmHg.1l-1).min; P = 0.038) vascular resistance, 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. 108 +/- 3 mmHg; P = 0.006) systolic pressures before flight. This is the first study to clearly document these differences among presyncopal and nonpresyncopal astronauts after spaceflight and also offer the possibility of preflight prediction of postflight susceptibility. These results clearly point to hypoadrenergic responsiveness, possibly centrally mediated, as a contributing factor in postflight orthostatic intolerance. They may provide insights into autonomic dysfunction in Earthbound patients.

The sympathetic nervous system and the physiologic consequences of spaceflight: a hypothesis

Many of the physiologic consequences of weightlessness and the cardiovascular abnormalities on return from space could be due, at least in part, to alterations in the regulation of the autonomic nervous system. In this article, the authors review the rationale and evidence for an autonomic mediation of diverse changes that occur with spaceflight, including the anemia and hypovolemia of weightlessness and the tachycardia and orthostatic intolerance on return from space. This hypothesis is supported by studies of two groups of persons known to have low catecholamine levels: persons subjected to prolonged bedrest and persons with syndromes characterized by low circulating catecholamines (Bradbury-Eggleston syndrome and dopamine beta-hydroxylase deficiency). Both groups exhibit the symptoms mentioned. The increasing evidence that autonomic mechanisms underlie many of the physiologic consequences of weightlessness suggests that new pharmacologic approaches (such as administration of beta-blockers and/or sympathomimetic amines) based on these findings may attenuate these unwanted effects.