Submitted on March 25, 2004
Accepted on June 23, 2004
Sleep Restriction Does Not Affect Orthostatic Tolerance in the Simulated Microgravity Environment
S. Marlene Grenon1, Shelley Hurwitz2, Natalie Sheynberg2, Xinshu Xiao3, Brad Judson2, Craig D Ramsdell2, Christine Kim2, Richard J Cohen3, and Gordon H Williams2*
1 Division of Endocrinology, Hypertension, and Diabetes, Brigham and Women's Hospital, Boston, MA, USA; Department of Cardio-Thoracic Surgery, McGill University, Montreal, PQ, Canada
2 Division of Endocrinology, Hypertension, and Diabetes, Brigham and Women's Hospital, Boston, MA, USA
3 Harvard-MIT Division of Health Sciences and Technology, Massachusetts Institute of Technology, Cambridge, MA, USA
* To whom correspondence should be addressed. E-mail: gwilliams{at}partners.org.
Orthostatic intolerance (OI) is a major problem following spaceflight, and reports have indicated that astronauts also experience sleep restriction during flight. Because sleep restriction has been associated with a variety of cardiovascular abnormalities, we hypothesized that sleep restriction will compound the risk and severity of OI following simulated microgravity and exaggerate the renal, cardio-endocrine, and cardiovascular adaptive responses to it. Nineteen healthy men were equilibrated on a constant diet, after which they underwent a tilt-stand test (pre-TST). They then completed 14-16 days of simulated microgravity [head-down tilt-bed rest (HDTB)], followed by repeat tilt-stand test (post-TST). During HDTB, 11 subjects were assigned to an 8-hour sleep protocol (NSR), and 8 were assigned to a sleep-restricted protocol with 6 hours of sleep/night (SR). During various phases the following were performed: 24-hour urine collections, hormonal measurements and cardiovascular system identification (a non-invasive method to assess autonomic function and separately quantify parasympathetic and sympathetic responsiveness). Development of pre-syncope or syncope defined OI. There was a significant decrease in time free of OI (p=0.02) and an increase in OI occurrence (p=0.06) after HDTB among all subjects. However, the increase in OI occurrence did not differ significantly between the two groups (p=0.60). The two groups also experienced similar physiological changes with HDTB (initial increase in sodium excretion; increased excretion of potassium at the end of HDTB; increase in PRA (plasma renin activity) secretion without a change in serum or urine aldosterone). No significant change in autonomic function or catecholamines was noted. Simulated microgravity leads to increased OI, and SR does not additively worsen OI in simulated microgravity. Further, conditions of SR and NSR are similar with respect to renal, cardio-endocrine and cardiovascular responses to simulated microgravity.
