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1 Department of Anesthesia and Critical Care, Massachusetts General Hospital, Boston 02114; 3 Brigham and Women's Hospital Endocrine-Hypertension Division, Boston 02115; 2 NASA Center for Quantitative Cardiovascular Physiology, Modeling and Data Analysis, Harvard- MIT Division of Health Sciences and Technology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139; 4 Royal Norwegian Air Force, Institute of Aviation Medicine, Oslo 0313; and Department of Physiology, University of Oslo, Oslo 0317, Norway; 5 NASA Johnson Space Center, Cardiovascular Laboratory, Houston, Texas 77058; and 6 Columbia University, New York, New York 10032
Many astronauts after being
weightless in space become hypotensive and presyncopal when they assume
an upright position. This phenomenon, known as orthostatic intolerance,
may interfere with astronaut function during reentry and after
spaceflight and may limit the ability of an astronaut to exit a landed
spacecraft unaided during an emergency. Orthostatic intolerance is more
pronounced after long-term spaceflight and is a major concern with
respect to the extended flights expected aboard the International Space Station and for interplanetary exploration class missions, such as a
human mission to Mars. Fully effective countermeasures to this problem
have not yet been developed. To test the hypothesis that
-adrenergic
stimulation might provide an effective countermeasure, we conducted a
16-day head-down-tilt bed-rest study (an analog of weightlessness)
using normal human volunteers and administered the
1-agonist drug midodrine at the end of the bed-rest
period. Midodrine was found to significantly ameliorate excessive
decreases in blood pressure and presyncope during a provocative tilt
test. We conclude that midodrine may be an effective countermeasure for
the prevention of orthostatic intolerance following spaceflight.
cardiovascular system; vasovagal syncope
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