CLINICAL-MODELS OF CARDIOVASCULAR REGULATION AFTER WEIGHTLESSNESS

After several days in microgravity, return to earth is attended by alt erations in cardiovascular function. The mechanisms underlying these e ffects are inadequately understood. Three clinical disorders of autono mic function represent possible models of this abnormal cardiovascular function after spaceflight. They are pure autonomic failure, barorefl ex failure, and orthostatic intolerance. In pure autonomic failure. vi rtually complete loss of sympathetic and parasympathetic function occu rs along with profound and immediate orthostatic hypotension. In baror eflex failure, various degrees of debuffering of blood pressure occur. In acute and complete baroreflex failure, there is usually severe hyp ertension and tachycardia, while with less complete and more chronic b aroreflex impairment, orthostatic abnormalities may be more apparent. In orthostatic intolerance, blood pressure fall is minor, but orthosta tic symptoms are prominent and tachycardia frequently occurs. Only car eful autonomic studies of human subjects in the microgravity environme nt will permit us to determine which of these models most closely refl ects the pathophysiology brought on by a period of time in the microgr avity environment.