Life Sciences in the Space Program 



status, and possibly cardiac function. However, reports of cardiac deconditioning 

 are based on either a small number of echocardiography data from Shuttle flights 

 or responses to orthostatic stress immediately following space flight. Inflight 

 echocardiographic data are consistent with changes in fluid compartmentalization, 

 but it is unclear if myocardial performance (e.g., inotropism or chronotropism) is 

 altered independent of changes in blood volumes. 



To supplement the scant data gathered from space missions, the cardiovascular 

 response to weightlessness has been studied by NASA using bed rest with head- 

 down tilt to simulate the effects of weightlessness on hemodynamics. When an 

 individual is placed in a head-down tilt for a number of days, it is hypothesized 

 that the person will experience the expansion of central blood volumes in the 

 thorax as the blood returned from the veins of the legs is increased. Bed-rest 

 studies are well controlled, can be done on Earth, represent a continued line of 

 investigations not dependent upon flight availability, and pose questions relevant 

 to clinical concerns beyond the space program. Although this method may be 

 effective in simulating exposure to microgravity, a significantly larger number of 

 inflight studies must be done to compare the reliability of using bed-rest protocols 

 to study the effects of prolonged exposure to microgravity (8,9). It cannot be 

 overemphasized that bed rest (even with head-down tilt) at 1 gravity (g) on Earth 

 does not mimic or reproduce the microgravity environment of space because 

 gravitational forces are still at work. Despite the recumbent posture, gravitational 

 forces still operate on the circulation, bones, and muscles. Such Earth-based 

 experiments on humans are merely a small first step toward understanding 

 microgravity deconditioning. 



Several countermeasures for cardiovascular deconditioning have been investigated. 

 These methods include the use of positive pressure suits, volume replacement 

 with water and salt, pharmacologic agents to promote the retention of electrolytes 

 and fluid, the use of applied lower body negative pressure, and exercise. However, 

 these prophylactic measures have not been uniformly effective at reducing the 

 incidence of near-syncope following Shuttle reentry. Accordingly since cardio- 

 vascular deconditioning and space adaptation syndrome may decrease crew 

 performance, and since the mechanisms causing these changes are poorly 

 understood, further efforts are needed to define and prevent or treat these 

 physiologic responses. 



In summary, significant changes occur in the cardiovascular system in microgravity, 

 and many questions remain unanswered. What is the role of the cardiovascular 

 system in the etiology of deconditioning? Is space travel associated with an 

 increase in morbidity from cardiovascular disease in flight crews? Does the degree 

 of cardiovascular deconditioning from short-term space flights predict incapac- 

 itating cardiovascular deconditioning with longer flights? Do cardiovascular 

 responses to microgravity detrimentally affect other organ systems? For example, 

 do the hemodynamic and hormonal responses to microgravity result in alterations 

 in vestibular function or cognitive function? Can improved countermeasures be 

 developed for the problems that occur with space travel? What is the role of 



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