A COMPREHENSIVE GUYTON MODEL ANALYSIS OF PHYSIOLOGICAL-RESPONSES TO PREADAPTING THE BLOOD-VOLUME AS A COUNTERMEASURE TO FLUID SHIFTS

The Guyton model of fluid, electrolyte, and circulatory regulation is an extensive mathematical model capable of simulating a variety of exp erimental conditions. It has been modified for use at NASA to simulate head-down tilt, a frequently used analog of weightlessness. Weightles sness causes a headward shift of body fluids that is believed to expan d central blood volume, triggering a series of physiologic responses r esulting in large losses of body fluids. We used the modified Guyton m odel to test the hypothesis that preadaptation of the blood volume bef ore weightless exposure could counteract the central volume expansion caused by fluid shifts, and thereby attenuate the circulatory and rena l responses that result in body fluid losses. Simulation results show that circulatory preadaptation, by a procedure resembling blood donati on immediately before head-down bedrest, is effective in damping the p hysiologic responses to fluid shifts and reducing body fluid losses. A fter 10 hours of head-down tilt, preadaptation also produces higher bl ood volume, extracellular volume, and total body water for 20 to 30 da ys of bedrest, compared with non-preadapted control. These results ind icate that circulatory preadaptation before current Space Shuttle miss ions may be beneficial for the maintenance of reentry and postflight o rthostatic tolerance in astronauts. This paper presents a comprehensiv e examination of the simulation results pertaining to changes in relev ant physiologic variables produced by blood volume reduction before a prolonged head-down tilt. The objectives were to study and develop the countermeasure theoretically, to aid in planning experimental studies of the countermeasure, and to identify potentially disadvantageous ph ysiologic responses that may be caused by the countermeasure.