Head-out immersion in the non-human primate: A model of cardiovascular deconditioning during microgravity

Background: Orthostatic intolerance is a common complication associated wit h spaceflight. It has been speculated that this is due to changes in blood volume and alterations in cardiovascular reflexes. The objective of the cur rent study was to develop a model that would allow us to study the cardiova scular system and the regulation of blood volume during short-term microgra vity exposure in the primate with the intent of eventually being able to el ucidate those factors responsible for the orthostatic intolerance. Hypothes is: Head-out water immersion in the conscious non-human primate simulates t he cardiovascular and volume regulatory responses observed in astronauts du ring exposure to microgravity. Methods: Four monkeys were chronically instr umented for measuring BP and heart rate and then conditioned to the primate restraint chair. They were then subjected to 72 h of head-out water immers ion (two immersions in three monkeys and one immersion in the fourth) in or der to simulate the cardiovascular and renal effects of the microgravity en vironment. Results: During the immersion, there was an increase in arterial BP (ABP) and central venous pressure (CVP) and a reflex decrease in heart rate (HR). Urine flow (UV) increased and water intake decreased, producing a negative water balance. This was not associated with an alteration in foo d intake. CVP and UV decreased following de-immersion. There was also reset ting of the arterial baroreflex control of HR. Significant tachycardia occu rred after the immersion that was associated with a decrease in ABP. Conclu sion: These results are similar to those observed in astronauts during and after spaceflight, suggesting that head-out water immersion of the non-huma n primate provides a good model for studying cardiovascular and renal adapt ations to spaceflight.