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

Orthostatic intolerance is a common complication associated with spaceflight. It has been speculated that this is due to changes in blood volume and alterations in cardiovascular reflexes. The objective of the current study was to develop a model that would allow us to study the cardiovascular system and the regulation of blood volume during short-term microgravity exposure in the primate with the intent of eventually being able to elucidate those factors responsible for the orthostatic intolerance.

Head-out water immersion in the conscious non-human primate simulates the cardiovascular and volume regulatory responses observed in astronauts during exposure to microgravity.

Four monkeys were chronically instrumented 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 immersion (two immersions in three monkeys and one immersion in the fourth) in order to simulate the cardiovascular and renal effects of the microgravity environment.

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 food intake. CVP and UV decreased following de-immersion. There was also resetting of the arterial baroreflex control of HR. Significant tachycardia occurred after the immersion that was associated with a decrease in ABP.

These results are similar to those observed in astronauts during and after spaceflight, suggesting that head-out water immersion of the non-human primate provides a good model for studying cardiovascular and renal adaptations to spaceflight.