Physiological Responses to Near-Vacuum

Unanesthetized dogs and trained subhuman primates, including chimpanzees, were decompressed in about 1 sec from 180 to less than 2 mm Hg. Exposure times at the low pressure ranged from 5 to 180 sec for the dogs and up to 150 sec for the chimpanzees. The animals usually became unconscious in about 9 to 12 sec after decompression. The effects of anoxia, water vapor, and other evolved gases were apparent, resulting in generalized muscle spasticity, a few gasps, transitory convulsive seizures, apnea, and gross swelling of the body. All dogs exposed for less than 120 sec survived despite evidence of transient lung damage. Respiration recommenced spontaneously either during or after recompression provided there was sufficient cardiovascular recovery to restore blood pressure, flow, and brain oxygenation. The longer the exposure time, the longer the recovery period, which ranged from a few minutes to a few hours, except for one dog, which showed a severe post-decompression paralysis with gradual recovery over a period of several weeks. In dogs, exposures of 120 to 180 sec resulted in about 15 to 18 per cent fatalities, respectively. Denitrogenation resulted in a significantly higher survival rate. Evidence of severe pathologic damage, except for the lungs and one case of paralysis, was essentially absent upon autopsy. Chimpanzees exposed for as long as 150 sec recovered with no apparent nervous system damage.