Cardiovascular Responses Following Explosive Decompression of Macaque Monkeys to Extreme Altitudes

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Abstract
Cardiovascular responses following sudden decompression to simulated altitudes of as high as 77,000 feet and free-fall descent to sea level are the result of mechanical effects of ambient pressure changes and the rapidly induced total anoxia. The following arterial pressure changes, recorded by strain gauge continuously throughout the expt., occur: (1) Momentary rise due to initial Valsalva effect at the instant of decompression; (2) immediate lowering due to carotid sinus and other reflex reactions to sudden rise in blood pressure with complete recovery within 10 sec. or less if factors 3 and/or 4 do not supervene; (3) additional lowering due to maintained impairment of venous return from lower half of body caused by positive intra-abdominal pressure, inevitably developing at altitudes above about 43,000 feet; the extent of this effect is related to altitude and recovery is prompt upon recompression; (4) following, fusing with and exacerbated by 3 is the precipitous drop due to the rapidly induced total anoxia at altitudes above 50,000 feet, even in 100% O2 (the mean anoxic drop in arterial pressure is to about 45% of control); (5) first sign of recovery is the rise in pulse pressure, due to improved diastolic filling resulting from intra-abdominal pressure equalization, which is complete by the time the animal reaches the 50,000 foot level during descent, and the reflex anoxic bradycardia occurring at about the same time. The liberation of epinephrine during anoxia is considered as a possible link between these 3 concurring events. With further descent in O2 the cardiovascular system responds promptly to the rapid reoxygenation of the blood resulting from the automatic rise in alveolar pO2 with increasing ambient pressure. Recovery is complete or nearby complete at the time of spontaneous resumption of respiration, which occurs at about 40,000 feet during descent or soon after. The pulse pressure, throughout the period beginning immediately upon decompression, never disappears, even at the highest final decompression altitudes.

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