Some cardiovascular responses in anesthetized dogs during repeated decompressions to a near-vacuum.

Abstract : Repeated, rapid decompressions of anesthetized dogs from ambient pressures of 250 or 200 mm. Hg abs. within 1 second to approximately 1 mm. Hg, followed by exposures from 55 to 240 seconds before recompression, carried out within a 4-hour period, have shown that the hearts of most animals tolerate such exposures well. A progressively severe degree of bradycardia was proportional to the length of each anoxic exposure. Following initial and repeated decompressions, the customary subatmospheric pressure in the superior vena cava was elevated to 60 or 70 mm. Hg within 2 seconds, and the arterial pressure approached comparable values within 4 to 10 seconds. To a great degree, former predecompression values were restored upon recompression as barometric pressure surpassed that of water vapor. A temporary high arterial and pulse pressure resulted in most animals after recompression, with a gradual return within 5 to 10 minutes. A surge of positive tracheal pressure occurred concurrently with each decompression, remaining slightly positive while the barometric pressure was below that of water vapor. Responses were similar in many respects to those following decompressions to 30 mm. Hg, although at vacuum conditions vaporization and outgassing effects become maximum. All such exposures are potentially dangerous and the possibility of cardiac damage, even for a short exposure, is ever present. Failure to survive such exposures, however, appears to be more dependent on damage to vital structures other than the heart. (Author)