Hyperbaric exposure to a 5 ATA He-N2-O2 atmosphere affects the cardiac function and organ blood flow distribution in awake trained rats.

Previous studies on rats have demonstrated an increased myocardial blood flow (MBF) in spite of an unchanged cardiac output (CO), mean arterial pressure (MAP), and heart rate (HR) when rats were exposed to a 5 ATA ambient pressure (PN2 = 4.8 ATA, PO2 = 0.2 ATA). We have investigated the changes in CO and organ blood flow by the microsphere method on 9 awake, trained rats exposed to a 5 ATA ambient pressure with surface pressure of N2 (= 0.8 ATA) and O2 (= 0.2 ATA), increasing the ambient pressure with He. The chamber temperature was 30-31 degrees C. CO and organ blood flow were measured in control and after 15 min under pressure. No changes were observed in CO, HR, or MAP during the hyperbaric exposure, indicating unchanged external work of the heart when calculated as MAP X HR. The pulse pressure however increased significantly (P less than 0.05). MBF increased at the same time by 36% (P less than 0.01), equally distributed in the right and left ventricles (P less than 0.01). The fact that myocardial perfusion increases when ambient pressure is elevated, independent of atmospheric inert gas composition, invites the conclusion that this change is due to the ambient pressure itself and not to a specific effect of N2 or He as inert gas at 5 ATA. The blood flow to the adrenals increased by 77% (P less than 0.01), the liver by 105% (P less than 0.05), while the blood flow to the skeletal muscles decreased by 30% (P less than 0.05). The blood flow to the lungs, the paws, and the outer ear increased, most likely due to the high ambient temperature in the chamber. We may conclude that HR does not change in a normoxic hyperbaric atmosphere. High ambient pressure increases the pulse pressure, but the MAP and CO remain unchanged. These changes and the changes in organ blood flow need well-balanced neuronal or hormonal reflexes to distribute an unchanged CO under pressure. The exact explanation is not known.