Effects of hyperbaric oxygen exposure at 31.3 ATA on spontaneously beating cat hearts.

Cardiovascular responses to raised ambient pressure and acute hyperbaric oxygen (HBO) exposure [HBO, partial pressure of oxygen (PO2) = 2 ATA)] were examined in anesthetized cats. Relatively normoxic [PO2 = 0.35 ATA] helium compression to 31.3 ATA decreased spontaneous heart rate, prolonged the P-R and Q-T intervals of the ECG independently of changes in rate, and significantly increased ventricular contractility. HBO while at depth produced further decreases in heart rate, with no change in the P-R interval. The Q-T interval decreased significantly in the presence of bradycardia, particularly during the first 15 min of exposure. During the first 10-15 min of HBO exposure myocardial contractility and developed ventricular pressure were reduced, but contractility increased significantly at 30 min. These results indicate that HBO interacts with depth effects to produce hyperbaric bradycardia but reverses pressure actions on ventricular repolarization. The inotropic effects of HBO are time dependent, with slightly negative effects occurring initially, followed by positive inotropic actions at 30 min.