Hyperbaric oxygen increases parasympathetic activity in professional divers

The role of autonomic nervous system in hyperoxic bradycardia was evaluated by using the power-spectral analysis of heart-rate variability (HRV). Ten professional divers went through two hyperbaric hyperoxic experiments: (li hyperbaric oxygen (HBO), 100% oxygen at 2.5 ATA, (2) hyperbaric air (HBAIR) , O-2 21% at 2.5 ATA. Four-minute traces of ECG were registered and subject ed to power-spectral analysis. Cardiac conduction parameters were evaluated by a diagnostic 12-lead ECG and arrhythmias by a continuous 3-lead EGG. St atistical analysis was made using analysis of variance for repeated measure ments. Heart rate decreased (P < 0.001), but the response was similar durin g both treatments (P = 0.14). Total power increased significantly more duri ng HBO than HBAIR (P = 0.003). High-frequency (HF) power (P < 0.001), Hayan o's index (P = 0.001) and normalized units of HF power (P = 0.002) increase d and LF/HF index (P < 0.001) decreased more during HBO than HBAIR. There w ere no changes in cardiac conduction or incidence of arrhythmias. In conclu sion, 100% oxygen at 2.5 ATA caused marked increase in parasympathetic tone compared with 21% oxygen at 2.5 ATA.