Background: Cardiovascular disease is the second leading cause of diving deaths. Scarcity of 12 lead ECG recordings during dives leaves many questions unanswered about cardiac function during ascent. I hypothesized that decreased oxygen partial pressure (PpO2) initiates cardiac arrhythmia on ascent. I examined heart rate variability (HRV), rhythm and circulating markers of cardiac damage in the blood in response to submersion, increased pressure and reduction in the partial pressure of oxygen during the ascent. Methods: Experiments were performed in the wet pot of the hyperbaric chamber. Participants (N=19) were occupational and scientific divers, age 39.3 years, BMI 26.5 kg/m2, 3 females. Each completed two dives in a drysuit while swimming in 8oC water outfitted with a 12 lead ECG holter recorder. A 30-minute swim was performed at ambient pressure followed by a dive to 5 atmospheres absolute (ATA) with a direct ascent to surface pressure. The experimental exposure held the PpO2 at 1.0 ATA for the ascent. Blood samples were drawn at baseline, immediately after the dive and one-hour post dive. ECG analysis was performed for 5 epochs of 5 minutes each. Results: Diving increased heart rate and decreased HRV (p
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