Cardiac arrhythmias and heart rate changes in prolonged hyperbaric air exposures

Daily resting and exercise electrocardiograms from 81 healthy subjects taking part in eight different air saturation chamber experiments at depths of 2-132 ft sea water gauge (fswg) were recorded and interpreted. Scalar and planar characteristics were measured and analyzed in addition to noting changes in rhythm. Of the measured variables, only the heart rate and QT-interval changed significantly in the majority of exposures. Heart rate decrements of 14%-23% were observed, generally with normalization after several days at depth, and in some cases a rebound immediately postexposure. The observed increase in QT-interval can be explained entirely by the bradycardia, which seems to be due to a combination of pressure, hyperoxia, and a chamber effect. Further distinction among these factors was not possible. Associated with the decrements in heart rate were asymptomatic supraventricular arrhythmias, generally AV-nodal escape rhythms or beats, in 10% of the subjects. One incident of symptomatic ventricular tachycardia was noted 20 h after surfacing during an exercise test. Otherwise, only infrequent, isolated premature ventricular beats were detected at depth, but were not more frequent than in the predive periods. These findings are consistent with the premise that exposure to hyperbaric air and other nitrogen-oxygen mixtures causes an increase in parasympathetic tone of sufficient magnitude to cause cardiac arrhythmias. That autonomic imbalance in diving activities may cause more malignant arrhythmias is discussed.