Electrocardiogram (ECG) analysis was performed in three human divers d
uring a 71 atm (7,200 kPa) saturation dive (COMEX HYDRA 10 experiment)
. The inhaled gas mixture was slightly hyperoxic; its composition was
basically helium and oxygen. Hydrogen was introduced during compressio
n and its partial pressure reached 20 atm. ECG changes were the same i
n the three divers. Marked bradycardia rapidly appeared at the beginni
ng of compression, then this response adapted throughout the dive. P-R
, QRS, and Q-T intervals and the S-T segment did not change significan
tly. The QRS axis remained stable. However, a rightward shift occurred
in P and T vector angles. These changes were correlated with time and
gas density, respectively. The modifications of ventricular repolariz
ation during compression are similar to those we observed during the H
YDRA 9 COMEX dive. They may correspond to changes in duration of myoca
rdial cell repolarization due to increased intrathoracic pressure chan
ges with dense-gas breathing. A marked global diminution of voltage oc
curred during the decompression period. This suggests that accumulatio
n of micro bubbles in tissues may influence the impedance, causing an
artifact in the amplitude of ECG complexes.