Hyperbaric exposures alter cardiac excitation-contraction coupling.

Cardiac excitation-contraction (E-C) coupling variables were measured in anesthetized cats during helium-oxygen dives to 1000 fsw (305 msw). At constant pacing rates of 190-300/min the delay between onset of ventricular excitation and onset of developed pressure increased with depth, the larger increases occurring at faster rates. The electrocardiographic Q-T interval increased with depth at all rates, but rate-related shortening of Q-T was less at depth. At a rate of 190, the indexes of contractile performance were enhanced by increases in depth, but they showed little change or were lessened at faster rates. Predive changes in contractility, time to peak pressure, and systolic duration correlated positively with rate-induced changes in the Q-T interval. The correlations were reduced or reversed at depth, however, suggesting a dissociation of E-C coupling. Several cases of conduction arrhythmias or pulsus alternans were encountered with increases in depth and heart rate. These results suggest cardiac E-C coupling is altered by interactions of depth and heart rate. Further, these findings may have relevance to the capacity of the cardiovascular system to support periods of work in a hyperbaric environment.