Bradycardia in human divers

Electrocardiograms showed bradycardia developing in men and women during apnea in air and more markedly when submerged in water. Intervals between heartbeats reached 2 sec. The onset and degree of bradycardia were not reduced by vigorous swimming underwater. The bradycardia appeared with more regularity and in greater degree in practiced and capable swimmers. The rapidity of onset of this bradycardia indicates that, as in many natural diving animals, it is reflexly initiated.