Roles of stress and adaptation in the elicitation of face-immersion bradycardia.

Thirteen young healthy human volunteers immersed their faces in warm or cold water on one day while breathing through a snorkel and on another day while breath holding. The magnitude of the elicited bradycardia was most prominently due to water temperature, with apnea playing a less important role. Perceived stress could affect the magnitude of the response, but it was less important than the other variables. Thus statistically significant nonparametric correlations were found for the group but not for most individuals between a scoring technique that assessed perceived stress and heart rate. In contrast to animals, these data indicate that bradycardia may be reliably elicited in humans by face immersion in cold water and that stress is neither necessary nor sufficient to produce this phenomenon. Adaptation did not seem to play a role in the development of this physiological response.