The attenuating effect of heat and hypovolemia on the human diving response.

Seven healthy adult volunteers underwent 30-second facial immersions in ice water before and during 36 minutes exposure to intense ambient heat (68 degrees C) on separate, non-consecutive days: one normovolemic and the second with thiazide-induced, 10% hypovolemia. The normovolemic, preheat immersion induced a heart rate (H.R.) decrease from 74 +/- 6 to 48 +/- 5 bpm (-35%) and a forearm blood flow (F.B.F.) decrease from 3.5 +/- 1.6 to 0.8 +/- 0.3 ml/(100 ml min) (-77%). Pre-heat changes were similar for hypovolemic subjects. After 36 minutes of heat exposure there was a 07 degrees C increase in esophageal temperature and a four-fold increase in F.B.F. for normovolemic and a two-fold increase in F.B.F. for hypovolemic subjects. Heating reduced diving bradycardia more than did superimposed hypovolemia. Four subjects had diving tachycardia after hypovolemic heating. We concluded that the bradycardia response to facial immersion can be overridden by reducing the central circulating blood volume by heat and that this effect is enhanced by hypovolemia.