The effect of peripheral vasodilators on the human diving response.

Five healthy subjects performed seated, 30-second mid-inspiratory breath hold, facial immersions in water at 1 degree C, 10 degrees C and 25 degrees C under control conditions and under the influence of three separate vasodilators. A temperature dependent bradycardia and decrease in forearm blood flow were observed during control dives. Nitropaste produced little change in diving bradycardia. Hydralazine reduced diving bradycardia only in 1 degrees C immersion. Prazosin produced consistent reductions in temperature dependent bradycardia. There was a temperature dependent change in dive forearm blood flow which was not affected by vasodilators. This data suggest the importance of acute peripheral vasoconstriction to the bradycardia of the human diving response.