Extremes in human breath hold, facial immersion bradycardia.

Although the average human response to apneic facial immersion in ice water is a reduction in heart rate from 70 to 45 beats/min, a small proportion of healthy subjects develop diving bradycardia to less than 20 beats/min. Twenty-seven healthy subjects performed resting, seated, 30-s mid-inspiratory breath hold, facial immersion in a basin of water. Heart rate dropped more when the water temperature was 1 degree C than at 24 degrees C. Five subjects developed asymptomatic diving bradycardia to less than 15 beats/min. One physically active individual consistently had dive heart rates as low as 5.6 beats/min.