Diving bradycardia and breath-holding time in man

The hypothesis that the diving response, recorded as diving bradycardia during submersed breath holding in man, would enhance his breath-holding time was tested. Five certified scuba divers served as subjects. They performed breath holds of maximal duration while nonimmersed and during submersion in cool (32 degrees C), cold (20 degrees C), and thermoneutral (35 degrees C) water. The mean breath-holding time and heart rate during the nonimmersed (control) condition were, respectively, 111.2 +/- 14.1 (SE) s and 64.1 +/- 4.7 (SE) beats/min, the relatively long breath-holding times being due primarily to the so-called short-term training effect. Compared to the control values the breath-holding time in 20 degrees C water was 54.9% shorter and heart rate 25.9% lower, in 32 degrees C water the breath-holding time was not different and heart rate was 28.1% lower, and in 35 degrees C water the breath-holding time was longer by 25.6% while there was no difference in heart rate. In all conditions the breath-hold breaking point alveolar PCO2 was the same at about 52 mmHg. The shortening of the breath holds in cold water was ascribed to a 256% increase (over nonimmersed control) in metabolic rate as well as a respiratory drive due to stimulation of skin cold receptors. As for the prolongation of breath holds in thermoneutral water, it was hypothesized that immersion caused a delay in the build-up of chemical stimuli at the chemoreceptors.