Diving reflex bradycardia in man; effect of different temperatures

ABSTRACT

62 male subjects from Karachi were selected and divided in four groups according to four levels of water temperature at 10°C, 20°C, 30°C and 40°C. Subjects were asked for face immersion, with breath holding for 30 seconds, and resulted in reflex bradycardia, the effect was maximum at 10°C, while at 40°C, the bradycardia was less pronounced. It is understood that bradycardia was not produced by asphyxia, as breath holding was only for 30 seconds. It is suggested that cold receptors in face were the triggering factor for reflex bradycardia. Prolongation of QT interval in ECG records in experimental groups, indicated that cardiac aspect of diving reflex was governed by vagus and withdrawal of cardiac sympathetic tone