Cardiac Rhythm during Breath‐Holding and Water Immersion in Man

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Abstract
A report is given of the pulse alterations in 30 subjects during face immersion with and without apnea.The breath was held after one mamixal inspiration, at a transmural, thoracic pressure of zero to +5 mm Hg.Apnea in itself produced a decrease in pulse averaging 9 per cent of the control pulse (mean: 75pLS.E.2.2 beats/min).Apnea plus immersion of the face in 15d̀ C water produced a 14 per cent fall, and face immersion in 15d̀ C water without apnea produced a 7 per cent fall.—In 11 supine subjects the pulse response during apnea was found to be a function of the transmural, thoracic pressure, with little difference between the response in air and in 36d̀ C water.—Both the bradycardia provoked by apnea plus face immersion and that provoked by apnea during whole body immersion is temperature dependent.It is concluded that the bradycardia observed during apneic diving in man, originates from two independent factors.One of these is an ‘apnea bradycardia’ elicited from the intrathoracic, high pressure baroreceptors, and the other is a bradycardia elicited by cold stimulation of the skin.Thus, the validity of Irvings concept of a general vertebrate diving reflex (with unknown receptors) common to animals and man, seems dubious.

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