1. The changes in heart rate surrounding a period of apnoeic asphyxia, produced by either central or obstructive sleep apnoea, are characteristically a progressive bradycardia during the apnoeic period followed by an abrupt reversal to tachycardia with resumption of lung inflation.

2. In a subject who had undergone bilateral pulmonary vagotomy with sparing of the cardiac branches on the left side, the bradycardia is still present during the apnoeic period; however, there is no abrupt response to lung inflation.

3. A subject who had undergone unilateral right-sided pulmonary vagotomy demonstrated a normal response, the increase in heart rate coinciding with resumption of airflow.

4. These findings are consistent with the hypothesis that the bradycardia of sleep apnoea is a hypoxia-induced chemoreflex and that reversal to tachycardia with resumption of lung inflation is a reflex with afferent fibres in the pulmonary vagus nerves.

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