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1378 Apnoea and bradycardia in pre-term babies

Bradycardia occurring in association with prolonged apnoea is well documented. However, on many occasions the bradycardia occurs early, before there is any detectable change in arterial saturation. It is also known that obstructive apnoea frequently occurs. We therefore set out to investigate whether bradycardia occurred more commonly in association with obstructive apnoea. Seven pre-term babies with recurrent apnoea were studied on nine occasions in a total body plethysmograph for periods of up to 60 minutes. Tidal flow and volume were measured by a pneumotachograph and intrathoracic pressure with an oesophageal balloon. The presence or absence of the cardiac artefact on the flow trace was used to identify whether the airway was open or closed. A total of 172 apnoeic epidodes were analysed, of which 50% were closed, 27.9% open and 22.1% showed a mixed picture. Bradycardia, defined as heart rate of less than 100 per minute, occurred in 33.9% of closed apnoea, 17.8% of open apnoea and 20% with the mixed picture. Bradycardia occurring with a closed airway tended to be more severe. Respiratory efforts against a closed airway produced a striking bradycardia within two seconds, explaining the excess of bradycardia in this group. Our conclusion is that bradycardia occurring in apnoea is often secondary to mechanical changes occurring within the lung, rather than a response to hypoxia.