SUMMARY Bradycardia occurred during 363 of 1520 apnoeas of 10 seconds' duration recorded in 28 preterm infants. The incidence increased with increasing duration of apnoea (10% of 10-14 seconds, 34% of 15-20 seconds, and 75%/O of >20 seconds, p<0-001). This was similar for each type of apnoea-central, mixed, and obstructive. During 133 apnoeas in five infants the time from the start of the apnoea to the onset in the fall in oxygen saturation (mean 6-9 seconds) was significantly related to the onset of the fall in heart rate (mean 9-3 seconds) (r=0-67, p<0U001). Recovery in heart rate coincided with resumption of air flow rather than breathing efforts and preceded the recovery in oxygen saturation. These results suggest that bradycardia occurs during apnoea as a response to falling oxygen saturation, probably through a peripheral chemoreceptor reflex that is manifest when breathing efforts are absent or ineffective.

SUMMARY The incidence, duration, and type of apnoea were determined in 28 preterm infants born at 27 to 34 weeks' gestation, using polygraphic records of abdominal breathing movements and nasal airflow. Of the 1520 episodes of apnoea of 10 or more seconds duration, 1002 (66%) lasted 10 to 14 seconds, 311 (20%) lasted 15 to 20 seconds, and 207 (14%) lasted more than 20 seconds. Overall, 69% were central in type, 20% were mixed, and 11% were purely obstructive. With increasing duration of apnoea, the proportion of episodes of central apnoea decreased (69 to 29%) while that of mixed apnoea increased (20 to 60%). Eight infants had obstructive apnoea of more than 20 seconds duration. When they were compared with the 10 infants of similar gestational age and birthweight who had central or mixed apnoea, they had a higher incidence of intraventricular haemorrhage, hydrocephalus, positive pressure ventilation via an endotracheal tube, and abnormal neurological development during the first year of life.