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% of greater than 20 seconds, p less than 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 less than 0.001). 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.
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