The relationship between bradycardia, apnea, and hypoxemia in preterm infants

The pathogenesis of bradycardias in preterm infants is poorly understood. Because their pathogenesis may involve both apnea and hypoxemia, we set out to analyze the proportion of bradycardias that were associated with an apneic pause and/or a fall in arterial oxygen saturation (SaO2), and the temporal sequence of the three phenomena, in overnight tape recordings of SaO2 (Nellcor N100 in beat-to-beat mode), breathing movements, nasal airflow, and ECG in 80 preterm infants at the time of discharge from hospital. A bradycardia was defined as a fall in heart rate of > or = 33% from baseline for > or = 4 s, an apneic pause as a cessation of breathing movements and/or airflow for > or = 4 s, and a desaturation as a fall in SaO2 to < or = 80%. A total of 193 bradycardias were found in 46 (58%) of the recordings (median, three per recording; range 1-18). There was a close relationship between bradycardias, apneic pauses, and desaturations: 83% of bradycardias were associated with apneic pauses and 86% with desaturations. Where all three phenomena occurred in combination, the time from the onset of apnea to the onset of the fall in SaO2 was shorter (median interval, 0.8 s; range -4.9-+ 11.5 s) than that from the onset of apnea to the onset of bradycardia (median, 4.8 s; range -4.0-+ 14.0 s). Hence, most bradycardias (86%) commenced after the onset of the fall in SaO2. We conclude that bradycardia, apnea, and hypoxemia are closely linked phenomena in preterm infants.