EVIDENCE OF A CRITICAL PERIOD OF AIRWAY INSTABILITY DURING CENTRAL APNEAS IN PRETERM INFANTS

The timing and magnitude of airway narrowing in central apneas is unkn own, We have developed a method of apnea classification that relies on the transmission of cardiac airflow oscillation to indicate airway pa tency. Using a theoretical model, we showed that the amplitude of the cardiac airflow oscillation is proportional to airway diameter for sma ll lumens. While in the majority of central apneas the amplitude of th e cardiac airflow oscillation remains nearly constant, in a subset of events the waveform decreases with time, suggesting airway narrowing. We hypothesized that this is not a random occurrence but reflects a cr itical period of airway instability during central apnea. To test this hypothesis we studied 41 preterm infants. Of 4,456 central apneas, 58 5 had a decrease in the amplitude of the cardiac oscillation. The ampl itude of the cardiac airflow oscillation during an apnea was recorded to provide a dynamic measure of changes in airway diameter with time. To allow for comparisons between patients the amplitude of each cardia c airflow oscillation was expressed as a proportion of the maximum amp litude observed in each infant. We then compared the amplitude at mult iple successive 0.5 s intervals with the amplitude of the cardiac airf low oscillation observed at the apnea outset using ANOVA. We found a s ignificant decrease in cardiac airflow oscillation after only 1 s irre spective of the apnea duration (3 to 16 s). We conclude that airway na rrowing during central apnea is not a random occurrence but appears sh ortly after the onset of the apnea. We speculate that the phenomenon i s secondary to passive airway relaxation.