EVIDENCE OF A CRITICAL PERIOD OF AIRWAY INSTABILITY DURING CENTRAL APNEAS IN PRETERM INFANTS. • 2314

We have developed a method of apnea classification which relies on the transmission of cardiac airflow oscillation to indicate airway patency. Using a theoretical model we have previously shown that the amplitude of the cardiac airflow oscillation is proportional to airway diameter for small lumens. While in the majority of central apneas, the amplitude of the cardiac airflow oscillation remains constant, there is a subset of events where the waveform decreases with time. We hypothesized that this is not a random occurrence but reflects a critical period of airway instability during a central apnea. To test this we examined 212/1257 central apneas in 12 infants [birth weight 1625±240 g (mean±SE), study weight 2065±270 g, gestational age 30±1 wk, postnatal age 35±9 d] referred to our laboratory. The amplitude of the cardiac airflow oscillation produced by each individual heart beat during an apnea was measured and tabulated to provide a dynamic measure of changes in airway diameter with time. To allow for comparisons between patients the amplitude of each cardiac airflow oscillation was expressed as a proportion of the maximum amplitude observed in each infant. We then compared the amplitude at multiple successive 0.5 second intervals with the amplitude of the cardiac airflow oscillation observed at the apnea outset using an ANOVA. We found a significant decrease in cardiac airflow oscillation after only one second (initial=0.609±0.215, vs. 0.5s=0.611±0.213, p=0.53; vs. 1s=0.589±0.235, p≤0.01; vs. 2s=0.471±0.301, p≤0.001) irrespective of the apnea duration (3 to 16 s). We conclude that airway narrowing during central apnea is not a random occurrence but occur shortly after the onset of the apnea. We speculate that the phenomenon is secondary to passive airway relaxation. Supported by Canadian Lung Association and Children's Hospital of Winnipeg Research Foundation.