AIRWAY-CLOSURE DURING MIXED APNEAS IN PRETERM INFANTS - IS RESPIRATORY EFFORT NECESSARY

Airway closure during mixed apneas in preterm infants may be due to la ck of tone in the upper airway followed by collapse and obstruction or diaphragmatic action inducing obstruction. We examine whether respira tory efforts are necessary for airway closure using a new method of de tecting airway obstruction, based on the disappearance of an amplified cardiac pulse observed on the respiratory flow tracing. We analyzed 1 98 episodes of mixed apnea of various lengths (greater than or equal t o 3 seconds) observed in 33 preterm infants (birth weight, 1.4 +/- 0.1 kg [mean +/- SEM]; study weight, 1.7 +/- 0.1 kg; gestational age, 29 +/- 1 weeks; postnatal age, 33 +/- 4 days). The great majority of thes e episodes (88%) had a central, followed by an obstructive, component. Infants were studied by using a nosepiece and a flow-through system. Respiratory efforts (abdominal and chest movements) were recorded. Of the apneas, 20 were <5 seconds; 78, 5 to <10 seconds; 45, 10 to <15 se conds; 27, 15 to <20 seconds; and 28, greater than or equal to 20 seco nds. Of the 198 mired apneas, 151 (76%) occurred in the absence of any respiratory effort; 43 (22%) showed a simultaneous cessation of the c ardiac oscillation and respiratory effort; and 4 (2%) showed diaphragm atic activity appearing after cessation of the cardiac oscillation (ai rway occlusion). Respiratory efforts never preceded the cessation of t he cardiac oscillation. The findings suggest that diaphragmatic action is not needed to occlude the airway in mixed apneas. The simultaneous cessation of cardiac oscillations (airway occlusion) and onset of res piratory efforts may indicate that such effort contributes to closure or is induced by the same stimulus that closes the airway. We speculat e that the mechanism for airway closure in mixed apneas is most likely a lack of upper airway tone, which normally occurs with the cessation of a central drive to breathe.