HYPERCAPNIA INCREASES EXPIRATORY BRAKING IN PRETERM INFANTS

In contrast to adults, newborn infants breathe from an elevated end-ex piratory lung volume, determined by the interaction of airflow retarda tion (braking) by the diaphragm and larynx, and expiratory duration. T o determine the effect of hypercapnia on this strategy, we examined ch anges in respiratory muscle activity and the ventilatory response to C O2 breathing in eight premature infants 33-34 wk gestational age in th e first 3 postnatal days. We recorded tidal volume, airflow, and elect romyograms (EMG) of the laryngeal abductor [posterior cricoarytenoid ( PCA)I, which abducts the vocal cords, and diaphragm during behaviorall y determined quiet sleep in room air and during steady-state inhalatio n of 2% CO2 in air. As expected, tidal volume increased (P < 0.0005) w ithout a change in inspiratory duration with hypercapnia. Unexpectedly , in all subjects, expiratory duration was longer during CO2 inhalatio n (P < 0.001), accompanied by marked changes in expiratory flow patter ns consistent with increased expiratory braking. Diaphragm postinspira tory EMG activity increased with hypercapnia (P < 0.005) with no chang e in baseline diaphragm or PCA EMG activity. Peak inspiratory EMG acti vity of the diaphragm and PCA increased with CO2 (10 and 37%, respecti vely; P < 0.05). We conclude that the mechanisms used to elevate end-e xpiratory lung volume are enhanced during hypercapnia in premature inf ants. This breathing strategy may be important in maintaining gas exch ange in infants with lung disease.