1175 ACTIVE EXPIRATORY WORK OF BREATHING IN THE NEWBORN

Abstract

The small lung volume and low total lung compliance of the newborn lung leads to a different pattern of respiration from that of the older child and adult. Respiratory rates are higher and inspiration-expiration times shorter in order to maintain adequate alveolar ventilation. These parameters are best mediated through a work of breathing pattern which requires active expiratory mechanical work in order to prevent air trapping and to stabilize resting lung volume. Transpulmonary pressure (esophageal balloon), air flow (pneumotachograph), and tidal volume enabled·determination of dynamic pressure volume loops. Expiratory work of breathing was studied by planimetry on pressure-volume diagrams obtained from 100 normal newborn infants in the first three days of life. Mean birthweight was 3.39 kg (range 2.91-5.26 kg.), gestational age 38.6 wks. (range 37-42 wks.). Dynamic lung compliance was 1.68 ± 0.06 ml/cm H2O/kg mean ± S.E., resting lung volume 26.1 ± 0.7 ml/kg, respiratory rate 49 ± 1 per min., total work of breathing 32.7 ± 0.4 gm/cm/kg and active expiratory work per breath 2.9 ± 0.3 gm/cm/kg. Active expiratory work was performed in 98% of infants studied. It is concluded that active mechanical work is required even in the normal newborn during quiet respiration in order to overcome the resistive forces to expiration and that insufficient potential energy is stored during inspiration to perform this passively.