Pulmonary outcome in extremely low birth weight infants

Objective. To determine whether infants with hyaline membrane disease (HMD) superimposed on immature lung disease (ILD) have more abnormal lung functi on and respiratory drive during the evolution of chronic neonatal lung dise ase (CNLD) in extremely low birth weight infants (ELBW; <1000 g). Methods. We measured lung mechanics (respiratory frequency, tidal volume, m inute ventilation, lung resistance, lung compliance, lung impedance, and wo rk of breathing per minute) and respiratory drive (airway opening pressure 100 milliseconds after initiation of breath [P-0.1] and maximal inspiratory pressure generated during airway occlusion) on 3 occasions before term in 24 ELBW infants. Results. Ten infants with ILD (mean [95% CI] gestation: 24.3 weeks [23.1,25 .4]; birth weight: 675 g [553,798]) were studied at 27, 31, and 35 weeks of postconceptional age and 14 infants with HMD superimposed on ILD (gestatio n: 25.1 weeks [24.4,25.9]; birth weight: 687 g [601,773]) were studied at 2 8, 32, and 35 weeks of postconceptional age. There were no statistically si gnificant differences between the groups for respiratory frequency, tidal v olume, minute ventilation, lung resistance, lung compliance, lung impedance , work of breathing per minute, P-0.1, and maximal inspiratory pressure gen erated during airway occlusion. With increasing age, both groups demonstrat ed increased respiratory drive as measured by P-0.1 without significant cha nges in respiratory frequency or CO2. Work of breathing per minute increase d in the HMD group with age and was higher in extubated subjects. A similar trend with age was demonstrated in ILD infants. Regardless of whether the initial lung disease was ILD alone or HMD + ILD, ELBW infants developed a m ildly reduced lung compliance/kg (.8-1.1 mL/cm . H2O/kg) and high lung resi stance (75-125 cm . H2O/L/second) pattern of CNLD, which changed little aft er 3 weeks of age. Survival to 6 months was 23/24 (96%). Oxygen dependency was 16/24 (67%) at 35 weeks, yet only 5/23 (22%) survivors required oxygen at discharge from the neonatal unit (43 weeks). Conclusions. The visco-elastic and flow-resistive properties of the lungs i n ELBW infants with CNLD remain only mildly abnormal, suggesting a more fav orable prognosis for lung function in later years than previously reported.