Objective.

To determine whether infants with hyaline membrane disease (HMD) superimposed on immature lung disease (ILD) have more abnormal lung function and respiratory drive during the evolution of chronic neonatal lung disease (CNLD) in extremely low birth weight infants (ELBW; <1000 g).

Methods.

We measured lung mechanics (respiratory frequency, tidal volume, minute ventilation, lung resistance, lung compliance, lung impedance, and work of breathing per minute) and respiratory drive (airway opening pressure 100 milliseconds after initiation of breath [P0.1] 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 (gestation: 25.1 weeks [24.4,25.9]; birth weight: 687 g [601,773]) were studied at 28, 32, and 35 weeks of postconceptional age. There were no statistically significant differences between the groups for respiratory frequency, tidal volume, minute ventilation, lung resistance, lung compliance, lung impedance, work of breathing per minute, P0.1, and maximal inspiratory pressure generated during airway occlusion. With increasing age, both groups demonstrated increased respiratory drive as measured by P0.1 without significant changes in respiratory frequency or CO2. Work of breathing per minute increased 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 mildly reduced lung compliance/kg (.8–1.1 mL/cm·H2O/kg) and high lung resistance (75–125 cm·H2O/L/second) pattern of CNLD, which changed little after 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 in ELBW infants with CNLD remain only mildly abnormal, suggesting a more favorable prognosis for lung function in later years than previously reported.

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