Pulmonary mechanics and the energetics of breathing in healthy infants.

In order to understand the change in the pulmonary mechanics and energetics of breathing in infants, dynamic pulmonary function tests were studied on 84 occasions in 70 normal healthy infants between birth and seven months of age. All of the study infants were born at full-term and had no lower respiratory infections after birth. Airflow and transpulmonary pressure were measured simultaneously and utilized to calculate values of pulmonary mechanics and energetics of breathing. Variables of ventilation (respiratory frequency, tidal volume, minute ventilation, and inspiratory time) had good correlation with age. Lung compliance increased with age. However, when it was expressed in body weight the lung compliance was independent of age (1.34 +/- 0.37 mL/cmH2O/kg for infants studied). Pulmonary resistance did not differ statistically between different age-groups and it correlated with the level of lung compliance (r = 0.3411, p < 0.01). Older infants had a higher flow rate and esophageal pressure change. This might imply that the older the infant is, the more forceful the breath becomes. The work of breathing was higher in infants of older age and in infants with higher inspiratory pulmonary resistance or lower lung compliance (r = 0.6257, 0.3825 and 0.4579, respectively; p < 0.05). This study establishes reference values for the pulmonary mechanics and energetics of breathing in infancy which may be of assistance in the objective evaluation of respiratory status in infants.