Early measurement of lung volume--a useful discriminator of neonatal respiratory failure severity

Respiratory distress syndrome (RDS) is characterized by lungs having collapsed alveoli (atelectasis) which reduces the volume of the gas-containing spaces of the lung. It seems likely, therefore, that measurement of lung volume might discriminate between infants with severe respiratory failure due to RDS and those with minimal respiratory distress. To test this hypothesis, lung volume was measured at end expiration, that is functional residual capacity (FRC), in 40 infants (median gestational age 29 weeks, range 24-35) all mechanically ventilated from birth. FRC was measured using a helium gas dilution technique at a median of 3 h of age. The infants were divided into two groups according to their FRC results: group A (n = 29) low FRC (FRC < 24 ml kg-1) and group B (n = 11) normal FRC (FRC > or = 24 ml kg-1). The clinicians were unaware of the FRC results. There was no significant difference in the gestational age or birthweight of the two groups, but group A were characterized by a significantly greater proportion requiring surfactant replacement therapy (p < 0.01), a higher maximum peak inspiratory pressure (p < 0.01) and inspired oxygen requirement (p < 0.01). A low FRC had 79% sensitivity and 91% specificity in predicting a requirement for surfactant replacement therapy. We conclude that measurement of FRC in the first hours of life does allow discrimination of disease severity.