Functional residual capacity to thoracic gas volume (FRC:TGV) ratio in healthy neonates

Gas trapping has been suggested to be common in healthy newborns in the immediate postnatal period. To determine the veracity of that finding, functional residual capacity (FRC) and thoracic gas volume (TGV) were measured in such a population and the FRC:TGV ratios were related to measurements of airway resistance (RAW). FRC was assessed by a helium gas dilution technique, TGV and RAW by plethysmography. Twenty-four healthy infants born at term were studied at a median age of 2 days (range 1-5 days). None had respiratory problems, nor had their mothers undergone invasive antenatal procedures. Their median FRC, which was 27.1 (range 23.8-32.0) ml kg-1, was significantly lower than their TGV (median 29.8, range 26-33 ml kg-1, P < 0.01). The mean 'within subject' difference between FRC and TGV was 2.5 (range 0.3-5.5) ml kg-1. The median FRC:TGV ratio was 0.93 (range 0.82-0.99). Eight infants had an FRC:TGV ratio less than 0.9, two of whom were studied on the first postnatal day. No infant with a low (< 0.9) FRC:TGV ratio had an abnormal RAW. The differences between FRC and TGV demonstrated in this study were smaller than documented in earlier series, suggesting the degree of gas trapping may previously have been over-estimated.