Method for assessment of volume of trapped gas in infants during multiple-breath inert gas washout.

A breath-by-breath inert gas washout method for assessment of the volume of trapped gas in the lungs (V(TG)) in sedated sleeping infants is described. It is based on washin using a gas mixture containing 4% sulfur hexafluoride (V(TG,SF6)) and washout with air. A mass spectrometer was used for continuous gas concentration measurements, and a Fleisch no. 0 pneumotachometer for flow measurement. When equilibration of the tracer gas was achieved with tidal breathing washin, five passive inflations with a maximum positive airway pressure of 20 cm of H(2)O were performed to ensure filling of lung spaces not communicating during tidal breathing. After tidal washout of the tracer gas by air until the end-tidal concentration was 1/40th of its starting concentration, five passive inflations with air were instituted again. The V(TG,SF6) was calculated from the volume of SF(6) mobilized by these large breaths, and expressed as the corresponding volume of air. Triplets of V(TG,SF6) determination in 8 infants aged 9-31 months with varying degrees of airway obstruction showed an average volume of 13.7 mL (range, 4.7-25.0). The average SD of the triplets was 2.1 mL (range, 0.1-5.5 mL). Subjects with high V(TG,SF6)/FRC results demonstrated lower maximal expiratory flow at FRC (V'(max)FRC) results (Z-scores) and greater inhomogeneity of ventilation distribution than those with low trapped gas volumes. It is concluded that gas trapping can be assessed with acceptable precision with this washout method. Further studies are needed to establish the sensitivity and usefulness of the method in infants with various types of airway pathology.