Spirometry with positive airway pressure. A simple method of evaluating obstructive lung disease in children.

In children with obstructive lung disease, the determination of functional resident capacity by the conventional helium dilution method [FRC(He)] often results in gross underestimations. This is due to exceptionally poor He mixing in children, which may be related to characteristics of the developing lung. We added a low level (2.5 cmH2O) of continuous positive pressure to the airway (CPAP) for 2 min beyond the end of conventional He rebreathing to evaluate its effect on gas mixing. The FRC was determined again from He concentration 1 min after CPAP was stopped. We recorded the results as an increase in measured FRC after CPAP (delta FRC) as percent of FRC(He). A total of 46 children (mean age: 13.5 yr) were studied, including those with normal lung function, those with central airway disease, and those with peripheral airway obstruction. With CPAP, delta FRC increased markedly in those with peripheral airway obstruction and was significantly correlated with volume of isoflow and the volume of trapped gas. Thus, the measurement of delta FRC is simple yet useful in detecting peripheral airway obstruction and air trapping.