Validation of a nitrogen washout system to measure functional residual capacity in premature infants with hyaline membrane disease

A multiple-breath nitrogen washout system designed to measure lung volume in mechanically ventilated infants was validated by assessing three performance criteria: 1) accuracy of lung volume measurements in the presence of an endotracheal tube leak was assessed by comparing the measurements of functional residual capacity (FRC) in a mechanical lung model with and without airway leak; 2) in vivo accuracy was assessed in rabbits by comparing FRC measurements obtained by this system with measurements obtained by helium dilution; and 3) in vivo precision was assessed by analyzing measurements of FRC obtained in replicate measurements at different times in ventilator-dependent premature infants with hyaline membrane disease. The average difference between the measurements of FRC in a mechanical lung model with airway leak and without leak was 3.0 +/- 9.4% (mean +/- SD, P > 0.2), and no difference was greater than 20%. There was a significant correlation between the measurements of FRC in rabbits by nitrogen washout and by helium dilution (r = 0.93, P < 0.0001), and 65.4% of the paired measurements were within 20% of their average. The 95% limits of agreement within pairs of measurements by the two techniques ranged from -4.0 to + 6.5 mL/kg. FRC measured by helium dilution was slightly higher (1.3 +/- 2.7 mL/kg, P < 0.01) than FRC measured by nitrogen washout, and positive end-expiratory pressure was a significant predictor of this difference (P < 0.0001). The regression between the individual FRC measurements obtained in premature infants and the average of the other replicates was significant (r2 > 0.98, P < 0.0001). The coefficient of variation was 12.3%. These findings provide further validation of this multiple-breath nitrogen washout system for measuring FRC in premature infants during mechanical ventilation.