Tidal volume forced expiration in asthmatic infants: reproducibility and reversibility tests

The tidal volume forced expiration technique used in infants is considered as the first practical noninvasive method of assessing airway physiology in infants. However, its role has been discussed mainly due to the high variability of the derived parameters.

The aim of the study was to assess the reproducibility of a complete measurement with the tidal volume forced expiration technique in infants as measured by the maximal flow at FRC (V(max)FRC). A second aim was to evaluate the bronchial reversibility test in infant asthma.

Thirty infants with asthma were investigated with the tidal volume forced expiration technique twice with 10 min in between and a third time 10 min after inhalation of terbutalin 0.5 mg.

The mean V(max)FRC in the first investigation was 285 ml.s(-1) (coefficient of variation 57%), unchanged in the second investigation and significantly lower than the mean predicted value of 404 ml.s(-1). The relative difference between the 2 investigations of V(max)FRC was mean 10.5% (SD 8.4) of the absolute V(max)FRC value and independent of the size of this V(max)FRC value. The 95% confidence interval for individual changes would then be up to 27% (mean + 2 SD). The infants with the lowest V(max)FRC percent predicted decreased further in V(max)FRC after inhalation of the bronchodilator (p < 0.05).

The tidal volume forced expiration technique was able to measure flow at late expiration with the same reproducibility as seen with spirometry in adults, even if the flow was low. We found the technique acceptable for clinical practice and research, but the results from reversibility tests are difficult to interpret. A significant change of V(max)FRC would, however, be 27% or more.