Assessment of airway function using partial expiratory flow-volume curves - How reliable are measurements of maximal expiratory flow at FRC during early infancy?

We investigated the extent to which measurements of maximal expiratory flow at FRC ((V) over dot Emax(FRC)) are influenced by the dynamic increase of FRC in young infants by superimposing partial forced expiratory flow-volume curves on those obtained after lung inflation to 2 kPa (20 cm H2O) in 12 infants during the first month of life. The elastic equilibrium volume (EEV) of the respiratory system was estimated by extrapolating the passive expiratory time constant (obtained after lung inflation but prior to forced deflation) to zero flow. There was a very strong relationship between (V) over dot Emax(FRC) (which ranged from 11 to 190 ml/s) and the extent to which FRC was dynamically increased above EEV (range: 0 to 5 ml/kg), r(2) = 0.88. The results of this study suggest that, although (V) over dot Emax(FRC) remains a useful means of measuring peripheral airway function in infants, its values should be interpreted with caution during the neonatal period. In particular, the relatively high (V) over dot Emax(FRC) values reported in healthy newborn infants may reflect differences in breathing strategy rather than airway structure. More meaningful within- and between-infant comparisons of peripheral airway function may be obtained by calculating forced expiratory flows at a fixed interval (e.g., 3 ml/kg) above EEV, rather than at the FRC that is operational at the time of measurement.