[Helium wash-in time as a critical factor in the determination of functional residual capacity in children].

The wash-in time (twi), defined as time until gaseous equilibrium between lungs and spirometer is reached during helium dilution rebreathing, and cumulative inspired volume (CIV) ventilated during this time were measured in 64 healthy children (30 boys and 34 girls, aged 6.5-14.7 years). The interrelationship between twi, CIV, respiratory frequency, tidal volume and lung growth was studied. Functional residual capacity (FRC) was compared with the plethysmographically determined thoracic gas volume. CIV is linearly related to standing height, and the ratio CIV/FRC is an age-independent constant (5.05 +/- 0.75). twi was found to be multi-linearly dependent of standing height and minute ventilation. A close relationship was found between FRC and thoracic gas volume (r = 0.94). It is concluded that FRC determination in children with lung disease should be standardized with respect to the predicted age-dependent twi for helium.