Dynamic versus steady-state determination of lung transfer factor in infants

The present study aimed at comparing two techniques which measure lung transfer factor for carbon monoxide (TL_CO) in infants. The gold standard deals with the Fick mass-balance applied when steady state for ventilation and CO gas fractions is achieved. The second technique models CO uptake in dynamic conditions where gas fractions are changed by a rebreathing maneuver. This procedure increases inspired CO ₂ fraction and decreases that of O₂ that can change the ventilation. Our goal was to test whether the changing ventilation biases TL_CO accuracy. The experimental setup includes a fast respiratory valve suitable to infants (low dead-space and short time response) which switches the ventilation from air to a Douglas bag. Eight infants (age: 1-5 months; body mass: 2.2-8.2 kg) participated to the study. Results show a strong correlation between the two methods determining TL_CO and no significant difference in the magnitudes of this parameter TL_COdyn=0.18±0.02 and TL_COss=0.19±0.04 mmol/min/kPa/kg; r=0.95. TL_COdyn accuracy is not modified by the changes in CO₂ and O ₂. As the test requires signal recordings of only 30 s duration, we recommend the rebreathing maneuver which shortens the test duration