Influence of respiratory dead space on lung clearance index in preterm infants.

Lung clearance index (LCI), a marker of ventilation inhomogeneity derived from multiple breath washout (MBW), is used for clinical monitoring and as a key outcome of clinical trials in infants and children with cystic fibrosis. Utility of LCI is controversial in preterm infants with bronchopulmonary dysplasia (BPD) who tend to have high dead space to tidal volume ratio (VD/VT). We investigated the effect of VD/VT on LCI in a cohort of preterm infants with and without BPD and term healthy controls. We analyzed MBW data from 455 infants at a mean (SD) of 43.4 (3.5) w postmenstrual age. VD was estimated from the molar mass signal of an ultrasonic flowmeter (VD,MM). LCI was associated with VD,MM/VT (r(2)=0.13, p<0.001) but was not associated with BPD. Adjusting for VD,MM/VT did not reveal an association between LCI and BPD. We conclude that VD,MM/VT is a relevant factor when interpreting LCI in this population but the effect size of this association is moderate.