Passive Expiration from Total Lung Capacity Can Estimate Expiratory Function in Infants: A retrospective study.

Background: Evaluating expiratory airway function in infants is challenging, as the gold standard, the raised-volume rapid thoraco-abdominal compression technique is technically difficult and has a high failure rate.

Research question: Are measurements obtained during passive expiration from total lung capacity correlated with forced expiration measurements obtained by the raised-volume technique in infants?

Study design and methods: This observational retrospective analysis included infants born ≥ 36 weeks gestation who underwent pulmonary function testing using the raised-volume rapid thoraco-abdominal compression technique at Hadassah Medical Centre between January 2011 and December 2019. Technically acceptable forced and passive flow-volume curve measurements were included in the analysis.

Results: Out of 296 eligible infants, 276 (93%) had technically acceptable passive flow-volume curves while 226 (76%) had acceptable forced curves (p<0.001). The success rate of producing an acceptable curve was 70% for the passive curves and 39% for forced curves (p < 0.001). The Spearman correlation coefficients of vital capacity, expiratory volumes at 0.5 second, maximal expiratory flows, and expiratory flows at 50%, 75% and 85% of vital capacity were 0.92, 0.72, 0.83, 0.66, 0.67, 0.68, respectively (n= 226; p<0.001 for all). The correlation remained high regardless of the level of expiratory airway obstruction, gender or age. The mean inter-maneuver coefficients of variation were fairly low for both methods (5.2% vs 5.4%, p=NS).

Interpretation: The passive flow-volume curve offers reliable and reproducible data with high correlation to the forced flow-volume curve. Therefore, the passive flow-volume curve can serve as an alternative tool in evaluating expiratory airway function in infants.