Intrasubject variability of repeated pulmonary function measurements in preterm ventilated infants

This study set out to describe the variability and assess the reproducibility of repeated pulmonary function measurements in ventilated preterm infants. We measured tidal volume (VT), lung compliance (CL), and resistance (RL) in 16 infants (mean +/- SD: birthweight 1222 +/- 343 g) during spontaneous breathing and during mechanical ventilation, suppressing breathing efforts by mild hyperventilation. CL and RL were calculated from the equation of motion using linear regression analysis (LR), and by the Mead and Wittenberger method (MW). Flow and transpulmonary pressure were recorded for at least two consecutive periods, after which the esophageal tube was removed and replaced 1 hour later for a second set of recordings. The mean percent change (% delta) between the initial and the repeated measurements with their respective 95% confidence intervals were calculated. Reproducibility was assessed by the intraclass correlation coefficient (ICC) (total agreement = 1, good reproducibility > or = 0.75). The mean % delta between initial and repeat measurements during spontaneous breathing ranged from 11% to 14% for CL and VT, and from 22% to 32% for RL. The variation for RL was even higher when the analysis was done separately for the inspiratory and expiratory phase. CL and VT had good reproducibility (ICC > 0.9), while RL was significantly less reproducible (ICC < 0.75). Measurements obtained from mechanical breaths had less variability than from spontaneous breaths, ranging from 8% to 15% for CL and VT, and from 13% to 21% for RL. Reproducibility assessed by the ICC was good for most measurements during mechanical breaths. The variability and reproducibility of measurements were similar for both methods of analysis during mechanical ventilation, but during spontaneous breathing variability was larger with the MW method than with LR analysis. We concluded that VT and CL were reproducible during spontaneous and mechanical breathing. However, RL measurements were reproducible only during mechanical ventilation. The high variability of RL in spontaneously breathing preterm infants may reduce the clinical usefulness of this measurement for individual patients.