INTRASUBJECT VARIABILITY OF REPEATED PULMONARY-FUNCTION MEASUREMENTS IN PRETERM VENTILATED INFANTS

This study set out to describe the variability and assess the reproduc ibility of repeated pulmonary function measurements in ventilated pret erm infants. We measured tidal volume (V-T), lung compliance (C-L), an d resistance (F-L) in 16 infants (mean +/- SD: birthweight 1222 +/- 34 3 g) during spontaneous breathing and during mechanical ventilation, s uppressing breathing efforts by mild hyperventilation. C-L and R(L) we re calculated from the equation of motion using linear regression anal ysis (LR), and by the Mead and Wittenberger method (MW). Flow and tran spulmonary pressure were recorded for at least two consecutive periods , after which the esophageal tube was removed and replaced 1 hour late r for a second set of recordings. The mean percent change (% Delta) be tween 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 greater than or equal to 0.75). The mean % Delta between initial and repeat measurements during spontaneous breathing ranged from 11% to 14% for C-L and V-T, and from 22% to 32% for R(L). The variation for R(L) was even higher when the analysis was done sepa rately for the inspiratory and expiratory phase. C-L and V-T had good reproducibility (ICC >0.9), while R(L) was significantly less reproduc ible (ICC <0.75). Measurements obtained from mechanical breaths had le ss variability than from spontaneous breaths, ranging from 8% to 15% f or C-L and V-T, and from 13% to 21% for R(L). Reproducibility assessed by the ICC was good for most measurements during mechanical breaths. The variability and reproducibility of measurements were similar for b oth methods of analysis during mechanical ventilation, but during spon taneous breathing variability was larger with the MW method than with LR analysis. We concluded that V-T and C-L were reproducible during sp ontaneous and mechanical breathing. However, R(L) measurements were re producible only during mechanical ventilation. The high variability of R(L) in spontaneously breathing preterm infants may reduce the clinic al usefulness of this measurement for individual patients. (C) 1996 Wi ley-Liss. Inc.