Lung resistance and elastance in spontaneously breathing preterm infants: effects of breathing pattern and demographics

Reported values of lung resistance (RL) and elastance (EL) in spontaneously breathing preterm neonates vary widely. We hypothesized that this variabil ity in lung properties can be largely explained by both inter- and intrasub ject variability in breathing pattern and demographics. Thirty-three neonat es receiving nasal continuous positive airway pressure [weight 606-1,792 g, gestational age (GA) of 25-33 wk, 2-49 days old] were studied. Transpulmon ary pressure was measured by esophageal manometry and airway flow by face m ask pneumotachography. Breath-to-breath changes in RL and EL in each infant were estimated by Fourier analysis of impedance (Z) and by multiple linear regression (MLR). RLMLR (RLMLR = 0.85 X RLZ -0.43; r(2) = 0.95) and ELMLR (ELMLR = 0.97 X ELZ + 8.4; r(2) = 0.98) were highly correlated to RLZ and E LZ, respectively. Both RL (mean +/- SD; RLZ = 76 +/- 38, RLMLR = 59 +/- 36 cmH(2)O . s . I-1) and EL (ELZ = 434 +/- 212, ELMLR = 436 +/- 210 cmH(2)O/l ) exhibited wide intra- and intersubject variability. Regardless of computa tion method, RL was found to decrease as a function of weight, age, respira tory rate (RR), and tidal volume (VT) whereas it increased as a function of RR.VT and inspiratory-to-expiratory time ratio (TI/TE). EL decreased with increasing weight, age, VT and female gender and increased as RR and TI/TE increased. We conclude that accounting for the effects of breathing pattern variability and demographic parameters on estimates of RL and EL is essent ial if they are to be of clinical value. Multivariate statistical models of RL and EL may facilitate the interpretation of lung mechanics measurements in spontaneously breathing infants.