Respiratory mechanics in spontaneously breathing term and preterm neonates

The compliance (Crs), resistance (Rrs) and passive time constant (τrs) of the respiratory system were determined by the single-breath method (sb) in 24 healthy term and preterm newborns. In 22 of them, Crs(sb) was compared to the slope of the pressure-volume curve determined by the multiple occlusion technique (mo), which is not dependent on the assumption of a linear flow-volume relationship. Crs(sb) and Rrs correlated significantly with body weight (kg): Crs(sb) = 0.56 X kg + 1.22 (r = 0.67); Rrs = -20.1 X kg + 134.6 (r = -0.68). No difference in Crs and Rrs between prone and supine positions was found. τrs was not significantly different between premature (0.21 ± 0.06 s) and full-term infants (0.21 ± 0.05 s). Crs(sb) was significantly higher than Crs(mo) in premature babies (2.27 ± 0.41 ml·cm H₂O^-1 vs. 1.98 ± 0.47 ml·cm H₂O^-1). This difference may be explained by a continuous braking of expiratory airflow after release of the occlusion or more likely, by a difference in the lung volume at which Crs(sb) and Crs(mo) are measured. However, the difference between Crs(sb) and Crs(mo) (~ 15%) is in the same range as the intrasubject variability, and is meaningless compared to the alterations of respiratory mechanics observed during neonatal ventilatory disorders. Therefore, the single-breath method appears to be a suitable and noninvasive method to measure respiratory mechanics in nonintubated prematures.