Comparison of dynamic and passive measurements of respiratory mechanics in ventilated newborn infants

Pulmonary mechanics may differ in intubated and ventilated infants depending on whether they are measured by a dynamic or passive method. The objective of this study was to compare respiratory mechanics measured by a dynamic technique with those obtained by a single-breath occlusion technique in ventilated newborn infants. Thirty-one preterm and 15 term infants (mean ± SD: gestational age, 29.3 ± 2.3 and 39.5 ± 1.4 weeks; birth weight, 1.2 ± 0.5 and 3.4 ± 0.4 kg; postnatal age, 12 ± 13 and 5 ± 4 days, respectively) were studied. Flows were measured through a pneumotachometer placed between the endotracheal tube and the ventilator circuit: tidal volume by integration of flow, and airway pressure directly with a pressure transducer. Airway occlusion was performed with a Neonatal Occlusion Valve (Bicore pulmonary monitor) at the end of inspiration, and the following relaxed exhalation was analyzed to give passive respiratory system compliance (C_rs) and resistance (R_rs). These values were compared with dynamic respiratory system compliance (C_dyn) and dynamic expiratory resistance (R_e) obtained with the PEDS system (P) within 1 hour, without an esophageal balloon and on the same ventilator settings. Dynamic respiratory system compliance and resistance measured with the PEDS and the Bicore systems did not differ significantly and were well correlated. Mean C_dyn(P) values in preterm and term infants were 77% and 77% of C_rs the equation of the regression line was c_dyn = 0.75 c_rs + 0.02 and C_dyn = 0.78 c_dyn ? 0.02; and standard error of the estimate (SEE) was 0.2 and 0.3 mL/cmH₂O with a correlation coefficient (r) of 0.89 and 0.89 (P < 0.0001), respectively. The mean R_e (P) values in preterm and term infants were 68% and 64% of R_rs and the equation of the regression line was R_e = 0.3 R_rs + 63 and R_e = 0.5 R_rs + 20, with SEE of 25 and 20 cmH₂O/Lsec, and r of 0.65 and 0.69 (P < 0.0001, P < 0.005). respectively. The two methods are non-invasive and were well tolerated. We conclude that passive and dynamic respiratory compliance and resistance measured in intubated infants are highly correlated, although the values measured by the passive technique are higher than those obtained by the dynamic technique. Pediatr Pulmonol. 1995; 20:258–264 . © 1995 Wiley-Liss, Inc.