COMPARISON OF DYNAMIC AND PASSIVE MEASUREMENTS OF RESPIRATORY MECHANICS IN VENTILATED NEWBORN-INFANTS

Pulmonary mechanics may differ in intubated and ventilated infants dep ending on whether they are measured by a dynamic or passive method. Th e objective of this study was to compare respiratory mechanics measure d by a dynamic technique with those obtained by a single-breath occlus ion 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 ag e, 12 +/- 13 and 5 +/- 4 days, respectively) were studied. Flows were measured through a pneumotachometer placed between the endotracheal tu be and the ventilator circuit: tidal volume by integration of flow, an d airway pressure directly with a pressure transducer. Airway occlusio n was performed with a Neonatal Occlusion Valve (Bicore pulmonary moni tor) at the end of inspiration, and the following relaxed exhalation w as analyzed to give passive respiratory system compliance (C-rs) and r esistance (R(rs)). These values were compared with dynamic respiratory system compliance (C-dyn) and dynamic expiratory resistance (R(e)) ob tained with the PEDS system (P) within 1 hour, without an esophageal b alloon and on the same ventilator settings. Dynamic respiratory system compliance and resistance measured with the PEDS and the Bicore syste ms 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 eq uation of the regression line was C-dyn = 0.75 C-rs + 0.02 and C-dyn = 0.78 C-rs - 0.02; and standard error of the estimate (SEE) was 0.2 an d 0.3 mL/cmH(2)O with a correlation coefficient (r) of 0.89 and 0.89 ( P < 0.0001), respectively. The mean R(e) (P) values in preterm and ter m infants were 68% and 64% of R(rs), and the equation of the regressio n line was R(e) = 0.3 R(rs) + 63 and R(e) = 0.5 R(rs) + 20, with SEE o f 25 and 20 cmH(2)O/L/sec, and r of 0.65 and 0.69 (P < 0.0001, P < 0.0 05), respectively. The two methods are non-invasive and were well tole rated. We conclude that passive and dynamic respiratory compliance and resistance measured in intubated infants are highly correlated, altho ugh the values measured by the passive technique are higher than those obtained by the dynamic technique. (C) 1995 Wiley-Liss, Inc.