LUNG-VOLUMES AND PRESSURE-VOLUME RELATIONS OF THE RESPIRATORY SYSTEM IN SMALL VENTILATED NEONATES WITH SEVERE RESPIRATORY-DISTRESS SYNDROME

Total lung capacity (TLC), inspiratory capacity (IC), functional resid ual capacity (FRC), and deflation pressure-volume (P-V) curves were st udied in 16 intubated neonates (540-3300 g), 10 with severe respirator y distress syndrome (RDS) and 6 air-ventilated with normal chest radio grams. FRC was measured using washout of a tracer gas (sulfur hexafluo ride), and TLC and IC were calculated after inflating the lungs to 30 cm H2O. P-V curves were obtained during expiration from TLC using an i nterrupter technique, and the steepest slope of the curve, i.e. the ma ximum compliance (C-rs-max), was calculated. In addition, an index of ventilation inhomogeneity (pulmonary clearance delay, PCD) was compute d from the shape of the SF6 washout curve. TLC/body weight was less in the RDS group than in the air-ventilated group (median 19 and range 1 6-43 mL/kg versus 48 and 43-52 mL/kg, respectively; p < 0.01), mainly because of a marked reduction in IC (median 11 and range 8-24 mL/kg ve rsus 29 and 28-40 mL/kg; p < 0.01). The flatter P-V curve in the RDS g roup was reflected also in a lower C-rs-max (median 0.7 and range 0.4- 1.7 cm H2O-1 kg(-1)) than in the air-ventilated group (2.3 and 2.0-3.1 mt cm H2O-1 kg(-1), respectively; p < 0.01). Thus, there was no overl ap in IC or C-rs-max between the groups, suggesting that reductions in these measures may be main characteristics of RDS. On the other hand, no difference in PCD was found, indicating that, in infants with RDS, the tidal volume is distributed fairly homogeneously to the ventilate d parts of the lungs.