Effects of Continuous Positive Airway Pressure on Pulmonary Function and Blood Gases of Infants with Respiratory Distress Syndrome

Abstract

SummaryNitrogen washout measurements and blood-gas analyses were made on 32 newborn infants with severe RDS at continuous positive airway pressures (CPAP) of 5, 10, and 15 cm H2O. Increases in airway pressure resulted in significant increases in Pa02 and functional residual capacity (FRC). It also produced significant decreases in alveolar turnover rates of the "fast" and "slow" alveolar spaces of a two-space lung model. Changes in CPAP did not significantly affect the distribution of ventilation.The changes in PaO2. due to changes in CPAP, did not correlate well with changes in &/wt nor wi& changes in'alveolar turnover rates. Thus, the effects of increasing CPAP on Pa02 were not simply due to increases in FRC. The changes in Pa02 are due to a complex relationship between changes in FRC, alveolar turnover rates, and to other alterations in cardiopulmonary function that are yet to be fully understood. SpeculationResults from this study show that large portions of the lung have alveolar turnover rates below normal. Increasing airway pressures could lead to decreases in the rate of pulmonary perfusion and to an improvement in the ventilation-perfusion ratio of these alveolar units. This could partially account for the observation that increases in CPAP generally produce increases in PaO2.CPAP (4, 12) has become a mainstay of therapy in the treatment of newborn infants with RDS. In order to optimally manage these the infants were 33.7 -+ 0.4 wk and 2013 + 93 g. Some infants were studied several times during the course of their disease so that 52 sets of data were gathered at infant postnatal ages ranging from 4 to 152 h (mean 38 h). Four of the infants required mechanical ventilation due to the inability to maintain Paon of 60 mm Hg with airway pressure greater than 15 cm HzO and FiOz of 1.0. Three of these four infants died of intraventricular hemorrhages. A fourth infant died of necrotizing enterocolitis. All four infants that died also had pneumothoraces, and were the only infants with pneumothoraces. EQUIPMENTOpen circuit nitrogen washout techniques were used to determine FRC, alveolar ventilation (VA), distribution of ventilation, and alveolar turnover rates. Respiratory nitrogen concentrations were measured by placing the Nitralyzer (model 391 nitrogen analyzer, Warren E. Collins, Inc., Braintree, MA) needle of the pickup head directly in the endotracheal tube; thus, the nitrogen signal to recording delay was only 15 msec for 90% response. Flow rates of respiration were measured by a heated Fleisch "00" pneumotachometer (Instrumentation Association, Inc., New York, NY).and integrated electronically (IC-RESP respiratory flow rate integrator, Gilson Medical Electronics, Inc., Middleton, WI) to obtain tidal volumes. The logarithmic output of the nitrogen analyzer and exhaled tidal volumes were recorded on an X-Y plotter (model 815 X-Y recorder, MFE Corp., Salem, NH). Calculations of VA and FRC and multiple space curve analyses were made directly on the recorded X-Y washout traces.infants the clinician needs to kn...

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