Serial studies of lung volume and VA-Q in hyaline membrane disease.

Abstract

Extract: Twenty infants with a clinical and radiologic diagnosis of hyaline membrane disease were studied serially by measurement of functional residual capacity (FRC) and arterial and alveolar gradients for O2, CO2, and N2. Nitrogen washouts were determined as well in nine infants. The FRC, alveolar-arterial O2 gradients (AaDo2) in 100% O2, and true shunt (Qs/Qt), which are different ways of quantitating atelectasis and shunt in hyaline membrane disease, were shown to be statistically related. The FRC was below 1.0 ml/cm in all infants when sick and returned to the usual range for gestational age, generally around 15 days. Extensive cardiopulmonary shunting was recorded both at the height of illness and for a considerable time during recovery. However, during recovery, not all of the venous admixture was due to shunt. As maldistribution of ventilation was excluded, the possibility of a diffusion barrier was raised. Alveoli with a high VA/Q were identified by large arterial-alveolar CO2 gradients (aADCO2) both during illness and recovery. This indicates that maldistribution of pulmonary blood flow is an important physiologic abnormality in hyaline membrane disease. Speculation: The FRC was shown to correlate inversely with AaDo2 and Qs/Qt ratios. As a low FRC is the only true unique characteristic of hyaline membrane disease, it is suggested that in the future the disease be defined in terms of the lung volume as well as in terms of the usual clinical and biochemical parameters which appear in other forms of respiratory distress in neonates. Constant positive airway pressure is an attempt to approach treatment from this point of view.