ExtractTwenty 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 O 2 , CO 2 , and N 2 . Nitrogen washouts were determined as well in nine infants.The FRC, alveolar-arterial O 2 gradients (AaD 02 ) in 100% O 2 , and true shunt (Q, s /Q,t), 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 CO 2 gradients (aAD C o 2 ) both during illness and recovery. This indicates that maldistribution of pulmonary blood flow is an important physiologic abnormality in hyaline membrane disease. Speculation

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