Relationship of arterial-alveolar nitrogen tension to alveolar-arterial oxygen tension, lung volume, flow measurements, and diffusing capacity in cystic fibrosis.

Pulmonary function in children with cystic fibrosis was assessed by the arterial-alveolar PN2 difference adjusted to sublingual temperature. The resulting values were compared with the alveolar-arterial PO2 difference, arterial PCO2, and standard measurements of lung volume, flow, and diffusing capacity. The arterial-alveolar PN2 difference was nearly one half of the PO2 difference, both early in the disease and at a more advanced stage. Analysis taking into account the O2 dissociation curve and the possibility that alveolar temperature is higher than sublingual temperature suggested that all of the PO2 difference could be explained in terms of ventilation-perfusion imbalance in gas-filled units of the lung. Reduction of fractional CO uptake with increasing PN2 difference suggested that the decrease in diffusing capacity in cystic fibrosis may be explained by ventilation-perfusion inequality. A significant relationship between arterial PCO2 and the PN2 difference supported the view that ventilation-perfusion inequality is the cause of CO2 retention when present. The PN2 and PO2 differences were abnormal before the standard tests of lung volume and flow, but in general, the correlation was excellent. Because the PN2 difference was not superior to the PO2 difference in detecting early disease, and because the technical problems in its measurement are considerable, it is not recommended as a routine measurement.