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. 
