Variability of the respiratory gas exchange ratio during arterial puncture

The value of R, the respiratory gas exchange ratio, was determined in 70 unselected outpatients while undergoing arterial blood sampling as part of routine pulmonary function testing. We wished to determine whether acute changes in alveolar ventilation occurring during blood drawing were of sufficient magnitude to cause a significant error in the calculation of alveolar PO2 if the value of 0.80 for R was assumed. In the majority of subjects, R ranged from 0.70 to 0.90. In approximately 25% of the cases, the assumption of the value of 0.80 for R would have led to an error equal to or greater than 10 mm Hg in the estimation of the alveolar-arterial PO2 difference.