Gradiente alveolo-arterial de O2 en la EPOC con hipercapnia

Objective: This study was conducted to assess whether or not calculation of the alveolar-arterial oxygen tension difference (AaPO 2) from arterial blood gas measurements, breathing air, is useful to discriminate patients with hypecapnia associated with obstructive lung disease. Patients and methods: The effect of different values for the respiratory exchange ratio (R) was also analyzed. We calculated the AaPO 2 in 45 patients with chronic airway obstruction and hypercapnia (PaCO 2 in 45 mmHg) and hemodynamic stability using the standard alveolar gas equation, PAO 2 = PIO2 ‐ PACO 2 [FIO2 + (1-FIO 2)/R], with R assumed to vary from 0.6 to 1.1 and the simplified alveolar gas equation, PAO 2 = PI02 - PACO 2/R, with R assumed to be 0.8. It was considered that a patient was correctly classified when the calculated AaPO 2 was greater than predicted AaPO 2. Results: When the simplified alveolar gas equation was used, 9 (20%) of 45 patients were misclassified, whereas using the standard equation with R assumed to be 0.8, 41 (91%) of 45 patients were correctly classified. Modification of the value of R only allowed improving the classification of two further patients. Conclusions: We conclude that calculation of the AaPO 2 using the standard equation and R = 0.8 can be used to classify hypercapnic COPD patients. This laboratory parameter is of sufficient discriminant value to be used for clinical purposes.