Abstract

CARRION VALERO, F. et al. Alveolar-arterial gradient of o2 in copd with hypercapnia . An. Med. Interna (Madrid) [online]. 2001, vol.18, n.3, pp.9-14. ISSN 0212-7199.

Objective: This study was conducted to assess whether or not calculation of the alveolar-arterial oxygen tension difference (AaPO₂) 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 AaPO₂ in 45 patients with chronic airway obstruction and hypercapnia (PaCO₂ in 45 mmHg) and hemodynamic stability using the standard alveolar gas equation, PAO₂ = PIO₂ - PACO₂ [FIO₂ + (1-FIO₂)/R];, with R assumed to vary from 0.6 to 1.1 and the simplified alveolar gas equation, PAO₂ = PI0₂ - PACO₂/R, with R assumed to be 0.8. It was considered that a patient was correctly classified when the calculated AaPO₂ was greater than predicted AaPO₂. 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 AaPO₂ 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.

Keywords : Alveolar-arterial oxygen gradient; Chronic obstructive pulmonary disease (COPD); Hypercapnia.

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