[Alveolar-arterial gradient of O2 in COPD with hypercapnia].
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.


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 = PIO2 - 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.
Carrión Valero
F
F
Servicios de Medicina Interna y Neumología, Hospital de Sagunto y CE, Valencia.
Martínez Francés
M
M
Perpiñá Tordera
M
M
Pascual Izuel
J M
JM
Marín Pardo
J
J
D000368:Aged; D005260:Female; D006801:Humans; D006935:Hypercapnia / Q000209:etiology*; D008173:Lung Diseases, Obstructive / Q000503:physiopathology*; D008297:Male; D008875:Middle Aged; D010101:Oxygen Consumption; D011659:Pulmonary Gas Exchange