Pulmonary gas exchange under room air respiration in hypercapneic patients

Abstract

The alveolar-capillary oxygen difference, P(A-a)O2, is important in the study of gas exchange disorders, however does not give us reliable results when applied to hypercapneic patients. On the other hand the venous admixture calculation, seems to be able to diagnose alveolar-capillary gas exchange difficulty of any kind. Checking the reliability of the alveolar-capillary oxygen difference to diagnose the alveolar-capillary disorder in face of alveolar hypoventilation, by using the venous admixture calculation as standard. 83 blood gas analysis from patients with hypercapnia were submitted to venous admixture and P(A-a)O2 determination with the measured PaCO2 and a fixed PaCO2 of 40 mmHg. The results where compared by correlation test. The mean partial pressure of arterial oxygen and carbon dioxide were respectively, 50.8+/-10.4 and 51.7+/-6.4 mmHg; the mean value of P(A-a)O2, amended P(A-a)O2 and venous admixture were respectively 26.6+/-10.2 mmHg, 40.4+/-10.8 mmHg and 37+/-15.2%. The correlation coefficient between the venous admixture versus P(A-a)O2 was 0.61 p<0.00001 and the venous admixture versus "amended" P(A-a)O2 was 0.89 p<0.00001. The use of the oxygen alveolar-capillary gradient in chronically hypercapneic patient does not reach the diagnosis of alveolar-capillary gas exchange disorder which usually comes together with alveolar hypoventilation.