[The influence of the arterio-alveolar gradient of carbonic dioxide and of the real value of respiratory gas exchange on the estimation of alveolo-arterial gradient of oxygen].

In 46 patients with diverse respiratory alterations, the alveolus-arterial gradient for oxygen (PA-aO2) was calculated from a simplified equation and from the complete equation of alveolar gas. In order to be able to apply the latter, the composition of expired air, ventilation, and capnogram was measured in each patient upon taking an arterial blood sample. These values are later used in determining the alveolar fraction of carbon dioxide (CO2) and thus, calculate the PA-aO2 from the complete equation for alveolar gas. The values for PA-aO2 estimated from this equation were greater than those derived from the simplified equation. With the latter, the PA-aO2 was underestimated in 36 percent of the patients with abnormal respiratory function. This underestimate is due to the fact that, in the simplified equation, both the possible variability of respiratory exchange (R) as well as increases in the arterio-alveolar gradient of CO2 (Pa-ACO2), as commonly found in patients with bronchopulmonary problems are taken into account. The high percentage of falsely normal gradients for O2 found in our study, we believe, may justify, at least in some cases, the calculation of PA-aO2 from the complete equation, and thus, the measurement of alveolar pressure for CO2 upon arterial blood sample extraction for gasometry.