Alveolar arterial O2 gradient in patients with cardiopulmonary pathology. Its study at rest with respiration of environmental air

The arterial oxygen pressure (PAO2) and the arterial carbon dioxide pressure (PACO2) are the mirror of the whole stage in alveolar ventilation, because there is a numerical correlation between them, in the alveolar air equation. In our material no difference was found when the respiratory cocient is used to calculate the equation modifying the PACO2 value. On the other hand, the PaO depends on a great amount of variables, i.e., the rationship V/Q. Qs/Qt and the arteriovenous oxygen difference in volume percentage, which reflect the functional stage of the gaseous interchange; other variables depend essentially of technical factors as the methodology used in the obtention and management of the samples and the measurement of the PAO2 at the laboratory. Thus, the alveolo-arterial oxigen Difference delta (A-a)O2 is considered as a mirror of the fluctuations in gaseous interchange, only if the alveolar ventilation the cardiac output, the systemic arterial-vein oxygen difference and in minor importance the respiratory cocient (RP remain constant. The delta (A-a)O2 is not always correlated with other parameters far from the gaseous interchange at lung level in the critically ill patient, for this, it has not a pronostic mecaning by itself. Nevertheless, we believe that delta (A-a)O2 continues being a useful measurement to evaluate the gaseous interchange if at the same time all the factors which may have influence in it are analysed. We consider, for trying to be simple in the management of the acute respiratory failure at the bedside that the (A-a)O2 must not be linked to only one parameter of the respiratory function, i.e. the intrapulmonar veno-arterial shunt, without considering the cardiac output and the systemic arterial-vein oxygen difference.