Gas exchange indices--how valid are they?

OBJECTIVE

This study examined the arterial-alveolar oxygen tension difference (AaDO2), arterial oxygen tension to inspired oxygen fraction ratio (PaO2/FiO2) and alveolar to arterial oxygen tension ratio (PAO2/PaO2) with regard to: (i) their correlation with the calculated pulmonary shunt in critically ill patients; and (ii) the influence of the inspired oxygen fraction on these indices before, during and after general anaesthesia.

DESIGN

This study comprised two sections: (i) retrospective analyses of blood gas data retrieved from the intensive care computerised database; and (ii) analyses of arterial blood gases before, during and after abdominal and orthopaedic surgery in patients subjected to various inspired fractions of oxygen.

SETTING

The study was conducted at an academic hospital.

PATIENTS

The first section of the study was a retrospective analysis of blood gases retrieved from a computerised database from the surgical and respiratory intensive care units. Blood gases which indicated hypoxaemia (arterial haemoglobin saturation less than 90%) were collected from patients who suffered from adult respiratory distress syndrome. The calculated pulmonary shunt was correlated with the AaDO2, PaO2/FiO2 and PAO2/PaO2. In the second section of this study, 15 patients of American Society of Anesthesiologists status 1, scheduled to undergo peripheral orthopaedic and intra-abdominal surgery, were exposed to various concentrations of inspired oxygen before, during and after general anaesthesia. At the end of a 15-minute period of exposure to a particular level of inspired oxygen (which was varied at random), arterial blood gases were analysed. A correlation was attempted between the inspired oxygen fraction and the various indices of pulmonary gas exchange.

INTERVENTION

Patients were subjected to the various inspired fractions of oxygen before, during and after general anaesthesia. A radial artery cannula, inserted under local anaesthesia, allowed the researchers to collect arterial blood gas analysis.

RESULTS

The correlation between the calculated pulmonary shunt and indices of gas exchange showed r = 0.35 for the AaDO2, r = 0.08 for the PaO2/FiO2 and r = 0.40 for the PAO2/PaO2. Stepwise variable selection demonstrated that the FiO2, PaCO2, PAO2 and shunt were the main components of the final models. The inspired oxygen fraction had an effect on the indices of gas exchange inasmuch as they all varied directly with the change in inspired oxygen concentration. Furthermore, the slope of this relationship was less steep during anaesthesia than in the case of values obtained before and after anaesthesia.

CONCLUSIONS

The so-called non-invasive indices of pulmonary gas exchange do not correlate well with the calculated pulmonary shunt, which is regarded as the gold standard that reflects the various components of gas exchange. We speculate that the poor performance of these indices can be explained by the fact that they do not take into account the mixed venous saturation and, except for the alveolar to arterial oxygen tension ratio, ignore the effects of alveolar ventilation. The effect of the inspired oxygen fraction on these ratios makes them difficult to interpret if similar inspired oxygen fractions are not used. The effect of the FiO2 on these indices could possibly be explained by the denitrogenation and collapse of alveoli with low ventilation perfusion ratios. The change in the slope of the FiO2 and the indices that was demonstrated during anaesthesia could possibly be explained by the expected change in the mixed venous saturation that occurs during anaesthesia.