End-tidal to arterial oxygen tension difference as an oxygenation index

Background: "Ideal" alveolar oxygen tension (PAO(2)) is a calculated entity and the alveolar-arterial oxygen tension difference (PA-aO(2)) is used to evaluate gas exchange function of the lungs. Accurate calculations of PAO(2 ) necessitate measurements of the respiratory exchange ratio (RER), which i s less frequently done, and most often approximations are made. The measure d end-tidal oxygen tension (PETO2) is a reflection of the alveolar oxygen t ension. The aim was to study the relationship between PAO(2) and PETO2, and to see whether the end-tidal to arterial oxygen tension difference (PET-aO (2)) could give the same information about lung function as PA-aO(2). Methods: Twenty patients admitted for cardio-pulmonary exercise tests were studied. They bicycled for 4 min at each work load until maximum work load was reached. Arterial blood gases were analysed before, after 4 min at each work load, at maximum work load and after 2 min of recovery. A metabolic c omputer measured mixed expired gas concentrations. End-tidal gas concentrat ions were measured with a side stream gas analyser. Results: We measured major increases in oxygen uptake, carbon dioxide elimi nation and RER. PAO(2) and PETO2 increased at maximum exercise and during r ecovery. PAO(2) and PETO2 were closely correlated during the study, through great changes in oxygen uptake and RER (r=0.88). When correction was made for wet gas the median difference was 0.12 kPa. Conclusions: At ambient air (FIO2=0.21), PET-aO(2) as a respiratory index m ay give equivalent information to PA-aO(2), without the need for measuremen ts of mixed expired gas tensions or the hazard of an assumed RER.