MEAN ALVEOLAR GASES AND ALVEOLAR-ARTERIAL GRADIENTS IN PULMONARY PATIENTS

In view of uncertainties about the best way to estimate mean alveolar gases in patients with ventilation-perfusion inequalities, three different methods were evaluated on 54 patients. 1) O2 and CO2 were recorded by mass spectrometer on an O2 (x)-CO2 (y) diagram. The coordinates at the intersect of the expiratory record with the mixed expired R line (RE) gives the mean alveolar values (PA(O2) and PA(CO2)). 2) PA'(CO2) was calculated with the Bohr equation using a predicted anatomic dead space and PA'(O2) was derived with the alveolar equation. 3) End-tidal (ET) P(O2) and P(CO2) were averaged over 1 min at rest in steady state. Mean RET calculated from 3 was identical with RE. Mean values for PA(CO2), PA'(CO2), and PET(CO2) differed by less than 1 Torr, but the variance was least with the end-tidal method. There was a highly significant correlation between ΔaAP(CO2) using PET(CO2) and VD/VT, better than with either of the other methods. The end-tidal measurement appears to give the best approximation of mean alveolar gas in pulmonary patients.