Mean alveolar gases and alveolar-arterial gradients in pulmonary patients

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Abstract
In view of uncertainties about the best way to estimate mean alveolar gases in patients with ventilation-perfusion inequalities, 3 different methods were evaluated on 54 patients. In the 1st method 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 (R.hivin.E) gives the mean alveolar values P.hivin.AO2 [alveolar O2 tension] and P.hivin.ACO2 [alveolar CO2 tension. With the 2nd method PA''CO2 was calculated with the Bohr equation using a predicted anatomic dead space and PA''O2 was derived with the alveolar equation. In the 3rd method End-tidal (ET) PO2 [partial pressure of O2] and PCO2 [partial pressure of CO2] were averaged over 1 min at rest in steady state. Mean RET calculated from the 3rd method was identical with R.hivin.E. Mean values for P.hivin.ACO2, PA''CO2 and PETCO2 differed by less than 1 torr, but the variance was least with the end-tidal method. There was a highly significant correlation between .DELTA.aAPCO2 [alveolar-arterial CO2 gradient] using PETCO2 and VD[anatomic dead space]/VT [tidal volume], 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.

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