[The effects of ephedrine and phenylephrine on arterial partial pressure of oxygen]

The changes in arterial partial pressure of oxygen were studied following intravenous injections of either ephedrine, 0.2mg.kg-1, or phenylephrine 2.5 micrograms.kg-1, to restore arterial hypotension. Eighty one adult patients underwent general anesthesia, being mechanically ventilated, were divided into two groups; both lung ventilation group (68 patients) and one lung ventilation group (13). In both lung ventilation group, PaO2/FIO2 changed from 390 +/- 132 to 400 +/- 137 mmHg, and 428 +/- 124 to 438 +/- 136 following administrations of ephedrine and phenylephrine, respectively. The results were not statistically significant. In the one lung ventilation group, PaO2/FIO2 did not change significantly from 190 +/- 45 to 186 +/- 29 with ephedrine and 267 +/- 138 to 317 +/- 230 with phenylephrine. We observed neither arrhythmia, severe hypoxemia, nor decrease in PaO2/FIO2 by more than 100mmHg. These results indicate that with respect to changes in arterial partial pressure of oxygen, the clinical doses of ephedrine and phenylephrine can be safely administered to patients under general anesthesia to restore arterial hypotension.