Relationships between inspired oxygen concentration and venous admixture during nitrous oxide-oxygen-halothane anaesthesia.

The inspired oxygen concentration (F/O2) was changed on 43 occasions at about 30-min intervals in 13 patients during artificial ventilation with mixtures of nitrous oxide (N2O), oxygen and halothane. Ventilator settings remained unchanged for each patient and at the end of each period, samples of arterial and central venous blood (and, in six patients, pulmonary arterial blood) and inspired and expired gases were collected. Oxygen tension was measured with a dedicated electrode shown to be unaffected by N2O. Venous admixture (Qva/Qt) was calculated at each F/O2. There was a highly significant correlation between the direction of change of F/O2 and that of Qva/Qt, irrespective of whether F/O2 increased or decreased. In 10 patients, there was a progressive increase in Qva/Qt as F/O2 increased above 40%, and in all patients Qva/Qt on nearly 100% oxygen was greater than that measured at the next lowest concentration (60-80%). These results are at variance with the pattern of behaviour predicted from the "critical Va/Q" theory and support the concept of an oxygen-dependent redistribution of perfusion.