RELATIONSHIPS BETWEEN INSPIRED OXYGEN CONCENTRATION AND VENOUS ADMIXTURE DURING NITROUS OXIDE-OXYGEN-HALOTHANE ANAESTHESIA

The inspired oxygen concentration (Fl_O²) was changed on 43 occasionsat about 30-min intervals in 13 patients during artificial ventilationwith mixtures of nitrous oxide (N₂O), oxygen and halothane.Ventilator settings remained unchanged for each patient andat the end of each period, samples of arterial and central venousblood (and, in six patients, pulmonary arterial blood) and inspiredand expired gases were collected. Oxygen tension was measuredwith a dedicated electrode shown to be unaffected by N₂O. Venousadmixture (Qva/Qt) was calculated at each Fl_O². There was ahighly significant correlation between the direction of changeof Fl_O² and that of Qva/Qt, irrespective of whether Fl_O² increasedor decreased. In 10 patients, there was a progressive increasein Qva/Qt as Fl_O² increased above 40%, and in all patients Qva/Qton nearly 100% oxygen was greater than that measured at thenext lowest concentration (60-80%). These results are at variancewith the pattern of behaviour predicted from the "critical Va/Q"theory and support the concept of an oxygen-dependent redistributionof perfusion. ^*Present address: Department of Anaesthesia, Prince Henry Hospital,Sydney, Australia