A reassessment of the validity of the iso-shunt graph.

The iso-shunt diagram was introduced in 1973 to simplify the selection of the optimal inspired oxygen concentration in patients undergoing intensive therapy. A further study of 25 patients in a District General Hospital showed that plots of arterial PO2 against inspired oxygen concentration closely follow predicted iso-shunt lines. There was no significant difference in calculated mean values for shunt with inspired oxygen concentrations between 30 and 100%. Results are compared with other studies and it is suggested that some reported discrepancies may be explained by the reduced response of many polarographic electrodes when measuring high values for arterial PO2.