Modification of the iso-shunt lines for low inspired oxygen concentrations

Arterial PO2 (PaO2) has been related to inspired oxygen concentration (FIO2 range 0.21-0.60) in 20 patients, breathing spontaneously, while recovering from a range of conditions treated in an intensive therapy unit. There was a highly significant linear trend (P < 0.001) for the mean venous admixture ("virtual shunt"), calculated from the iso-shunt diagram (model A), to increase from 0.11 to 0.19 as FIO2 decreased from 0.60 to 0.21. As this appeared to be caused by relative ventilation/perfusion (V/Q) mismatch, we prepared a second model (model B) with a variable two-compartment relative V/Q mismatch in addition to a shunt. Regression of the degree of mismatch on the shunt gave good agreement between predicted and observed PaO2 values (mean bias -0.17 kPa, pooled within patient SD 1.22). Model B is now proposed to extend the applicability of the iso-shunt diagram for FIO2 values less than 0.35. Published postoperative PaO2 data from other workers showed good agreement with our final model (mean bias 0.10 kPa, pooled SD 0.41). The new diagram may therefore have wide applicability in the field of anaesthesia and intensive care.