The shunt equation quantifies the amount of blood in the three-compartment model that reaches the left heart without exchanging any gases. The equation quantitates venous admixture as a fraction of total lung blood flow, i.e. cardiac output. Breathing an FIO2 of 1.0 can be used to eliminate the impact of low V′A/Q′ units and, thus, calculate the true shunt. The equation is derived from principles of mass conservation, see for example [12] and figure 7. Notations are explained in table 1. CaO2 and Cv¯O2 are obtained from samples of arterial and mixed venous blood, respectively; the latter requires a pulmonary artery catheter. CecO2 is calculated assuming that PecO2 equals that of ideal alveolar gas according to the alveolar gas equation. This equation can be conceptualised and graphically presented as shown in the vertical bar of figure 7. CecO2 at the top of the bar is the best the CaO2 could be if there is no shunt at all. Cv¯O2 at the bottom of the bar is the worst the CaO2 could be if all of the blood flow was through a shunt. The resultant CaO2 is somewhere between the Cv¯O2 and the CecO2. The fraction of blood flow through the shunt (Q′s/Q′t) is, therefore, the observed difference between the CecO2 and CaO2 divided by the potential greatest difference between CecO2 and Cv¯O2. |