The oxygen tension in the arterial blood (PaO2) is less than that in the alveolus (pa02). This difference (p\<)2 - pa02) is due to the diffusion gradient across the alveolar capillary membrane, dilution of the pulmonary capillary blood by blood which has bypassed the lungs (bronchial circulation, thebesian veins and cardiac anomalies) and that which has come from areas of the lung with a V/Q ratio of < 1. For convenience, the different causes of an A-a difference can be aggregated and the lung regarded as perfect but with a proportion of the cardiac output (^s) bypassing or shunting past the lungs (Qt). The ratio Qs/Qr (virtual shunt fraction) is derived in a similar way to the Bohr equation:

where Cc>O2, Ca02 and CyO2 are the oxygen contents (ml 100 ml-1 blood) of end pulmonary capillary, arterial and mixed venous blood, respectively. The concept of virtual shunt, which is normally less than 4% of cardiac output, is useful in critically ill patients.

If the lungs are normal, decreases in mixed venous oxygenation, e.g. due to a decrease in cardiac output, have little effect on arterial oxygenation. However, in the presence of a shunt, such decreases can result in significant hypoxaemia.

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