The shunt equation

The purpose of the shunt equation is to give a ratio of shunt blood flow to total blood flow. The normal ratio is 0.3. Under abnormal conditions, the ratio will tend to increase and so markedly reduce the Pao2. 

Those areas of the lung that are perfused but not ventilated  

Start with the theoretical lungs shown above and remember that blood entering the systemic circulation has a component that is shunted past the pulmonary circulation (Qs) and another component that passes through it (Qt - Qs). 

Now consider the blood flow generated in a single beat. The 02 delivered in this volume of blood is equal to (Qr.Cao2). This must be made up of shunted blood (Qs.Cvo2) and capillary blood ([Qt - Qs].Cc o2). 

Qs needs to be moved to the left, aiming for Qs/Qt in the final equation. 

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