Shunt Equation

• When shunt is caused by addition of mixed venous blood to blood draining from the capillaries, shunt flow can be calculated:
• Total O₂ leaving the system is total blood flow Q_T x O₂ concentration in arterial blood Ca_O2
• This equals the sum of the O₂ in shunted blood Q_S x C_VO2 and end-capillary blood (Q_T - Q_S) x Cc_O

• O₂ concentration of end-capillary blood is calculated from alveolar P_O2 and the oxygen dissociation curve
• This equation only holds up if all the decrease in O₂ is the result of shunted blood - other factors like V/Q mismatch, bronchial vein blood etc will affect this