Shunt Equation

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  • 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 QT x O2 concentration in arterial blood CaO2
  • This equals the sum of the O2 in shunted blood QS x CVO2 and end-capillary blood (QT - QS) x CcO

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  • O2 concentration of end-capillary blood is calculated from alveolar PO2 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