Calculation of shunt flow:
Shunt flow can be calculated using the three equations shown, by knowing whole body oxygen uptake and the oxygen content of mixed venous, pulmonary artery, pulmonary venous, and aortic blood samples (Figure 4.07). Left to right shunt is equal to pulmonary blood flow minus 'effective' blood flow. Right to left shunt is equal to systemic blood flow minus 'effective' blood flow. Shunt flow divided by pulmonic blood flow times 100 is equal to percent of left to right shunt. In the example this works out to 46%.
A second and far simpler method is also shown. It is based on the methods of mixtures concept. The difference between the oxygen content of pulmonary artery blood and mixed venous blood is divided by the difference between aortic and mixed venous blood, all multiplied by 100. The answer comes out 45%. It is not necessary to know oxygen consumption values and the calculation can be done roughly in ones head. It is a method used by pediatric cardiologists in the catheterization laboratory.
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