Which expression correctly defines the pulmonary shunt fraction (Qs/Qt)?

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The pulmonary shunt fraction measures how much blood bypasses oxygenation in the lungs. It uses three oxygen content terms: CaO2 is the arterial oxygen content, CvO2 is the mixed venous oxygen content, and CcO2 is the end-capillary (fully oxygenated) content after gas exchange. The correct expression compares the amount of oxygen content lost to the arterial blood (CcO2 minus CaO2) to the total potential oxygen content gain from venous blood (CcO2 minus CvO2). So the shunt fraction is (CcO2 - CaO2) / (CcO2 - CvO2). This captures the portion of cardiac output that reaches the systemic circulation without being oxygenated, relative to the maximum possible oxygenation from venous blood. If there were no shunting, CaO2 would equal CcO2 and the ratio would be near zero; large shunting would push the ratio upward toward higher values.