Which expression defines Qs/Qt?

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Qs/Qt is the shunt fraction: the portion of total pulmonary blood flow that bypasses oxygenation in the lungs. It is defined using three oxygen contents: CaO2 (arterial), CvO2 (mixed venous), and CcO2 (end-capillary, the oxygen content the blood would have if fully equilibrated with alveolar gas).

The correct expression is the ratio of (CcO2 minus CaO2) to (CcO2 minus CvO2). The numerator represents the oxygen content that would be gained if the blood were properly oxygenated but is not, reflecting the shunted blood. The denominator represents the maximum possible rise in oxygen content from venous to end-capillary blood, giving a scale for how much oxygen transfer could occur. Thus the ratio quantifies the fraction of blood flow that remains unoxygenated as it passes through the lungs.

If CaO2 equals CcO2, the numerator is zero, so the shunt fraction is zero (no shunt). If there is a true shunt, the numerator is positive, yielding a positive shunt fraction.