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Qs/Qt = (CcO₂ − CaO₂) ÷ (CcO₂ − CvO₂)
Normal: < 5% (normal); 5–20% = mild-moderate shunt; > 30% = severe shunt (ARDS territory)
The shunt fraction represents the proportion of cardiac output that passes through non-ventilated lung (or anatomic shunts) without participating in gas exchange. True shunt is distinguished from V/Q mismatch by failure to improve with 100% oxygen — if PaO₂ doesn't improve on FiO₂ 1.0, true shunt is present.
Quantify in severe refractory hypoxemia to guide therapy. Requires mixed venous blood from a pulmonary artery catheter (true Qs/Qt). A simplified estimate using the '1 mmHg rule' (shunt % ≈ A-a gradient / 20 on 100% O₂) is commonly used clinically.
→ ~10% shunt
Moderate shunt fraction — consistent with moderate V/Q mismatch or partial shunt
Qs/Qt > 30% correlates with severe ARDS and very high mortality. Classic causes: ARDS, pneumonia, pulmonary edema, hepatopulmonary syndrome, intracardiac shunt (ASD, VSD). Unlike V/Q mismatch, intrapulmonary shunt does NOT improve with supplemental O₂. PEEP reduces shunt by recruiting collapsed alveoli.