Intrapulmonary Shunt Fraction

Qs/Qt = (CcO₂ − CaO₂) ÷ (CcO₂ − CvO₂)

Normal: < 5% (normal); 5–20% = mild-moderate shunt; > 30% = severe shunt (ARDS territory)

Variables

Qs/Qt

Shunt fraction(%)

Normal: < 5%

CcO₂

Pulmonary capillary O₂ content (ideal)(mL/dL)

Normal: ~20 mL/dL

CaO₂

Arterial O₂ content(mL/dL)

Normal: 18–20 mL/dL

CvO₂

Mixed venous O₂ content(mL/dL)

Normal: 13–15 mL/dL

What It Measures

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.

When to Use

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.

Interactive Calculator

Capillary O₂ Content (CcO₂)(mL/dL)

Arterial O₂ Content (CaO₂)(mL/dL)

Mixed Venous O₂ Content (CvO₂)(mL/dL)

Worked Examples

Simplified estimate (100% FiO₂)

→ ~10% shunt

Moderate shunt fraction — consistent with moderate V/Q mismatch or partial shunt

Clinical Significance

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.

Memory Tips

💡Shunt = blood bypasses the lung — like traffic taking a shortcut that avoids the toll booth (O₂ exchange)
💡True shunt does NOT improve with O₂ — distinguish from V/Q mismatch which DOES improve with O₂
💡Quick estimate: Qs/Qt (%) ≈ A-a gradient / 20 on 100% FiO₂

NBRC Pearls

Simplified shunt estimate: 1% shunt for every 20 mmHg of A-a gradient on FiO₂ 1.0
PEEP is the primary ventilator tool to reduce shunt (recruits alveoli)
Shunt > 30% = severe ARDS; unlikely to respond to supplemental O₂ alone

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What It Measures

Clinical Significance