The shunt fraction estimates what percentage of blood passing through the lungs never picks up oxygen at all, quantifying how much of a patient's hypoxemia is due to true shunt versus other causes.
Physiologic shunt — normal.
Mildly elevated.
Clinically significant shunt.
Severe shunt — refractory to supplemental oxygen.
Given: CcO₂ 20.4, CaO₂ 19.5, CvO₂ 14.5 mL/dL.
0.9 ÷ 5.9Blood past a collapsed or fluid-filled alveolus stays deoxygenated no matter how much oxygen is given.
Shunted blood never contacts ventilated alveoli, so raising FiO₂ can't oxygenate blood that bypasses gas exchange entirely. This refractory hypoxemia is a hallmark of true shunt.
A physiologic shunt of less than about 5% is normal. Above roughly 20% is clinically significant, and above 30% represents a severe shunt.
The end-capillary oxygen content, calculated assuming the blood leaving a ventilated alveolus is 100% saturated at the alveolar oxygen tension (PAO₂). It's the theoretical best-case content used as the reference.
ARDS, severe pneumonia, atelectasis, and pulmonary edema all flood or collapse alveoli so blood passes without oxygenating. Structural cardiac shunts do the same at the heart level.