In pulmonary shunting, the primary effect on oxygenation is best described as?

• A. Refractory hypoxemia
• B. Hypercapnia with respiratory acidosis
• C. Normal oxygenation with increased FiO2
• D. Improved oxygenation with PEEP

Answer: (A)

Pulmonary shunting creates venous admixture: blood passes through parts of the lung where gas exchange doesn’t occur. That unoxygenated blood mixes with oxygenated blood, so arterial oxygen remains low. Because that portion of blood never encounters ventilated alveoli, simply giving more oxygen can’t fully correct the oxygenation deficit—you get hypoxemia that is refractory to high inspired oxygen. Recruitment with PEEP can help by reopening collapsed units and reducing the shunt, but the defining effect is persistent, oxygenation-insensitive hypoxemia. Hypercapnia isn’t the primary consequence, and normal oxygenation with higher FiO2 wouldn’t occur with a true shunt.