A patient breathing 100% O2 develops complete right lower lobe atelectasis. The PaO2 remains 90 mmHg despite 100% FiO2. The physiological mechanism responsible for the persistent hypoxemia is:

• A Diffusion limitation due to thickened alveolar membrane in the atelectatic zone
• B Hypoventilation of the remaining lung reducing overall oxygen delivery
• C Reduced cardiac output reducing mixed venous PO2 causing hypoxemia
• D True right-to-left intrapulmonary shunt (Qs/Qt) — blood perfusing the atelectatic unventilated lung bypasses oxygenation entirely; shunted blood cannot be corrected by increasing FiO2 ✓

Explanation

Intrapulmonary shunt (true shunt, Qs/Qt) occurs when blood passes through the pulmonary circulation without contacting ventilated alveoli—as in atelectasis, consolidation, AVM, or intracardiac right-to-left shunt. This blood is not exposed to oxygen regardless of FiO2. The shunted blood (with mixed venous PO2 ~40 mmHg, saturation ~75%) mixes with oxygenated blood from ventilated alveoli, reducing PaO2. Critically, shunt does NOT respond to 100% O2 supplementation (only ventilated alveoli increase their PO2; the shunted fraction remains unchanged). This is the classic clinical test: failure of PaO2 to rise above 550 mmHg on 100% O2 suggests shunt >15–20%. V/Q mismatch (not pure shunt) is partially correctable by 100% O2.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.