During one-lung ventilation (OLV) for thoracic surgery, hypoxaemia occurs primarily due to which physiological mechanism?

• A Hypoxic pulmonary vasoconstriction (HPV) diverts blood away from the dependent lung
• B Atelectasis of the dependent (ventilated) lung causes V/Q mismatch
• C Mediastinal shift compresses the dependent bronchus reducing tidal volume delivery
• D Intrapulmonary shunt: blood continues to perfuse the non-ventilated (operative) lung without oxygenation ✓

Explanation

During OLV, the non-dependent (operative, collapsed) lung continues to receive blood flow (~40–50% of right heart output) while receiving no ventilation — this constitutes an obligate intrapulmonary shunt, producing hypoxaemia. HPV is actually a protective mechanism: it vasoconstricts the hypoxic non-ventilated lung to reduce shunt fraction. Volatile anaesthetics at >1 MAC attenuate HPV, worsening OLV hypoxaemia; propofol-based TIVA preserves HPV better. OLV management includes increasing FiO2, CPAP to the non-ventilated lung, and PEEP to the ventilated lung.

Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.