During one-lung ventilation (OLV) for a left pneumonectomy, hypoxaemia develops (SpO2 88%). CPAP 5 cmH2O is applied to the non-dependent (operative) lung. What is the mechanism by which this improves oxygenation while still allowing surgical access?

• A CPAP recruits collapsed alveoli in the non-dependent lung and allows passive O2 diffusion via apnoeic oxygenation ✓
• B CPAP converts the non-dependent lung to a controlled-ventilation state restoring V/Q matching
• C CPAP eliminates HPV in the non-dependent lung, redistributing blood flow to the dependent lung
• D CPAP forces nitrogen washout from the non-dependent lung, increasing FiO2 in alveolar gas

Explanation

During OLV, the non-dependent (operative) lung is collapsed and receives no ventilation; blood flowing through it represents true intrapulmonary shunt (Q without V). Applying low-level CPAP (5 cmH2O) to the operative lung re-recruits some alveoli without causing gross lung inflation that impairs surgical access. The recruited alveoli allow passive O2 diffusion (apnoeic oxygenation) — O2 placed in the alveoli is absorbed into perfusing blood down a concentration gradient, reducing shunt fraction and improving SpO2. It does not restore normal V/Q ratio or eliminate hypoxic pulmonary vasoconstriction (HPV). Option C is opposite: CPAP without ventilation does not abolish HPV.

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

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