One-Lung Ventilation Shunt Physiology — FRCA Final MCQ

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HardCardiothoracic AnaesthesiaOne-Lung Ventilation Shunt PhysiologyFRCA Final

A 60-year-old man (ASA III) is undergoing a right pneumonectomy. After right PA clamping, his SpO2 drops to 85% despite 100% FiO2 on one-lung ventilation. What is the physiological explanation?

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Correct answer: E — Increased intrapulmonary shunt through the non-ventilated lung

During one-lung ventilation, the non-ventilated (operative) lung continues to be perfused, creating an intrapulmonary shunt (blood passing through unventilated lung does not undergo gas exchange). Normally, hypoxic pulmonary vasoconstriction (HPV) diverts blood away from the non-ventilated lung, reducing shunt. However, shunt may still be 20-30% of cardiac output. If HPV is inhibited (by volatile agents at high concentrations, vasodilators, or sepsis), shunt increases further. After PA clamping (pneumonectomy), the shunt through the operative lung is eliminated, which should improve oxygenation. If SpO2 drops AFTER PA clamping, other causes must be considered.

Reference: Lumb AB – 2022 – Nunn's Applied Respiratory Physiology; RCOA – 2023 – Cardiothoracic module