Venous Air Embolism — FRCA Final MCQ

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ModerateNeuroanaesthesiaVenous Air EmbolismFRCA Final

A 62-year-old man undergoes posterior fossa craniotomy in the sitting position for excision of a vestibular schwannoma. During tumour dissection, the precordial Doppler signal changes abruptly. Despite unchanged ventilation, end-tidal carbon dioxide falls from 4.0 to 2.1 kPa and arterial blood pressure falls to 70/40 mmHg. What is the most likely diagnosis?

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Correct answer: A — Venous air embolism

The diagnosis is **venous air embolism**. In sitting posterior fossa surgery, exposed veins may lie above the right atrium, creating a subatmospheric pressure gradient that entrains air. Air reaching the pulmonary circulation acutely increases pulmonary vascular resistance and alveolar dead space, reducing carbon dioxide delivery to ventilated alveoli and therefore causing a sudden fall in end-tidal carbon dioxide. A substantial embolic load impairs right ventricular output and causes hypotension. A thrombotic pulmonary embolism can produce similar capnographic and haemodynamic changes, but the operative setting and abrupt precordial Doppler change strongly favour air entrainment. Tension pneumothorax would usually increase airway pressure; anaphylaxis commonly causes bronchospasm or cutaneous signs; myocardial infarction does not explain the Doppler change.

Reference: Bedford RF, Marshall WK, Butler A, Welsh JE. Cardiac catheters for diagnosis and treatment of venous air embolism: a prospective study in man. Journal of Neurosurgery. 1981;55:610–614. https://www.rcoa.ac.uk/gpas/chapter-14