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Beach-Chair Position BP Correction — FRCA Final MCQ

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ModerateAnaesthetic Equipment & SafetyBeach-Chair Position BP CorrectionFRCA Final

A 45-year-old man is undergoing total shoulder replacement under general anaesthesia in the beach-chair position. A radial arterial catheter displays a blood pressure of 105/65 mmHg, with the transducer levelled at the right atrium. The external auditory meatus is 20 cm vertically above the transducer. What adjustment should be applied to estimate the mean arterial pressure at cerebral level?

Educational content. Not a substitute for clinical judgement or local policy.

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Correct answer: D — Subtract approximately 15 mmHg

Option D is correct. Because the external auditory meatus is above the heart-level transducer, cerebral-level arterial pressure is lower owing to the hydrostatic column of blood. The gradient is approximately 0.74 mmHg per centimetre, so a 20 cm height difference requires subtraction of about 15 mmHg. The displayed MAP is approximately 78 mmHg, giving a cerebral-level MAP of about 63 mmHg. No correction would overestimate cerebral pressure, while adding 15 mmHg applies the gradient in the wrong direction. Subtracting 30 mmHg approximately doubles the required correction, and multiplying the MAP has no physiological basis. Strictly, this estimates cerebral-level MAP, not cerebral perfusion pressure; calculating CPP would also require intracranial pressure.

Reference: McCulloch TJ et al. Relative hypotension in the beach-chair position: effects on middle cerebral artery blood velocity. Anaesthesia and Intensive Care. 2010. https://pubmed.ncbi.nlm.nih.gov/20514957/