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Hyperventilation Carpopedal Spasm Mechanism — FRCA Final MCQ

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EasyGeneral AnaesthesiaHyperventilation Carpopedal Spasm MechanismFRCA Final

A 48-year-old woman (ASA II) with severe needle phobia is having a dental procedure. She hyperventilates and develops perioral tingling, carpopedal spasm, and lightheadedness. PaCO2 is 2.5 kPa. What is the mechanism of the carpopedal spasm?

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Correct answer: E — Respiratory alkalosis causes reduced ionised calcium by increasing calcium-protein binding

Hyperventilation causes respiratory alkalosis (low PaCO2, high pH). The alkalosis increases the binding of calcium to plasma albumin, reducing the concentration of ionised (free/physiologically active) calcium in the blood. This relative hypocalcaemia increases neuromuscular excitability, causing paraesthesiae, carpopedal spasm, and potentially seizures. Total calcium is unchanged – only the ionised fraction decreases. Treatment: re-breathing into a paper bag (increases PaCO2), anxiolysis, and reassurance. The carpopedal spasm resolves as pH normalises.

Reference: RCOA – 2023 – Final FRCA curriculum: acid-base and calcium physiology