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Hyperventilation physiology — MRCPsych Paper A MCQ

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ModerateNeurophysiology and neurochemistryHyperventilation physiologyMRCPsych Paper A

A 34-year-old man develops sudden intense fear, perioral tingling and painful carpopedal spasm while breathing rapidly during a panic attack. Arterial blood gas analysis during the episode shows pH 7.55 and PaCO2 3.0 kPa; his total serum calcium is normal. Which mechanism best explains his neuromuscular symptoms?

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Correct answer: B — Alkalosis increases calcium binding to albumin, reducing ionised calcium

The correct answer is B. Hyperventilation blows off CO2, producing acute respiratory alkalosis (here pH 7.55, PaCO2 3.0 kPa). At an alkaline pH, albumin binds calcium more strongly, so ionised (free) calcium falls even though total serum calcium remains normal. Reduced ionised calcium increases neuromuscular excitability, producing perioral paraesthesia, tetany and carpopedal spasm. Option C is a real consequence of hypocapnia and can explain dizziness or light-headedness, but not peripheral tetany. Option A reverses the true direction of the pH effect on protein binding. Option D is wrong in direction: alkalosis drives potassium into cells, tending toward hypokalaemia, not hyperkalaemia. Option E is inconsistent with panic physiology, in which catecholamines raise rather than lower glucose, and does not explain the blood gas findings or tetany.

Reference: Kanjee Z et al. The Effect of Hyperventilation Syndrome on Ionized and Serum Calcium: A Case Presentation (Cureus, 2023). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10442464/