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Infant airway resistance — MRCPCH TAS MCQ

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HardPhysiologyInfant airway resistanceMRCPCH TAS

A 5-month-old with bronchiolitis has subcostal recession and a small increase in airway oedema. Work of breathing rises markedly despite modest wheeze. What physiological principle best explains this?

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Correct answer: A — Resistance is inversely proportional to radius to the fourth power

Poiseuille's law explains how a small reduction in airway radius can greatly increase resistance, especially in infants with narrow bronchioles. Diffusion principles do not explain the sharp rise in airway resistance from mucosal oedema. The pearl is that small airways make infants vulnerable to respiratory distress from apparently minor obstruction.

Reference: Ganong's Review of Medical Physiology; Nelson Textbook of Pediatrics; RCPCH Theory examination syllabi