ETT Resistance r⁴ — FRCA Primary MCQ
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Correct answer: E — 16
Resistance ∝ 1/r⁴ (Hagen-Poiseuille). Halving the radius: (1/(0.5))⁴ = 2⁴ = 16-fold increase. This is why small reductions in airway calibre (e.g. from oedema or downsizing ETT) dramatically increase resistance and work of breathing.
Reference: Al-Shaikh Stacey 2019 Essentials of Anaesthetic Equipment