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Endotracheal Tube Resistance — FRCA Primary MCQ

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HardPhysics & Clinical MeasurementEndotracheal Tube ResistanceFRCA Primary

A bronchoscope is inserted through an already narrow tracheal tube and peak inspiratory pressure rises markedly at unchanged tidal flow. Which explanation is most accurate?

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Correct answer: D — A steep, potentially nonlinear resistance increase as effective radius falls

The best answer is “A steep, potentially nonlinear resistance increase as effective radius falls”. A bronchoscope reduces the effective lumen, and resistance rises steeply as radius falls. Poiseuille behaviour predicts an inverse fourth-power relationship for laminar flow, while clinically relevant tracheal-tube flow may also include turbulent components and become nonlinear. Maintaining tidal flow therefore demands a substantially greater driving pressure.

Reference: Calderon et al., Model of single-sized endotracheal tube for adults: https://pmc.ncbi.nlm.nih.gov/articles/PMC6896656/