Abstract
There appears to be a recent trend, at least in the UK, toward inserting smaller sizes of endo-tracheal and tracheostomy tubes. Given that flow through these tubes is laminar, small changes in diameter will have large effects on flow and work of breathing. Work of breathing can be further increased by secretions, biofilm and the placement of disposable inner cannula. We contend that placement of larger tubes may facilitate rapid weaning in some patients.
Highlights
Recent experience has compelled us to write this letter as several patients, referred to our hospital and labelled slow-weaners, made rapid progress when upsized to a larger internal diameter (ID) tracheostomy
The radius of a tube has the predominant influence on gas flow; small changes in tracheostomy/endotracheal tube (ETT) ID can exert a large effect on flow
Work performed in the 1960s demonstrated that work of breathing (WoB) increases as tracheostomy ID reduces, and WoB through a tracheostomy is only less than WoB through the mouth with ID ≥10 mm [1]
Summary
Recent experience has compelled us to write this letter as several patients, referred to our hospital and labelled slow-weaners, made rapid progress when upsized to a larger internal diameter (ID) tracheostomy. Flow through the airways (natural and artificial) is both turbulent and laminar. Flow through a tracheostomy/ endotracheal tube (ETT) is mainly laminar and governed by the Hagen–Poiseuille equation: Flow = pressure×radius4×π / length×viscosity×8 The radius of a tube has the predominant influence on gas flow; small changes in tracheostomy/ETT ID can exert a large effect on flow.
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