An endotracheal tube may have a reduction in its effective internal diameter the longer it is in use in a patient. This may be due to dried secretions or other debris. This reduced diameter will affect airway resistance.
Air Flow
Resistance
laminar
proportionate to 1 / ((internal radius of ET tube)^4)
turbulent
proportionate to 1 / ((internal radius of ET tube)^5)
relative increase in airway resistance due to accumulation of intraluminal material =
= ((original internal radius in mm)^(power for air flow)) / (((original internal radius in mm) - (thickness of debris layer in mm))^(power for air flow))
where:
• It is assumed that the debris accumulates evenly around the entire circumference of the tube, which is an oversimplification.
If sufficient material accumulates then air flow resistance may be significantly affected, especially for smaller endotracheal tubes.
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