Describe the factors that affect airways resistance
Candidates who used a structured approach of using formulae that describe resistance fluid
flow scored well. Poiseuille’s law describes the determinates of resistance to laminar fluid flow
and provides a useful answer structure. The most common mistakes were confusion between
resistance and compliance as well as failure to describe turbulent as well as laminar flow.
The question asked specifically for airways resistance, and the candidates would have probably not earned any marks if they discussed other contributing elements (eg. inertance, tissue resistance and the effect of gas compression). Ergo, only airway stuff is listed here:
A main determinant of airway resistance is whether the flow is laminar or turbulent. This depends on the Reynolds number, which is a dimensionless metric determined by:
- Tube diameter
- Tube length
- Flow rate
- Gas density
- Gas viscosity
Thus, the factors which affect airway resistance are:
- Gas properties which affect the type of flow
- Gas density (increased density leads to increased turbulence and hence increased resistance)
- Gas viscosity (increased viscosity promotes laminar flow and hence decreases resistance)
- Factors which affect airway diameter
- Lung volume (resistance decreases with higher volume)
- Physiological variation in airway diameter
- Pathological conditions which affect airway diameter:
- Increased smooth muscle tone
- Irritants, eg. histamine
- Parasympathetic nervous system agonists
- Decreased smooth muscle tone
- Sympathetic nervous system agonists
- Decreased internal crossection
- Mucosal or smooth muscle hypertrophy
- Encrusted secretions
- Mechanical obstruction or compression
- Extrinsic, eg. by tumour
- Dynamic compression, eg. due to gas trapping or forceful expiratory effort
- Artificial airways and their complications, eg. endotracheal tube becoming kinked
- Increased smooth muscle tone
- Factors which affect airway length
- Lung volume (increasing volume stretches and elongates the bronchi)
- Artificial airways (increase the length in the case of an ETT, or decrease it in the case of a tracheostomy)
- Factors which affect flow rate
- Respiratory rate (increased respiratory rate produces an increase in the flow rate for each breath)
- Inspiratory and expiratory work (eg. voluntary forced expiration for spirometry)
- Inspiratory flow pattern generated by a mechanical ventilator
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