College Answer

This topic required a definition and understanding of airways resistance. It was expected candidates could identify that issues around the nature of flow (turbulent vs. laminar) and airway diameter were central determinants. It was expected candidates would describe the determinants of turbulent flow.The provision of formula and comments about Reynolds number helped demonstrate an understanding of this. Better answers discussed the transitional point in the airway and the paradox about size vs. total cross sectional area and its influence on total resistance. Several candidates confused pulmonary vascular resistance with airways resistance. Using graphs to help illustrate certain concepts would have been helpful. Measurement of resistance (indirectly via measurement of flow and pressure difference by a body plethysmography, spirometry) and detection of flow (spirometry, capnography) was in general poorly understood.

Discussion

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
    • The "paradox" alluded to by the college is probably referring to the fact that the airways get narrower, and one might expect the resistance to increase because of this, but because their total crossectional area becomes exponentially greater the flow in them slows down to the point where all the airways distal to Generation 10 contribute less than 16% to the total airway resistance.
  • Pathological conditions which affect airway diameter:
    • Increased smooth muscle tone
      • Bronchospasm
      • Irritants, eg. histamine
      • Parasympathetic nervous system agonists
    • Decreased smooth muscle tone
      • Bronchodilators
      • Sympathetic nervous system agonists
    • Decreased internal crossection
      • Oedema
      • 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 
  • 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

Measurement of respiratory resistance ...and/or... detection of .. flow changes? It is hard to tell exactly what the examiners wanted by reading the question, even if one stares at it for a really long time without blinking. From the examiners' comments, it would appear that they were looking for a list of methods of measuring respiratory resistance and detecting gas flow in the respiratory system. It is unclear what is meant by "poorly understood", as one would not have enough time to demonstrate a good understanding of these matters in a question weighing only 20%. Ergo, one might surmise that they were looking for a regurgitated list, such as this one:

• Measurement of respiratory resistance:
  • Direct measurement of air flow, airway pressure and alveolar pressure or less invasive surrogate (eg. oesophageal pressure
  • Body plethysmography
  • Forced oscillation technique
  • Airway interrupter resistance measurement
  • Inspiratory hold (in a mechanically ventilated patient)
  • Rhinomanometry
• Methods of determining flow and detecting increased resistance to flow:
  • Spirometry
  • Mechanical ventilator data (eg. flow and volume waveforms), detected by:
    • Hot wire anemometry
    • Variable orifice flowmeters
    • Screen pneumotachography
    • Ultrasonic flowmeters
  • End-tidal gas monitoring (eg. EtCO₂ monitoring)