Question 6(p.2)

College Answer

Important factors to be discussed in this answer were anatomical site, laminar versus
turbulent flow, airway calibre and factors that affect it such as oedema and sympathetic tone.
The effect of lung volume on airway resistance is usefully described in a diagram.
The differences in infants earned extra marks
Syllabus B1d, 2h
Reference: Nunn 6th edition p39-47.

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)
    Usefully described in a diagram, it looks like this:
    
  • Physiological variation in airway diameter
  • 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

Additionally, for extra marks, it may be mentioned that airway resistance decreases markedly with growth from infancy to adulthood. Marciniak (2019) lists a resistance of 19 to 28 cm H₂O/L per second in neonates, whereas the figure is closer to 2 cm H₂O/L per second in adults. 

Kaminsky, David A. "What does airway resistance tell us about lung function?." Respiratory care 57.1 (2012): 85-99.

Macklem, PETER T., and Jere Mead. "Resistance of central and peripheral airways measured by a retrograde catheter." Journal of Applied Physiology 22.3 (1967): 395-401.

Briscoe, William A., and Arthur B. Dubois. "The relationship between airway resistance, airway conductance and lung volume in subjects of different age and body size." The Journal of clinical investigation 37.9 (1958): 1279-1285.

Hoppin Jr, FREDERIC G., M. A. L. C. O. L. M. Green, and MICHAEL S. Morgan. "Relationship of central and peripheral airway resistance to lung volume in dogs." Journal of Applied Physiology 44.5 (1978): 728-737.

Nakagawa, Misa, et al. "Effect of increasing respiratory rate on airway resistance and reactance in COPD patients." Respirology 20.1 (2015): 87-94.

Bruno Marciniak, in A Practice of Anesthesia for Infants and Children (Sixth Edition), 2019