Question 12

College Answer

It was expected candidates cover the breadth of the factors that affect airway resistance. Generally, as a concept the type of flow (laminar vs turbulent) was answered well by most candidates, however many failed to mention the other factors that affect airway resistance. Airway diameter as a primary determinant of airway resistance was commonly omitted. Better answers which covered the factors affecting airway diameter classified them broadly and included examples such as physical compression/external obstruction, broncho-motor tone and local cellular mechanisms. Some answers did not explain these factors in enough detail and often with incorrect facts.

Discussion

Factors which affect airway resistance

• 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:
    • 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 
    • Decreased internal crossection
      • Oedema
      • Mucosal or smooth muscle hypertrophy
      • Encrusted secretions
    • Decreased smooth muscle tone
      • Bronchodilators
      • Sympathetic nervous system agonists
    • Increased smooth muscle tone
      • Bronchospasm
      • Irritants, eg. histamine
      • Parasympathetic nervous system agonists
• 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

Other factors which affect respiratory resistance as a whole:

• Resistance from deformation of the tissues (important at all flow rates)
  • Tissue resistance from lung parenchyma (~70%)
  • Tissue resistance from chest wall (~30% )
• Inertance of air and thoracic tissues (important at high respiratory rates) 
• Compression of intrathoracic gas (important mainly with high respiratory pressures)

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