Describe the factors that affect airway resistance.
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.
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:
Thus, the factors which affect airway resistance are:
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