What factors affect airway resistance? (80% of marks)
Briefly outline how it may be measured and/or changes in flow are detected. (20% of marks)
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.
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:
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:
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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.
Urbankowski, Tomasz, and Tadeusz Przybyłowski. "Methods of airway resistance assessment." Advances in Respiratory Medicine 84.2 (2016): 134-141.
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Goldman, M. D., H. J. Smith, and W. T. Ulmer. "Whole-body plethysmography." European Respiratory Monograph 31 (2005): 15.
Beydon, Nicole. "Interrupter resistance: what's feasible?." Paediatric respiratory reviews 7 (2006): S5-S7.
DuBois, Arthur B., et al. "Oscillation mechanics of lungs and chest in man." Journal of applied physiology 8.6 (1956): 587-594.
Clement, P. A., and F. Gordts. "Consensus report on acoustic rhinometry and rhinomanometry." Rhinology 43.3 (2005): 169-179.