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Measuring small airway function

Although widely used in the diagnosis of asthma, spirometry primarily measures large airway function. Spirometry has limited value in recording the function of the small airways, which also play an important role in asthma.

A universally accepted method of measuring small airway function is not currently available,1 but biological, functional and imaging techniques can be used to assess small airway abnormalities (Figure 1):

  • Parameters such as residual volume and forced expiratory flow between 25% and 75% of forced vital capacity (FEF25–75) have been shown to reflect peripheral abnormalities. Residual volume (RV) also has a close relationship to peripheral resistance; FVC improvements correlate with reductions in small airway obstruction.1
  • Impulse oscillometry (IOS) can be used to measure peripheral resistance.2
  • Nitrogen washout can distinguish inconsistencies in ventilation between the peripheral and proximal conducting airways. In the single-breath nitrogen washout test, an increase in the phase III slope of the washout curve and an increased closing volume (CV) or closing capacity (CC) correlate with air trapping because of small airway closure or near closure.1,3
  • Exhaled nitric oxide (eNO) and alveolar eNO have been investigated as biomarkers of small airway inflammation.4 
  • High-resolution computed tomography (HRCT) can also help to visualise ventilation heterogeneity in the lungs.1

121 fig 1
Figure 1: Assessment of air trapping as a result of small airway closure. Small airway abnormalities (1) lead to small airway closure or near closure (2), causing peripheral air trapping (3). This can be assessed by single-breath nitrogen washout test, lung function tests and imaging techniques such as HRCT1
Reproduced with permission from Wiley & Sons.

Small airway inflammation is present in a wide variety of asthma patients. For more information, visit the Patient profiles section.

References
  1. Contoli M, Bousquet J, Fabbri LM, et al. The small airways and distal lung compartment in asthma and COPD: a time for reappraisal. Allergy 2010;65:141-51.
  2. Kaminsky DA, Irvin CG, Lundblad L, et al. Oscillation mechanics of the human lung periphery in asthma. J Appl Physiol 2004;97:1849-58.
  3. in 't Veen JC, Beekman AJ, Bel EH, et al. Recurrent exacerbations in severe asthma are associated with enhanced airway closure during stable episodes. Am J Respir Crit Care Med 2000;161:1902-6.
  4. van Veen IH, Sterk PJ, Schot R, et al. Alveolar nitric oxide versus measures of peripheral airway dysfunction in severe asthma. Eur Respir J 2006;27:951-6.

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