Diagnosing asthma

Classifying asthma

Asthma has previously been classified according to its severity. However, asthma severity can be highly variable and may also depend on responsiveness to treatment.

For this reason, GINA now promotes a classification system based on control rather than severity¹ (Table 1).

Table 1: Levels of asthma control¹

Lung function tests

An accurate diagnosis of requires an assessment of clinical symptoms, measurement of airway function and response to therapy, and occasionally, the use of provocation tests. Lung function tests, such as spirometry, are routinely performed in the clinic to measure changes in pulmonary function and these include¹:

• Forced expiratory volume in 1 second (FEV₁)
• Forced vital capacity (FVC) measurements.
• Peak expiratory flow (PEF) measurement.

Predicted values of FEV₁, FVC and PEF based on age, sex and height have been obtained from population studies, and with the exception of PEF (for which there is wide variability), are useful for judging whether lung function is abnormal. Hand-held PEF-meters are, however, used by patients to obtain objective daily measurements of airflow limitation.¹

Measuring small airway function

Although spirometry is widely used in the diagnosis of asthma, it primarily measures large airway function. Therefore, spirometry has limited value in recording the function of the small airways, which also play a role in asthma.

A universally accepted method of measuring small airway function is not currently available,² but biological, functional and imaging parameters 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 (FEF_25–75) 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.²
  
  
• Impulse oscillometry (IOS) is used to measure peripheral resistance.³
  
  
• 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.^2,4
  
  
•  Exhaled nitric oxide (eNO) and alveolar eNO have been investigated as biomarkers of small airway inflammation.⁵ High-resolution computed tomography (HRCT) has also been investigated as a potential method.²

Figure 1: Assessment of air trapping as a result of small airways closure. Small airways abnormalities 1) lead to small airways 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 HRCT²

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

 

References:

1. From the Global Strategy for Asthma Management and Prevention, Global Initiative for Asthma (GINA) 2011. Available from: http://www.ginasthma.org/
2. 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;65:141-151
3. Kaminsky DA, Irvin CG, Lundblad L, et al. Oscillation mechanics of the human lung periphery in asthma. J Appl Physiol 2004;97:1849-58
4. 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
5. 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