C4. Assessing acute response to bronchodilators

The response to bronchodilators is determined to:

  • assign a level of severity of airflow obstruction (post- bronchodilator); and
  • help confirm asthma.

The details for this assessment are outlined in Box 5.

The change in FEV1 after an acute bronchodilator revers­ibility test indicates the degree of reversibility of airflow limitation. This is often expressed as a percentage of the baseline measurement (eg, 12% increase). An increase in FEV1 of more than 12% and 200mL is greater than average day-to-day variability and is unlikely to occur by chance (Sourk 1983, Pellegrino 2005). An analysis of cross-sectional data from 3,922 healthy never smokers in the BOLD study (Tan 2012) found that the 95th percentiles (95% CI) for bronchodilator response were 284 ml (263 to 305) absolute change in forced expiratory volume in 1 second from baseline. However,this degree of reversibility is not diagnostic of asthma and is frequently seen in patients with COPD (eg, the FEV1 increases from 0.8L to 1.0L when the predicted value is, say, 3.5L). The diagnosis of asthma relies on an appropriate history and complete, or at least substantial, reversibility of airflow limitation (see also below).

Box 5: Assessment of acute response to inhaled beta-agonist at diagnosis

Preparation

  • Patients should be clinically stable and free of respiratory infection.
  • Withhold inhaled short-acting bronchodilators in the previous six hours, long-acting beta-agonists in the previous 12 hours, or sustained-release theophyllines in the previous 24 hours.

Spirometry

  • Measure baseline spirometry (pre-bronchodilator). An FEV1 <80% predicted and FEV1/FVC ratio <0.70 shows airflow limitation.
  • Give the bronchodilator by metered dose inhaler (MDI) through a spacer device or by nebuliser.
  • Give short-acting beta-agonist, at a dose selected to be high on the dose– response curve (eg, 200–-400mcg salbutamol from MDI and spacer).

Repeat spirometry 15–-30 minutes after bronchodilator is given and calculate reversibility.

FEV1=forced expiratory flow in one second.
FVC=forced vital capacity.