Quantifying bronchodilator responsiveness
The improvement in airway obstruction is commonly expressed in terms of the increase in FEV1. Responsiveness is assessed from two indices:
- The increase expressed as a percentage
of the predicted value, according to the following formula:
[value after – value before] x 100 / predicted value
This was the index recommended by ECSC/ERS , because it does not suffer from the 'regression to the mean', but the ATS/ERS  now recommend the following index:
- The increase expressed as a percentage of the initial value:
[value after - value before] x 100 / initial value
- The second criterion is the increase
[value after - value before]
What does regression to the mean imply, and why should it be avoided? Measurements are affected by errors of all sorts. Thus the initial measurement may be lower than, or conversely higher than the true mean. If the initial measurement is lower than the true mean, there is a greater likelihood that upon repeating the measurements you will find a higher value than the same or an even higher value; you are therefore likely to record a spurious increase. Conversely, a value greater than the true mean is more likely to lead to a lower value upon repeating the measurement. Therefore the change correlates negatively with the initial value. It follows that one should always avoid relating change to initial value, and this obviously also holds for assessing bronchodilator responsiveness. Waalkens et al.  demonstrated how such spurious effects may give rise to erroneous conclusions in assessing bronchodilator responsiveness.
Obviously it is important to assess whether the FVC or the IVC increased after administration of the bronchodilator. This would signify a decreased residual volume due to less extensive airway closure.
The use of the FEV1/VC ratio to assess bronchodilatation should therefore be discouraged: both the numerator and the denominator in this ratio may increase.
|1||Quanjer PhH, Tammeling GJ, Cotes JE, Pedersen OF, Peslin R, Yernault JC. Lung volumes and forced ventilatory flows. Eur Respir J 1993; 6 suppl. 16: 5-40. Erratum Eur Respir J 1995; 8: 1629.|
|2||Pellegrino R, Viegi G, Brusasco V, Crapo RO, Burgos F, et al. Interpretative strategies for lung function tests. Eur Respir J 2005; 26: 948-968.|
|3||Waalkens HJ, Merkus PJFM, van Essen-Zandvliet EEM, Brand PLP, Gerritsen J, Duiverman EJ, Kerrebijn KF, Knol K, Quanjer PhH. Dutch CNSLD Study Group. Assessment of bronchodilator response in children with asthma. Eur Respir J 1993; 6: 645-651.|