Assessing responsiveness in practice
In practice airway responsiveness to bronchoconstrictors (most often histamine or methacholine) and bronchodilators (e.g. ß2-sympathomimetic or anticholinergic drugs) is usually assessed from changes in airways resistance or changes in forced expiratory volume in one second (FEV1). The multiple steps between delivery of the agonist to the mucosal surface and the resulting airway narrowing are not individually assessed, rather the end response as measured by a change in pulmonary function is assessed. For bronchial challenges
- First the baseline FEV1 or airway resistance is assesssed ....
- ... then the solute for the agonist is administered as an aerosol, to exclude reaction to the solute rather than to the agonist,
- ... followed by increasing concentrations or dosages (usually the concentration is doubled at each successive challenge) of the agonist.
- The challenge is continued until the maximal concentration has been administered or either FEV1 falls by 20% or airways resistance doubles.
- The PC20 or PC200 are established by interpolation from the concentration of the agonist leading to the required response. For PD20 the dosage is computed from the change in volume (weight) of the nebulizer during a standard administration time, and the agonist concentration in the nebulizer solution.