Expressing airway responsiveness
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The airway response to an agonist is most often assessed from the change in the volume which can be exhaled in one second during a forced expiration (FEV1) from total lung capacity. Less often the airway response is assessed more directly with the measurement of airways resistance by whole body plethysmography. An increase in airways resistance is associated with a decrease in FEV1.
Bronchial responsiveness is expressed as either the Provocative Concentration or the Provocative Dose which produces a decrease in FEV1 by 20% from the initial value or baseline value prior to inhalation of the agonist (PC20 and PD20, respectively). If the desired response is obtained at x µmol/L agonist concentration in the nebulizer, this will be reported as the PC20 (the provocative concentration required to decrease FEV1 by 20% from baseline). For the response to be expressed as PD20 (the provocative dose to decrease FEV1 by 20% from baseline), the µmol of agonist delivered by the nebulizers needs to be known. Alternatively the responsiveness can be expressed as the concentration or the dose of agonist which causes airways resistance to double from the initial value, i.e. increase to 200% (PC200 and PD200, respectively).
Left: Relationship between airway resistance assessed in a body plethysmograph, and FEV1 in patients with COPD or asthma. They are not linearly related, suggesting that their sensitivity to changes in airway diameter is not the same. FEV1 is the most frequently used index in assessing bronchial hyperresponsiveness.
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