RESPIRATORY SENSATION DURING BRONCHIAL CHALLENGE TESTING WITH METHACHOLINE, SODIUM METABISULFITE, AND ADENOSINE-MONOPHOSPHATE

Background - There is some evidence that the perception of bronchocons triction may vary according to the nature of the provoking stimulus. T he aims of this study were, firstly, to develop a method for measuring dyspnoea during induced bronchoconstriction in patients with asthma a nd, secondly, to apply this method to testing differences between dire ctly and indirectly acting bronchoconstricting stimuli. Methods - Desc riptive terms suitable for quantifying respiratory discomfort due to b ronchoconstriction in patients with asthma were identified in a prelim inary investigation. The relation between reduction in forced expirato ry volume in one second (FEV(1)) and respiratory discomfort, measured using a visual analogue scale (VAS), was then studied during challenge s with three different inhaled stimuli: methacholine (MCH), sodium met abisulphite (MBS), and adenosine monophosphate (AMP). Three indices we re calculated to describe the relation: the VAS value associated with a 20% fall in FEV(1) (FEV(20) VAS); the ratio of the final VAS value t o the final percentage fall in FEV1( )(VAS-FEV(1) ratio); and the regr ession coefficient for predicting VAS from the percentage fall in FEV1 ( )within each challenge (beta VAS FEV(1)). Results - ''Difficulty in breathing'' and ''chest tightness'' were selected as suitable terms fo r quantifying respiratory discomfort. There were no differences betwee n the three agonists in the qualitative aspects of the respiratory sen sation. In paired challenges with the same agonist the three indices w ere all found to be reproducible for both sensations measured. MCH ind uced less intense difficulty in breathing and chest tightness for a gi ven fall in FEV(1) than did AMP. There was a trend in the same directi on for the comparison between MCH and MBS. There were no differences b etween AMP and MBS. FEV(20) VAS was less powerful in discriminating be tween agonists than the two slope indices. Conclusions - The relation between induced reduction in FEV(1) and the intensity of respiratory d iscomfort can be measured reliably. The indirectly acting bronchoconst ricting agonists AMP and MBS induced more intense respiratory discomfo rt for a given fall in FEV, than the direct agonist MCH. This may be d ue to differences in unmeasured mechanical changes in the lungs or to an additional action on airway sensory nerves.