BACKGROUND--In asthmatic subjects bronchoconstriction is induced by inhalation of the common food preservatives sulphur dioxide (SO2) and metabisulphite (MBS). SO2 and MBS challenges share many similarities, but it is not known whether they are equivalent. In this study of subjects with mild clinical asthma equivalence was assessed by comparing SO2 and MBS reactivity by estimating the total dose of SO2 inhaled during SO2 and MBS challenges, and by calculating SO2 uptake during both challenges. In addition, as the MBS solutions inhaled were acidic and hyperosmolar, the effect of these factors on MBS responsiveness was investigated. METHODS--Fifteen subjects were challenged on separate days with doubling (0.5 to 8.0 ppm) concentrations of SO2 gas inhaled during three minute periods of isocapnic hyperventilation and MBS administered in doses ranging from 0.1 to 12.8 mumol using the Wright protocol. On two other days SO2 and MBS challenges were preceded by a challenge with phosphate buffered saline (PBS) solutions of pH and osmolarity similar to MBS solutions. Response was measured as the dose or concentration causing a 20% fall in FEV1 (PD20 or PC20). RESULTS--All subjects reacted to MBS and 14 responded to SO2. Geometric mean histamine PD20 was 1.61 mumol (95% confidence interval 0.72 to 3.60). MBS and SO2 airway responsiveness were not significantly related. Estimates of the mean concentration of SO2 inhaled during SO2 and MBS challenges differed, as did estimates of the mean SO2 uptake during both challenges. MBS and SO2 reactivity were not affected by prior challenge with PBS solutions. CONCLUSIONS--SO2 and MBS challenges are not comparable. MBS reactivity was not affected by the hyperosmolar, acidic nature of its solutions.
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