Provocholine (Methacholine) Challenge

Methacholine challenge - The A series of methacholine chloride solutions are prepared, ranging from approximately 0.03 mg/mL (the most dilute) to 25 mg/mL (the most concentrated). These solutions are usually prepared in two-fold dilutions.

After baseline spirometry (that meets criteria noted above), either diluent (most common) or the most dilute concentration of methacholine is administered by nebulizer, using either a tidal breathing or dosimeter method. Both of these methods are acceptable under ATS guidelines; however, the tidal breathing method appears to result in lower PC20 values (more sensitive) than the dosimeter method, possibly due to a bronchodilator effect resulting from the deep inhalation and breathhold of the dosimeter method. Based on this observation, adjusting the dosimeter method to use inhalations that are half of inspiratory capacity, rather than total lung capacity, may improve the sensitivity of this method.

After inhalation of the aerosol by one of these methods, the FEV1 is measured at 30 to 90 seconds with careful coaching of the subject to obtain an acceptable quality FEV1. The concentration is increased one step sequentially, following this procedure, until a decrease in FEV1 greater than 20 percent or a 35 or 40 percent decrease in specific airways conductance (SGaw) is observed. As determined by interpolation, the dose of the inhaled antagonist that provokes a 20 percent drop in FEV1 is referred to as the PC20. Generally, a methacholine PC20 of 8 mg/ml (< 4 mg/mL, for SGaw) or less is considered a positive test.

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