Comparison of Responses to Methacholine and Cold Air in Patients Suspected of Having Asthma
In recent years bronchoprovocation tests have become well established in making the diagnosis of asthma. The most commonly used tests involve the inhalation of aerosols of bronchoconstrictor agents such as methacholine and histamine, but isocapnic cold-air hyperventilation has been suggested as a more physiologic test, since it appears to operate by mechanisms similar to exercise-induced asthma. In known asthmatic patients and normal subjects, results of methacholine and cold-air testing have been shown to correlate well. To our knowledge, methods of bronchoprovocation have not been compared in patients in whom the diagnosis of asthma is suspected but not established, although it is in this setting that the tests are thought to be of greatest value. We therefore sought to make such a comparison. http://cfp-for-you.com/
Material and Methods
Patients who were referred to our laboratories to establish or rule out a diagnosis of asthma were studied, provided they had an FEV, >60 percent predicted normal and were at least 17 years old and gave informed consent.
Methacholine testing was carried out using the method of Hargreave et al. Control and methacholine aerosols were generated by a Wright nebulizer with a gas flow rate of 10 L/min, so that 0.13 ml/min of solution was nebulized. Aerosols were inhaled from the nebulizer via face mask during 2 min of tidal breathing. The aerosols used were a control composed of diluent, and then successively increasing concentrations of methacholine: 0.03, 0.125, 0.5, 2.0, 4.0, 8.0, and 16.0 mg/ml. Using a waterless spirometer that met ATS specifications (model 822, Sensor Medics), the FEV, was measured three times before testing and again 0.5 and 1.5 min after exposure to each aerosol. Postexposure values of FEV, were compared with the largest preexposure FEV,. Aerosol administration was discontinued when the FEV, fell by 20 percent or more compared with the baseline value. Change in FEV, (% FEV,) was plotted against the log of methacholine concentration, and the methacholine dose associated with a 20 percent change in FEV, (PC20) was calculated by linear interpolation between the last two points. A positive methacholine response was defined as PC^ <8 mg/ml.