Comparison of Responses to Methacholine and Cold Air in Patients Suspected of Having Asthma: Material and Methods
Cold-air challenges were done as previously reported from this laboratory, using methods developed by Deal et al. A mixture of compressed air and 5 percent C02, which was dry, was passed over a cooling coil and supplied to a Hans Rudolph breathing valve. Inspired volume was measured with a dry gas meter and the expired side of the valve sampled for C02. End-tidal COz was maintained constant by adding C02 to the inspirate when necessary. Inspired gas temperature was —17° to — 20°C. After three baseline measurements of FEV,, the patients hyperventilated cold air for 6 min. The target ventilation was 30 times the FEV,. The FEV, was measured 1, 3, 5, 7, and 9 min after hyperventilation and compared with the largest of the baseline values. Decreases of FEV, of 10 percent were regarded as significant provided there was at least a 7 percent decrease measured directly preceding or following the critical maneuver. Decreases of 20 percent or more were also specifically noted.
In most patients both challenges were accomplished on the same day, the cold-air challenge always being first. Methacholine challenges were carried out at least 30 min after the cold-air test and after any cold-air-induced changes in FEV, had returned to control. In a few subjects the cold-air response was large enough to merit bronchodilator therapy, and in these subjects methacholine testing was carried out on a subsequent day. No testing was done unless the subject reported had not taken theophylline for 12 h or inhaled bronchodilators for 6 h.
Statistical analysis was done to determine significance, using unpaired t tests and correlation coefficients. read
A total of 140 adults with FE Vx ^60 percent predicted were studied (Table 1). Of these, 77 had PC^ >8 mg/ml, ie, had a negative methacholine challenge, while the remaining 63 demonstrated PC20 ^8 mg/ml. Baseline lung function test results did not differ between the two groups, nor did sex distribution, but those with positive responses were slightly and significantly younger than those with negative responses.
Table 1—Results of Methacholine Challenge
|Dose||No.||% Female||Age, yr||FEVj, % Pred|
|PCjo >8 mg/ml||77||47||42.2f (16.7)||110.7(19.3)|
|PCgo ^8 mg/ml||63||41||37.3(15.8)||98.7(20.9)|