Comparison of Responses to Methacholine and Cold Air in Patients Suspected of Having Asthma: Outcome
We therefore compared the dose of methacholine that produced a fixed decline in FEVi with the decline in FEV! measured after the cold-air stimulus. Although we tried to standardize the stimulus, we found it to be practically impossible in all patients; the level of ventilation achieved during cold-air testing varied substantially from patient to patient and was usually submaximal. It is very likely that these factors contributed to the variability shown in Figure 1. When all patients with a positive response to either challenge were considered, the cold-air response related weakly to the absolute level of ventilation obtained (r = 0.21, p<0.10) and to the ventilation expressed as a fraction of the expected maximum (r = 0.24, p<0.05).
Although in one case the relationship was statistically significant, the level of ventilation attained accounted for less than 6 percent of the variation in cold-air responses. Among patients with PC20 ^8 mg/ml we reexamined the relationship between PC20 and cold-air response, after discarding all subjects whose ventilation was less than 60 L/min during cold air testing and also after discarding all subjects whose ventilations during the test were less than 45 L/min or more than 75 L/min. Neither of these efforts substantially reduced the variance shown in Figure 1. We conclude that, while our submaximal and variable cold-air stimulus may have contributed to the scatter shown in Figure 1, it by no means explained it. in detail
We did not exclude patients with positive smoking histories, and it might be argued that our relatively poor correlation between cold-air and methacholine sensitivity was due to the inclusion of patients with chronic bronchitis. However, in such patients methacholine response is related to the initial FEVl5 and positive methacholine responses with negative cold-air responses are common. We excluded patients with FEVi <60 percent predicted, with the result that our patients had, on average, normal FEVi (Tables 1 and 2). The initial FEV1 did not differ between patients with positive and negative methacholine challenges, and in those with PC20 ^8 mg/ml, the initial FEWl did not correlate with PC). Thus, while our patient group may have included some patients with chronic bronchitis, they did not appear to influence the main trends of our results.