Comparison of airway hyperreactivity in chronic obstructive pulmonary disease and asthma

Airway hyperreactivity (AHR) has been described in patients with chronic obstructive pulmonary disease (COPD). However, the nature and characteristics of AHR in this disease have not been fully investigated.

AHR was examined in a sample of 33 patients with COPD and 25 with asthma and compared during continuous inhalation of stepwise increased concentrations of methacholine. Respiratory resistance (Rrs) was measured by the forced oscillation technique and the dose-response curves were recorded.

The mean values for both forced vital capacity (FVC) and forced expiratory volume in 1s (FEV1) were well-preserved in subjects with asthma. In contrast, there was an obstructive ventilatory defect in patients with COPD, as evidenced by the FEV1/FVC ratio, which fell below 70%. Upon methacholine challenge, only 54.5% (18/33) of the patients with COPD had AHR, compared with 100% (25/25) of those with asthma. Analysis of the dose-response curves revealed that the patients with COPD had a significantly higher baseline Rrs, and thus lower baseline respiratory conductance (Grs), than those with asthma. The cumulative dose of methacholine capable of provoking a positive reaction was significantly higher in patients with COPD. The slope of the Grs was also less steep in responders with COPD. There was good correlation between the severity of AHR and the initial level of airway narrowing in patients with COPD (r=0.623, p<0.01), but not in those with asthma.

AHR is not uncommon in COPD, and it has different characteristics from that occurring in asthma.