Characteristics of bronchial hyperresponsiveness in chronic obstructive pulmonary disease and in asthma

By studying the characteristics of BHR in subjects clearly defined as having asthma or COPD, it is apparent that there are both similarities and differences. These responses can be broadly explained in terms of present knowledge of the pathologic features and the mechanisms causing the abnormalities in the two diseases, and they highlight the important differences between the diseases. It seems likely that tests of BHR with histamine and methacholine cannot be used to distinguish asthma from COPD in subjects with a mild decrease in FEV1 and symptoms that do not clearly suggest asthma or COPD. However, a dose-response curve to methacholine is helpful. If there is a plateau, and the PD20 FEV1 is more than 4.0 mumol, it is unlikely to be asthma. If there is no plateau, a test with propranolol or SO2 may be useful to discriminate the two diseases. The similarities in the responses-especially those to histamine-explain why there is frequently difficulty in deciding the nature of the disease present in an individual subject. Is it important to distinguish the two diseases? In the introduction it was suggested that it is important for understanding these diseases, for prognosis, for treatment, and ultimately for prevention. It remains difficult to determine prognosis from any single test of bronchial responsiveness, and response to prophylaxis (stopping smoking, allergen avoidance) plus drug treatment over several years is probably needed to predict outcome in an individual patient. Finally, despite the valid hypothesis proposed by Orie and coworkers (2), it seems unlikely that studying the characteristics of BHR will shed light on the "host" factors present in both diseases that remain poorly understood.