Do subjects with asthma have greater perception of acute bronchoconstriction than smokers with airflow limitation?

Smokers who develop chronic airflow limitation (CAL) do not usually present for medical attention until their lung disease is well advanced. In contrast, asthmatic subjects experience acute symptoms and present for care early in the course of their disease. The aim of this study was to determine whether subjects with asthma differ from smokers with CAL in their ability to perceive acute methacholine-induced bronchoconstriction.

Thirteen subjects with diagnosed asthma and 10 current smokers with CAL, defined as forced expiratory volume in 1 s (FEV1) < 75% predicted and FEV1/forced vital capacity < 80%, with no previous diagnosis of asthma, were challenged with methacholine. Symptom severity was recorded on a Borg scale. Lung volumes were measured before challenge and after the FEV1 had fallen by 20%.

After methacholine falls in FEV1 were similar in the asthmatic subjects and smokers. The regression lines relating change in FEV1 to symptom score were significantly steeper in asthmatic subjects than smokers (0.13 +/- 0.04, 0.03 +/- 0.04, respectively, P < 0.01). At 20% fall in FEV1 there were no significant differences between asthmatic subjects and smokers in the magnitude of change of lung volumes.

In asthmatic subjects, symptoms are closely related to change in FEV1. In smokers with CAL, symptoms change little during bronchial challenge despite large changes in FEV1. The differences in perception between the two subject groups are not due to differences in acute hyperinflation during challenge. We propose that heavy smokers may adapt to poor lung function, or may have damaged sensory nerves as a result of prolonged cigarette smoking.