Effects of chronic smoking on respiratory muscle function

To investigate the effects of chronic smoking on respiratory muscle function, 20 heavy smokers and 20 nonsmokers were studied. After familiarization, respiratory muscle strength, inspiratory muscle endurance and the spirometric function (including FVC, FEV 1, FEV1/FVC, tidal volume, breathing rate and duty cycle) were measured. The respiratory muscle strength was indicated by the maximal static inspiratory and expiratory pressures (PIMmax and PEMmax). Inspiratory muscle endurance was determined by a modified Nickerson-Keens method. The results showed that long-term smoking drastically enhanced expiratory muscle strength (p less than 0.01) and slightly increased inspiratory muscle strength (p less than 0.05). The inspiratory muscle endurance, in contrast, was reduced in the smokers (p less than 0.05). We also observed the tendency of decreases in the duty cycle and FEV1/FVC in the smokers. We, therefore, conclude that chronic smoking can raise respiratory muscle strength but impair the inspiratory muscle endurance.