Effect of inspiratory muscle fatigue on perception of effort during loaded breathing

The present study examined the relationship between the intensity of the sense of effort during inspiratory threshold loading and the severity of inspiratory muscle fatigue. Studies were performed on normal subjects in whom the magnitude of airway pressure developed (Pm) and the duty cycle of breathing (TI/TT) were constrained to achieve a pressure-time integral (i.e., Pm/Pmax x TI/TT) 24% of maximum. In separate trials, the same pressure-index (24%) was achieved using two widely different patterns of pressure magnitude and duty cycle to allow the effects of changes in pattern of inspiratory muscle contraction on sensation and fatigue to be assessed. The intensity of the sense of effort was assessed using a category (Borg) scale. The severity of inspiratory muscle fatigue was assessed both from changes in the centroid frequency of the diaphragm electromyogram and from changes in the maximum static inspiratory pressure. Loaded breathing produced inspiratory muscle fatigue and a progressive increase in the sense of effort over time in all subjects. The rate at which the inspiratory muscles fatigued was the same with the two patterns of loading. In contrast, the rate of growth in the intensity of the sense of effort varied significantly as a function of the pattern of loaded breathing. The sense of effort increased at a faster rate with the high pressure-short duty cycle pattern of contraction as compared with the low pressure-long duty cycle pattern. As a result, the intensity of the sense of effort was not uniquely related to the severity of inspiratory muscle fatigue. We speculate that the sense of dyspnea in patients with lung disease may depend more on the intensity of muscle contraction than its duration.