The mechanisms underlying dyspnea in interstitial lung disease (ILD) and chronic obstructive pulmonary disease (COPD) are unknown.

OBJECTIVE: To examine whether the relationship between inspiratory neural drive to the diaphragm and exertional dyspnea intensity is different in ILD and COPD, given the marked differences in static respiratory mechanics between these conditions.

METHODS: We compared sensory-mechanical relationships in ILD, COPD and healthy controls (n=16 each) during incremental cycle exercise with diaphragmatic electromyography (EMGdi) and respiratory pressure measurements.

RESULTS: In mild-to-moderate ILD and COPD patients with similarly reduced inspiratory capacity (IC), the peak oxygen uptake, work-rate and ventilation were lower (P<0.05) than controls. EMGdi expressed as percent of maximum (EMGdi/EMGdi,max), respiratory effort (esophageal pressure expressed as percent of maximum) and ventilation were higher (P<0.05) at rest and during exercise in both ILD and COPD compared with controls; each of these measurements was similar in the ILD and COPD groups. A tidal volume (VT) inflection and critically reduced inspiratory reserve volume occurred at a lower (P<0.05) ventilation in ILD and COPD than controls. ILD patients had greater diaphragmatic activity, while COPD patients had greater expiratory muscle activity. The relationship between dyspnea intensity and EMGdi/EMGdi,max during exercise was similar in all three groups. In ILD and COPD, descriptors alluding to inspiratory difficulty were selected more frequently, with a greater disparity between EMGdi/EMGdi,max and VT.

CONCLUSION: Disease-specific differences in mechanics and respiratory muscle activity did not influence the key association between dyspnea intensity and inspiratory neural drive to the diaphragm.