Common Mechanisms of Dyspnea in Chronic Interstitial and Obstructive Lung Disorders

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

Objectives: 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 patients with ILD, patients with COPD, and healthy control subjects (n = 16 each) during incremental cycle exercise with diaphragmatic electromyography (EMGdi) and respiratory pressure measurements.

Measurements and main results: In patients with mild to moderate ILD or COPD with similarly reduced inspiratory capacity, the peak oxygen uptake, work rate, and ventilation were lower (P < 0.05) than in healthy control subjects. EMGdi expressed as a percentage of the maximum (EMGdi/EMGdi,max), respiratory effort (esophageal pressure expressed as percentage of the maximum), and ventilation were higher (P < 0.05) at rest and during exercise in both patients with ILD and patients with COPD than in control subjects. Each of these measurements was similar in the ILD and COPD groups. A Vt inflection and critically reduced inspiratory reserve volume occurred at a lower (P < 0.05) ventilation in the ILD and COPD groups than in control subjects. Patients with ILD had greater diaphragmatic activity, whereas patients with COPD 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.

Conclusions: 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.