Dyspnea and Respiratory EMG in Patients with COPD: Results of the COPD Monitoring Study

Exertional dyspnea is the main symptom of COPD. Based on the positive relationship between the levels of dyspnea (Borg-score) and the electrical activity of respiratory muscles (electromyogram, EMG) during an incremental cardiopulmonary exercise test, it has been suggested that respiratory EMG can provide a physiological biomarker for dyspnea. This study aimed to characterize the relationship between dyspnea and EMG during exercises simulating daily activities. Surface EMG was measured at 2 locations on the chest of 28 COPD patients while they were performing constant-work rate cycling tests and walking/cycling exercises that were part of their rehabilitation program. Simultaneously, the level of dyspnea was assessed using the Borg-score at several timepoints throughout the exercise sessions, along with respiration rate (RR), heart rate (HR) and transcutaneous oxygen saturation (SpO 2 ). Patients completed each up to 10 such study visits during their 8-week stay at the rehabilitation center (CIRO, the Netherlands). In total, 1981 Borg-scores with associated EMG measurements were recorded during 263 study visits. A linear mixed model was used to assess the relation of the Borg-score with EMG while controlling for RR, HR, (type of) exercise, SpO 2 , age and sex. Random effects for patient and visit were included to account for correlation in the measurements. EMG had a highly significant association with the Borg-score (p<0.0001). Respiratory EMG and Borg-score showed consistent positive correlations, of which the magnitude varied between patients. These results indicate that respiratory EMG can provide a physiological biomarker for dyspnea during activities of daily living in COPD patients.