Dyspnea and respiratory EMG in patients with COPD: results of the COPD monitoring study

Exertional dyspnea is the main symptom of chronic obstructive pulmonary disease (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 two locations on the chest of 28 patients with COPD 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 (Sp<inf>O2</inf>). Patients completed each up to 10 such study visits during their 8-wk 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, Sp<inf>O2</inf>, 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 patients with COPD.