Discharge Frequencies of Parasternal Intercostal and Scalene Motor Units During Breathing in Normal and COPD Subjects

To determine whether patients with chronic obstructive pulmonary disease (COPD) contract the inspiratory muscles of the rib cage more strongly than do healthy subjects, we recorded the discharge frequencies of single motor units in the scalene and second parasternal intercostal muscles of seven patients with stable COPD (FEV1 = 33 +/- 13% predicted, mean +/- SD) and seven control subjects. Recordings were made with insulated monopolar electrodes during resting breathing, and single motor-unit discharges were identified with a customized method based on "template" matching. A total of 211 motor units were recorded in the control subjects and 260 in the patients. The inspiratory discharge frequencies were greater in the COPD patients than in the control subjects for both the parasternal (13.4 versus 10.1 Hz, p < 0.05) and scalene (11.4 versus 8.5 Hz, p < 0.02) muscles. Recording sites at which no motor units were recruited were more common in the control subjects than in the patients (p < 0.001). The sternomastoid muscle was silent in both subject groups. Therefore, effective central neural drive is increased to both the scalene and parasternal intercostal muscles but not to the sternomastoid muscle in patients with COPD.