DISCHARGE FREQUENCIES OF PARASTERNAL INTERCOSTAL AND SCALENE MOTOR UNITS DURING BREATHING IN NORMAL AND COPD SUBJECTS

To determine whether patients with chronic obstructive pulmonary disea se (COPD) contract the inspiratory muscles of the rib cage more strong ly than do healthy subjects, we recorded the discharge frequencies of single motor units in the scalene and second parasternal intercostal m uscles of seven patients with stable COPD (FEV(1) = 33 +/- 13% predict ed, mean +/- SD) and seven control subjects. Recordings were made with insulated monopolar electrodes during resting breathing, and single m otor-unit discharges were identified with a customized method based on ''template'' matching. A total of 211 motor units were recorded in th e control subjects and 260 in the patients. The inspiratory discharge frequencies were greater in the COPD patients than in the control subj ects for both the parasternal (13.4 versus 10.1 Hz, p < 0.05) and scal ene (11.4 versus 8.5 Hz, p < 0.02) muscles. Recording sites at which n o 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 i ncreased to both the scalene and parasternal intercostal muscles but n ot to the sternomastoid muscle in patients with COPD.