NEURAL DRIVE TO THE DIAPHRAGM IN PATIENTS WITH SEVERE COPD

Patients with severe chronic obstructive pulmonary disease (COPD) have a greater neural drive to the parasternal intercostal and scalene mus cles and greater inspiratory expansion of the rib cage than do healthy individuals. However, such patients also have a reduced outward displ acement or a paradoxical inward displacement of the ventral abdominal wall during inspiration. This has led to the suggestion that they may have less use of the diaphragm, possibly secondary to chronic muscle f atigue. To assess the effect of COPD on the neural drive to the diaphr agm, we inserted needle electrodes into the costal part of the right h emidiaphragm in eight patients with severe disease (mean [+/- SD] FEV( 1): 0.82 [+/- 0.27] L) and six control subjects of similar age, and me asured the discharge frequencies of single motor units during resting breathing. A total of 115 diaphragmatic motor units were recorded in t he control subjects and 122 in the patients. All motor units discharge d rhythmically in phase with inspiration. However, whereas 95% of the units in the control subjects had a peak discharge frequency between 7 and 14 Hz, 79% of the units in the COPD patients had a peak discharge frequency greater than 15 Hz. As a result, the discharge frequency of all units averaged 10.5 [+/- 2.4] Hz in the control subjects, but 17. 9 [+/- 4.3] Hz in the patients (p < 0.001). These observations indicat e that patients with severe COPD have an increased neural drive not on ly to the rib cage inspiratory muscles, but also to the diaphragm. Con sequently, the reduced inspiratory expansion of the abdomen in severe COPD results from mechanical factors alone.