Canine diaphragm muscle after 1 yr of continuous electrical stimulation: its potential as a myocardial substitute

Skeletal muscle has been rendered fatigue resistant by chronic stimulation and therefore has potential as an active substitute for damaged myocardium. It is therefore important to know whether stimulation produces any deleterious effects in the long term. Hemidiaphragm muscles of four dogs were examined after chronic stimulation for 1 yr at either 2 or 4 Hz. The stimulated hemidiaphragms appeared normal on gross inspection and were still contracting vigorously. By histochemical and immunohistochemical criteria, they had acquired a uniformly type I character, in contrast to the mixed fiber type composition of the unstimulated hemidiaphragms. This transformation was also reflected in their complement of myosin isozymes. There was some enzymatic evidence of an associated shift towards aerobic pathways of energy generation. Histological examination revealed no evidence of degenerative changes. Trends, observed in the shorter term (6-8 wk), toward a decrease in fiber area and an increase in connective tissue showed no further progression at 1 yr. Thus hemidiaphragm muscle stimulated at frequencies at or above the normal heart rate does not appear to undergo adverse long-term changes that would constrain its use in a myocardial assist role.