Effect of electrical stimulation on diaphragmatic muscle used to enlarge right ventricle.

The effect of chronic electrical stimulation on muscle mass of pedicled diaphragmatic muscle grafts (DMGs) implanted into the right ventricle was studied in 21 dogs. Nineteen dogs, followed up to 70 days (mean, 34 days), were divided into three groups. In six dogs (control) the DMG was not stimulated. In six other dogs the DMG was stimulated at 50 stimuli/min with a fixed-rate unipolar pacemaker. In seven animals the graft was paced 1:1 with the dog's own rhythm with an R-wave synchronous pacemaker. Surface area of the DMG decreased 34% to 43% (P less than 0.05) in all dogs. DMG thickness increased in all dogs (P less than 0.05). The increase was 29% for control animals and 132% and 134% for the two paced groups. The volume of the DMG decreased 24% in the nonpaced animals but increased 29% and 51% in the two paced groups. Histologic studies showed some degree of muscle degeneration and atrophy in all grafts. However, morphometric studies showed relatively normal fiber diversity in paced grafts. Fiber size decreased in nonpaced grafts, indicating muscle atrophy. Perfusion studies with labeled microspheres and silicone injections demonstrated collateral circulation with the host ventricle as early as 28 days. However, total blood flow to DMGs implanted for 4 to 6 weeks averaged only 32% of the blood flow to normal ventricular myocardium. Chronic electrical stimulation of DMGs reduces muscle degeneration and atrophy and is associated with an increase in graft thickness and volume.