The tissue response to epimysial electrodes for diaphragm pacing in dogs.

Epimysial electrodes stapled to the abdominal surface of the diaphragm produced a chronic inflammatory response that appeared to be mediated by mechanical stresses placed on the encapsulation tissue by periodic diaphragm contraction. The tissue response surrounding 34 epimysial electrodes implanted in 11 dogs was studied three months post implant. The tissue response was characterized by a capsule having a mean thickness of 1.24 mm between the electrode and the muscle, while having only a very thin capsule on the back, or abdominal side of the electrode. The tissue response between the electrode and the muscle was comprised of two tissue layers: a layer of granulation tissue and a layer of collagen. The granulation tissue layer contained evidence of acute inflammatory processes including the presence of polymorphonuclear leukocytes in 68% of the samples. Granulation layer thickness was inversely correlated with back encapsulation indicating a reduction in granulation tissue for mechanically stabilized electrodes. In addition, encapsulation tissue surrounding the granulation layer was comprised of collagen fibers with an oblique orientation and an extraperitoneal locale suggesting mechanical load transfers between the electrode and the surrounding tissue. As a result, the histological response to epimysial electrodes implanted on the diaphragm suggests that mechanical loading, induced by movement associated with the contraction of adjacent muscle, must be a consideration for devices that employ epimysial electrodes.