Along the twenty odd year journey that ultimately lead to the NeuRx DPS® diaphragm pacing system, which is now used by several thousand patients, we had an experience that I want to bring to your attention. This experience underscores the deleterious effects of unconstrained electrode movement relative to the implant tissues.
Shown here is a picture of an epimysial electrode that had been stapled to the abdominal aspect of animal’s diaphragm and had been in place for several months. When stimulation was applied to induce inspiration a momentary glitch, like a hiccup, was noted. When viewed with a laparoscope in the abdominal cavity a slight buckling of the electrode was note at about the same time of the “hiccup”. When the experiment was ended the tissues surrounding the epimysial electrode were excised and histological sections were made along such a line. The two open spaces shown, are the abdominal cavity, and space that had been occupied by the epimysial electrode. A thin layer of encapsulation tissue separates the two spaces. The diaphragm muscle fiber are stained red. But most surprising, and I might say saddening, was the build up of connective tissue between the electrode and the target nerves in the diaphragm. The connective tissue growth was believed to be a result of relative motion between the electrode carrier and the muscle during the electrically induced muscle contraction. The hiccup was a result of the stimulating contact lifting away during the contraction. Continued growth of the connective tissues here, separates the electrode contact from the target giving rise to an escalating need to increase the magnitude of the stimulation current. This result doomed the epimysial approach, to the chagrin of the surgeon, and launched the next effort in our successful pursuit of a diaphragm pacing system using a the Peterson type intramuscular electrode1.
1Aiyar, Harish, Electrical Activation of the Diaphragm for Ventilatory Assist, PhD. Thesis, 2000, CWRU.