MYOCARDIAL AND ENDOCARDIAC ELECTRODES FOR CHRONIC IMPLANTATION

The experimental and clinical use of an implantable electronic prosthesis operating through myocardial electrodes for the long-term correction of heart block was reported by this group in 1959l and 19602 respectively. Since then, “electrical pacemaking” of the abnormally slow heart rate characteristic of the disorder has been widely accepted as the treatment of choice for this disease. Briefly, the pacemaker system consists of a subcutaneously implanted pulse generator which delivers an appropriate repetitive stimulus to the heart. This stimulus is delivered either through electrode structures attached to the outer surface of the myocardium (myocardial electrode) or through an endocardiac electrode, lying within the cavity of the right ventricular chamber of the heart (FIGURE 1). Installation of myocardial electrodes usually requires general anesthesia and a surgical procedure which entails opening of the chest, whereas endocardiac electrodes are positioned into the right ventricular cavity by threading through one of the jugular veins, accessible through a small skin incision in the neck. Installation of this latter system does not require entrance into the chest and the entire procedure is performed under local anesthesia. The electrode system of either type is attached to a subcutaneously placed pulse generator. When myocardial electrodes are used, the generator is usually placed beneath the skin of the upper abdomen just beneath the costal arch or in a convenient location on the chest wall. For endocardiac electrodes, the method of choice is to place the pulse generator under the skin of the anterior chest wall beneath the clavicle. The pulse generator is powered by mercury cells and must be periodically replaced-currently at intervals of approximately three years. This requires only an incision of skin and subcutaneous tissue under local anesthesia. The electrodes remain undisturbed. In order to operate continuously for years, the electrode system must possess mechanical as well as electrical stability. Within the body, the electrodes are exposed to mechanical stresses, mostly of the flexing type, principally from motion of the heart but also from respiration and from general body motion. Tolerance of moderate elongation stresses is also desirable. At a stimulation rate of approximately 60-70 per minute, approximately 30 million duty cycles are encountered by the electrodes during one year of operation and considerable structural strength is required to withstand such stresses. In addition to mechanical stability, electrode systems suitable for long-term stimulation of the heart must exhibit electrical stability. They must be corrosion-resistant when submitted to the passage of electrical cwrents in a saline medium and must be constructed of materials suitable for long-term implantation in human tissue without generating an increasing reaction of the latter. Myocardial and endocardiac electrode sys-