This invention relates to medical electrical stimulation electrode leads in general and to epicardial defibrillation electrode leads in particular.
In the past years, there has been substantial activity directed toward development of a practical, implantable defibrillator. Some approaches, such as disclosed in U.S. Pat. No. 3,942,536, issued to Mirowski et al, and U.S. Pat. No. 4,161,952, issued to Kinney et al have employed only endocardial electrodes. However, more recent approaches to this problem have focused on systems employing one or more epicardial electrode leads as alternatives to or in addition to endocardial electrodes. Some such systems are disclosed in U.S. Pat. No. 4,030,509, issued to Heilman et al, and U.S. Pat. No. 4,291,707, issued to Heilman et al.
Generally, an epicardial defibrillation electrode must accomplish two important functions. First, it must deliver a relatively large amount of electrical energy to the heart with a minimal amount of tissue damage. For this reason, it is generally believed that epicardial defibrillation electrodes should have a large electrode surface area in order to distribute the energy over a wide area of the heart. The use of large surface electrodes reduces the density of the current applied to the heart, reducing the potential for damage to heart tissue. In addition, use of electrodes extending over a large area of the heart is believed to assist in improving current distribution through the heart tissue, reducing resistance and reducing the amount of energy which must be applied to the heart. For this reason, many prior art leads have employed electrodes having large surface areas and having individual, dispersed conductive areas.
The large size of epicardial defibrillation electrodes itself creates a problem. Motion of the heart during contraction is complex, and has been likened to a "wringing" action. A large surface area electrode must be capable of conforming to the contours of the heart and to changes in contours of the heart in order to continue to function properly. Typical prior art epicardial defibrillation leads, as disclosed in U.S. Pat. No. 4,030,509, issued to Heilman et al, however, employ large surface electrodes in the form of screens or plates which limit the flexibility of the electrode pad. One alternative to the use of screens and plates is disclosed in U.S. Pat. No. 4,641,656 issued to Smits et al which discloses electrode pads having spaced contact areas separated by perforations or indentations which allow the individual conductive areas to move with respect to one another.
In the above cited parent application to the present case, an epicardial defibrillation lead is disclosed employing a plurality of electrode coils mounted within grooves molded into a flexible, insulative base pad. In some embodiments, the electrode coils are simply glued into the grooves. In other embodiments, a solid, insulative core is provided and the insulative core is bonded to the electrode pad, holding the electrode coils in place.