It is well known that cardiac arrhythmias, such as atrial or ventricular fibrillation, can be overcome by applying electrical energy to the fibrillating myocardium. This procedure, defibrillation, can be accomplished by applying the electrical energy either to the chest of the patient by means of conductive-metal paddles held in place by medical personnel or, during the course of cardiac surgery, by holding conductive-metal paddles in direct contact with the surface of the heart. Such procedures are well known and have been found to be generally effective.
More recently, implantable defibrillators have been proposed for automatically detecting the onset of the cardiac arrhythmia and for automatically correcting such arrhythmia. These automatic defibrillators may employ conformal electrodes, which are maintained in contact with the surface of the heart or, electrodes on an intravascular catheter, or some combination of these. In any case, the electrodes act to impart the desired electrical energy to the heart muscle to achieve defibrillation.
With the intravascular catheter electrode approach, it has been found that although less electrical energy need be imparted to the heart than in the exterior chest paddles approach, more energy is needed than in the system wherein the electrodes are placed directly in contact with the heart surface. In other words, it has been found that physically placing the electrodes in contact with the exterior of the heart will provide a more efficient use of the electrical energy, thereby reducing the amount of energy required. Obviously, energy consumption is of the utmost importance in any implanted medical-electronic device.
In the automatic defibrillators, previously under consideration, the defibrillation electrodes have been designed for application to the heart by entering the chest cavity and by sewing the electrodes to the heart or positioning the electrodes on the surface of the heart. At times, such electrode implantation may be accomplished during the course of cardiac surgery, such as during a bypass operation. However, even when such heart surgery is not independently required, the previous surface electrodes required that the chest cavity be opened in order to implant the defibrillating electrodes. This surgical procedure requires intubation of the lungs and exposes the surfaces of the lungs to possible infection. Additionally, in order for the surgeon to have sufficient working space to effectively position and apply the electrodes, it may be necessary to perform an additional surgical procedure involving spreading two adjacent ribs or splitting the sternum. Accordingly, at the present time, in order to apply any type of cardiac electrodes to the surface of the heart, it is necessary to perform major surgery. Nevertheless, it is desirable to be able to implant the electrodes without the necessity of entering the pleural space, thereby maintaining the integrity of the pleural cavity.
Moreover, known surface electrodes suffer from the disadvantage that less than uniform energy density results from a discharge. Higher energy densities appear at the electrode edges, and at higher discharge levels, damaged tissue could result at the heart surface. It is, of course, desirable that the discharge be uniform over the entire electrode surface and that no regions of high energy density be present.