The last several decades have seen considerable advancement in the treatment of cardiac dysrhythmias encountered in the course of heart disease. Particularly dramatic results have been achieved in the treatment for ventricular fibrillation, the most serious form of cardiac dysrhythmia. Early cardiac defibrillation systems were externally applied and, as the efficacy of the system was proven, efforts focused on providing increasingly smaller defibrillation systems that would be suitable for implantation. Along with the advancement of electrical intervention to treat defibrillation, electrical treatment of other ventricular and atrial dysrhythmias was undertaken. Experience has now demonstrated that many cardiac dysrhythmias are amenable to treatment by electrical countershock intervention.
Examples of the various types of cardiac dysrhythmias are ventricular fibrillation, ventricular flutter, high rate ventricular tachycardia, low rate ventricular tachycardia, supraventricular tachycardias including atrial fibrillation and atrial flutter. These and other dysrhythmias have been demonstrated to be treatable by application of an electrical countershock of appropriate energy, size, and waveform such as to correct the cardiac dysrhythmia thereby converting the rhythm to a normal sinus rhythm or a rhythm of lesser morbidity.
Necessary components of any defibrillation system are the discharge electrodes used to deliver an electrical countershock to the myocardium. The very first defibrillation devices were external devices that utilized large patch or disc electrodes applied to the surface of the patient's skin, generally with one electrode at or near the low left antero-lateral aspect of the chest and the second electrode placed well up over the sternum or into the right upper anterior chest wall. An electrical countershock up to 400 Joules was then delivered between these two electrodes.
As implantable defibrillator devices have been developed, electrodes have been devised to allow for implantation within the patient's body, in and around the heart. The implantation of internal electrodes is obviously more complicated than the placement of external electrodes. The complexity of the surgical procedure required to implant the electrodes is closely associated with the degree of complexity for any given electrode configuration.
Several early internal electrodes required major surgery in the form of a thoracotomy to open the chest and place patch electrodes either on the pericardium or further opening the pericardium and placing electrodes directly onto the epicardial surface of the heart. The electrode leads are then tunneled out to an implanted device that usually needs to be implanted within the abdominal cavity due to the excessive size of the implantable cardioverter defibrillator (ICD).
Alternative patch electrode configurations have been developed in the form of electrodes suitable for implantation within the subcutaneous space of a patient. Ideal locations for placement of these subcutaneous patch electrodes has been in the left antero-lateral chest wall outside the rib cage proper. The metallic surface covering of an ICD housing is useful as an alternative subcutaneous electrode. Further development and refinement of ICDs have allowed attainment of sizes small enough to allow for comfortable implantation within a subcutaneous pocket in the infraclavicular space of the anterior chest wall. Convenience of implantation within the infraclavicular space allows for ready access to the venous vascular system via the subclavian veins.
The convenience of vascular access has seen a development in the art for intravascular catheters bearing one to several electrodes suitable for discharge into the myocardium. The arrangement is not unlike that arrangement used for cardiac pacemakers. The significant difference between ICD systems and cardiac pacemakers is the amount of energy delivered. Electrical cardioversion in general, and ventricular defibrillation in particular, can require upwards of 40 Joules of energy to be delivered over a 3 to 10 millisecond duration through the intravascular discharge electrodes. Initial peak currents can range as high as 25 to 30 amps and initial voltages are as high as 750 volts. Given this amount of energy, the practice has been to manufacture discharge electrodes with sufficient surface area so as to provide an electrical field density sufficiently low to avoid burning the immediately adjacent myocardial tissue.
The application of an electrical countershock depends in some part on the actual myocardial dysrhythmia detected. In general, the efficacy of any electrical countershock therapy will be directly dependent upon how well the myocardium is immersed within the electrical field generated by the countershock. For example, epicardial patches that have been positioned on the external surface of the myocardium must be positioned carefully to ensure that the space between the margins of the patches is uniform. Any position closer than another position will allow shunting of current at that localized position increasing the electrical field density in a focal fashion which may contribute to focal damage in that area and lack of treatment in the rest of the myocardium. With intravascular catheters significant impairment of electrical treatment is seen where countershock energy has shunted through the blood itself from the edge of one countershock electrode to the edge of the nearest adjacent countershock electrode. Such shunted countershock energy does not enter the myocardium and treatment efficacy deteriorates. Because of this phenomenon, the practice has been to employ discharge electrodes that allow positioning of the electrodes next to or very close to the inner myocardial surface, but are also sufficiently far removed from the nearest adjacent electrode so as to increase the amount of electrical energy that must travel through the myocardium in order to complete the electrical path from one discharge electrode to the other. To achieve a placement sufficiently far enough away has led to placement of the proximal intravascular electrode within the superior vena cava which is outside of the heart above the right atrium. To complete a discharge, a countershock flows from an electrode in the right ventricle up the lateral wall of the right ventricle to the right atrium and into the superior vena cava.
A typical intravascular defibrillation discharge catheter arrangement is seen in U.S. Pat. No. 3,942,536 issued to Mirowski et al. on Mar. 9, 1976, disclosing an intravascular catheter electrode system. The catheter as discussed uses two discharge electrodes separated from each other by at least 11/2 inches and up to 41/2 inches in order to develop the electrical field needed to effect defibrillation. The catheter system placed a distal electrode at or near the apex of the right ventricle and a proximal electrode outside of the heart proper in the superior vena cava, a large vein draining blood from the upper body into the heart. The concomitant electrical field generated by discharge of current from one electrode to the other would create an approximate ellipsoidal field from the apex of the right ventricle to the superior vena cava with significant amounts of current shunting through the blood within the right atrial and right ventricular chambers.
A second representative electrode system is disclosed in U.S. Pat. No. 4,603,705 issued to Speicher et al. on Aug. 5, 1986. As disclosed, the catheter carries two discharge electrodes. The distal discharge electrode is located near the apex of the right ventricle with a proximal discharge electrode near or in the superior vena cava but no closer than about three inches between the two discharge electrodes. This patent, being similar to the Mirowski patent, recognizes the need to separate the discharge electrodes in an effort to provide a countershock capable of achieving a therapeutic response. The Speicher patent preferred an inter-electrode distance of 41/3 inches or 11 centimeters.
U.S. Pat. No. 4,817,608 issued to Shapland et al. on Apr. 4, 1989 discussed the Speicher patent and pointed out a deficiency in the Speicher patent by noting that treatment outcome using the Speicher catheter is only 40 to 50% successful even with energy delivered as high as 40 Joules. The Shapland patent discloses a third patch electrode. As disclosed, the reason for adding the third patch electrode is an attempt to expand the electrical field to include more of the myocardium. The method as disclosed electrically connects in common the proximal catheter electrode to the patch electrode and discharges from this electrical combination to the second distal catheter electrode. The catheter inter-electrode distance is in a range from 8 to 14 centimeters or greater than 3 inches. The Shapland patent, however, still suffers from the same deficiency as Mirowski and Speicher because of the closer proximity of the two catheter electrodes by comparison to the distance from the distal catheter electrode to the patch electrode. The vast majority of current will still flow the shorter route having the lower resistance.
These patents, and others, teach that the proximal and distal electrodes mounted on a common catheter must be mounted to maintain an inter-electrode distance of at least 6 centimeters in order to accomplish some cardioversion or defibrillation and avoid excessive and significant shunting or short circuiting between the two electrodes. The distal electrode placed within the right ventricle must be constructed of sufficient size so as to ensure adequate surface area and contact with the inner ventricular wall. There is not adequate consensus as to the actual physical dimensions needed, however, on average the intra-ventricular electrodes are at least 5 centimeters in length. Allowing for an inter-electrode distance of 8 centimeters or 3 inches the proximal electrode is at the most at or near the superior vena cava, a structure outside the heart proper. One effect of this requirement is to force a sub-optimal electrode placement in order to avoid significant short circuiting. A second effect is that the placement of the proximal electrode outside of the heart above the right atrium renders the electrode useless for use in atrial defibrillation. As shown in FIG. 1, the energy that would be delivered to a heart to treat atrial fibrillation or flutter will miss the atria.