Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical energy applicator
Reexamination Certificate
1998-03-13
2001-09-25
Evanisko, George R. (Department: 3762)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical energy applicator
C607S120000
Reexamination Certificate
active
06295474
ABSTRACT:
BACKGROUND OF THE INVENTION
A. Field of the Invention
The present invention generally relates to electrodes for implantable cardiac stimulators, particularly defibrillators employing a “hot can” stimulus generator housing. More particularly, the invention relates to such stimulation electrodes having a coating that protects the electrode surface from oxidation, and still more particularly to such oxidation resistant coatings that contain a conduction-enhancing medium.
B. Description of the Related Art
Abnormal rhythms or arrhythmias can arise in the heart as a consequence of an impairment of the heart's electro-physiologic properties. Tachycardia, for example, is an arrhythmia characterized by rapid beating of the affected cardiac chamber which, in some instances, may lead to fibrillation. In other instances, fibrillation may arise in a diseased heart without the advance episode of tachycardia.
During fibrillation, sections of conductive cardiac tissue of the affected chamber undergo completely uncoordinated, random contractions, quickly resulting in a loss of the blood-pumping capability of that chamber. During ventricular fibrillation (i.e., fibrillation occurring in a ventricle), cardiac output ceases instantaneously. Unless cardiac output is restored almost immediately after the onset of ventricular fibrillation, tissue begins to die for lack of oxygenated blood, and death of the patient will occur within minutes.
Because ventricular fibrillation is frequently caused by ventricular tachycardia, various methods and devices have been proposed to treat and arrest the tachycardia before the onset of fibrillation. Conventional techniques for terminating tachycardia include pacing therapy and cardioversion. In the latter technique, the heart is shocked with one or more current or voltage pulses of considerably higher energy content than is delivered in pacing pulses from a pacemaker. Unfortunately, cardioversion itself presents a considerable risk of precipitating fibrillation, as a result commonly called “refibrillation.”
Defibrillation, that is, the method employed to terminate fibrillation, generally involves applying one or more high energy “countershocks” to the heart in an effort to overwhelm the chaotic contractions of individual tissue sections, thereby restoring the synchronized contractions of the atria and ventricles. Successful defibrillation requires the delivery to the heart of the afflicted person an electrical pulse containing energy at least adequate to terminate the fibrillation and to preclude refibrillation. Although high intensity defibrillation shocks are often successful in arresting fibrillation, they tend to precipitate cardiac arrhythmias, which themselves may accelerate into fibrillation, i.e., refibrillation. Additionally, high intensity shocks can cause permanent injury to the lining of the heart (myocardium).
In the conventional approach of external defibrillation, conducting paddles or electrodes are positioned on the patient's chest and electrical energy typically in the range of 100 to 400 joules is delivered to the chest area in the region of the heart. When fibrillation occurs during open heart surgery, internal paddles may be applied to opposite surfaces of the ventricular myocardium, and the energy required for defibrillation is considerably less, on the order of 20 to 40 joules.
More recently, implantable defibrillators have been developed for use in detecting and automatically treating ventricular fibrillation. In the last twenty years, a vast number of improvements in implantable defibrillators, including fibrillation detectors and high energy pulse generators with related electrode configurations, have been reported in the scientific literature and disclosed in patent publications.
Typically, electrodes for implantable defibrillators are made similarly to those developed for cardiac pacemakers, except defibrillation electrodes are larger than those used for cardiac pacing because a greater area of the heart tissue needs to be stimulated. These electrodes may be in the form of patches applied directly to the heart. The most common approach in the past has been to suture two patches to the epicardial tissue via thoracotomy. It has been theorized that electrodes with large surface areas are important for a wider distribution of current flow and a more uniform voltage gradient over the ventricles. Others have postulated that uniformity of current density is important since: (i) low gradient areas contribute to the continuation or reinitiation of ventricular fibrillation, and (ii) high current areas may induce temporary damage, that then may cause sensing difficulties, set-up areas of reinitiation of fibrillation, or even potentially cause permanent damage (new arrhythmias, decreased contractility, and myocardial necrosis).
For most patients, the best conventional devices and implantation methods are those that avoid surgical entry into the chest cavity and attachment of epicardial electrodes. Employing a less invasive surgical technique, one or more defibrillation electrodes are implanted proximate the pleural cavity and rib cage, and are used in combination with one or more coil electrodes positioned in the right atrium or right ventricle of the heart. This kind of defibrillator is described in U.S. Pat. No. 5,203,348, issued to Dahl, et al.
Stimulation of tissues requires that the charge be injected reversibly by a purely capacitive mechanism. In such a mechanism, the electrode behaves as a charge flow transducer whereby electrical discharge takes place in a uni-directional manner between media exhibiting different charge flow properties. The capacitive mechanism allows electrons to flow away from the stimulation electrode causing electrical charges at the electrode/electrolyte interface to orient themselves in order to cause a displacement current through the electrolyte. Since the electrolyte is an ionic medium, the slight displacement of the ions in reorientation creates a charge flow.
When irreversible chemical reactions begin to occur as a result of poor electrode selection or exceedingly high currents or other thermodynamic or kinetic limitations, the mechanism is no longer capacitive. Irreversible faradaic reactions may lead to water electrolysis, oxidation of soluble species, and metal dissolution from the electrode. In addition, some of the products of the reactions may be toxic. Neither gas evolution nor oxide formation reactions contribute to electrical stimulation of excitable tissue and stimulation energy is wasted in electrolyzing the aqueous phase of blood instead of carrying desirable charged species from one electrode to the other via the tissues. Because of the need for high energies in defibrillating the heart, higher currents are usually generated from the defibrillator than from a pacemaker. Under such circumstances, the efficiency of the electrode during high current generation is vital in not only reducing the defibrillation threshold, but in reducing the unwanted gas reactions or oxide formation. Excessive levels of gassing reactions may also lead to embolism in other vital organs such as the brain. Thus, stimulation electrodes should preferably allow a large charge flow across the electrode-tissue interface without the risk of irreversible faradaic reactions. Selection and design of the metal of the electrode is critical.
A metal of choice in electrode manufacturing has traditionally been titanium. On a fresh titanium surface, however, oxygen ions react with the titanium anode to form an oxide layer. Once a finite oxide thickness has been formed on the surface, polarization increases further. A point is reached when the oxygen ions reaching the surface of the titanium cannot be reduced further to form the oxide, and instead are reduced to elemental oxygen to form oxygen gas. The oxide film developed on the surface of a titanium electrode, either naturally or electrochemically, is irreversible. It cannot be reduced to the original metal by passing a charge in the reverse direction. Hence, it is clear that
Evanisko George R.
Intermedics Inc.
Schwegman Lundberg Woessner & Kluth P.A.
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