The present invention generally relates to an automatic implantable atrial defibrillator for delivering cardioverting or defibrillating electrical energy to the atria of a human heart. The present invention is more particularly directed to such an atrial defibrillator which has an intermittently activated pacing modality for assisting the heart in returning to normal sinus rhythm immediately following each attempted cardioversion of the atria. Because the pacing function is only activated for a finite period of time following each attempted cardioversion, the pacing function is provided without undue power consumption of a depletable power source, such as a battery, within the atrial defibrillator.
Atrial fibrillation is probably the most common cardiac arrhythmia. Although it is not usually a life threatening arrhythmia, it is associated with strokes thought to be caused by blood clots forming in areas of stagnant blood flow as a result of prolonged atrial fibrillation. In addition, patients afflicted with atrial fibrillation generally experience palpitations of the heart and may even experience dizziness or even loss of consciousness.
Atrial fibrillation occurs suddenly and many times can only be corrected by a discharge of electrical energy to the heart through the skin of the patient by way of an external defibrillator of the type well known in the art. This treatment is commonly referred to as synchronized cardioversion and, as its name implies, involves applying electrical defibrillating energy to the heart in synchronism with a detected electrical activation (R wave) of the heart. The treatment is very painful and, unfortunately, most often only results in temporary relief for patients, lasting but a few weeks.
Drugs are available for reducing the incidence of atrial fibrillation. However, these drugs have many side effects and many patients are resistant to them which greatly reduces their therapeutic effect.
Implantable atrial defibrillators have been proposed to provide patients suffering from occurrences of atrial fibrillation with relief. Unfortunately, to the detriment of such patients, none of these atrial defibrillators have become a commercial reality.
Implantable atrial defibrillators proposed in the past have exhibited a number of disadvantages which probably has been the cause of these defibrillators from becoming a commercial reality. Two such defibrillators, although represented as being implantable, were not fully automatic, requiring human interaction for cardioverting or defibrillating the heart. Both of these defibrillators require the patient to recognize the symptoms of atrial fibrillation with one defibrillator requiring a visit to a physician to activate the defibrillator and the other defibrillator requiring the patient to activate the defibrillator from external to the patient's skin with a magnet.
Implantable ventricular defibrillators for applying defibrillating electrical energy to the ventricles of the heart are well known and have been commercially available for a number of years. Because ventricular fibrillation is life threatening, resulting in unconsciousness in just a few seconds and leading to death in just a few minutes, implantable ventricular defibrillators are fully automatic for detecting ventricular fibrillation and quickly applying the defibrillating electrical energy to the ventricles. Such defibrillators are quite large in physical size as compared to the size of a pacemaker, for example, because of the rather large battery and storage capacitors required for providing defibrillating energies of ten joules of more. Due to their rather large size, these devices must be implanted in an abdominal region of the human body.
Any form of implantable device must be powered by a portable, depletable power source, such as a battery. When the battery is depleted of its energy, it is necessary to explant the device and implant a replacement. As a result, for an implantable device to be considered commercially viable, it is generally believed that the device should have a predicted lifetime of a number of years, such as five years, for example.
Predicted lifetimes of less than five years for ventricular defibrillators have not diminished the commercial nature of these devices because ventricular fibrillation is life threatening. However, since atrial fibrillation is not generally considered to be life threatening, it is generally believed that atrial defibrillators should have lifetimes on the order of five years to render such devices commercial in nature. To further enhance the commercial nature of these devices, it is desirable to limit the physical size of an atrial defibrillator to the size of a large pacemaker, for example, to permit the atrial defibrillator to be implanted, like a pacemaker, within the chest of the human body. While predicted lifetime and physical size have not adversely affected the commercial nature of ventricular defibrillators, such constraints have probably been the cause of an atrial defibrillator not being commercially available to date.
It has long been believed that as much electrical energy is required to cardiovert or defibrillate the atria of the heart as is required to cardiovert or defibrillate the ventricles of the heart, on the order of ten joules or more. Furthermore, episodes of atrial fibrillation occur much more frequently than do episodes of ventricular fibrillation. As a result, due to the contemplated required cardioverting or defibrillating energy levels for cardioverting or defibrillating the atria of the heart and the predicted required frequency of delivering such energies, it has long been believed that an implantable atrial defibrillator would either have an unreasonably short and commercially unacceptable predicted lifetime or a battery and storage capacitor of such a large size that the atrial defibrillator would be too large in physical size. Fortunately, a lead system has been discovered for an atrial defibrillator which greatly reduces the amount of energy required to defibrillate or cardiovert the atria. This lead system is fully described in U.S. Pat. No. 5,279,291 which issued on Jan. 18, 1994 for "Method for Atrial Defibrillation" which is assigned to the assignee of the present invention, and which is incorporated herein by reference. The lead system described in that patent includes a first electrode in the coronary sinus or great cardiac vein of the heart and a second electrode in the right atrium or superior vena cava of the heart. With such electrode placement, cardioverting energy applied to these electrodes is substantially confined to the atria, reducing the amount of energy required to cardiovert the atria to on the order of one joule or less.
It has also long been believed that an atrial defibrillator, like a ventricular defibrillator, should charge its storage capacitor quickly to permit essentially immediate cardioversion. Such quick storage capacitor charging places an extreme drain on the battery thereby further limiting the predicted lifetime of an implantable atrial defibrillator and further adding to the heretofore perceived non-commercial nature of these devices.
Recently, it has been recognized that, since atrial fibrillation is not life threatening, the storage capacitor of an atrial defibrillator need not be charged as quickly as the storage capacitor of a ventricular defibrillator. That recognition has led to another improvement in an atrial defibrillator fully described in U.S. Pat. No. 5,251,624 for "Pulse Generator for Use in an Implantable Atrial Defibrillator" which issued on Oct. 12, 1993, which is assigned to the assignee of the present invention and which is also incorporated herein by reference. The pulse generator described in that patent conserves battery power while still providing adequate electrical energy to cardiovert or defibrillate the atria of the heart to arrest atrial fibrillation. This is achieved by charging the storage capacitor comparatively slowly to minimize drain on the defibrillator battery but in sufficient time to arrest the atrial fibrillation. In accordance with the described preferred embodiment, this is accomplished by converting the rather low voltage of the battery to a pulsating high voltage of 300 to 400 volts, for example, with a flyback transformer having a primary winding coupled to an oscillator which provides the primary winding with a high frequency, low duty cycle input. By virtue of this arrangement, sufficient electrical energy for cardioverting or defibrillating the heart is stored in the storage capacitor without imposing a high drain on the defibrillator battery. Even though a minute may be required to fully charge the storage capacitor, this is sufficient to arrest the atrial fibrillation and bring comfort to the patient.
Further, as is well known in the art, the sinus node of the heart is the normal pacemaker of the heart and may be rendered dysfunctional by the application of cardioverting electrical energy to the atria. When such sinus node dysfunction occurs following an attempted cardioversion, the heart is caused to pause for a few seconds. It is also known that patients who suffer from atrial fibrillation may have an increased risk of sinus node dysfunction due to disease and/or drug therapy. Hence, it has been proposed in the past to provide an implantable atrial defibrillator with a demand ventricular pacing modality for pacing the ventricles when required.
Unfortunately, demand pacing requires the maintenance of a pacing output in readiness for pacing and the sensing of heart activity, and more particularly, R waves of the heart. This sensing is generally provided by one or more sense amplifiers and at least one R wave detector.
While sense amplifiers and R wave detectors used to sense heart activity and pacer output circuits are generally perceived as consuming little power when held in a fully biased readiness condition, the power consumed by these circuits under such conditions in a continuous manner over periods of months and years as is contemplated by the prior art, is considerable. Hence, the power consumed by continuously maintaining a demand pacing modality of an implantable atrial defibrillator in readiness is still another factor which limits the predicted lifetimes of these devices.
The present invention overcomes the power consumption problem of maintaining continuous demand pacing readiness in a fully automatic atrial defibrillator. This is accomplished by effectively providing power to the pacing circuitry for only a short finite time period following each attempted cardioversion. As a result, demand pacing is provided during those times when sinus node dysfunction may be present while conserving precious battery power. In fact, by practicing the present invention as described hereinafter, a twenty percent (20%) savings in battery power may be realized.