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 inhibits therapy delivery when the time span between detected atrial events exceeds a predetermined limit to avoid delivering therapy when the heart reverts to normal sinus rhythm or during a normal sinus rhythm cycle of the heart.
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 ventricular 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.
Early implantable atrial defibrillators were proposed to provide patients suffering from occurrences of atrial fibrillation with relief. Unfortunately, to the detriment of such patients, these early atrial defibrillators never became a commercial reality.
Two such proposed defibrillators, although represented as being implantable, were not fully automatic, requiring human interaction for cardioverting or defibrillating the heart. Both of these proposed defibrillators required the patient to recognize the symptoms of atrial fibrillation. One defibrillator required a visit to a physician for activation of the defibrillator and the other defibrillator required the patient to activate the defibrillator from external to the patient's skin with a magnet.
An improved atrial defibrillator which provides automatic operation is fully disclosed in U.S. Pat. No. 5,282,837 entitled "IMPROVED ATRIAL DEFIBRILLATOR AND METHOD," and which issued on Feb. 1, 1994 in the names of John M. Adams and Clifton A. Alferness. This patent is assigned to the assignee of the present invention and is incorporated herein by reference.
In addition to being automatic in operation, the atrial defibrillator of the above-referenced patent includes further features to assure the safe operation of the device. For example, to assure that the cardioverting electrical energy is not applied during the T wave vulnerable period of the heart, the atrial defibrillator provides R wave detection of increased reliability which is utilized to advantage in synchronizing the delivery of the cardioverting electrical energy to the atria with an R wave of the heart. Further, as another feature, an electrode system is utilized which minimizes the amount of energy that is required to cardiovert the atria. This is achieved by locating the cardioverting electrodes in or near the atria of the heart to provide a cardioverting energy path which confines substantially all of the cardioverting electrical energy to the atria of the heart.
Further improvements directed to the safe operation of an implantable automatic atrial defibrillator are described in U.S. Pat. No. 5,207,219 which issued on May 4, 1993 for "ATRIAL DEFIBRILLATOR AND METHOD FOR PROVIDING INTERVAL TIMING PRIOR TO CARDIOVERSION," and which is also assigned to the assignee of the present invention and incorporated herein by reference. The atrial defibrillator there disclosed provides an answer to the observation that during episodes of atrial fibrillation, the cardiac rate increases to a high rate and/or becomes extremely variable. At high or variable cardiac rates, the R wave of a cardiac cycle may become closely spaced from the T wave of the immediately preceding cardiac cycle. This creates a condition known in the art as an "R on T" condition which is believed to contribute to induced ventricular fibrillation if the atria are cardioverted in synchronism with the R wave close to the preceding T wave. In order to prevent cardioversion of the atria during an R on T condition, the atrial defibrillator described in U.S. Pat. No. 5,207,219 detects for a cardiac interval longer than a minimum interval prior to delivering the cardioverting electrical energy to the atria. This assures that the cardioverting electrical energy is not delivered during an R on T condition.
In addition to the foregoing, there is certain electrogram data related to atrial fibrillation detection and cardioversion from which the cardiologist would benefit. Such information includes electrograms of the heart during fibrillation to confirm proper operation of the atrial fibrillation detector, electrograms of the heart prior to cardioversion and electrograms of the heart from immediately prior to and ending after the deliverance of the cardioverting electrical energy to the atria to confirm that the application of the cardioverting electrical energy was synchronized with an R wave and not on a T wave and to also confirm that the cardioversion was successful by returning the heart to normal sinus rhythm. One such implantable automatic atrial defibrillator capable of storing such electrogram data for later recall by the cardiologist is fully disclosed in U.S. Pat. No. 5,522,850 which issued on Jun. 4, 1996 for "DEFIBRILLATION AND METHOD FOR CARDIOVERTING A HEART AND STORING RELATED ACTIVITY DATA," which is assigned to the assignee the present invention, and which is incorporated herein by reference.
One problem with storing electrogram data related to the cardioversion of atrial fibrillation is that during atrial fibrillation, the heart may spontaneously convert to normal sinus rhythm or do so for at least one or more cardiac cycles. Such a cardiac cycle would most likely satisfy electrical energy therapy criteria, including a minimum interval criteria. Hence, it is possible for the defibrillator to correctly detect the presence of atrial fibrillation and then deliver cardioverting energy on cycle which, when later reviewed by the cardiologist, would appear to be a normal sinus rhythm cardiac cycle.