BACKGROUND OF THE INVENTION 
   The present invention relates to implantable cardioverters/defibrillators generally, and more particularly to an implantable bi-atrial pacemaker/defibrillator with capability of atrial fibrillation reoccurrence prevention. 
   Early concepts of implantable defibrillators, such as those disclosed in Reissue Pat. No. 27,652 by Mirowski, et al, envision an electrode system employing a ventricular endocardial electrode and a plate electrode mounted to the heart directly, subcutaneously, or to the skin. However, it has long been recognized that a totally transvenous system would be desirable in order to simply the use of implantable defibrillators. One such system is suggested in Mirowski, et al U.S. Pat. No. 3,942,536, which discloses a transvenous lead having electrodes intended for location in the right ventricular apex and in the superior vena cava. This electrode system is disclosed as useful for either ventricular or atrial defibrillation. 
   While systems employing epicardial patch electrodes are workable, a thoracotomy is required in order to apply the epicardial electrodes. It is generally believed that it would be desirable to produce an implantable defibrillation system that entirely avoids the necessity of a thoracotomy. To this end, there has been substantial work directed towards development of such systems, as disclosed in Kallok U.S. Pat. No. 4,727,877, Tacker, et al, U.S. Pat. No. 4,708,145, and U.S. Pat. No. 5,014,696 by Mehra, for an “Endocardial Defibrillation Electrode System”. Other types of endocardial defibrillation electrodes are disclosed in Gold et al U.S. Pat. No. 4,481,953, Kinney, et al U.S. Pat. No. 4,161,952, Kiekhafer et al U.S. Pat. No. 4,934,049, and in U.S. Pat. No. 5,042,143 by Holleman, et al, for an “Method for Fabrication of Implantable Electrode”. The Kinney, Gold, Kiekhafer, and Holleman patents all disclose endocardial defibrillation leads employing defibrillation electrodes fabricated from elongated coils of biocompatible metal, mounted exposed to the exterior of the defibrillation lead, for placement in the right ventricle and other locations within the heart. The above-cited Mehra patent discloses a variety of endocardial defibrillation electrodes intended for use in the atrium, ventricle and coronary sinus, all of which employ electrodes in the form of elongated coils of conductive biocompatible metals. 
   Concurrent with the development of lead systems adapted to treat ventricular fibrillation, there has also been some work directed to the development of lead systems to treat atrial fibrillation. Synchronized cardioversion using two electrodes located on a lead located in the right atrium is disclosed in Charms U.S. Pat. No. 3,738,370. A later system is disclosed in Mirowski et al U.S. Pat. No. 3,952,750, employing one electrode in the atrium and presumably a second electrode at an unspecified location. 
   An electrode lead system specifically designed for atrial defibrillation is disclosed in the article “Elective Countershock in atrial Fibrillation With an Intracardiac Electrode—A Preliminary Report, by Jain, et al, published in the Journal of the Association of Physicians of India, Vol. 18, pp 821–824, 1970. The lead in the system was provided with a 10 mm silver electrode for location in the right atrium and was tested in conjunction with either a second electrode located in the right atrium or a second, cutaneous electrode located on the left side of the chest wall. A second electrode system specifically designed for use in atrial cardioversion is disclosed in the article “Safety and feasibility of transvenous cardioversion in atrial tachycardia”, by Blanc et al, published in Cardiac Pacing, edited by Gomez, Futura Pub. Co., 1985, pp 1526–1529. This electrode system employed a single lead with electrodes located in the atrium and pulmonary artery. 
   Atrial defibrillators are also known in the art. Specifically U.S. Pat. No. 5,165,403 “Defibrillation lead system and method of use” to Mehra describes a method of cardioverting the atrium of a human heart that includes insertion of first and second elongated electrodes tranvenously into the heart and associated vessels. One electrode is preferably located in the coronary sinus and great vein of the heart. The other electrode is preferably located in the vicinity of the atrium of the heart, spaced from the electrode located in the coronary sinus. In response to detection of fibrillation or in response to manual triggering, a defibrillation pulse is applied between the first and second electrodes to effect atrial cardioversion. Additionally, U.S. Pat. No. 5,433,729 “Atrial defibrillator, lead systems, and method” to Adams describes an implantable atrial defibrillator providing a pulse of defibrillating electrical energy to the atria of the heart in synchronism with sensed R waves in response to non-coincident sensing of an R wave at first and second areas of the heart. The defibrillating pulse is provided after a predetermined number of consecutive R waves are non-coincidently sensed to assure reliable synchronization. The atrial fibrillation detector of the defibrillator is normally disabled and is activated when the sensed ventricular activity indicates a probability of atrial fibrillation to conserve a depletable power source. A plurality of lead systems are also described for use with the atrial defibrillator which reduce the quantity of electrical energy required to defibrillate the heart and ensure that the delivered atrial defibrillating electrical energy is substantially confined to the atria of the heart. The &#39;403 and &#39;729 are incorporated by reference herein in their entireties. 
   In many patients, upon successful termination of atrial fibrillation by cardioversion and/or defibrillation, the immediate or near immediate reoccurrence of atrial fibrillation is a known problem. For example, Tse, et. al. describes a difference in p-wave width after low energy shocks in some patients in “Reoccurrence of Atrial Fibrillation after Successful Cardioversion with Transvenous Atrial Defibrillation is Associated with Prolonged P Wave”, European Journal of Cardiology, Pacing and Electrophysiology, Vol 6, No 1, Page 55, June 1996. The authors&#39; hypothesis is that some patients have a propensity to have a temporary increase in dispersion of Intra-Atrial Conduction Delay (IACD) that is an indicator of re-initiation of atrial fibrillation. 
   Standard defibrillators as described above (Mehra &#39;403 and Adams &#39;729) would re-detect the re-initiated fibrillation and re-shock the patient. If multiple re-initiations occurred, this would cause multiple fibrillation detections and treatments causing shortened defibrillator life, patient discomfort, and reduced quality of life. What is needed is a defibrillator that initiates preventive therapy after a successful cardioversion/defibrillation shock to prevent reoccurrence or re-initiation of fibrillation upon successful termination of an arrhythmic episode. 
   SUMMARY OF THE INVENTION 
   The present invention is directed toward the provision of an entirely endocardial defibrillation lead system particularly optimized for use in defibrillation or cardioversion of the atrium. The system disclosed includes coronary sinus, right atrial, and right ventricular electrodes, and may be embodied with the electrodes located on the same or different lead bodies. Additionally, bi-atrial pacing is incorporated to prevent the reoccurrence or re-initiation of atrial fibrillation after a successful cardioversion or defibrillation therapy. 
   According to the present invention, an implantable medical device for delivering a first therapy in response to variations in heart rhythm detected by electrodes inserted within a patient includes an amplifier generating p-waves in response to a signal between the electrodes, and a pacer timing/control circuit. In response to the first therapy being successful, the pacer timing/control circuit detects variations in subsequently generated p-waves, so that a second therapy is delivered in response to the detected variations in subsequently generated p-waves. According to a preferred embodiment of the present invention, the pacer timing/control circuit determines, subsequent to delivery of the second therapy, whether a width of a subsequent p-wave from the amplifier is greater than a predetermined threshold width, and delivery of the second therapy is repeated in response to the width of the p-wave being greater than the predetermined threshold width. According to another embodiment of the present invention, the pacer timing/control circuit determines, subsequent to delivery of the second therapy, whether a width of a p-wave is greater than a predetermined threshold width, and delivery of the second therapy is repeated in response to the width of the p-wave being greater than the predetermined threshold width. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by making reference to the following description, taken in conjunction with the accompanying drawings, in the several figures of which like reference numerals identify like elements, and wherein: 
       FIG. 1  illustrates a first embodiment of a cardioversion/defibrillation lead system according to the present invention employing a J-shaped atrial electrode located on a lead also provided with a pair of electrodes including a helical electrode for affixing the lead to the atrial wall and a separate lead carrying the coronary sinus electrode; 
       FIG. 2  is a schematic block diagram of a fully implantable atrial defibrillator embodying the present invention for applying defibrillating electrical energy to the atria of a human heart and which is shown in association with a human heart in need of atrial fibrillation monitoring and potential cardioversion of the atria; 
       FIG. 3A  is a graphical representation of a normal sinus rhythm as depicted on an electrocardiogram (ECG); 
       FIG. 3B  is a graphical representation of the effects of conduction delay on a P-wave of a waveform representation of a cardiac cycle; 
       FIG. 4  is a flow diagram illustrating the manner in which the atrial defibrillator of  FIG. 1  may be implemented in accordance with the present invention; a method and apparatus for determining the width of depolarization of the atrium and for providing pacing of the left atrium of the heart if they exceed a predetermined width; 
       FIG. 5  is an alternative flow diagram illustrating the manner in which the atrial defibrillator of  FIG. 1  may be implemented in accordance with the present invention; a method and apparatus for determining the width of depolarization of the atrium and for providing pacing of the left atrium of the heart if they exceed a predetermined width. 
   


   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  is a cutaway view of the human heart in which a lead system suitable for use in practicing the present invention has been implanted. The lead system includes three leads. A right atrial lead  101  includes an elongated insulative lead body  102 , to which a coiled defibrillation electrode  106  has been mounted. The distal end of the lead includes an electrode head  111 , carrying a helical electrode  110  and a ring electrode  108 . The electrodes  106 ,  108  and  110  are coupled to an implantable pacemaker/cardioverter/defibrillator  100  by means of conductors  116 ,  118  and  120 . Electrodes  108  and  110  are employed for cardiac pacing and sensing in the atrium. 
   A coronary sinus lead  103  also has an elongated insulative lead body  104  which carries a coiled defibrillation electrode at its distal end, located as generally indicated at  112 . Electrode  112  is located within the coronary sinus and the great cardiac vein. Preferably the proximal end of the electrode  112  is spaced about 3 to 8 cm from the opening of the coronary sinus into the right atrium. Electrode  112  is coupled to implantable pacemaker/cardioverter/defibrillator  100  by means of conductor  122 . Additionally, ring electrodes  129  and  131  located proximally to electrode  112 , are connected to pacemaker/cardioverter/defibrillator  100  via insulated conductors  133  and  135 , are used for left atrial sensing and pacing. 
   A right ventricular pacing lead  105  also includes an elongated insulative lead body  124 . The distal end of the lead  105  carries a tip electrode  126  and a ring electrode  128 . The electrodes  126  and  128  are coupled to implantable pacemaker/cardioverter/defibrillator  100  by means of conductors  132  and  134 . Electrodes  126  and  128  are used for cardiac pacing and sensing in the ventricle. Tines  130  anchor the distal end of the lead in the right ventricular apex. An additional electrode may be located on the housing of the device  100  and may be employed to accomplish pacing and sensing functions in conjunction with the other illustrated pacing and sensing electrodes. Although not illustrated, lead  124  may include an additional defibrillation electrode for location in the right ventricle. This ventricular defibrillation electrode may also be employed for atrial defibrillation. If the invention is embodied in a device capable of detecting ventricular tachyarrhythmias, such a ventricular defibrillation electrode would also be used for ventricular cardioversion and defibrillation. 
   Electrodes pairs  108 / 110 , and  129 / 131  and, optionally, electrodes  126  and  128  are used by pacemaker/cardioverter/defibrillator  100  to sense the electrical activity of the atrium and optionally the ventricle to diagnose the presence of atrial tachycardias or fibrillation requiring delivery of high voltage cardioversion or defibrillation pulses, respectively. In response to the detection of tachycardia or fibrillation, pacemaker/cardioverter/defibrillator  100  generates a high voltage pulse between electrodes  106  and  112 . Electrodes  108 / 110 ,  129 / 131  and  126 / 128 , along with an electrode located on the housing of the device  100 , may also be employed in order to deliver any of the various conventional pacing therapies, including pacing in single chamber (VVI, All, etc.) modes or dual chamber (VDD, DVI, DDD, DDI, etc) modes. 
   Electrodes  140 / 142 / 144  are electrodes mounted on the case of pacemaker/cardioverter/defibrillator  100  to sense a subcutaneous ECG as substantially described in U.S. patent application Ser. No. 09/721,275 filed Nov. 22, 2000 “System and Method for Deriving a Virtual ECG or EGM Signal” incorporated herein by reference in its entirety. Additionally, the subcutaneous electrode array sensing of ECG is described in “Electrogram Signals Recorded from Acute and Chronic Pacemaker Implantation Sites in Pacemaker Patients”, by Theres, et al, in PACE, Vol. 21, Jan. 1998, Part 1, pg 11–17. 
   In the embodiments disclosed in detail below, the pacemaker/cardioverter/defibrillator  100  operates in VVI pacing mode, using electrode  126  and an electrode mounted on the housing of the device for delivery of cardiac pacing pulses. Electrodes  128  and  126  are employed for sensing during VVI pacing. Electrode  128  and an electrode on the device housing may be employed for capture detection following delivery of overdrive pacing pulses during the synchronization sequence. The electrode on the housing may take the form of some or all of a conductive housing. Electrodes  108  and  110  are employed for atrial tachyarrhythmia detection and may be employed for atrial sensing and pacing if a dual chamber pacing mode is desired. 
     FIG. 2  is a functional schematic diagram of an implantable pacemaker/cardioverter/defibrillator in which the present invention may usefully be practiced. This diagram should be taken as exemplary of the type of device in which the invention may be embodied, and not as limiting, as it is believed that the invention may usefully be practiced in a wide variety of device implementations, including cardioverter and defibrillators, which do not provide anti-tachycardia pacing therapies. 
   The device is provided with an electrode system including electrodes as illustrated in  FIG. 1 , with the addition of electrode  107 , located on the housing of the implanted device. Electrode  112  is a defibrillation electrode located in the coronary sinus and great vein. Electrode  106  is a defibrillation electrode located in the right atrium and/or the superior vena cava. Both electrodes  112  and  106  are coupled to high voltage output circuit  234 . Electrodes  128  and  126  are located on or in the right ventricle and are coupled to the R-wave amplifier  200 , which preferably takes the form of an automatic gain controlled amplifier providing an adjustable sensing threshold as a function of the measured R-wave amplitude. A signal is generated on R-out line  202  whenever the signal sensed between electrodes  126  and  128  exceeds the present sensing threshold. 
   Electrodes  108  and  110  are located on or in the right atrium and are coupled to the P-wave amplifier  204 , which preferably also takes the form of an automatic gain controlled amplifier providing an adjustable sensing threshold as a function of the measured P-wave amplitude. A signal is generated on P-out line  206  whenever the signal sensed between electrodes  108  and  110  exceeds the present sensing threshold. The general operation of the R-wave and P-wave amplifiers  200  and  204  may correspond to that disclosed in allowed, commonly assigned, co-pending U.S. Pat. No. 5,117,824, by Keimel, et al., for an Apparatus for Monitoring Electrical Physiologic Signals, incorporated herein by reference in its entirety. 
   An alternate embodiment incorporates subcutaneous electrode array electrodes  140 / 142 / 144  that are connected via ADDR/DATA bus  218  to amplifier  146  to increase the signal level for conversion via A/D converter  222  as substantially described in U.S. patent application Ser. No. 09/736,640 filed Dec. 14, 2000, “Atrial Aware VVI—A Method for Atrial Synchronous Ventricular (VDD/R) Pacing Using the Subcutaneous Electrode Array and a Standard Pacing Lead” incorporated herein by reference in its entirety. The p-wave width of the converted signal is measured via a PACER TIMING and CONTROL circuit  212 , which includes various circuit elements, including timing circuitry  213  and control circuitry  215 . 
   Switch matrix  208  is used to select which of the available electrodes are coupled to wide band (0.5–200 Hz) amplifier  210  for use in digital signal analysis. Selection of electrodes is controlled by the microprocessor  224  via data/address bus  218 , which selections may be varied as desired. Signals from the electrodes selected for coupling to bandpass amplifier  210  are provided to multiplexer  220 , and thereafter converted to multi-bit digital signals by A/D converter  222 , for storage in random access memory  226  under control of direct memory access circuit  228 . Microprocessor  224  may employ digital signal analysis techniques to characterize the digitized signals stored in random access memory  226  to recognize and classify the patient&#39;s heart rhythm employing any of the numerous signal-processing methodologies known to the art. One representative example is U.S. Pat. No. 6,029,087 to Wohlgemuth, incorporated herein in its entirety by reference. 
   Switch matrix  208  is also used to select electrodes for use in performing the bi-atrial pacing function following delivery of cardioversion or defibrillation therapy pulses. Additionally, electrode  128  and electrode  107  located on the housing of the device may be so employed. Other electrode configurations are also easily selected. The signal from wide band amplifier  210 , as digitized by A/D converter  222  may be stored and analyzed by microprocessor  224  during the delay period. 
   Bi-atrial pacing of the present invention may be as described in U.S. Pat. Nos. 6,070,101, 6,081,748 and 6,122,545, all to Struble, et al and U.S. Pat. No. 5,902,324 to Thompson, et al all incorporated by reference in their entireties. The Struble &#39;101, &#39;748 and &#39;545 patents describe multi-chamber cardiac pacing systems for providing synchronous pacing to at least the two upper heart chambers or the two lower heart chambers or to three heart chambers or to all four heart chambers employing programmable conduction delay window (CDW) times timed out from paced and sensed events occurring in each heart chamber. The synchronous pacing of one of the right and left heart chambers is provided on demand following expiration of programmable pace and sense CDWs that are started by both a paced event and a sensed event first occurring in the other of the right and left heart chambers. The delivery of the pacing pulse is inhibited by a sensed event detected in the other of the right and left heart chambers before the expiration of the corresponding CDW. In a four channel atrial and ventricular pacing system, the right and left atrial chambers are sensed and paced as necessary upon at the end of a V-A escape interval and right and left AV delays are commenced for sensing ventricular depolarizations in the right and left ventricles. The four channel system is programmable to pace and sense in three selected heart chambers. 
   The Thompson &#39;324 patent describes a multi-chamber cardiac pacing system for providing synchronous pacing to at least the two upper heart chambers or the two lower heart chambers or to three heart chambers or to all four heart chambers employing one or more field density clamp (FDC) sense amplifiers for accurately sensing and timing cardiac depolarizations of the right and left heart chambers. 
   The remainder of the circuitry of  FIG. 2  is dedicated to the provision of cardiac pacing, cardioversion and defibrillation therapies, and, for purposes of the present invention may correspond to circuitry known in the prior art. The pacer timing/control circuitry  212  includes programmable digital counters which control the basic time intervals associated with DDD, VVI, DVI, VDD, AAI, DDI and other modes of single and dual chamber pacing well known to the art. Circuitry  212  also controls escape intervals associated with anti-tachyarrhythmia pacing in both the atrium and the ventricle, employing any anti-tachyarrhythmia pacing therapies known to the art. 
   Intervals defined by pacing circuitry  212  include atrial and ventricular pacing escape intervals, the refractory periods during which sensed P-waves and R-waves are ineffective to restart timing of the escape intervals and the pulse widths of the pacing pulses. The durations of these intervals are determined by microprocessor  224 , in response to stored data in memory  226  and are communicated to the pacing circuitry  212  via address/data bus  218 . Pacer circuitry  212  also determines the amplitude of the cardiac pacing pulses under control of microprocessor  224 . 
   During pacing, the escape interval counters within pacer timing/control circuitry  212  are reset upon sensing of R-waves and P-waves as indicated by a signals on lines  202  and  206 , and in accordance with the selected mode of pacing on timeout trigger generation of pacing pulses by pacer output circuitry  214  and  216 , which are coupled to electrodes  126  and  107 , for ventricular pacing and to electrodes  108  and  110  for atrial pacing. The escape interval counters are also reset on generation of pacing pulses, and thereby control the basic timing of cardiac pacing functions, including anti-tachyarrhythmia pacing. The durations of the intervals defined by the escape interval timers are determined by microprocessor  224 , via data/address bus  218 . The value of the count present in the escape interval counters when reset by sensed R-waves and P-waves may be used to measure the durations of R-R intervals, P-P intervals, P-R intervals and R-P intervals, which measurements are stored in memory  226  and used to detect the presence of tachyarrhythmias. 
   Microprocessor  224  operates as an interrupt driven device, and is awakened by interrupts from pacer timing/control circuitry  212  corresponding to the occurrence sensed P-waves and R-waves and corresponding to the generation of cardiac pacing pulses. These interrupts are provided via data/address bus  218 . Any necessary mathematical calculations to be performed by microprocessor  224  and any updating of the values or intervals controlled by pacer timing/control circuitry  212  take place following such interrupts. 
   For example, using pacer timing/control circuitry  212 , R-wave amplifier  206 , electrodes  107 ,  126  and  128 , and pacer output circuitry  216 , a pace or a sense interrupt is generated in response to a sensed or paced ventricular depolarization or R-wave, the intervals separating that R-wave from the immediately preceding R-wave, paced or sensed (R-R interval) and the interval separating the paced or sensed R-wave from the preceding atrial depolarization, paced or sensed (P-R interval), all of which may be stored. Similarly, using pacer timing/control circuitry  212 , P-wave amplifier  204 , electrodes  208  and  110 , and pacer output circuitry  214 , a pace or sense interrupt is generated in response to the occurrence of a sensed or paced atrial depolarization (P-wave), the intervals separating the sensed P-wave from the immediately preceding paced of sensed atrial contraction (P-P Interval) and the interval separating the sensed P-wave from the immediately preceding sensed or paced ventricular depolarization (R-P interval) may be stored. Preferably, a portion of the memory  226  is configured as a plurality of recirculating buffers, capable of holding a preceding series of measured intervals, which may be analyzed in response to the occurrence of a pace or sense interrupt to determine whether the patient&#39;s heart is presently exhibiting an atrial tachyarrhythmia. 
   Detection of atrial tachyarrhythmias, as employed in the present invention, may correspond to tachyarrhythmia detection algorithms known to the art. For example, presence of atrial tachyarrhythmia may be confirmed by means of detection of a sustained series of short P-P intervals of an average rate indicative of tachyarrhythmia or an unbroken series of short P-P intervals. The suddenness of onset of the detected high rates, the stability of the high rates, or a number of other factors known to the art may also be measured at this time. Appropriate detection methodologies measuring such factors are described in U.S. Pat. No. 4,726,380, issued to Vollmann, U.S. Pat. No. 4,880,005, issued to Pless et al and U.S. Pat. No. 4,830,006, issued to Haluska et al. Alternatively, the tachyarrhythmia analysis and recognition methodology disclosed in the article “Automatic Tachycardia Recognition” by Arzbaecher et al., published in Pace, May-June, 1984, pp. 541–547. However, one of the advantages of the present invention is that it is believed practicable in conjunction with all prior art atrial tachycardia detection algorithms. 
   The device also detects termination of atrial tachyarrhythmias, for example by detection of a sequence sequential P-P intervals greater than a preset duration, or by detection of a average atrial rate, over a series of p-waves, which is less than a preset value. Detection of termination is preferably operative during charging of the high voltage output capacitors  246  and  248 , as well as during the cardioversion or defibrillation synchronization sequence according to the present invention. 
   In the event that an atrial tachyarrhythmia is detected, and an anti-tachyarrhythmia pacing regimen is desired, appropriate timing intervals for controlling generation of anti-tachyarrhythmia pacing therapies are loaded from microprocessor  224  into the pacer timing and control circuitry  212 , to control the operation of the escape interval counters therein and to define refractory periods during which detection of R-waves and P-waves is ineffective to restart the escape interval counters. 
   Alternatively, circuitry for controlling the timing and generation of antitachycardia pacing pulses as described in U.S. Pat. No. 4,577,633, issued to Berkovits et al on Mar. 25, 1986, U.S. Pat. No. 4,880,005, issued to Pless et al on Nov. 14, 1989, U.S. Pat. No. 7,726,380, issued to Vollmann et al on Feb. 23, 1988 and U.S. Pat. No. 4,587,970, issued to Holley et al on May 13, 1986, all of which are incorporated herein by reference in their entireties may also be used. 
   In the event that generation of a cardioversion or defibrillation pulse is required, microprocessor  224  employs the counters in circuitry  212  to control timing of such cardioversion and defibrillation pulses, as well as associated refractory periods. In response to the detection of an atrial tachyarrhythmia requiring a cardioversion or defibrillation pulse, microprocessor  224  activates cardioversion/defibrillation control circuitry  230 , which initiates charging of the high voltage capacitors  246  and  248  via charging circuit  236 , under control of high voltage charging control line  240 . The voltage on the high voltage capacitors is monitored via VCAP line  244 , which is passed through multiplexer  220  and in response to reaching a predetermined value set by microprocessor  224 , results in generation of a logic signal on Cap Full (CF) line  254 , terminating charging. Thereafter, timing of the delivery of the defibrillation or cardioversion pulse is controlled by pacer timing/control circuitry  212 . Following delivery of the fibrillation or tachycardia therapy the microprocessor then returns the device to pacing and awaits the next successive interrupt due to pacing or the occurrence of a sensed atrial or ventricular depolarization. 
     FIG. 3A  is a graphical representation of a normal sinus rhythm as depicted on an electrocardiogram (ECG). As illustrated in  FIG. 3A , a normal cardiac cycle is a repetitive waveform characterized by a periodic PQRST electrical activation sequence of the upper and lower heart chambers. The PQRST sequence is associated with the sequential depolarization and contraction of the atria followed by the depolarization and contraction of the ventricles, and successive PQRST complexes are separated by a baseline or isoelectric region. The PQRST electrical activation sequence commences with a P-wave  260  indicative of the depolarization and contraction of the atria. In a normal sinus rhythm, P-wave  260  has a rounded shape which does not include notches or peaks and is typically less than or equal to 80 msec in width. P-wave  260  is followed by a QRS complex, indicative of the depolarization and contraction of the ventricles, and which includes a Q-wave  264 , an R-wave  266  and an S-wave  268 . Q-wave  264  and S-wave  268  represent negative (downward) deflections, while R-wave  266  represents positive (upward) deflections. Finally, QRS complex  262  is followed by a rounded asymmetrically shaped T-wave  270 , which results from ventricular repolarization, and a U-wave  272 , which follows and has the same polarity as T-wave  270  and tends to become more evident with hypokalemia, bradycardia, and age. 
     FIG. 3B  is a graphical representation of the effects of conduction delay on a P-wave of a waveform representation of a cardiac cycle. As illustrated in  FIG. 3B , when dispersion of intra-atrial conduction delay increases, notches  280  tend to be formed on P-wave  260 , and a width  282  of P-wave  260  tends to increase. The present invention utilizes these tendencies in the morphology of P-wave  260  resulting from increased conduction delay, indicating potential spontaneous re-initiation of atrial fibrillation, to temporarily initiate bi-atrial pacing to prevent the immediate or near immediate reoccurrence of atrial fibrillation subsequent to a successful termination of atrial fibrillation by cardioversion and/or defibrillation, as will be described below. 
     FIG. 4  is a functional flowchart illustrating the overall operation of the device according to the present invention. The flowchart begins at step  302 . Sense amp  204  senses p-waves at step  304 , and using the sensed p-waves from p-wave amplifier  204 , pacer timing/control circuitry  212  detects the presence of atrial tachyarrhythmia at step  306  by sensing a sustained series of short intervals between the sensed p-waves, i.e., sustained short P-P intervals, less than an average rate. For example, pacer timing/control circuit  212  detects that atrial tachyarrhythmia is present when sustained short P-P intervals less than 500 msec. are sensed, or when an unbroken series of short P-P intervals is detected. In addition, the suddenness of onset of the detected high rates, the stability of the high rates, or a number of other factors known to the art may also be measured by pacer timing/control circuit  212 . 
   If an arrhythmia is detected, therapy is provided at step  308  via output circuit  234 , HV charge circuit  236 , and CV/defib control  230 . If no arrhythmia is detected, pacer timing/control circuitry  212  continues to monitor intrinsic p-waves from p-wave amplifier  204 . If a therapy is delivered at step  308 , a determination is made by pacer timing/control circuitry  212  of the success of the therapy delivered at step  310 , for example, by detecting a sequence of sequential P-P intervals from p-wave amplifier  204  greater than a preset duration, or by detection of an average atrial rate, over a series of the received p-waves, which is less than a preset value. It is understood that according to the present invention, any method may be used for detecting the presence of atrial tachyarrthythmia and for detecting the success of a delivered therapy in steps  306  and  308 , respectively. 
   If not successful, the therapy is retried or increased to a higher level as is well known in the art. If the therapy is successful, pacer timing/control circuit  212  continues to sense p-waves from amplifier  204  at step  312 . Pacer timing/control circuit  212  then determines, using the methods for detecting the presence of atrial tachyarrhythmia described in step  306 , whether atrial tachyarrhythmia is detected in step  313 . If an arrhythmia is detected in step  313 , the process returns to step  308  and a therapy is provided. If an arrhythmia is not detected in step  313 , the intrinsic p-wave widths of the p-waves from amplifier  204  are compared to a preset, or pre-programmed threshold width by pacer timing and control circuit  212  at step  314 . This threshold width may be a preset or programmed msec. value (e.g., 10–50 msec) over the normal intrinsic width of a p-wave, i.e., 80 msec., or, alternatively, may be of a proportionately greater measure over a preset width of a p-wave, such as a preset or programmed percent (e.g., 5–20%) increase over the normal intrinsic width of a p-wave. If the measured p-wave widths do not exceed the threshold width, the process is completed at step  320 . However, if at step  314 , the threshold width is exceeded, bi-atrial pacing begins via A pace  214  and switch matrix  208  at a preset or pre-programmed delay at step  316 . Bi-atrial pacing continues for a preset, programmed time (e.g., for 1–5 minutes) via timer  212  at step  318  with the process again completing at step  320 . 
   Turning now to  FIG. 5 , an alternative embodiment of the functional flowchart of  FIG. 4  is shown. The flowchart begins at step  402 . Sense amp  204  senses p-waves at step  404 , and using the sensed p-waves from p-wave amplifier  204 , pacer timing/control circuitry  212  detects the presence of atrial tachyarrhythmia at step  406  by sensing a sustained series of short intervals between the sensed p-waves, i.e., sustained short P-P intervals less than an average rate. For example, pacer timing/control circuit  212  detects that atrial tachyarrhythmia is present when sustained short P-P intervals less than 500 msec. are detected, or when an unbroken series of short P-P intervals is detected. In addition, the suddenness of onset of the detected high rates, the stability of the high rates, or a number of other factors known to the art may also be measured and utilized to detect the presence of atrial tachyarrhythmia in step  406 . 
   If an arrhythmia is detected by pacer timing/control circuitry  212  at step  406 , therapy is provided at step  408  via output circuit  234 , RV charge circuit  236 , and CV/defib control  230 . If no arrhythmia is detected, pacer timing/control circuitry  212  continues to monitor intrinsic p-waves from p-wave amplifier  204 . If a therapy is delivered at step  408 , a determination of the success of the therapy is made by pacer timing/control circuitry  212  at step  410  by detecting a sequence of sequential P-P intervals from p-wave amplifier  204  greater than a preset duration, for example, or by detection of an average atrial rate over a series of the received p-waves which is less than a preset value. It is understood that, according to the present invention, any method may be used for detecting the presence atrial tachyarrhythmia and for detecting the success of a delivered therapy in steps  406  and  410 , respectively. 
   If pacer timing/control circuitry  212  determines in step  410  that the therapy is not successful, the therapy is retried or increased to a higher level as is well known in the art. If the therapy is successful, pacer timing/control circuit  212  continues to monitor p-waves from amplifier  204  at step  412 . Pacer timing/control circuit  212  then determines, using the methods for detecting the presence of atrial tachyarrhythmia described in step  406 , whether atrial tachyarrhythmia is detected in step  413 . If an arrhythmia is detected in step  413 , the process returns to step  408  and a therapy is provided. If an arrhythmia is not detected in step  413 , the intrinsic p-wave widths of the p-waves from amplifier  204  are compared to a preset or pre-programmed threshold width by pacer timing and control circuit  212  at step  414 . If the intrinsic p-wave widths do not exceed the threshold, the process is completed at step  420 . However, if at step  414 , the threshold width is exceeded, bi-atrial pacing begins via A pace  214  and switch matrix  208  at a pre-programmed delay at step  416 . According to the present invention, bi-atrial pacing continues for a preset time (e.g., 1–5 minutes) by timing circuitry  213  of pacer timing/control circuit  212  whereby the p-wave width is measured and again compared to the preset or pre-programmed threshold width at step  418 . This threshold width may be a preset or programmed msec value (e.g., 10–50 msec) over the normal intrinsic width of a p-wave, i.e., 80 msec., or, alternatively, may be of a proportionately greater measure over a normal width of a p-wave, such as a preset or programmed percent (e.g., 5–20%) increase over the normal intrinsic width of a p-wave. Once the measured p-wave width is determined to be below the threshold width, the process terminates at step  420 . If, on the other hand, the measured p-wave width is determined to be greater than the preset or pre-programmed width threshold width in step  418 , timing circuit  213  of pacer timing control circuit  212  is reset and bi-atrial pacing continues at step  416 .In this way, according to the present invention, once bi-atrial pacing is initially delivered for the preset time period, if the width of the p-wave continues to be greater than the threshold width, indicating a continued propensity for re-initiation of atrial fibrillation, delivery of bi-atrial pacing is repeated for the preset time period. As a result, the bi-atrial pacing is continued until this propensity for re-initiation is reduced. 
   It is understood that, while the present invention has been described using electrodes  108  and  110 , place on or within the right atrium and connected to p-wave amplifier  204 , to generate p-waves, the present invention is not limited use of electrodes  108  and  110  placed on within the right atrium. Rather, it is envisioned that the present invention could utilize other methods for sensing, recording and acquiring electrocardiographic data (ECG) and tracings from an implanted medical device. For example, the present invention can be utilized using subcutaneous electrode array electrodes  140 ,  142  and  144 , with pacer timing/control circuit  212  comparing p-waves received from electrodes  140 ,  142  and  144  and comparing the p-waves to the threshold to determine whether to initiate bi-atrial pacing subsequent to a successfully delivered therapy, as described above. 
   A method of collecting ECG tracings from a set of subcutaneous electrodes, or a subcutaneous electrode array (SEA), has been described in U.S. Pat. No. 5,331,966 to Bennett, incorporated herein by reference, which discloses a method and apparatus for providing an enhanced capability of detecting and gathering electrical cardiac signals via an array of relatively closely spaced subcutaneous electrodes (located on the body of an implanted device). More recently, a patent application entitled “ Surround Shroud Connector and Electrode Housings for a Subcutaneous Electrode Array and Leadless ECGs ”, by Ceballos, et al., filed on Oct. 26, 2000, Ser. No. 09/697,438, incorporated herein by reference in its totality, discloses an alternate method and apparatus for detecting electrical cardiac signals via an array of subcutaneous electrodes located on a shroud circumferentially placed on the perimeter of an implanted pacemaker. Similarly, a patent application entitled “ Subcutaneous Electrode for Sensing Electrical Signals of the Heart ”, by Brabec et al, filed Oct. 31, 2000, Ser. No. 09/703,152, incorporated herein by reference in its totality, discloses the use of a spiral electrode using in conjunction with the shroud described in the Ceballos et al disclosure. In addition, two applications, entitled “ Multilayer Ceramic Electrodes for Sensing Cardiac Depolarization Signals ”, by Guck et al, filed Oct. 25, 2000, Ser. No. 09/696,365 and “ Thin Film electrodes for Sensing Cardiac Depolarization Signals ” by Guck and Donders, filed Dec. 13, 2000, Ser. No. 09/736,046, both incorporated herein by reference in their totality, disclosed the use of sensing electrodes placed into recesses incorporated along and into the peripheral edge of the implantable pacemaker. Finally, the submission entitled, “ Subcutaneous Electrode Array Virtual ECG Lead ” by Panken and Reinke, filed Nov. 22, 2000, Ser. No. 09/721,275, also incorporated by reference to its entirety, describes the algorithm used by the implanted device that compiles the ECG from any two subcutaneous electrodes found in the SEA. The preceding specific embodiments are illustrative of the practice of the invention. It is to be understood, therefore, that other expedients known to those of skill in the art or disclosed herein may be employed. In the following claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures. For example, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw are equivalent structures. It is therefore to be understood, that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described without actually departing from the spirit and scope of the present invention.
A method and apparatus for cardioverting the atrium of a human heart that includes insertion of first and second elongated electrodes tranvenously into the heart and associated vessels. One electrode is preferably located in the coronary sinus and great vein of the heart. The other electrode is preferably located in the vicinity of the right atrium of the heart, spaced from the electrode located in the coronary sinus. In response to detection of fibrillation or in response to manual triggering, a defibrillation pulse is applied between the first and second electrodes to effect atrial cardioversion. Further, after delivery of a successful defibrillation shock, the width of intrinsic p-waves are monitored and bi-atrial pacing is temporarily initiated if the width exceeds a preset or programmable threshold.