Abstract:
An implantable ventricular cardioverter/defibrillator applies a quantity of electrical energy to a heart for terminating a ventricular arrhythmia while preventing the induction of atrial fibrillation. The cardioverter/defibrillator includes a ventricular arrhythmia detector and an atrial pacer that delivers atrial pacing pulses to an atrium of the heart when the ventricular arrhythmia detector detects a ventricular arrhythmia. A generator applies a quantity of electrical energy to the heart in timed relation to a delivered atrial pacing pulse to terminate the ventricular arrhythmia to avoid inducing atrial fibrillation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 09/452,493, filed Dec. 1, 1999, titled “Implantable Ventricular Cardioverter/Defibrillator Employing Atrial Pacing for Preventing Atrial Fibrillation from Ventricular Cardioversion and Defibrillation Shocks,” which has now matured to U.S. Pat. No. 6,442,426 B1. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to an implantable ventricular cardioverter/defibrillator. The present invention more particularly relates to such a cardioverter/defibrillator which employs atrial pacing for preventing the induction of atrial fibrillation from ventricular cardioversion and defibrillation shocks. 
     BACKGROUND OF THE INVENTION 
     There is an increasing problem with ventricular defibrillation and cardioversion shocks causing atrial fibrillation. This is due to the tendency towards the use of a “single-pass” lead, the use of a “hot can” and the progressive decreasing of energy requirements for ventricular cardioversion and defibrillation. The single-pass lead, of the type known in the art, includes an atrial shock coil for positioning in the right atrium and a ventricular shock coil for positioning in the right ventricle. Hot can usage encompasses the use of the electrically conductive device enclosure as a common electrode wherein the cardioversion or defibrillation shocks are delivered from the atrial and ventricular shock coils to the electrically conductive device enclosure. Prior to the hot can approach, a subcutaneous patch electrode was used as the common electrode. With the single pass lead, the hot can approach causes more current flow through the atria than the subcutaneous patch electrode approach. The higher current though the atria can increase the probability of atrial fibrillation induction as a result of ventricular cardioversion and defibrillation 
     Extremely high-energy shocks cardiovert or defibrillate the entire heart so as to cardiovert or defibrillate both the atria and the ventricles to thus preclude induction of atrial fibrillation during ventricular cardioversion or defibrillation. However, the modern trend is to employ more moderate energy shocks for ventricular cardioversion and defibrillation. These energy levels may not cardiovert or defibrillate the atria during ventricular cardioversion and defibrillation thus frequently the atria is in fibrillation after ventricular cardioversion or defibrillation. 
     The induction of atrial fibrillation by ventricular arrhythmia shock therapy causes a cascading sequence of unfortunate problems. The delivery of the ventricular shock usually occurs during a period of patient unconsciousness and is not felt. However, after atrial fibrillation is induced, the patient is left with significant anxiety that there is still an arrhythmia. This can lead to inappropriate decisions on the part of the patient, as well as the implantable ventricular cardioverter/defibrillator. For example, the implanted device can mistake the atrial fibrillation for a ventricular arrhythmia and thus cause another shock to be delivered to the patient. This second shock is often extremely painful, because the patient will now be conscious. The second delivered shock, moreover, will most likely merely serve to ensure that the patient remains in atrial fibrillation. 
     SUMMARY OF THE INVENTION 
     The invention provides an implantable ventricular cardioverter/defibrillator for applying a quantity of electrical energy to a heart for terminating a ventricular arrhythmia while preventing the induction of atrial fibrillation. The cardioverter/defibrillator includes a ventricular arrhythmia detector that detects a ventricular arrhythmia, an atrial pacer that delivers atrial pacing pulses to an atrium of the heart responsive to the ventricular arrhythmia detector detecting a ventricular arrhythmia, and a generator that applies the electrical energy to the heart in timed relation to an atrial pacing pulse delivered by the atrial pacing means. The relative timing of the generator application and the atrial pacing pulse prevents the induction of atrial fibrillation. 
     The implantable ventricular cardioverter-defibrillator may further include a synchronizer that synchronizes the application of the electrical energy to the heart by the generator with a delivered atrial pacing pulse. 
     In accordance with a further aspect of the present invention, the ventricular arrhythmia detector may include a ventricular fibrillation detector and a ventricular tachycardia detector. As a further aspect of the present invention, a ventricular antitachycardia pacer, responsive to the ventricular tachycardia detector detecting ventricular tachycardia of the heart, applies antitachycardia pacing pulses to a ventricle of the heart. An analyzer determines if the antitachycardia pacing pulses terminate the detected ventricular tachycardia. If the ventricular tachycardia is not terminated by the antitachycardia pacing, the analyzer activates the atrial pacer and generator. 
     In accordance with a further aspect of the present invention, the implantable ventricular cardioverter/defibrillator may include a ventricular activation detector adapted to detect ventricular activations of the heart. The atrial pacer, responsive to the ventricular activation detector, delivers the atrial pacing pulses synchronized to detected ventricular activations. 
     In accordance with a further aspect of the present invention, if the ventricular fibrillation detector detects ventricular fibrillation, the atrial pacer delivers the atrial pacing pulses at a substantially fixed rate. Further, the generator may include a storage capacitor and a charger that begins charging the storage capacitor to a given energy upon the detection of ventricular fibrillation and the generator applies the electrical energy to the heart from the storage capacitor when the storage capacitor is charged to the given energy. 
     The invention further provides a method of terminating a ventricular arrhythmia of a heart while preventing the induction of atrial fibrillation. The method includes the steps of detecting a ventricular arrhythmia, delivering atrial pacing pulses to an atrium of the heart upon detecting a ventricular arrhythmia, and applying a quantity of electrical energy to the heart in timed relation to a delivered atrial pacing pulse. 
    
    
     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 characters identify identical elements, and wherein: 
     FIG. 1 is a schematic illustration of a human heart in need of ventricular arrhythmia cardioversion/defibrillation shown in association with an implantable ventricular cardioverter/defibrillator embodying the present invention; 
     FIG. 2 is a block diagram of the implantable ventricular cardioverter/defibrillator of FIG. 1; and 
     FIG. 3 is flow diagram illustrating operative steps that the device of FIGS. 1 and 2 may implement in accordance with a preferred embodiment of the present invention for providing ventricular arrhythmia therapy while preventing atrial fibrillation induction. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 1, it illustrates a heart  10  in need of ventricular arrhythmia cardioversion/defibrillation and an associated implantable ventricular cardioverter/defibrillator  30  embodying the present invention. The portions of the heart  10  illustrated in FIG. 1 are the right ventricle  12 , the left ventricle  14 , the right atrium  16  and the left atrium  18 . Also illustrated are the superior vena cava  20  and inferior vena cava  27 . As is well known in the art, the cardioverter/defibrillator  30  is arranged to be implanted in an upper left chest portion of a patient within a subcutaneous pocket. 
     The implantable device  30  includes a first endocardial lead  32  which is of the “single-pass” type. To that end, the lead  32  includes a first shock coil  34  arranged to be disposed within the right ventricle  12 , a second shock coil  36  proximal to the shock coil  34  and arranged to be disposed within the right atrium  16  or superior vena cava  20 , and a distal tip pacing electrode  38 . The implantable device  30  further includes a second endocardial lead  42  having an electrode pair including a distal electrode  44  and a proximal electrode  46 . 
     The implantable cardioverter/defibrillator  30  includes a hermetically sealed, electrically conductive enclosure  50 . When a quantity of cardioverting or defibrillating electrical energy is applied to the heart  10 , in accordance with this preferred embodiment, the electrodes  50  and  36  are connected in parallel and the quantity of electrical energy is applied between the parallel connected electrodes  50  and  36  and the electrode  34 . Alternatively, the cardioverting and defibrillating quantity of electrical energy may be applied between electrode  34  and the electrically conductive enclosure  50  without employing electrode  36 . All such cardioverting and defibrillating methodologies apply cardioverting and defibrillating electrical energy to the heart and are thus deemed to be alternative structures and methods for practicing the present invention. Electrodes  44  and  46  of lead  42  support sensing of right atrial electrical activity and, in accordance with the present invention, delivery of atrial pacing pulses to the right atrium  16 . 
     As illustrated in FIG. 2, the implantable cardioverter/defibrillator  30  includes within the enclosure  50  a ventricular sense channel  52 , an atrial sense channel  62 , and a pacing pulse generator  70  including a first or atrial pacing pulse generator  72  for providing atrial pacing pulses and a second or ventricle pacing pulse generator  74  for providing ventricle pacing pulses. The device  30  further includes a microprocessor  80 , a memory  110 , and a telemetry stage  150 . The device  30  still further includes a cardioversion/defibrillation generator  166  including a charging circuit  160 , a storage capacitor  162  and a switch  164 . 
     The ventricular sense channel  52  includes a sense amplifier  54  and a threshold detector  56 . The sense amplifier  54  has an input coupled to electrode  38  of lead  32  by a conductor  138  of the lead  32 . The sense amplifier  54  has another input which is coupled to electrode  34  of lead  32  by another conductor  134  of the lead  32 . The sense amplifier  54  further includes an output which forms an input to the threshold detector  56 . As further illustrated, the threshold detector  56  has an output which is coupled to the microprocessor  80 . 
     The sense amplifier  54 , together with electrodes  38  and  34  sense electrical activity in the right ventricle  12 . When the output from the amplifier  54  transitions through a programmed threshold of the threshold detector  56 , the threshold detector  56  provides an input signal to the microprocessor  80  indicating that a ventricular activation or R-wave has been detected. Such detection is well known in the art. 
     Similarly, the atrial sense channel  62  includes a sense amplifier  64  and a threshold detector  66 . The sense amplifier  64  has an input which is coupled to electrode  44  of lead  42  by a conductor  144  of lead  42 . The sense amplifier  64  has another input which is coupled to electrode  46  of lead  42  by another conductor  146  of lead  42 . As further illustrated, the sense amplifier has an output which forms an input to the threshold detector  66  and the threshold detector  66  has an output which is coupled to the microprocessor  80 . 
     The sense amplifier  64 , together with electrodes  44  and  46 , sense electrical activity in the right atrium. When the output of the sense amplifier  64  transitions through a programmed threshold of the threshold detector  66 , the threshold detector  66  provides an input signal to the microprocessor  80  indicating that an atrial activation or P-wave has been detected. Again, such detection is also well known in the art. 
     The first or atrial pulse generator  72  has outputs coupled to electrodes  44  and  46  of lead  42  by conductors  144  and  146  respectively of lead  42 . This permits atrial pacing pulses produced by the atrial pacer  72  to be applied to the right atrium  16 . The second or ventricular pulse generator  74  has outputs coupled to electrodes  34  and  38  of lead  32  by conductors  134  and  138  respectively of lead  32 . This permits ventricular pacing pulses produced by the ventricular pacer  74  to be applied to the right ventricle  12 . 
     The cardioversion/defibrillation generator  166  applies a quantity of cardioverting or defibrillating electrical energy to the heart  10 . To that end, the charging circuit  160  charges the storage capacitor  162  with the quantity of electrical energy to be applied to the heart. The switch  164  applies the quantity of electrical energy from the storage capacitor  162  to the heart. As can be seen in FIG. 2, the switch has an output coupled to electrode  34  of lead  32  by the conductor  134  of lead  32  and another output which is coupled to electrode  36  by a conductor  136  of lead  32 . Also, another output of the switch  164  is coupled to the electrically conductive enclosure  50 . As a result, when cardioverting and defibrillating electrical energy is applied to the heart  10 , the electrodes  50  and  36  may be coupled in parallel such that the quantity of electrical energy applied to the heart  10  is applied between the parallel coupled electrodes  50  and  36  and the electrode  34 . 
     The microprocessor  80  controls the overall functioning of the implantable cardioverter/defibrillator  30 . To implement such control, the microprocessor executes operating instructions stored in the memory  110  and utilizes various parameters also stored in memory  110 . For example, the memory  110  stores the operating instructions defining the various pacing modalities which may be provided by the device  30  in a storage location  112 . Pacing parameters may be stored in a storage location  114  and detection parameters for both pacing and cardioversion and defibrillation may be stored in storage location  116 . Lastly, the operating instructions defining the therapy to be provided for ventricular cardioversion and defibrillation may be stored in a storage location  118 . 
     The telemetry stage  150  permits pacing mode selections and storage of pacing, detection, and cardioversion/defibrillation parameters in the memory  110  to be made through the use of an external programmer (not shown) of the type well known in the art. The telemetry stage includes a receiver  152  which receives telemetry commands including mode selection and parameter commands from the programmer. The receiver  152  conveys the commands to the microprocessor  80  which then stores them in the memory  110 . The telemetry stage  150  also includes a transmitter  154 . The transmitter may be used for transmitting data to the programmer. The transmitted data may include sensed electrograms or status information, for example, as is well known in the art. 
     The microprocessor  80  is coupled to the memory  100  by a multiple-bit address bus  120  and a bi-directional, multiple-bit data bus  122 . The microprocessor  80  uses the address bus  120  to fetch operating instructions or programmable parameters from the memory at address locations defined on the address bus  120 . The fetched instructions and parameters are conveyed to the microprocessor  80  over the data bus  122 . Similarly, the microprocessor  80  may store data in the memory  110  at memory locations defined on the address bus  120 . The microprocessor  80  conveys the data to the memory over the data bus  122 . Such microprocessor and memory operation are conventional in the art. 
     When executing the operating instructions stored in memory  110 , the microprocessor  80  implements a number of functional stages in accordance with the present invention. Those stages include an arrhythmia detector  82  which detects the presence of a ventricular arrhythmia and which may further include a ventricular tachycardia detector  84  and a ventricular fibrillation detector  86 . As will be seen hereinafter, when the arrhythmia detector  82  detects a ventricular arrhythmia, the ventricular tachycardia  84  and ventricular fibrillation detector  86  determine whether the arrhythmia is a ventricular tachycardia or a ventricular fibrillation respectively. The functional stages of microprocessor  80  further include an antitachycardia pacing analyzer stage  88 , an averaging stage  90 , and an antitachycardia pacing control stage  92 . The functional stages of microprocessor  80  still further include an atrial pace control  94 , an autocapture stage  96 , a synchronizing stage  98 , and a charge control stage  100 . 
     In accordance with a primary aspect of the present invention, when the heart  10  is to receive a quantity of cardioverting or defibrillating electrical energy, the atrial pace control  94  causes the atrial pacer  72  to apply atrial pacing pulses to the atria and, more specifically, the right atrium  16 . If the ventricular arrhythmia is ventricular tachycardia, the atrial pacing pulses are applied at one-half the ventricular rate and are synchronized to every other R-wave detected by the ventricular sense channel  52 . If the ventricular arrhythmia is ventricular fibrillation, the atrial pacing pulses are applied by the atrial pacer  72  at a fixed rate such as, for example, 120 beats per minute. For either ventricular tachycardia or ventricular fibrillation therapy, when the required quantity of electrical energy is to be applied to heart  10 , the synchronizing stage  98  operates the switch  164  for applying the cardioverting or defibrillating electrical energy to the heart synchronized to an atrial pacing pulse. In this manner, atrial fibrillation is prevented even though the cardioverting or defibrillating electrical energy is applied between the electrically conductive enclosure  50  of the device  30  (in parallel with the right atrial shock coil  36 ) and the right ventricular shock coil  34 . 
     The foregoing and other aspects of the present invention may best be appreciated by making reference to FIG. 3 which is a flow diagram illustrating a preferred implementation of the present invention. Therapy to the heart  10  begins at step  200  when the arrhythmia detector  82  detects a ventricular arrhythmia. The arrhythmia detection criteria may be based upon ventricular rate, for example, or other criteria well known in the art. 
     When a ventricular arrhythmia is detected in step  200 , the arrhythmia detector  82  causes a ventricular tachycardia detector  84  and ventricular fibrillation  86  to determine the nature of the ventricular arrhythmia in step  202 . If the arrhythmia is ventricular tachycardia, in step  204 , the antitachycardia pacing control  92  causes the ventricular pacer  74  to apply antitachycardia pacing pulses to electrodes  38  and  34  of lead  32 . After a period of, for example, 10-20 seconds, the process continues to step  206  wherein the antitachycardia pacing analyzer  88  determines if the antitachycardia pacing was successful. The criteria utilized in this step  206  may include intrinsic ventricular rate. If the intrinsic ventricular rate of the heart  10  is within normal limits, the antitachycardia pacing will be considered successful and the process returns. If however it is determined that the antitachycardia pacing was not successful, the process then moves to step  208  wherein atrial pacing is initiated. In accordance with this preferred embodiment, the atrial pace control  94  causes the atrial pacer  72  to pace the right atrium  16  at one-half the ventricular rate. The pacing pulses are synchronized to every other detected R-wave. During the atrial pacing, the charge control  100  causes the charger  160  to charge the storage capacitor  162  with a predetermined quantity of energy for cardioverting the heart  10 . The quantity of cardioverting energy for cardioverting the ventricular tachycardia may be, for example, on the order of five joules. 
     After the storage capacitor  162  is charged with the desired amount of electrical energy, the process moves to step  210  wherein the synchronizing stage  98  operates the switch  164  to cause the storage capacitor  162  to be discharged synchronized to an atrial pacing pulse. The cardioverting energy is applied using the electrically conductive enclosure  50  of the device  30  and the shock coils  34  and  36 . When step  210  is completed, the process returns. 
     If in step  202  ventricular fibrillation is detected, the charge control  100  immediately causes the charger  160  to charge the storage capacitor  162  with an amount electrical energy required to defibrillate the heart  10 . That quantity of energy may be, for example, on the order of 15 joules. 
     Immediately after charging of the storage capacitor is begun in step  214  and thus responsive to ventricular fibrillation being detected, the atrial pacer  72  is caused by the atrial pace control  94  to pace the right atrium in step  216 . Here, the atrial pacing pulses are applied at a fixed rate and at a rate of, on the order of, 120 beats per minute. Fixed rate pacing is employed for ventricular fibrillation as discernable R-waves most likely will not be detectable. 
     As the atrial pacing pulses are applied by the atrial pacer  72 , the process continually checks to determine if the storage capacitor  162  is charged with the desired amount of energy in step  218 . Step  218  is repeated until the storage capacitor  162  is charged. When the storage capacitor is charged, which should be accomplished in a matter of a few seconds, the synchronizing stage  98  in step  220  operates the switch  164  for discharging the energy stored in storage capacitor  162  into the heart  10 . The defibrillation electrical energy is synchronized to an atrial pacing pulse and is applied using the electrically conductive enclosure  50  of device  30  and the shock coils  34  and  36  of lead  32 . Once the defibrillating electrical energy is applied to the heart, the process returns. 
     In addition to the foregoing, an additional step may be interposed in the flow diagram of FIG. 3 wherein immediately after the beginning of the atrial pacing of step  208 , the autocapture stage  96  determines if the atria are being captured by the atrial pacing pulses applied by the atrial pacer  72 . Such autocapture is well known in the art. If the atria are captured by the atrial pacing pulses, the atrial pacing will be permitted to continue until it is time to deliver the cardioverting energy in step  210 . However, if the atrial pacing pulses are not capturing the atria, the process may immediately go to step  210  for delivering the cardioversion energy synchronized to an atrial pacing pulse. 
     In accordance with a further aspect of the present invention, if ventricular tachycardia is detected, the atrial pacing may be conducted in an anticipatory manner to improve the cardiac output of the heart  10 . For example, if the ventricular tachycardia has a rate of 180 beats per minute, there will be a significantly reduced cardiac output. However, pacing the atrium at 90 beats per minute, just before the R-waves of the heart, could add significant cardiac output. This is due to the fact that one of the problems with ventricular tachycardia is that there is AV disynchrony, and therefore, even greater reduction of output than from the ventricular tachycardia itself. To accomplish the anticipatory pacing, the averaging stage  90  may average the time spans between a successive number of R-waves to determine the average heart rate interval. Since ventricular tachycardia is a stable arrhythmia in terms of rate, the average heart rate interval may be utilized to predict when the atrium may be paced in anticipation of the R-waves. The atrial pacing control  94  will cause the atrial pacer  72  to pace the right atrium just prior to every other R-wave. The pacing of the right atrium before every other R-wave is expected will provide improved cardiac output during the ventricular tachycardia before the heart receives a quantity of cardioverting energy using the electrically conductive enclosure  50  of the device  30  and the shock coils  34  and  36  of lead  32 . 
     While particular embodiments of the present invention have been shown and described, modifications may be made, and it is therefore intended in the appended claims to cover all such changes and modifications which fall within the true spirit and scope of the invention.