FIELD OF THE INVENTION 
   This invention relates to an implantable cardiac stimulation device and method for responding to a detected premature ventricular complex (PVC) to prevent a pacemaker mediated tachycardia (PMT). More specifically the present invention relates to such a stimulation device that discriminates between retrograde P waves and sinus P waves before providing atrial pacing to prevent a PMT. 
   BACKGROUND OF THE INVENTION 
   Implantable cardiac devices are well known in the art. They may take the form of implantable defibrillators or cardioverters which treat accelerated rhythms of the heart such as fibrillation or implantable pacemakers which maintain the heart rate above a prescribed limit, such as, for example, to treat a bradycardia. Implantable cardiac devices are also known which incorporate both a pacemaker and a defibrillator. 
   A pacemaker may be considered to be comprised of two major components. One component is a pulse generator which generates the pacing stimulation pulses and includes the electronic circuitry and the power cell or battery. The other component is the lead, or leads, having electrodes which electrically couple the pacemaker to the heart. A lead may provide both unipolar and bipolar pacing and/or sensing electrode configurations. In the unipolar configuration, the pacing stimulation pulses are applied or intrinsic responses are sensed between a single electrode carried by the lead, in electrical contact with the desired heart chamber, and the pulse generator case. The electrode serves as the cathode (negative pole) and the case serves as the anode (positive pole). In the bipolar configuration, the pacing stimulation pulses are applied or intrinsic responses are sensed between a pair of closely spaced electrodes carried by the lead, in electrical contact with the desired heart chamber, with the most proximal electrode serving as the anode and the most distal electrode serving as the cathode. 
   Pacemakers deliver pacing pulses to the heart to induce a depolarization and a mechanical contraction of that chamber when the patient&#39;s own intrinsic rhythm fails. To this end, pacemakers include sensing circuits that sense cardiac activity for the detection of intrinsic cardiac events such as intrinsic atrial events (P waves) and intrinsic ventricular events (R waves). By monitoring such P waves and/or R waves, the pacemaker circuits are able to determine the intrinsic rhythm of the heart and provide stimulation pacing pulses that force atrial and/or ventricular depolarizations at appropriate times in the cardiac cycle when required to help stabilize the electrical rhythm of the heart. 
   Pacemakers are described as single-chamber or dual-chamber systems. A single-chamber system stimulates and senses in one chamber of the heart (atrium or ventricle). A dual-chamber system stimulates and/or senses in both chambers of the heart (atrium and ventricle). Dual-chamber systems may typically be programmed to operate in either a dual-chamber mode or a single-chamber mode. 
   Recently, there has been the introduction of pacing systems that stimulate in corresponding chambers of the heart as, for example, the right ventricle (RV) and left ventricle (LV). These are termed biventricular stimulation devices. 
   Pacemaker Mediated Tachycardia (PMT) also called “endless-loop tachycardia”, or “pacemaker reentrant tachycardia”, is a recognized pacemaker related rhythm anomaly. PMT can result in any pacemaker capable of sensing and responding to atrial depolarizations when A-V synchrony is dissociated, typically by a premature ventricular complex (PVC). A PVC, as is known, is a native depolarization arising from an ectopic location in the ventricle and occurring early with respect to the next expected conducted ventricular depolarization. Basically, it is an R wave that is not preceded by an atrial event. Such ventricular events may conduct in a retrograde direction to the atria and cause atrial depolarizations. If this atrial depolarization occurs after completion of the PVARP, the device is able to sense this retrograde atrial depolarization, classify it as an alert event, and then, after the appropriate AV delay, delivers a stimulus to the ventricle. Thus, the device provides the anterograde conduction pathway for the reentrant circuit and the intrinsic conduction system of the heart provides the retrograde pathway. A repetitive cycle of ventricular pacing synchronized to the retrograde P-wave can ensue. 
   Premature ventricular complex (PVC) events are actually the most common trigger for a pacemaker mediated tachycardia (PMT). In the patient whose conduction system can support retrograde conduction, the other prerequisite is that the AV node and atrium be physiologically recovered and hence, not refractory. This occurs when there is AV dissociation. The most common setting for this is the presence of a PVC. In the art, several different methods and hence algorithms, such as extending PVARP, “A pace on PMT” and “A pace on PVC”, etc., have been implemented to deal with this inappropriate rhythm disorder. Each method has its unique limitations. In fact, still newer methods and algorithms have been designed to mitigate the limitations of the previous methods and algorithms. 
   For example, the PVARP extension may simply postpone development of a PMT. In addition, PVARP extensions predispose the heart to sustained loss of atrial tracking when there is intact conduction with a first degree AV block. 
   The “A pace on PMT” algorithm is intended to terminate a PMT that has already developed. It calls for the intentional delivery of an atrial pacing stimulus after retrograde P-waves are confirmed but at a time when the atrial myocardium should no longer be physiologically refractory. The atrial stimulus will capture the atrium and AV node in the anterograde direction blocking retrograde conduction from the ensuing ventricular paced complex and terminating the PMT. 
   The “A pace on PVC” algorithm, on the other hand, calls for the delivery of an atrial pacing pulse after a PVC event with the objective of preventing a PMT. An atrial pacing stimulus is delivered after a preset delay when a P-wave (presumed a retrograde P wave) is detected within the PVARP after the PVC event. The atrial stimulus will capture the atrium and AV node in the anterograde direction in order to prevent the PMT from happening. However, the “A pace on PVC” algorithm can create a long short sequence that may initiate another type of tachycardia known as a supraventricular tachycardia. 
   In one instance, for example, an R-wave was properly detected as a PVC. As a result, an atrial pacing pulse was provided 319 ms after the P-wave (presumed to be a retrograde P wave) according to the “A Pace on PVC” algorithm. While this prevented a PMT, it initiated a non-sustained atrial tachycardia at a cycle length of approximately 270 ms. While the system correctly identified the PVC event, this PVC event was not associated with a retrograde P wave but in fact, a sinus P wave. Had the P wave actually been a retrograde P wave, an undesired intrinsic tachycardia would not have resulted in this case. 
   Hence, there is a need in the art to improve the specificity of the A-pace on PVC response to insure that the P wave detected after a PVC event is a retrograde P wave. The invention addresses these and other issues. 
   SUMMARY 
   What is described herein is an implantable cardiac stimulation device comprising an atrial pulse generator that provides atrial pacing pulses, a premature ventricular complex detector that detects premature ventricular complexes in the heart, and a sensing circuit that senses atrial events. The device further comprises a P wave discriminator that identifies an atrial event sensed by the sensing circuit following detection of a premature ventricular complex as one of a retrograde P wave and a normal sinus P wave. The atrial pulse generator delivers an atrial pacing pulse responsive to the discriminator identifying an atrial event sensed by the sensing circuit following detection of a premature ventricular complex as a retrograde P wave. 
   The device may further comprise a timer that times atrial event intervals including the atrial event interval completed by the atrial event sensed by the sensing circuit following detection of a premature ventricular complex. The discriminator may include an interval compare that compares the atrial event interval completed by the atrial event sensed by the sensing circuit following detection of the premature ventricular complex to a predetermined interval standard to identify the atrial event sensed by the sensing circuit following detection of a premature ventricular complex as a retrograde P wave or a normal sinus P wave. The predetermined interval standard may be an average P wave to P wave interval. The predetermined interval standard may alternatively be a predicted P wave to P wave interval. The discriminator may identify an atrial event as a retrograde P wave when the atrial event interval completed by the atrial event sensed by the sensing circuit following detection of a premature ventricular complex is shorter than the predetermined interval standard by at least a preset interval difference. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further features and advantages of the present invention may be more readily understood by reference to the following description taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a simplified diagram illustrating an implantable stimulation device according to an embodiment of the invention; 
       FIG. 2  is a functional block diagram of the implantable stimulation device of  FIG. 1  illustrating the basic elements thereof to provide cardioversion, defibrillation and pacing stimulation in four chambers of the heart as well as a PVC response according to an embodiment of the invention; 
       FIG. 3  is a flow chart describing an overview of the operation of one embodiment of the present invention wherein P wave discrimination is carried out by timing interval comparison; 
       FIG. 4  is a flow chart illustrating an overview of the operation of another embodiment of the present invention wherein P wave discrimination is carried out by timing interval and morphology comparison; and 
       FIG. 5  is a flow chart illustrating an overview of the operation of still another embodiment of the present invention wherein P wave discrimination is carried out by morphology comparison. 
   


   DETAILED DESCRIPTION 
   The following description includes the best mode presently contemplated for practicing the invention. This description is not to be taken in a limiting sense but is made merely for the purpose of describing the general principles of the invention. The scope of the invention should be ascertained with reference to the issued claims. In the description that follows, like numerals or reference designators will be used to refer to like parts or elements throughout. 
   As shown in  FIG. 1 , there is a stimulation device  10  in electrical communication with a patient&#39;s heart  12  by way of three leads,  20 ,  24  and  30 , suitable for delivering multi-chamber stimulation and shock therapy. To sense atrial cardiac signals and to provide right atrial chamber stimulation therapy, the stimulation device  10  is coupled to an implantable right atrial lead  20  having at least an atrial tip electrode  22 , which typically is implanted in the patient&#39;s right atrial appendage. 
   To sense left atrial and ventricular cardiac signals and to provide left chamber pacing therapy, the stimulation device  10  is coupled to a “coronary sinus” lead  24  designed for placement in the “coronary sinus region” via the coronary sinus ostium for positioning a distal electrode adjacent to the left ventricle and/or additional electrode(s) adjacent to the left atrium. As used herein, the phrase “coronary sinus region” refers to the venous vasculature of the left ventricle, including any portion of the coronary sinus, great cardiac vein, left marginal vein, left posterior ventricular vein, middle cardiac vein, and/or small cardiac vein or any other cardiac vein accessible by the coronary sinus. Accordingly, an exemplary coronary sinus lead  24  is designed to receive atrial and ventricular cardiac signals and to deliver left ventricular pacing therapy using at least a left ventricular tip electrode  26 , left atrial pacing therapy using at least a left atrial ring electrode  27 , and shocking therapy using at least a left atrial coil electrode  28 . 
   The stimulation device  10  is also shown in electrical communication with the patient&#39;s heart  12  by way of an implantable right ventricular lead  30  having, in this embodiment, a right ventricular tip electrode  32 , a right ventricular ring electrode  34 , a right ventricular (RV) coil electrode  36 , and an SVC coil electrode  38 . Typically, the right ventricular lead  30  is transvenously inserted into the heart  12  so as to place the right ventricular tip electrode  32  in the right ventricular apex so that the RV coil electrode will be positioned in the right ventricle and the SVC coil electrode  38  will be positioned in the superior vena cava. Accordingly, the right ventricular lead  30  is capable of receiving cardiac signals, and delivering stimulation in the form of pacing and shock therapy to the right ventricle. 
   As illustrated in  FIG. 2 , a simplified block diagram is shown of the multi-chamber implantable stimulation device  10 , which is capable of treating both fast and slow arrhythmias with stimulation therapy, including cardioversion, defibrillation, and pacing stimulation. While a particular multi-chamber device is shown, this is for illustration purposes only, and one of skill in the art could readily duplicate, eliminate or disable the appropriate circuitry in any desired combination to provide a device capable of treating the appropriate chamber(s) with cardioversion, defibrillation and pacing stimulation. 
   The housing  40  for the stimulation device  10 , shown schematically in  FIG. 2 , is often referred to as the “can”, “case” or “case electrode” and may be programmably selected to act as the return electrode for all “unipolar” modes. The housing  40  may further be used as a return electrode alone or in combination with one or more of the coil electrodes,  28 ,  36  and  38 , for shocking purposes. The housing  40  further includes a connector (not shown) having a plurality of terminals,  42 ,  44 ,  46 ,  48 ,  52 ,  54 ,  56 , and  58  (shown schematically and, for convenience, the names of the electrodes to which they are connected are shown next to the terminals). As such, to achieve right atrial sensing and pacing, the connector includes at least a right atrial tip terminal (A R  TIP)  42  adapted for connection to the atrial tip electrode  22 . 
   To achieve left chamber sensing, pacing and shocking, the connector includes at least a left ventricular tip terminal (V L  TIP)  44 , a left atrial ring terminal (A L  RING)  46 , and a left atrial shocking terminal (A L  COIL)  48 , which are adapted for connection to the left ventricular ring electrode  26 , the left atrial tip electrode  27 , and the left atrial coil electrode  28 , respectively. 
   To support right chamber sensing, pacing and shocking, the connector further includes a right ventricular tip terminal (V R  TIP)  52 , a right ventricular ring terminal (V R  RING)  54 , a right ventricular shocking terminal (R V  COIL)  56 , and an SVC shocking terminal (SVC COIL)  58 , which are adapted for connection to the right ventricular tip electrode  32 , right ventricular ring electrode  34 , the RV coil electrode  36 , and the SVC coil electrode  38 , respectively. 
   At the core of the stimulation device  10  is a programmable microcontroller  60  which controls the various modes of stimulation therapy. As is well known in the art, the microcontroller  60  typically includes a microprocessor, or equivalent control circuitry, designed specifically for controlling the delivery of stimulation therapy and may further include RAM or ROM memory, logic and timing circuitry, state machine circuitry, and I/O circuitry. Typically, the microcontroller  60  includes the ability to process or monitor input signals (data) as controlled by a program code stored in a designated block of memory. The details of the design and operation of the microcontroller  60  are not critical to the present invention. Rather, any suitable microcontroller  60  may be used that carries out the functions described herein. The use of microprocessor-based control circuits for performing timing and data analysis functions are well known in the art. 
   As shown in  FIG. 2 , an atrial pulse generator  70  and a ventricular pulse generator  72  generate pacing stimulation pulses for delivery by the right atrial lead  20 , the right ventricular lead  30 , and/or the coronary sinus lead  24  via an electrode configuration switch  74 . It is understood that in order to provide stimulation therapy in each of the four chambers of the heart, the atrial and ventricular pulse generators,  70  and  72 , may include dedicated, independent pulse generators, multiplexed pulse generators, or shared pulse generators. The pulse generators,  70  and  72 , are controlled by the microcontroller  60  via appropriate control signals,  76  and  78 , respectively, to trigger or inhibit the stimulation pulses. 
   The microcontroller  60  further includes timing control circuitry  79  which is used to control the timing of such stimulation pulses (e.g., pacing rate, atrio-ventricular (AV) delay, atrial interconduction (A-A) delay, or ventricular interconduction (V-V) delay, etc.) as well as to keep track of the timing of refractory periods, blanking intervals, noise detection windows, evoked response windows, alert intervals, marker channel timing, etc., which is well known in the art. 
   The switch  74  includes a plurality of switches for connecting the desired electrodes to the appropriate I/O circuits, thereby providing complete electrode programmability. Accordingly, the switch  74 , in response to a control signal  80  from the microcontroller  60 , determines the polarity of the stimulation pulses (e.g., unipolar, bipolar, combipolar, etc.) by selectively closing the appropriate combination of switches (not shown) as is known in the art. 
   Atrial sensing circuits  82  and ventricular sensing circuits  84  may also be selectively coupled to the right atrial lead  20 , coronary sinus lead  24 , and the right ventricular lead  30 , through the switch  74  for detecting the presence of cardiac activity in each of the four chambers of the heart. Accordingly, the atrial (ATR. SENSE) and ventricular (VTR. SENSE) sensing circuits,  82  and  84 , may include dedicated sense amplifiers, multiplexed amplifiers, or shared amplifiers. The switch  74  determines the “sensing polarity” of the cardiac signal by selectively closing the appropriate switches, as is also known in the art. In this way, the clinician may program the sensing polarity independent of the stimulation polarity. 
   Each sensing circuit,  82  and  84 , preferably employs one or more low power, precision amplifiers with programmable gain and/or automatic gain control, bandpass filtering, and a threshold detection circuit, as known in the art, to selectively sense the cardiac signal of interest. The automatic gain control enables the device  10  to deal effectively with the difficult problem of sensing the low amplitude signal characteristics of atrial or ventricular fibrillation. The outputs of the atrial and ventricular sensing circuits,  82  and  84 , are connected to the microcontroller  60  which, in turn, are able to trigger or inhibit the atrial and ventricular pulse generators,  70  and  72 , respectively, in a demand fashion in response to the absence or presence of cardiac activity in the appropriate chambers of the heart. 
   For arrhythmia detection, the device  10  utilizes the atrial and ventricular sensing circuits,  82  and  84 , to sense cardiac signals to determine whether a rhythm is physiologic or pathologic. As used herein “sensing” is reserved for the noting of an electrical signal, and “detection” is the processing of these sensed signals and noting the presence of an arrhythmia. The timing intervals between sensed events (e.g., P-waves, R-waves, and depolarization signals associated with fibrillation which are sometimes referred to as “F-waves” or “Fib-waves”) are then classified by the microcontroller  60  by comparing them to a predefined rate zone limit (i.e., bradycardia, normal, low rate VT, high rate VT, and fibrillation rate zones) and various other characteristics (e.g., sudden onset, stability, physiologic sensors, and morphology, etc.) in order to determine the type of remedial therapy that is needed (e.g., bradycardia pacing, anti-tachycardia pacing, cardioversion shocks or defibrillation shocks, collectively referred to as “tiered therapy”). 
   Cardiac signals are also applied to the inputs of an analog-to-digital (A/D) data acquisition system  90 . The data acquisition system  90  is configured to acquire intracardiac electrogram signals, convert the raw analog data into a digital signal, and store the digital signals for later processing and/or telemetric transmission to an external device  102 . The data acquisition system  90  is coupled to the right atrial lead  20 , the coronary sinus lead  24 , and the right ventricular lead  30  through the switch  74  to sample cardiac signals across any pair of desired electrodes. 
   The microcontroller  60  is further coupled to a memory  94  by a suitable data/address bus  96 , wherein the programmable operating parameters used by the microcontroller  60  are stored and modified, as required, in order to customize the operation of the stimulation device  10  to suit the needs of a particular patient. Such operating parameters define, for example, pacing pulse amplitude, pulse duration, electrode polarity, rate, sensitivity, automatic features, arrhythmia detection criteria, and the amplitude, waveshape and vector of each shocking pulse to be delivered to the patient&#39;s heart  12  within each respective tier of therapy. Advantageously, the operating parameters of the implantable device  10  may be non-invasively programmed into the memory  94  through a telemetry circuit  100  in telemetric communication with the external device  102 , such as a programmer, transtelephonic transceiver, or a diagnostic system analyzer. The telemetry circuit  100  is activated by the microcontroller by a control signal  106 . The telemetry circuit  100  advantageously allows intracardiac electrograms and status information relating to the operation of the device  10  (as contained in the microcontroller  60  or memory  94 ) to be sent to the external device  102  through an established communication link  104 . 
   In the preferred embodiment, the stimulation device  10  further includes a physiologic sensor  108 , commonly referred to as a “rate-responsive” sensor because it is typically used to adjust pacing stimulation rate according to the exercise state of the patient. However, the physiological sensor  108  may further be used to detect changes in cardiac output, changes in the physiological condition of the heart, or diurnal changes in activity (e.g., detecting sleep and wake states). Accordingly, the microcontroller  60  responds by adjusting the various pacing parameters (such as rate, AV Delay, V-V Delay, etc.) at which the atrial and ventricular pulse generators,  70  and  72 , generate stimulation pulses. 
   The stimulation device additionally includes a battery  110  which provides operating power to all of the circuits shown in  FIG. 2 . For the stimulation device  10 , which employs shocking therapy, the battery  110  must be capable of operating at low current drains for long periods of time and then be capable of providing high-current pulses (for capacitor charging) when the patient requires a shock pulse. The battery  110  must also have a predictable discharge characteristic so that elective replacement time can be detected. Accordingly, the device  10  may employ lithium/silver vanadium oxide batteries, as are known in the art. 
   As further shown in  FIG. 2 , the device  10  is shown as having an impedance measuring circuit  112  which is enabled by the microcontroller  60  via a control signal  114 . The impedance measuring circuit  112  is not critical to the present invention and is shown for only completeness. 
   In the case where the stimulation device  10  is intended to operate as an implantable cardioverter/defibrillator (ICD) device, it must detect the occurrence of an arrhythmia, and automatically apply an appropriate electrical shock therapy to the heart aimed at terminating the detected arrhythmia. To this end, the microcontroller  60  further controls a shocking circuit  116  by way of a control signal  118 . The shocking circuit  116  generates shocking pulses of low (up to 0.5 joules), moderate (0.5-10 joules), or high energy (11 to 40 joules), as controlled by the microcontroller  60 . Such shocking pulses are applied to the patient&#39;s heart  12  through at least two shocking electrodes, and as shown in this embodiment, selected from the left atrial coil electrode  28 , the RV coil electrode  36 , and/or the SVC coil electrode  38 . As noted above, the housing  40  may act as an active electrode in combination with the RV electrode  36 , or as part of a split electrical vector using the SVC coil electrode  38  or the left atrial coil electrode  28  (i.e., using the RV electrode as a common electrode). 
   Cardioversion shocks are generally considered to be of low to moderate energy level (so as to minimize pain felt by the patient), and/or synchronized with an R-wave and/or pertaining to the treatment of tachycardia. Defibrillation shocks are generally of moderate to high energy level (i.e., corresponding to thresholds in the range of 5-40 joules), delivered asynchronously (since R-waves may be too disorganized), and pertaining exclusively to the treatment of fibrillation. Accordingly, the microcontroller  60  is capable of controlling the synchronous or asynchronous delivery of the shocking pulses. 
   With further reference to  FIG. 2 , and according to this embodiment, the device further includes a PVC detector  62 , an interval averager  64 , an interval predictor  65 , an interval compare  66 , and a P wave morphology averager  67 . The PVC detector  62  is provided to detect the occurrence of PVC&#39;s. The interval averager  64  is provided to average the P wave to P wave intervals for a number of P wave sinus cardiac cycles. The intervals may be timed by the timing control  79 . This average interval then becomes a predetermined interval standard (T o ) to which future atrial intervals will be compared to discriminate sinus P waves and retrograde P waves. 
   Alternatively, the interval predictor  65  is provided to provide a predetermined interval standard (T o ) that is a predicted next P wave interval that is dynamically updated for sinus rhythms. Hence, for a steady sinus rhythm, T o  would equal the average. For an increasing sinus rate, T o  would be shorter or equal to the last interval. For a decreasing sinus rate, T o  would be longer than or equal to the last interval. Such interval predicting methods are known in the art. 
   The interval compare  66  is provided to compare an atrial event interval completed by an atrial event sensed after detection of a PVC to the predetermined interval standard (T o ) to identify the atrial event as a sinus P wave or a retrograde P wave. If, for example, the atrial event interval completed by an atrial event sensed after detection of a PVC is less than the predetermined interval standard by at least a preset interval difference, the atrial event is identified as a retrograde P wave. If not, the atrial event is identified as a sinus P wave. Each identification is followed by its own resulting device response. 
   The P wave averager  67  is a morphology averager. The morphology detector  68  may be employed to isolate sinus P wave electrograms which are stored in memory  94 . When a sufficient number are stored, the P wave averager averages the electrograms to provide a P wave template (Po). The morphology detector  68  may then compare the morphology of an atrial event sensed after detection of a PVC to the P wave morphology template (Po) to identify the atrial event as either a retrograde P wave or sinus P wave. Electrogram template generation and comparison are techniques that are well know in the art. Preferably, the P wave template P o  is updated by each sinus P wave. Further aspects of the embodiments will become apparent as the flow charts of  FIGS. 3-5  are described. 
   In  FIG. 3 , a flow chart is shown describing an overview of the operation and novel features implemented in one embodiment of the device  10 . In this flow chart, and the other flow charts described herein, the various algorithmic steps are summarized in individual “blocks”. Such blocks describe specific actions or decisions that must be made or carried out as the algorithm proceeds. Where a microcontroller (or equivalent) is employed, the flow charts presented herein provide the basis for a “control program” that may be used by such a microcontroller (or equivalent) to effectuate the desired control of the stimulation device. Those skilled in the art may readily write such a control program based on the flow charts and other descriptions presented herein. 
   The process of  FIG. 3  initiates with activity block  302 . Here, the predetermined interval standard T o  is determined. The predetermined interval standard T o  may be determined as an average, in a manner as previously described, or a predicted interval, also as previously described. 
   Next, in decision block  304 , it is determined if the PVC detector  62  has detected a PVC. As long as it does not, the process repeatedly returns to decision block  304 . However, when a PVC is detected, the process advances to decision block  306  where the device  10  detects for an atrial event. The atrial event may be either a sinus P wave or a retrograde P wave. If an atrial event is not detected, the process proceeds to activity block  307  to deliver an atrial pacing pulse after the PVC atrial escape interval. The process then returns to decision block  304 . 
   The process advances to activity block  308  if atrial event is detected. Here, the timing control  79  determines the length of the atrial interval just completed by the atrial event sensed after the detection of the PVC. The process then moves on to decision block  310 . Here, the interval compare  66  compares the interval just completed by the atrial event sensed after the detection of the PVC to the predetermined interval standard T o  determine if the sensed atrial event is potentially a retrograde P wave. Here, T o  may be either the average interval or the predicted interval. In decision block  310  the interval compare  66  determines if the interval difference is less than a predefined threshold and thus is small enough to warrant identifying the atrial event as a sinus P wave. If it is, the process immediately proceeds to activity block  318  to inhibit an atrial pacing response, to activity block  320  to provide a properly timed ventricular pacing pulse, and then to activity block  322  to update the predetermined interval standard T o . The process then returns to decision block  304  for the next PVC detection. 
   The proper timing of the ventricular pacing pulse would preferably depend on the maximum tracking interval and the AV delay currently set. More specifically, the ventricular pacing pulse is preferably delivered at the last to expire of the AV delay or the maximum tracing interval. This will prevent a cardiac interval considered too short for the patient. 
   If the difference between the atrial interval just completed by the atrial event sensed after the detection of the PVC and T o  is greater than the predefined threshold, The sensed atrial event is potentially a retrograde P wave and the process moves to decision block  314 . Here, it is determined if the interval just completed by the atrial event sensed after the detection of the PVC is longer or shorter than the predetermined interval standard, T o . If it is shorter, the atrial event is confirmed and identified as a retrograde P wave and the process immediately proceeds to activity block  316  where the device provides an atrial pacing response. The atrial pacing response may be, for example, the A pace on PVC, previously referred to or another atrial pacing response. The process then returns to decision block  304 . 
   However, if the atrial interval just completed by the atrial event sensed after the detection of the PVC is longer than the predetermined interval standard, T o , the atrial event is treated as a sinus P wave. Hence, the process then proceeds to activity block  318  to inhibit an atrial pacing response, to activity block  320  to provide a properly timed ventricular pacing pulse, and then to activity block  322  to update the predetermined interval standard T o . The process then returns to decision block  304  for the next PVC detection. Again, the proper timing of the ventricular pacing pulse would preferably depend on the maximum tracking interval and the AV delay currently set. More specifically, the ventricular pacing pulse is preferably delivered at the last to expire of the AV delay or the maximum tracing interval. 
     FIG. 4  is a flow chart illustrating an overview of the operation of another embodiment of the present invention wherein P wave discrimination is carried out by timing interval and morphology comparison. The process of  FIG. 4  initiates with activity block  402 . Here, the predetermined interval standard T o  and the P wave template are determined. The predetermined interval standard T o  may again be determined as an average, in a manner as previously described, or a predicted interval, also as previously described. The P wave template may also be determined as previously described. 
   Next, in decision block  404 , it is determined if the PVC detector  62  has detected a PVC. As long as it does not, the process repeatedly returns to decision block  304 . However, when a PVC is detected, the process advances to decision block  406  where the device  10  detects for an atrial event. The atrial event may be either a sinus P wave or a retrograde P wave. If an atrial event is not detected, the process proceeds to activity block  407  to deliver an atrial pacing pulse after the PVC atrial escape interval. The process then returns to decision block  404 . 
   The process advances to activity block  408  if an atrial event is detected. Here, the timing control  79  determines the length of the atrial interval just completed by the atrial event sensed after the detection of the PVC and the morphology detector determines the morphology of the sensed atrial event. The process then moves on to decision block  410 . Here, the interval compare  66  compares the interval just completed by the atrial event sensed after the detection of the PVC to the predetermined interval standard T o  determine if the sensed atrial event is potentially a retrograde P wave. Again, T o  may be the average interval or the predicted interval. The interval compare  66  determines if the interval difference is less than a predefined threshold and thus is small enough to warrant identifying the atrial event as a sinus P wave. If it is, the process immediately proceeds to activity block  418  to inhibit an atrial pacing response, to activity block  420  to provide a properly timed ventricular pacing pulse, and then to activity block  422  to update the predetermined interval standard T o  and the P wave morphology template. The process then returns to decision block  304  for the next PVC detection. 
   The proper timing of the ventricular pacing pulse would again preferably depend on the maximum tracking interval and the AV delay currently set. More specifically, the ventricular pacing pulse is preferably delivered at the last to expire of the AV delay or the maximum tracing interval. This will prevent a cardiac interval considered too short for the patient. 
   If the difference between the atrial interval just completed by the atrial event sensed after the detection of the PVC and T o  is greater than the predefined threshold, the process moves to decision block  414 . Here, the morphology detector  68  compares the morphology of the sensed atrial event to the P wave template. If there is a mismatch, the atrial event is confirmed and identified as a retrograde P wave. The process then proceeds to activity block  416  where the device provides an atrial pacing response. The atrial pacing response may be, for example, the A pace on PVC, previously referred to or another atrial pacing response. The process then returns to decision block  404 . 
   However, if the morphology of the atrial event sensed after the detection of the PVC does match the P wave morphology template, the atrial event is confirmed as a sinus P wave. Hence, the process then proceeds to activity block  418  to inhibit an atrial pacing response, to activity block  420  to provide a properly timed ventricular pacing pulse, and then to activity block  422  to update the predetermined interval standard T o  and the P wave morphology template. The process then returns to decision block  404  for the next PVC detection. Again, the proper timing of the ventricular pacing pulse would preferably depend on the maximum tracking interval and the AV delay currently set. More specifically, the ventricular pacing pulse is preferably delivered at the last to expire of the AV delay or the maximum tracing interval. 
     FIG. 5  is a flow chart illustrating another embodiment of the present invention wherein P wave discrimination is carried out by morphology comparison. The process of  FIG. 5  initiates with activity block  502 . Here, the P wave template is determined. The P wave morphology template may be derived as previously described. 
   Next, in decision block  504 , it is determined if the PVC detector  62  has detected a PVC. As long as it does not, the process repeatedly returns to decision block  504 . However, when a PVC is detected, the process advances to decision block  506  where the device  10  detects for an atrial event. The atrial event may be either a sinus P wave or a retrograde P wave. If an atrial event is not detected, the process proceeds to activity block  507  to deliver an atrial pacing pulse after the PVC atrial escape interval. The process then returns to decision block  504 . 
   The process advances to activity block  508  if an atrial event is detected. Here, the morphology detector determines the morphology of the sensed atrial event. The process then moves to decision block  514 . Here, the morphology detector  68  compares the morphology of the sensed atrial event to the P wave template. If there is a mismatch, the atrial event is confirmed and identified as a retrograde P wave. The process then proceeds to activity block  516  where the device provides an atrial pacing response. The atrial pacing response may be, for example, the A pace on PVC, previously referred to or another atrial pacing response. The process then returns to decision block  504 . 
   However, if the morphology of the atrial event sensed after the detection of the PVC does match the P wave morphology template, the atrial event is confirmed as a sinus P wave. Hence, the process then proceeds to activity block  518  to inhibit an atrial pacing response, to activity block  520  to provide a properly timed ventricular pacing pulse, and then to activity block  522  to update the predetermined interval standard T o  and the P wave morphology template. The process then returns to decision block  504  for the next PVC detection. Again, the proper timing of the ventricular pacing pulse would preferably depend on the maximum tracking interval and the AV delay currently set. More specifically, the ventricular pacing pulse is preferably delivered at the last to expire of the AV delay or the maximum tracing interval. 
   While the invention has been described by means of specific embodiments and applications thereof, it is understood that numerous modifications and variations could be made thereto by those skilled in the art without departing from the spirit and scope of the invention. It is therefore to be understood that within the scope of the claims, the invention may be practiced otherwise than as specifically described herein.
An implantable cardiac stimulation device provides selective atrial pacing upon detection of a premature ventricular complex (PVC). The device comprises a premature ventricular complex detector that detects premature complexes of the heart, and a sensing circuit that senses atrial events. The device further comprises a P wave discriminator that identifies an atrial event sensed by the sensing circuit following detection of a premature ventricular complex as one of a retrograde P wave and a normal sinus P wave, and an atrial pulse generator that delivers an atrial pacing pulse responsive to the discriminator identifying an atrial event sensed by the sensing circuit following detection of a premature ventricular complex as a retrograde P wave.