Abstract:
A device and method for multi chamber pacing a patient&#39;s heart having heart failure and intrinsic conduction, wherein ventricular tracking is used to pace the ventricle when the sinus rate exceeds a preset atrial maximum tracking rate. The ventricular tracking pacemaker increases the range of pacing the ventricle. The ventricular tracking minimizes the loss of ventricular pacing caused by intrinsic conduction when the sinus rate is below an atrial maximum tracking rate, and it induces a new ventricular pacing behavior for sinus rates above the atrial maximum tracking rate without any pacing hysteresis as the sinus rate returns towards the lower rate limit.

Description:
BACKGROUND OF THE INVENTION 
     I. Field of the Invention 
     This invention relates generally to a device and method for pacing a patient&#39;s heart and more particularly to a device and method for improving the hemodynamic performance of patients suffering from heart failure through dual chamber pacing. The patient in need of improving hemodynamic performance may suffer from, for example, congestive heart failure (CHF), or other heart failure requiring pacing even though intrinsic PR conduction is present. The device includes a means for tracking an R-wave associated with intrinsic conduction in a ventricle of the patient&#39;s heart. The method includes tracking a sensed R-wave for a predetermined RV delay interval and then stimulating the ventricle if a P-wave is sensed during the preset simultaneous Post Ventricular Atrial Refractory Period (PVARP) interval. 
     II. Discussion of the Related Art 
     Typically, a patient suffering from a higher degree of AV-block or an AV conduction disorder is implanted with a conventional atrial tracking (DDD or VDD) pacemaker suited for pacing the ventricle. Such a pacemaker is designed to pace the ventricle after a pre-set AV delay, synchronous with the intrinsic atrial rate. The purpose of these pacemakers is to make sure that heart beats are properly timed and not omitted. Intrinsic rhythm is favorable over paced rhythm for both hemodynamic and economic (battery conservation) reasons. It is also important and necessary to prevent pacing the ventricle during the vulnerable period for inducing arrhythmia immediately following an intrinsic ventricular beat. Hence, ventricular pacing is inhibited when intrinsic conduction from the SA node causes an R-wave to be sensed prior to the scheduled ventricular pace. 
     Patients suffering from congestive heart failure (CHF), for example, either do not exhibit heart block at all or often only suffer from a first degree AV-block or a slightly prolonged delay interval between the depolarization of the atrium and the depolarization of the ventricle. Recent research has shown that acute hemodynamic performance, exercise tolerance and quality of life of CHF patients can be improved by a pre-excitation of the ventricles with pacing following normal sinus beats. Patients benefiting from pre-excitation of the ventricles experience a return of heart failure symptoms immediately upon omission of pre-excitation pacing. Thus, for these patients, it is important that the paced pre-excitation of the ventricle be performed continuously in order to improve the contraction pattern, even though intrinsic beats would occur slightly later if there were no pacing. When pacing a patient suffering from CHF, it is highly undesirable to omit pacing when it is supposed to occur. 
     When pacing the heart of a CHF patient having normal intrinsic (PR) conduction, although continuous pacing is desirable, use of a conventional atrial tracking dual chamber pacemaker completely inhibits ventricular pacing when the intrinsic atrial rate (hereinafter the sinus rate) rises above a preprogrammed atrial maximum tracking rate (AMTR). These pacemakers also exhibit an undesirable pronounced pacing hysteresis, wherein ventricular pacing is not resumed until the sinus rate falls below a ventricular inhibition threshold rate (VIR). Also, in these pacemakers, a premature ventricular contraction (PVC) may inhibit ventricular pacing when the sinus rate exceeds the VIR. 
     It would be advantageous to eliminate the pacing hysteresis, while extending the limit or MTR for pacing of the ventricle. However, this is not possible with the conventional atrial tracking multi-chamber pacemaker. A CHF patient, for example, has an elevated sinus rate in order to maintain normal cardiac output despite reduced cardiac pumping efficiency and, therefore, requires a high atrial MTR. Together with an elevated sinus rate, many such patients also have prolonged PR intervals and correspondingly prolonged retrograde conduction intervals requiring long PVARP intervals. The required long PVARP intervals prevent tracking of retrograde P-waves, thereby reducing pacemaker mediated tachycardia (PMT). In conventional atrial tracking pacemakers, the highest allowable atrial MTR is determined in part by the length of the PVARP interval, which may limit the atrial MTR to a rate that is below the normal range of sinus rates in the CHF patient. Hence, use of a conventional atrial tracking dual chamber pacemaker would not allow continuous ventricular pacing above the atrial MTR. Therefore, there is a need for a dual chamber pacemaker and a method of operating the same that may be used to pace the failing heart of a patient having intrinsic conduction, wherein the pacemaker provides for continuous pacing of the ventricle at a sinus rate that exceeds an atrial maximum tracking rate and does not exhibit pacing hysteresis below the MTR. The present invention addresses this need. 
     SUMMARY OF THE INVENTION 
     The purpose of the present invention is to provide a device and method of pacing continuously, without hysteresis, the ventricles of a patient&#39;s failing heart having intrinsic conduction even when the sinus rate rises above a preset atrial maximum tracking rate. Conventional dual chamber pacemakers commonly have a combination of dual chamber sensing, atrial sensing, ventricular sensing, dual chamber pacing, atrial pacing, ventricular pacing, and atrial tracking. A conventional dual chamber pacemaker may be modified according to the present invention to include a ventricular tracking mode and thereby increase the range of pacing therapy. When used with a patient having intrinsic (PR) conduction, the ventricular tracking mode minimizes the loss of ventricular pacing output as the sinus rate rises above a preset atrial MTR. As the atrial MTR is exceeded by the sinus rate, the ventricular tracking pacemaker restores a Wenckebach-like pacing behavior, thereby continuing ventricular pacing. 
     During this Wenckebach-like pacing, the ventricular tracking pacemaker paces the ventricle due to atrial tracking unless a legitimate P-wave is sensed during a preset post ventricular atrial refractory period (PVARP). A legitimate P-wave refers to a signal or wave that corresponds to an intrinsic atrial depolarization. When a legitimate P-wave is sensed during the PVARP, the ventricular tracking pacemaker tracks a preceding sensed R-wave (due to intrinsic PR conduction) and paces the ventricle after a preset delay interval, hereinafter referred to as the RV delay. Alternatively the ventricular pace can occur after a preset delay from the P-wave sensed during PVARP, hereinafter referred to as the refractory atrial to ventricular (RAV) delay. As the sinus rate continues to increase, the sinus rate reaches a 2:1 ventricular tracking rate up to a ventricular MTR or limit at which point ventricular pacing is inhibited. The conventional dual chamber pacemaker ignores legitimate P-waves sensed during PVARP and does not pace the ventricle above the atrial MTR. With the ventricular tracking pacemaker, as the sinus rate decreases from the ventricular MTR, there is no pacing hysteresis commonly found in the conventional atrial tracking pacemakers. 
     The ventricular tracking pacemaker includes a means for sensing an atrial event and transmitting a signal associated with the sensed atrial event, means for sensing a ventricular event and transmitting a signal associated with the sensed ventricular event, means for tracking a P-wave, means for tracking an R-wave, means for selectively stimulating a preselected ventricle of the patient&#39;s heart, and a means for controlling the selective stimulation of the ventricle. The means for controlling the stimulation is electrically coupled to the sensing, tracking and stimulating means. The means for controlling the stimulation controls the stimulation to the preselected ventricle in accordance with a timing sequence which is dependent upon the tracked ventricular and tracked atrial events. 
     In the preferred embodiment, the means for controlling determines the intrinsic ventricular rate from the signal corresponding to sensed ventricular events. If the intrinsic ventricular rate is greater than the preset ventricular maximum tracking rate, the means for controlling inhibits the stimulation to the ventricle. 
     OBJECTS 
     It is accordingly a principal object of the present invention to provide a multi chamber pacemaker for pacing the selected ventricles of a patient suffering from heart failure but having intrinsic PR conduction, wherein the ventricle may be paced at a rate that is above an atrial maximum tracking rate. 
     A further object of the present invention is to provide a multi chamber pacemaker for pacing the selected ventricles of a patient, wherein the selected ventricles are paced a preset time after an R-wave is tracked by the pacemaker. 
     Another object of the present invention is to provide a method of multi chamber pacing that paces the selected ventricles a preset time after detecting an intrinsic conduction transmitted from the ventricles. 
     Yet another object of the present invention is to provide a method of multi chamber pacing which paces the ventricles a preset time after a P-wave is sensed during a PVARP interval following a sensed R-wave, wherein pacemaker-mediated tachycardia is prevented if the P-wave is due to retrograde conduction. 
     Still another object of the present invention is to provide a pacemaker that may pace the ventricle above the atrial MTR, wherein there is no significant pacing hysteresis when the atrial rate returns to a lower rate limit. 
     These and other objects, as well as these and other features and advantages of the present invention will become readily apparent to those skilled in the art from a review of the following detailed description of the preferred embodiment in conjunction with the accompanying drawings and claims and in which like numerals in the several views refer to corresponding parts. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph showing the intrinsic atrial (sinus) rate in relation to the paced ventricular rate of a conventional atrial tracking pacemaker, employed when a patient has complete heart block; 
     FIG. 2 is a graph showing the intrinsic atrial rate in relation to the paced ventricular rate of a conventional atrial tracking pacemaker where the patient has normal intrinsic (PR) conduction; 
     FIG. 3 is a graph showing the intrinsic atrial rate in relation to the paced ventricular rate of a conventional atrial tracking pacemaker where the patient has experienced a Premature Ventricular Contraction (PVC); 
     FIG. 4 is a graph showing the intrinsic atrial rate in relation to the paced ventricular rate of a ventricular tracking pacemaker where the patient has normal intrinsic (PR) conduction; 
     FIG. 5 is a comparison graph illustrating the increased range of pacing therapy utilizing a ventricular tracking pacemaker compared to the conventional atrial tracking pacemaker; 
     FIG. 6 is a plot showing a sensed P-wave and tracked R-wave in conjunction with a ventricular tracking pacemaker&#39;s PVARP and RV intervals; 
     FIG. 7 is a plot showing a sensed P-wave and tracked R-wave in conjunction with a ventricular tracking pacemaker&#39;s PVARP and RV intervals, wherein a P-wave is tracked during the RV interval; 
     FIG. 8 is a plot showing a tracked R-wave in conjunction with a ventricular tracking pacemaker&#39;s PVARP and RV intervals, wherein a P-wave is neither sensed nor tracked during the RV interval; 
     FIG. 9 is a plot showing a tracked R-wave in conjunction with a ventricular tracking pacemaker&#39;s PVARP and RV intervals, wherein a second R-wave is tracked during the RV interval; 
     FIG. 10 is a plot showing a sensed P-wave and tracked PVC in conjunction with a ventricular tracking pacemaker&#39;s PVARP and RV intervals; 
     FIG. 11 is a plot showing a sensed P-wave and tracked PVC in conjunction with a ventricular tracking pacemaker&#39;s PVARP and RV intervals, wherein a second R-wave is tracked during the RV interval; 
     FIG. 12 is a plot showing a sensed retrograde P-wave and tracked PVC in conjunction with a ventricular tracking pacemaker&#39;s PVARP and RV intervals; 
     FIG. 13 is a plot showing a sensed P-wave and tracked R-wave in conjunction with a ventricular tracking pacemaker&#39;s PVARP and RV intervals, wherein the sinus rate is between the VIR and AMTR; 
     FIG. 14 is a plot showing a sensed P-wave and tracked PVC in conjunction with a ventricular tracking pacemaker&#39;s PVARP and RV intervals, wherein the sinus rate is between the VIR and AMTR; 
     FIG. 15 is a plot showing a sensed P-wave and tracked R-wave in conjunction with a ventricular tracking pacemaker&#39;s PVARP and RV intervals, wherein the sinus rate is between the AMTR and VMTR; 
     FIG. 16 is a plot showing a sensed P-wave and tracked PVC in conjunction with a ventricular tracking pacemaker&#39;s PVARP and RV intervals, wherein the sinus rate is between the AMTR and VMTR; 
     FIG. 17 is a plot showing a sensed P-wave and tracked R-wave in conjunction with a ventricular tracking pacemaker&#39;s PVARP and RV intervals, wherein the sinus rate is between the AMTR and VMTR and the ventricular tracking reaches a 2:1 blocking point; 
     FIG. 18 is a plot showing a sensed P-wave and tracked R-wave in conjunction with a ventricular tracking pacemaker&#39;s PVARP and RV intervals, wherein the sinus rate exceeds the VMTR; 
     FIG. 19 is a plot showing a sensed P-wave and tracked R-wave in conjunction with a ventricular tracking pacemaker&#39;s PVARP, RV, and RAV intervals, when delay priority timing is in effect; 
     FIG. 20 is a plot showing a sensed P-wave and tracked R-wave in conjunction with a ventricular tracking pacemaker&#39;s PVARP, RV, and RAV intervals, when delay priority timing is in effect and a P-wave is sensed during the PVARP interval and after the RV interval; 
     FIG. 21 is a plot showing a sensed P-wave and tracked R-wave in conjunction with a ventricular tracking pacemaker&#39;s PVARP, RV, and RAV intervals, wherein the RAV interval expires before the RV interval; 
     FIG. 22 is a plot showing a sensed P-wave and tracked R-wave in conjunction with a ventricular tracking pacemaker&#39;s PVARP, RV, and RAV intervals, wherein the RAV interval expires before the RV interval and a second R-wave is tracked during the RV interval; 
     FIG. 23 is a block diagram showing the components of the ventricular tracking pacemaker of the present invention; 
     FIG. 24 is a flowchart showing the algorithm used by the ventricular tracking pacemaker of the present invention to track intrinsic conduction from the ventricle and accordingly pace the ventricle when rate priority timing is in effect; and 
     FIGS. 25 and 26 are flowcharts showing two algorithms that may be used by the ventricular tracking pacemaker of the present invention to track intrinsic conduction from the ventricle and accordingly pace the ventricle when delay priority ventricular tracking is in effect. 
     
         ______________________________________DEFINITIONS______________________________________PVC         Premature Ventricular ContractionLRL          Lower Rate LimitPVARP      Post Ventricuiar Atrial Refractory PeriodPR            time between depolarization of the atrium and ventricieVlR          Ventricular Inhibition RateAMTR        Atrial Maximum Tracking RateVMTR        Ventricuiar Maximium Tracking RateCHF          Congestive Heart FailureSAV          Sensed Atrial to Ventricular delayRAV          Refractory Atrial to Ventricular delayRV            time delay between sensing ventricular conduction and  pacing               the ventricleURL          Upper Rate LimitMTR          Maximum Tracking Rate______________________________________ 
    
    
    
     DETAILED DESCRIPTION 
     Referring first to FIGS. 1-3, these graphs show the intrinsic atrial rate or sinus rate of a patient in relation to the paced ventricular rate or output of a conventional atrial tracking pacemaker when used in a variety of patient conditions. FIG. 1 illustrates the ventricular pacing rate by a conventional atrial tracking pacemaker, pacing the heart of a patient having a complete block of all intrinsic conduction. As the sinus rate increases, the pacemaker&#39;s pacing rate tracks the atrial rate until the pacing rate reaches a pre-programmed atrial maximum tracking rate (AMTR) at 15. At this point, as the atrial rate continues to increase, there is a falloff in the ventricular pacing rate attributable to a pacemaker mediated atrial Wenckebach behavior at 12. During this period, some of the pacemaker&#39;s ventricular pacing pulses are inhibited by the pacemaker to prevent pacing the ventricle at a rate above the AMTR. As the sinus rate at 16 continues to increase above the AMTR, the average ventricular pacing rate slowly decreases until a 2:1 ratio between the atrial rate and ventricular pacing occurs, as at 14. 
     FIG. 2 illustrates the pacing behavior of a conventional atrial tracking pacemaker, when the patient&#39;s heart has normal intrinsic (PR) conduction. The ventricular pacing rate tracks the intrinsic atrial rate until the atrial rate reaches an AMTR as at 15. Once the sinus rate exceeds the pre-programmed AMTR, the pacemaker inhibits ventricular pacing until the intrinsic atrial rate decreases to a rate below the AMTR corresponding with the Ventricular Inhibition Rate (VIR). The VIR is equal to the rate corresponding to the sum of the PVARP and PR intervals. When the sinus rate decreases to a rate equal to the VIR, ventricular pacing resumes until the intrinsic atrial rate again reaches the AMTR. As seen in FIG. 2, when pacing a patient (having intrinsic conduction) with a conventional atrial tracking pacemaker, there is a pronounced pacing hysteresis as represented at 18. This pacing hysteresis limits the range of pacing therapy to the patient. Unlike pacing a patient having complete heart block, the atrial tracking pacemaker will not pace a patient having intrinsic conduction when the sinus rate exceeds the AMTR. This is significant because many of the patients having intrinsic conduction who could benefit from uninterrupted ventricular pacing typically have elevated sinus rates (above the AMTR) to compensate for the reduced pumping efficiency of their failing heart. 
     Additionally, when the patient&#39;s heart has intrinsic conduction, the ventricular pacing rate by the pacemaker may even be inhibited at a rate below the AMTR. FIG. 3 illustrates that the range of pacing therapy of a conventional atrial tracking pacemaker may be further limited by a premature ventricular contraction (PVC). When a PVC occurs, and the sinus rate is above the VIR, ventricular pacing may be inhibited by the atrial tracking pacemaker until the sinus rate falls below the VIR, thereby further limiting the range of available pacing therapy. 
     Referring to FIGS. 4 and 5, the sinus rate of a patient having normal intrinsic conduction is shown in relation to the resulting paced ventricular rate of a ventricular tracking pacemaker of the present invention. Significantly, the range of pacing therapy is expanded using a ventricular tracking pacemaker of the present invention on a patient having intrinsic conduction. When the sinus rate exceeds a preset atrial maximum tracking rate (AMTR) as at 15, ventricular tracking restores a Wenckebach-like behavior at 22 (hereinafter referred to as ventricular Wenckebach). During ventricular Wenckebach, ventricular pacing occurs due to atrial tracking, until a sensed atrial event falls within the preset PVARP allowing an intrinsically conducted R-wave to occur, which inhibits ventricular pacing for that sinus beat. To restore ventricular pacing when the sensed atrial event falls within the preset PVARP, the subsequent intrinsically conducted R wave is tracked and the ventricle is paced after a preset pacing delay, subject to other variables discussed in further detail below. As the sinus rate (represented by dotted line 16) increases above the AMTR as at 15, the average ventricular pacing rate slowly decreases due to Wenckebaching at 22 until the sinus rate reaches a 2:1 ventricular tracking rate as at 24. Once the sinus rate exceeds the preset ventricular maximum tracking rate (VMTR), ventricular pacing is inhibited. Importantly, unlike the resulting behavior of an atrial tracking pacemaker, there is no significant hysteresis as the sinus rate falls below the VMTR or AMTR of the ventricular tracking pacemaker. 
     FIGS. 6-22 are various plots illustrating the occurrence of a sensed P-wave and the resulting tracked R-wave plotted over time with respect to the ventricular tracking pacemaker&#39;s preset timing intervals and varying sinus rates. The time of occurrence of the ventricular pacing stimulation is indicated by labeled block &#34;V&#34; at 30. The time of occurrence of a sensed P-wave is indicated by labeled block &#34;P&#34; at 32. A tracked P-wave is indicated by labeled block &#34;P *  &#34; at 34. A time of occurrence of a sensed R-wave due to intrinsic conduction is indicated by labeled block &#34;R&#34; at 36 and a time of occurrence of a sensed PVC is indicated by labeled block &#34;PVC&#34; as at 38 and the time of occurrence of a sensed retrograde P-wave (Retro P) is indicated at 46 (see FIGS. 10-12, 14, and 16). Several preset timing intervals are positioned relative to the time at which the P wave, R-wave or a PVC are sensed. The preset post ventricular atrial refractory period (PVARP) timing interval is indicated at 40, the preset ventricular pacing delay interval (RV) at 42, the sensed atrial to ventricular delay (SAV) at 44, the intrinsic conduction (PR) interval is indicated at 48, the preset atrial maximum tracking rate interval (AMTRI) is indicated at 50 and the preset refractory atrial to ventricular delay (RAV) is indicated at 51. 
     Two alternative timing sequences of the ventricular tracking pacemaker are illustrated by the various Figures. Only one timing sequence can be in effect during a particular cardiac cycle, however, those skilled in the art will appreciate that alternative timing sequences can be used on different cardiac cycles. The choice and use of the alternate timing sequence is programmable in the ventricular tracking pacemaker. The first timing sequence, referred to as the &#34;rate priority&#34; timing sequence or &#34;rate priority&#34; ventricular tracking, paces the ventricles after a tracked R-wave at a preset ventricular tracking rate. An alternate timing sequence, referred to as the &#34;delay priority&#34; timing sequence or &#34;delay priority&#34; ventricular tracking, paces the ventricles after a tracked R-wave to achieve a preset delay between a refractory P-wave sense and the ventricular pace, but not faster than a preset ventricular tracking rate. The delay priority timing sequence is an alternate of the more general rate priority timing sequence. As further described below, FIGS. 6-18 illustrates the general &#34;rate priority&#34; timing sequence, which applies also to &#34;delay priority&#34; ventricular tracking, except that the ventricular tracking rate is not constant for &#34;delay priority&#34; timing. FIGS. 19-22 illustrate the alternate &#34;delay priority&#34; timing sequence. 
     Referring to FIG. 6, the rate priority timing sequence of the ventricular tracking pacemaker is illustrated. An R-wave 36 is sensed, initiating the PVARP 40 and RV 42 intervals. Since a P-wave 32 is sensed during the PVARP 40 interval, the ventricle is paced after a tracked RV delay 42. If a P-wave 34 is sensed after the PVARP 40 but before the expiration of the RV 42 delay, then the ventricle is not paced until the expiration of a sensed atrial to ventricular delay 44 as shown in FIG. 7. Further, when a P-wave 32 is not sensed during the PVARP 40 interval, the ventricle is not paced at the end of the RV 42 interval, (see FIG. 8). Also, if an R-wave 36 is sensed during the RV delay 42 interval, then the RV delay 42 interval is restarted (see FIG. 9). Hence, normally the ventricle will be paced according to rate priority ventricular tracking a predetermined amount of time after an R-wave is sensed if a P-wave is also sensed during a PVARP and RV interval, unless other conditions are present. 
     FIGS. 10-12 show the tracking behavior of the ventricular tracking pacemaker when the sinus rate is between the lower rate limit (LRL) and the ventricular inhibition rate (VIR). When the sinus rate is in this range, the ventricular tracking pacemaker may be programmed to pace the ventricle according to 1:1 atrial tracking, unless a PVC causes a sensed P-wave to fall in PVARP or causes a sensed retrograde P-wave within PVARP. In such a case, when a PVC causes a sensed P-wave to fall in PVARP or causes a sensed retrograde P-wave within PVARP, ventricular tracking takes over for pacing of the ventricle. FIG. 10 illustrates a sensed PVC 38 starting the preset PVARP 40 and RV delay 42. Since a legitimate P-wave 32 is sensed during the PVARP 40 and the intrinsic PR 48 conduction delay extends beyond the RV delay 42, the ventricle is paced at the end of the RV delay 42 interval. FIG. 11 illustrates a PVC causing a sensing of an R-wave 36 during the RV delay 42 interval and restarting of the RV delay 42 interval. When the sinus rate is between the lower rate limit (LRL) and the ventricular inhibition rate (VIR), a PVC 38 may cause a sensed retrograde P-wave 46. As shown in FIG. 12, a PVC cannot cause a pacemaker mediated tachycardia due to ventricular tracking. The PVC starts an RV 42 interval and the sensed retrograde P-wave 46 enables a ventricular tracking pace at 30, which in turn causes a second retrograde P-wave that is not followed by a ventricular pace because a second RV 42 interval is not initiated at the ventricular 30 pace. 
     Referring now to FIGS. 13 and 14, these plots illustrate the general reaction of the ventricular tracking pacemaker when the sinus rate is between the ventricular inhibition rate and the atrial MTR. Again in this rate range the ventricular tracking pacemaker may pace the ventricle in accordance with 1:1 atrial tracking, however since a single PVC 38 could cause continuous inhibition of ventricular pacing, it is necessary to pace the ventricle in accordance with ventricular tracking, unless a P *  -wave 34 is sensed outside of the PVARP 40. When an R-wave 36 due to intrinsic PR 48 conduction is sensed, wherein the sinus rate is greater than the ventricular inhibition rate, the ventricular tracking is triggered to restore ventricular pacing. A sensed R-wave 36 and P-wave 32 during PVARP 40 causes the pacing of the ventricle after the preset RV delay 42 (see FIG. 13). When a PVC is sensed and causes the intrinsic P-wave to be sensed during PVARP, ventricular tracking is triggered (see FIG. 14). If a PVC occurs such that a P-wave is sensed outside the PVARP, pacing of the ventricle after the RV delay is inhibited and normal atrial tracking occurs (see FIG. 14). Hence, it can be seen that in this rate range the ventricular tracking pacemaker differs from a conventional DDD pacemaker. While a single PVC can cause an atrial tracking pacemaker to inhibit ventricular pacing, the ventricular tracking pacemaker will pace the ventricle after the RV delay as described above. 
     FIGS. 15-17 show the general tracking behavior of the ventricular tracking pacemaker when the sinus rate is between the atrial MTR (AMTR) and the ventricular maximum tracking rate (VMTR). When the sinus rate is in this range, the pacer goes into a ventricular Wenckebach behavior, during which an intrinsically conducted R-wave occasionally inhibits a ventricular pace. The ventricular tracking pacemaker will continue to generate Wenckebach ventricular pacing behavior as long as the RV interval is less than the AMTRI (see FIG. 15). FIG. 16 shows that a PVC in this range can cause an extra Wenckebach cycle, wherein the ventricular tracking eventually restores ventricular pacing. Eventually, ventricular tracking reaches a 2:1 ventricular rate tracking (see FIG. 17). As illustrated in FIG. 18, once the sinus rate exceeds the VMTR, ventricular pacing is inhibited, because every RV interval is restarted by an intrinsic R wave before pacing can occur. 
     FIGS. 19-22 illustrate the alternate timing sequence of the ventricular pacemaker that dynamically extends the time of ventricular pacing after the RV delay expires, to thereby maintain a preset delay between the atrial sense and ventricular pace due to ventricular tracking. This timing sequence of the ventricular pacemaker is referred to as delay priority ventricular tracking and is illustrated generally in FIGS. 19 and 20. An R-wave 36 is sensed, initiating the PVARP 40 and RV 42 intervals. When a P-wave 32 is sensed during the PVARP 40 interval, a preset refractory atrial to ventricular delay (RAV) is initiated. As shown in FIG. 19, when the RV 42 interval ends before the expiration of the RAV 51 delay, the ventricle is paced at the end of the RAV 51 delay, as long as the RAV 51 delay is less than the intrinsic PR 48 interval. This has the effect of extending the duration of the RV 42 interval by the RV extension 52 period indicated by the highlighted portion of the box. 
     FIG. 20 illustrates the delay priority timing sequence when the P-wave 32 is sensed during the PVARP 40 interval after the tracked RV delay 42 has ended. The RV delay 42 ends without pacing the ventricle because the P-wave 32 is not sensed during the RV delay 42. Instead, the ventricle is paced after the expiration of the RAV 51 interval, which is initiated by the P-wave 32 sensed during the PVARP 40 interval. Again, this has the effect of extending the duration of the RV 42 interval by the RV extension period. 
     FIGS. 21 and 22 illustrate that the delay priority timing sequence reverts to the rate priority timing sequence when the delay priority timing sequence would result in pacing the ventricle before the end of the RV interval. As shown in FIG. 21, an R-wave 36 is sensed, initiating the PVARP 40 and RV 42 intervals. When a P-wave is sensed during the PVARP 40 interval, the RAV 51 delay is initiated. Since the RAV 51 delay ends before the RV 42 interval, the ventricle is paced after the RV 42 interval, as long as it expires before the intrinsic PR 48 interval expires. When the RV 42 interval expires after the intrinsic PR 48 interval, as illustrated in FIG. 22, an intrinsic R-wave 36 is detected, which restarts the RV 42 interval. 
     Hence, the ventricle will be paced according to delay priority ventricular tracking a predetermined amount of time after a P-wave is sensed during PVARP following an R-wave, but not faster than the RV delay, unless other conditions are present. Those skilled in the art will appreciate that since delay priority ventricular tracking only has the effect of extending the RV interval, all the rate priority timing sequences shown in FIGS. 6-18 also apply to the ventricular pacemaker having a delay priority timing sequence when the RV interval in those Figures is replaced by an extended RV interval (the RV interval plus RV extension period) when it applies as shown in FIGS. 19 and 20. 
     Referring next to FIG. 23, the preferred embodiment of the invention is shown generally in block diagram, wherein the cardiac stimulator or ventricular tracking pacemaker 110 (enclosed by a dotted line) operatively connects to a patient&#39;s heart 112 by electrical conductors 114, 116, and 118 embodied in a pacing lead 120 for atrial and ventricular dual chamber pacing. The first end of the pacing lead 122, inerted into the patient&#39;s heart 112, branches into an atrial branch 124 and a ventricular branch 126. The atrial branch 124 connects to a first set of stimulating and sensing electrodes 128 which are adapted to be disposed in the right atrium of the heart 112 and is arranged to sense the occurrence of P-wave activity relating to atrial events. The ventricular branch 126 connects to a second set of stimulating and sensing electrodes 130, which are adapted to be disposed in either the right or left ventricle of the heart. Those skilled in the art will appreciate that other pacing and/or sensing leads of suitable known construction may be coupled to the cardiac stimulator and disposed in the patient&#39;s heart. 
     The atrial branch 124 is connected by electrical conductor 114 to an atrial sense amplifier 132 which detects P-waves associated with atrial events. The resulting atrial event signal is fed to an input of a microprocessor-based controller 134. In a similar fashion, the ventricular branch 126 is operatively coupled by conductor 116 to a ventricular sense amplifier 136. The ventricular sense amplifier 136 functions to detect R-wave activity relating to ventricular depolarization. The signal representing the R-wave activity is then fed to an input of a microprocessor-based controller 134. 
     The microprocessor-based controller 134 is programmed to operate in any one of a plurality of known pacing modes and includes a ventricular tracking mode of the present invention. Also coupled to the microprocessor 134 is a timing circuit 150, atrial tracking circuit 152, and ventricular tracking circuit 154. The microprocessor 134 has both RAM (random access memory) 138, and ROM (read only memory) 140 for storing programs and data, which allows: the processing of the sensed signals, triggering the pulse generator 142, determining a sinus rate from the sensed signals, analyzing the sensed signals, and storing various information derived from the analysis. While FIG. 23 depicts a pacing/sensing lead in the right atrium and right ventricle, those skilled in cardiac rhythm management systems will appreciate that other leads of known construction may be positioned in other areas of the heart and coupled to corresponding amplifiers and the Microprocessor-based controller. 
     The microprocessor 134 controls the cardiac stimulating pulses delivered by pulse generator 142 to one or both of the first and second stimulating electrodes 128 and 130 (depending upon the pacing mode selected). An external programmer 144 having a microprocessor and associated memory may transmit information in a conventional way through a telemetry link 146 and transmission receiver 148 of the cardiac stimulator&#39;s microprocessor. Using the programmer 144 and the telemetry link 146, operating parameter values for the pacemaker 110 can be delivered to it by a cardiologist for setting the cardiac cycle pacing parameter values to be utilized, including various timing intervals. Cardiac stimulating devices capable of telemetering various status information including selecting the pacing parameters and mode (determined by the physician) are commercially available from, for example, Cardiac Pacemakers, Inc., St. Paul, Minn. 
     FIG. 24 shows an algorithm that may be used by the ventricular tracking pacemaker of the present invention for rate priority ventricular tracking to track intrinsic conduction from the ventricle and accordingly pace the ventricle over a broader pacing range. Initially, a signal is transmitted through sensing lead 120 from the atrium of a patient&#39;s heart and P-waves from the signal are identified and tracked (see block 200). A signal is also transmitted through sensing lead 120 corresponding to events of the ventricle of a patient&#39;s heart and R-waves from the signal are identified (see block 202). The clock timer of the timing circuit 150, the RV delay interval and the PVARP interval are initialized in conjunction with the sensing of an R-wave (see blocks 204 and 206). Once the time on the clock timer exceeds the preset RV delay (see decision block 208) then the ventricle is paced at block 214 if: a P-wave is sensed during the PVARP interval, a sensed atrial to ventricular delay has not been started to track a P-wave sensed outside of PVARP (see decision blocks 210 and 212). If an R-wave is sensed during the RV delay interval, the timer is reset and the RV and PVARP intervals are re-initiated (see decision block 211 and loop 216). If a P-wave is tracked by an SAV delay, the sensing and tracking is re-initiated (see loop 218). The ventricles will not be paced according to this algorithm, unless a P-wave is sensed during PVARP (see loop 218). 
     FIGS. 25 and 26 show algorithms that may be used by the ventricular pacemaker of the present invention for &#34;delay priority&#34; ventricular tracking. The algorithm shown in FIG. 25 is implemented when the RV interval is constrained to be greater than or equal to the PVARP. The algorithm shown in FIG. 26 is implemented when the RV interval is allowed to be less than the PVARP. Referring to FIG. 25, initially a signal is transmitted through sensing lead 120 from the atrium of a patient&#39;s heart and P-waves from the signal are identified and tracked (see block 270). A signal is also transmitted through sensing lead 120 corresponding to events of the ventricle of the patient&#39;s heart and R-waves from the signal are identified (see block 272). The clock timer of the timing circuit 150, the RV delay interval and the PVARP interval are initialized in conjunction with the sensing of an R-wave (see blocks 274 and 276). During a time not greater than the RV interval (see decision block 278) it is determined whether a P-wave is sensed during the PVARP interval (see decision block 280). If a P-wave is sensed during the PVARP interval, then a RAV interval is initiated from the current time (see block 282). Also during the time when the time of the clock timer is not greater than the RV interval, if an R-wave is sensed (as at decision block 284), then the clock timer is reset (see loop 286). When the time on the timer exceeds the RV interval, as at 278, then it is determined whether the P-wave is being tracked by an SAV delay (see decision block 290) and whether a P-wave has been sensed during PVARP (see decision block 292). If the P-wave is being tracked by an SAV delay or a P-wave has not been sensed during PVARP, then ventricular tracking is reset (see loop 288). If the P-wave is not being tracked by an SAV delay and a P-wave has been sensed during PVARP, then the clock timer is compared to the RAV interval as at decision block 294. When the time of the clock timer exceeds the RAV interval, then the ventricle is paced as at block 298, unless an R-wave is sensed first (see block 296), which then resets the clock timer without pacing the ventricle and the algorithm is then repeated (see loop 286). 
     Referring now to FIG. 26, an algorithm is shown that may be used by the ventricular pacemaker of the present invention for &#34;delay priority&#34; ventricular tracking when the RV interval is allowed to be less than the PVARP. Initially, a signal is transmitted through sensing lead 120 from the atrium of a patient&#39;s heart and P-waves from the signal are identified and tracked (see block 230). A signal is also transmitted through sensing lead 120 corresponding to events of the ventricle of a patient&#39;s heart and R-waves from the signal are identified (see block 232). The clock timer of the timing circuit 150, the RV delay interval and the PVARP interval are initialized in conjunction with the sensing of an R-wave (see blocks 234 and 236). If the time on the timer is not greater than RV, and a P-wave is sensed during PVARP, then an RAV interval is initiated from the current time and it is then determined whether the time on the timer is greater than the RV as long as a new R-wave has not been sensed (see decision blocks 250, 252, 238 and 248). 
     Once the time on the clock timer exceeds the preset RV delay (see decision block 238) then it is detened whether the P-wave is being tracked by an SAV delay (see decision block 240). If the P-wave is being tracked by an SAV delay, then ventricular tracking is reset (see loop 264). If the P-wave is not being tracked by an SAV delay at decision block 240, then it is determined whether a P-wave has been sensed during PVARP (see decision block 242). If a P-wave is sensed during PVARP at 242 and the time is greater than the RAV interval (block 246), then the ventricle is paced at block 262. If a P-wave has not been sensed during PVARP at block 242, it is then determined whether the time is greater than PVARP (see decision block 244). When the time exceeds the PVARP as at 244, then ventricular pacing is reset (see loop 264) unless a P-wave is sensed during PVARP (see block 256). In that case, an RAV delay is initiated at the current time (see block 258), and after the time on the timer is greater than the RAV interval (see decision block 246), the ventricle is paced at 262 as long as a new R-wave has not been sensed (see decision block 260). If a new R-wave is sensed during the algorithm (see decision blocks 248, 254, and 260), then the clock timer is reset and the RV and PVARP delays are re-initiated (see loop 249). 
     Another algorithm that may be used by the ventricular tracking pacemaker of the present invention for delay priority ventricular tracking includes a pre-programmed modification of the conventional atrial tracking timing intervals after a sensed R-wave so that the atrial tracking rate is temporarily increased to the ventricular maximum tracking rate. When a pacing cycle follows a ventricular pace, the normal atrial tracking timing intervals are implemented by the pacemaker. However, when a pacing cycle follows a sensed R-wave, the atrial tracking timing interval resets for the next cycle to the following: the atrial maximum tracking rate interval is set to equal the preset RV interval, the SAV interval is set to equal the preset RAV interval, and the PVARP interval set to be less than the time of the RV interval minus the PR interval. With these reset timing intervals, a conventional atrial tracking algorithm is used to control ventricular pacing for the cycle (a conventional atrial tracking algorithm may include the following: when a P-wave is sensed outside of PVARP, the SAV interval is initiated and when it expires, the ventricle is paced unless the maximum tracking rate interval has not expired, in which case the ventricular pace is delayed until the end of the maximum tracking rate interval. After the cycle, the RV interval, SAV interval and PVARP intervals return to their preset intervals. Those skilled in the art will appreciate that this algorithm will produce the same pacing behavior as that described above in conjunction with FIG. 26. 
     It is recognized that the length of the RV delay and RAV intervals may be varied relative to other timing intervals of the pacemaker to control the ventricular tracking behavior of the ventricular tracking pacemaker. Further, the varied length of the RV delay and RAV intervals may also depend on features of a conventional atrial tracking pacemaker. 
     This invention has been described herein in considerable detail in order to comply with the patent statutes and to provide those skilled in the art with the information needed to apply the novel principles and to construct and use such specialized components as are required. However, it is to be understood that the invention can be carried out by specifically different devices, and that various modifications, both as to the equipment details and operating procedures, can be accomplished without departing from the scope of the invention itself.