Abstract:
Methods for improving detection of arrhythmias by adaptively increasing arrhythmia detection intervals. One method includes increasing the V2V, the overall cardiac cycle length, thereby decreasing the pacing rate in the presence of ventricular safety paces (VSPs). Another method includes shortening the trigger interval following the atrial pace event, during which time the pacemaker will detect V-sense events, while leaving the A2V VSP interval unchanged, at the end of which any required VSP will be generated. In yet another method, the interval from A-pace to V-pace, the PAV interval, is shortened, while leaving the overall V2V cycle interval unchanged. This increases the ventricular to artial V2A interval, increasing the detection window for arthythmias. The PAV interval can be shortened in response to a recent history of VSP events.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to cardiac pacemakers. More particularly, the present invention relates to cardiac pacemakers having improved methods for detecting arrhythmias.  
         BACKGROUND OF THE INVENTION  
         [0002]    An arrhythmia is a heart rhythm disorder which interferes with the life sustaining blood pumping action of the heart. Examples of arrhythmias include ventricular tachycardia and atrial tachycardia. Ventricular tachycardia effects the lower chambers of the heart, the ventricles, and atrial tachycardia effects the upper chambers of the heart, the atria. Ventricular tachycardia is a rapid heart beat initiated within the ventricles, characterized by three or more consecutive premature ventricular beats. Ventricular tachycardia is a potentially lethal arrhythmia, as it may cause the heart to become unable to pump adequate blood through the body. Companies such as Medtronic, Inc., have developed implantable pacemakers which may be used to successfully treat ventricular tachycardia by delivering ventricular pacing pulses to the heart when ventricular tachycardia is detected.  
           [0003]    Dual chamber pacing modes have been widely adopted for pacing therapy. Among the dual chamber operating modes is the “DDD” mode, which can pace an atrium and a ventricle, senses both the atrium and the ventricle, and can either inhibit or trigger pacing stimuli for both chambers. This mode has a sensor augmented variant mode called “DDDR”, where the “R” stands for rate-adaptive or rate modulation.  
           [0004]    A DDD pacemaker includes an atrial sense amplifier to detect atrial depolarizations of the heart, and a ventricular sense amplifier to detect ventricular depolarizations of the heart. If the atrium of the heart fails to beat within a predefined time interval (atrial escape interval), the pacemaker supplies an atrial stimulus to the atrium through an appropriate lead system. Following an atrial event (either sensed or paced) and an atrioventricular (A-V or A2V) interval, the pacemaker supplies a ventricular pacing stimulus to the ventricle through an appropriate lead system, if the ventricle fails to depolarize on its own. Pacemakers which perform this function have the capability of tracking the patient&#39;s natural sinus rhythm and preserving the hemodynamic contribution of the atrial contraction over a wide range of heart rates.  
           [0005]    Various types of pacemakers are disclosed in the prior art, and are presently in widespread use. The pacing literature has documented the different types of pacemakers and their characteristics extensively. A summary of the evolution and characteristics of pacemaker types, and specifically different types of dual chamber pacemakers, is set forth in U.S. Pat. No. 4,951,667, which is incorporated herein by reference.  
           [0006]    Another and more recent advance in the field of cardiac pacing systems is that of the rate responsive pacemaker which increases cardiac output in response to exercise or other body demands. Such pacemakers may control pacing rate based upon sensing any one or a combination of different body parameters such as body activity, blood pH, respiratory rate, QT interval or historical atrial activity. See, for example, U.S. Pat. No. 4,428,378, (Anderson et al.), disclosing a pacemaker which varies pacing rate in response to sensed patient activity; and U.S. Pat. No. 4,228,308, (Rickards), which discloses controlling pacing rate in response to Q-T interval. Additionally, rate responsive control has been integrated into dual chamber pacing systems, e.g., DDDR and DDIR systems. See “Rate Responsive Dual Chamber Pacing” in PACE, vol. 9, pp. 987-991; U.S. Pat. No. 4,467,807, Bornzin; and the above-noted U.S. Pat. No. 4,951,667.  
           [0007]    Background information directly related to the present invention may be discussed in greater detail. The atrium may be paced with an A-pace. The energy from the A-pace may be sensed by the ventricle amplifier as a V-sense event. This is referred to as an over-sense or cross-chamber sensing. It is not really a contraction of the ventricle, but is rather the electrical activity of the atrium being detected by the sensor in the ventricle. In this situation, the ventricle may not have actually contracted. If the V-sense event is too close to the A-pace event, a ventricular safety pace (VSP) stimulation pulse is given to the ventricle, in case the V-sense was actually an indication of a premature ventricular contraction, which might continue as ventricular tachycardia.  
           [0008]    In many patients, it would be desirable to wait until closer in time to the expected time of a natural ventricular contraction. However, waiting too long would put the VSP pulse at about the same point in time as the T-wave, which would be undesirable, as pacing in the middle of the T-wave may cause an arrhythmia. The VSP pulse is given because of a premature V-sense, which is believed to not be an indication of an actual ventricular contraction. If the V-sense reflected a real ventricular contraction, there would be nothing seen from the ventricle until the next natural event. Therefore, waiting a long period would gain nothing. If the V-sense was an over-sense, then waiting for the V-sense reflecting an actual ventricular contraction would require waiting too long, putting any required V-pace too close to the T-wave. Thus, in this situation, while it is not known that the V-sense reflected an actual premature ventricular contraction, it is desirable that the ventricle contract. Therefore the VSP pulse will be generated to ensure that the ventricle contracts.  
           [0009]    When a pacemaker is operated in DDD mode, the atrium is paced in the absence of a sensed natural event. After the A-pace, there is a time period, a trigger window, within which a V-sense may be detected. If a V-sense is detected during this window, then a VSP pulse will be scheduled, at the end of the VSP timing window or interval.  
           [0010]    In one example, where a desired pacing rate of about 120 beats per minute is desired, the VSP, if it is to occur at all, will be scheduled at about 60 milliseconds after the A-pace. In the example where a slower desired pacing rate of about 60 beats per minute is desired, the VSP, if it is to occur at all, is scheduled at about 110 milliseconds after the A-pace. The VSP is normally scheduled no longer than about 80 milliseconds after the V-sense, to avoid being too close to the T-wave. In the absence of any V-sense event within the trigger window after the A-pace, the next scheduled V-pace would not normally occur for a longer period, for example, about 150 milliseconds. This interval from the A-pace to the V-pace can be based on the PAV interval.  
           [0011]    In a paced, cardiac cycle, there may be three blanking periods where the pacemaker is unable to sense arrhythmias. The first blanking period follows the A-pace. The second blanking period follows a V-sense, as it is undesirable for the pacing device to double count the V-sense event. The third blanking period follows the V-pace. Thus, if there is a ventricular arrhythmia occurring at a fast rate, the pacing device may see only every other beat, resembling a normal heart beat. This is because every other beat may lie within a blanking interval. It would be desirable to have an improved time window for detecting arrhythmias. In particular, it would be desirable to have at least half of the window between ventricular events available for detecting arrhythmias, even at high pacing rates.  
         SUMMARY OF THE INVENTION  
         [0012]    The present invention provides improved methods for cardiac pacing that may find particular use in pacing situations having high pacing rates that would otherwise have substantially shortened windows for detecting arrhythmias, and/or pacing situations forced to accept slow pacing rates to maintain long windows for detecting arrhythmias. The present invention may be described with respect to a cardiac pacing cycle proceeding from a first atrial pace (A-pace) event, followed by a first ventricular pace (V-pace) event, followed by a second A-pace event, followed by a second V-pace event, followed by further A-pace and V-pace events. The A-pace event can be followed by an A-pace blanking interval which in turn is followed by a trigger zone. During the trigger zone, the pacemaker is able to detect V-sense events. The cardiac cycle also includes an atrial to ventricular (A2V) ventricular safety pace (VSP) timing interval, which can begin at the A-pace event. If a V-sense event is detected during the trigger zone, a VSP pace can be generated at the end of the A2V VSP timing window.  
           [0013]    The cardiac cycle further includes a V-pace blanking interval following the V-pace event, and a PAV interval giving the scheduled interval between an A-pace event and the following V-pace event. Finally, the cardiac cycle may be characterized by a cardiac overall pacing interval, the ventricular to ventricular (V2V) interval, giving the time from one ventricular event until the next ventricular event.  
           [0014]    The present invention may be used to avoid or substantially reduce events where the time available for detecting arrhythmias would otherwise be less than half the V2V interval, or half the overall cardiac cycle interval. This may otherwise be a problem, where the blanking intervals, which are substantially fixed in length, take over half of the V2V interval as the V2V interval decreases, as may occur during high physiological activity periods.  
           [0015]    One method according to the present invention reduces the length of the A2V trigger zone in response to recent occurrences of VSP paces. The trigger zone interval length may be decreased in response to a recent higher frequency of VSP paces, and increased in response to a decrease in recent VSP paces. The trigger zone may be decreased, such that the end of the trigger zone occurs substantially prior to the end of the A2V VSP interval. Another method according to the present invention adjusts the overall pacing rate or pacing interval in response to the recent history of VSP events. The overall cycle interval can be increased in response to a recent history of VSP events, and decreased in response to a lack of recent VSP events. The overall pacing rate may thus be decreased below that otherwise called for in the presence of VSP events in order to lengthen the ventricular to atrial (V2A) window for detecting arrhythmias. In yet another method according to the present invention, the arterial to ventricle interval (PAV) may be decreased in response to the recent occurrence of VSPs, while leaving the overall cycle interval unchanged. This will also increase the arrhythmia detection window between the V-pace and the A-pace events.  
           [0016]    One method according to the present invention includes increasing the overall pacing interval responsive to detecting V-sense events, and decreasing the overall pacing interval responsive to recent overall pacing intervals having a minimum V2V interval ending in a ventricular pace that is long enough so as to not interfere with arrhythmia detection. Another method according to the present invention includes waiting for detection of a ventricular event, either a V-sense or a V-pace event. Upon detection of the ventricular event, if the ventricular event is a V-sense detected during the trigger zone, the overall pacing interval is increased. Upon detection of the ventricular event, if the ventricular event is either not a V-sense, or the ventricular event is detected outside the trigger zone, and if a recent history indicates all recent pacing intervals have at least a DV2V interval, then the overall pacing interval is decreased. The DV2V interval represents the minimum V2V interval ending in a ventricular pace that is long enough so as to not interfere with arrhythmia detection.  
           [0017]    In yet another method according to the present invention, the trigger window interval is increased in response to detecting recent VSP events, and the trigger window interval is decreased in response to detecting VSP events. The detecting steps can include detecting N VSP events in the previous M time slots, where N is an integer and M has units of time.  
           [0018]    In still another method according to the present invention, the method includes waiting for detection of a ventricular event, either a V-sense or a V-pace. Upon detection of the ventricular event, if the most recent V2V interval is not less than the DV2V interval, and all of the most recent N V2V intervals were greater than or equal to DV2V, then the PAV value is increased, but not above an upper limit. Upon detection of the ventricular event, if the most recent V2V interval is less than the DV2V interval, then the PAV value is decreased, but not below a lower limit. The PAV value is then used to schedule the next V-pace.  
           [0019]    The present invention further includes computer programs for executing methods described herein. The present invention includes pacemakers containing programs and executing those programs for executing methods described in the present application. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1 is a cross-sectional view of a heart having ventricles and atria;  
         [0021]    [0021]FIG. 2 is a diagrammatic view of a pacing system in accordance with the present invention;  
         [0022]    [0022]FIG. 3 is a schematic, timing diagram of a cardiac pacing cycle having a fixed A2V VSP interval and a cross chamber blanking interval, followed by a trigger zone, where the trigger zone terminates at the same point in time as the A2V VSP interval;  
         [0023]    [0023]FIG. 4 is a schematic, timing diagram of a cardiac pacing cycle similar to that of FIG. 3, but having a shortened A2V VSP interval, and a shortened trigger zone which terminates at the same point in time as the A2V VSP interval;  
         [0024]    [0024]FIG. 5 is a timing diagram similar to that of FIG. 3, but having a shortened trigger zone which terminates earlier in time that the A2V VSP interval;  
         [0025]    [0025]FIG. 6 is a high level flow chart of a method for switching between the cardiac pacing methods of FIGS. 3, 4, and  5 ;  
         [0026]    [0026]FIG. 7 is a flow chart of a method for adapting the overall pacing interval in the presence of VSP;  
         [0027]    [0027]FIG. 8 is a timing diagram of a method adapting the paced atrial to ventricle interval (PAV) to short V2V intervals;  
         [0028]    [0028]FIG. 9 is a flow chart of a method for adapting the paced arterial to ventricle (PAV) interval in the presence of VSP; and  
         [0029]    [0029]FIG. 10 is a block diagram of a pacing system in accordance with an exemplary embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]    The following detailed description should be read with reference to the drawings, in which like elements in different drawings are numbered identically. The drawings, which are not necessarily to scale, depict selected embodiments and are not intended to limit the scope of the invention. Several forms of invention have been shown and described, and other forms will now be apparent to those skilled in art. It will be understood that embodiments shown in drawings and described above are merely for illustrative purposes, and are not intended to limit scope of the invention as defined in the claims which follow:  
         [0031]    [0031]FIG. 1 is a cross-sectional view of a heart  302  having ventricles  304  and atria  322 . Ventricles  304  of heart  302  include a left ventricle  308  and a right ventricle  320 , and atria  322  of heart  302  include a left atrium  324  and a right atrium  326 . In FIG. 1, it may be appreciated that heart  302  includes a conductive path  328  extending between atria  322  and ventricles  304 . In heart  302 , conductive path  328  includes an atrioventricular (AV) node  330 , a bundle of His  332 , a right bundle branch  334 , and a left bundle branch  336 .  
         [0032]    Heart  302  also includes a sinoatrial (SA) node  338 . In a healthy heart, the SA node acts as a natural pacemaker controlling the life sustaining blood pumping action of the heart. At an appropriate time, an electrical impulse arising from the SA node is transmitted to the right and left atrial chambers. This impulse causes muscle tissue surrounding the atrium to depolarize and contract which generates an electrical signal known as a P-wave. The same electrical impulse arising from the SA node also travels to the right and left ventricles through the atrioventricular (AV) node. The impulse received by the AV node is transmitted through the bundle of His, the right bundle branch, the left bundle branch, and a plurality of Purkinje fibers that encompass most of the endocardial surface of the ventricles. The ventricular muscle tissue depolarizes, then contracts. This forces blood held in the ventricles through the arteries and to various body locations. This action is repeated in a rhythmic cycle in which the atrial and ventricular chambers alternately contract and pump, then relax and fill.  
         [0033]    [0033]FIG. 2 is a diagrammatic view of a pacing system  340  in accordance with the present invention. Pacing system  340  includes a pacemaker  342  that is coupled to heart  302  of FIG. 1 by a plurality of leads  344  and electrodes  346 . Pacemaker  342  may be used to treat a heart in which the natural pacing system has ceased performing properly. Pacemaker  342  may have a single electrode operation in which pacing current flows between an electrode  346  and a housing of pacemaker  342 . Pacemaker  342  may also have a dual electrode operation in which pacing current flows between two or more electrodes.  
         [0034]    Some methods in accordance with the present invention may include the step of severing the conductive path between atria  322  and ventricles  304 . In some methods, the step of severing the conductive path may include the step of ablating the AV node of a heart. The step of ablating the AV node may be accomplished, for example, using a catheter including an ablation electrode coupled to a source of radio or other form of frequency energy. By comparing FIG. 1 and FIG. 2, it may be appreciated that the AV node of heart  302  has been ablated in the embodiment of FIG. 2.  
         [0035]    [0035]FIG. 3 illustrates a standard method of pacing using the ventricular safety pacing (VSP) feature. The ventricular safety pace is a ventricular pace event that is delivered after an atrial pace, if a ventricular sense is detected in a short window after the atrial pace. In this method, there is a fixed atrial-to-ventricular (A2V) ventricular safety pacing (VSP) interval. In the method illustrated, the A2V VSP interval  30  is 110 ms. long. The A2V interval includes a 30 ms. cross chamber blanking zone  32  and an 80 ms. trigger zone  34 . The A2V VSP interval  30  begins with an atrial pace  36 . If a ventricular sensed event occurs in the trigger zone, the device delivers a ventricular safety pace at the end of the A2V VSP interval  33 .  
         [0036]    [0036]FIG. 3 also illustrates the paced arterial to ventricular (PAV) interval at  37 , extending from Apace  36  to ventricular pace (Vpace)  31 . A Vpace blanking interval  35  follows Vpace  31 , which is followed by a V2A detection interval  38 . A second Apace  39  ends one cardiac cycle and can define the cardiac A2A interval, along with the V2V interval.  
         [0037]    The arrhythmia detection process may be blinded from the atrial pace  36  to the end of the ventricular pace blanking  35  that starts with the ventricular safety pace. This is not a problem as long as the interval between the ventricular safety pace at  33  and the next atrial pace  39  is long enough to allow an arrhythmic event to be sensed before the next atrial pace. One way of minimizing the impact of ventricular safety pacing on detection of a ventricular sensed event is to shorten the A2V VSP interval. In FIG. 3, the cross-chamber blanking interval  32  is 30 ms. long.  
         [0038]    [0038]FIG. 4 illustrates a method utilizing a shorter A2V VSP interval than that of FIG. 3. The blanking interval  42  is again 30 ms. long, followed in this method by a shorter, 40 ms. long trigger zone  44 . Together, blanking zone  42  and trigger zone  44  form an A2V VSP timing interval  40  having a duration of about 70 ms. One problem with this method is that at slower pacing rates, an A2V interval less than 110 ms. may not be desirable.  
         [0039]    It may be better yet for the patient to have longer A2V VSP intervals at the faster rates. In cases where there are very few VSP paces, detection will work with the longer VSP. In these cases, an algorithm that switched to a VSP like that of FIG. 5 below, at high pacing rates, would decrease the number of VSP events. If multiple VSP events occurred in a specified amount of time, the device would switch to the VSP method described in FIG. 4.  
         [0040]    [0040]FIG. 5 illustrates a ventricular safety pacing method having a short trigger zone. The ventricular safety pacing method of FIG. 5 includes an A2V VSP timing interval  50 , having a length of 110 ms., terminating at a VSP pace point  58 , where a VSP pace is delivered if a ventricular event is sensed by the device within the trigger zone. A blanking interval of 30 ms. is indicated at  52 , followed by a 40 ms. long trigger zone indicated at  54 . The blanking interval begins with atrial pacing event  56 , as before.  
         [0041]    One method according to the present invention may be briefly described with reference to FIG. 6. If a high rate pacing is not desired at step  60 , then the ventricular safety pacing method  61  of FIG. 3 is used. If a high rate of pacing is desired at step  60 , then it must be determined at step  62  whether N VSP paces have occurred in the last M seconds. In other words, determine whether a limit number N of paces occurred within a time window M. If a sufficient number of VSP paces have occurred within the time window, then the VSP method of FIG. 4 is used at  63 . On the other hand, if an insufficient number of VSP have not occurred in the time window, then the VSP method of FIG. 5 is used at  64 . This overall method can decrease the adverse effect, if any, that VSP events have on detection at high pacing rates. These adverse effects on detection can be eliminated if the device decreases the pacing rate in the presence of VSP events, and increases the pacing rate in the absence of VSP events.  
         [0042]    [0042]FIG. 7 illustrates a method for adapting the pacing rate in the presence of ventricular safety pacing. As used herein, “V2V interval” is defined to be the interval between the previous ventricular event to the current ventricular event. As used herein, “ventricular event” includes Vsenses and Vpaces, but not ventricular safety paces. In step  100 , the method waits for a ventricular event. Upon detection of a ventricular event, either a V-sense or a V-pace, but not a VSP, path  102  is followed, and step  104  executed. At step  104 , a determination is made as to whether the ventricular event was a V-sense and within the trigger zone. If the ventricular event detected was a V-sense within the trigger zone, then path  108  is followed. Otherwise, the ventricular event detected is paced, and path  106  is followed. If path  106  is followed, then step  124  is executed.  
         [0043]    In step  124 , a determination is made as to whether the dynamic upper activity interval (DUAI) is greater than the upper activity interval (UAI). The UAI is the programmed minimum V2V escape for brady pacing. The DUAI is the dynamic upper activity level. This interval is greater than or equal to UAI and less than or equal to VSPV2V. If the DUAI is not greater than UAI at  124 , then DUAI is already at minimum, so there is no need to check further with respect to decreasing DUAI. In this situation, path  126  is followed and step  100  is executed again, waiting for the next ventricular event. If the DUAI is greater than the UAI, then path  128  is followed from step  124  to step  130 . In step  130 , a determination is made as to whether all of the last N V2V intervals are greater than or equal to DV2V, the minimum V2V interval ending in a ventricular pace that is long enough such that it will not interfere with detection. If all of the last N V2V intervals are not greater than or equal to DV2V at  130 , then path  134  is followed and step  120  is executed.  
         [0044]    In step  120 , the dynamic upper activity interval (DUAI) is used as the upper activity interval (UAI) in calculating the current V2V escape. After execution of step  120 , path  122  is followed, returning control to step  100  to wait for another ventricular event.  
         [0045]    At decision step  104 , if the ventricular event is in a trigger zone, path  108  is followed to execute step  110 . In step  110 , the dynamic upper activity interval (DUAI) is increased by an increment VSPI, where VSPI is the increment to DUAI when a VSP event takes place. After incrementing DUAI, step  112  is executed.  
         [0046]    In decision step  112 , a determination is made as to whether the dynamic upper activity interval (DUAI) is less than VSPV2V, the minimum V2V escape following a ventricular safety pace that will insure the safety pace does not interfere with arrhythmia detection. If the dynamic upper activity interval (DUAI) is not less than this minimum V2V escape, VSPV2V at  112 , then path  116  is followed and step  118  is executed. In step  118 , the dynamic upper activity interval (DUAI) is set equal to the minimum V2V escape previously discussed VSPV2V. After execution of step  118 , step  120  is executed, followed by step  100  as previously discussed.  
         [0047]    Referring again to step  130 , execution is analyzed for the case where all of the previous N V2V intervals were greater than or equal to the minimum V2V interval DV2V, in which case path  132  is followed to step  136 . N is equal to 10 seconds in some methods. Step  130  may be viewed as determining whether there are all recent short V2V intervals, meaning normal sinus rhythm. If there has recently been normal sinus, then DUAI can be decreased, increasing the pacing rate. In step  136 , the dynamic upper activity interval (DAUI) is decreased by the amount VSPD. After DAUI is decreased in step  136 , execution proceeds to a decision step  138 . In decision step  138 , a determination is made as to whether the dynamic upper activity interval (DUAI) is greater than the upper activity interval that is the programmed minimum V2V escape for brady pacing UAI. If the dynamic upper activity interval (DUAI) is greater than the upper activity interval (UAI), then path  140  is followed and step  120  is executed as previously discussed. If the dynamic upper activity interval is not greater than the upper activity interval, then path  142  is followed, and step  144  is executed. In step  144 , the upper activity interval (DUAI) is set equal to the upper activity interval (UAI). After execution of step  144 , path  146  is followed, returning execution to step  100  to wait for the next ventricular event.  
         [0048]    The method of the present invention gradually slows the pacing rate to insure proper detection of arrhythmia. In the absence of arrhythmia and/or any cross-talk, the method allows the pacing rate to gradually return to the desired brady pacing rate. The rate at which the intervals are changed is controlled by setting the parameters VSPI and VSPD, where VSPI is the increment added when a VSP event takes place. VSPD is the decrement which is subtracted when a VSP does not take place.  
         [0049]    In another aspect of the invention, the PAV interval length may be adaptively varied. In some situations, it is advantageous to set the time from an atrial pace to the next scheduled ventricular pace PAV to a long value, even at fast pacing rates. By doing this, the patient is given a better chance to receive a conducted ventricular contraction. This may make detection arrhythmias difficult. Detection may not know whether a ventricular event with a short V2V interval and an atrial pacing event in the V2V interval, should be treated as a conducted event or as an arrhythmic event. If, in the presence of a short V2V interval, the atrial pace to ventricular pace, the PAV is decreased, while the ventricular event to a ventricular pace, the V2V escape is maintained, the device can prevent a string of short V2V intervals caused by conducting an atrial pace. Thus, the Apace, but not the Vpace, is delayed, increasing the V2A interval.  
         [0050]    [0050]FIG. 8 illustrates the aforementioned situation. FIG. 8 includes a time line  180  having thereon a series of three ventricular events, V1 at  182 , V2 at  184 , and V3 at  186 . Time line  180  also includes a first atrial pacing event, A1 at  188 , and a second atrial pacing event, A2 at  190 . As may be seen from inspection of FIG. 8, the interval between ventricular event V2 at  184  and ventricular event V3 at  186  is increased relative to the interval between ventricular event V1 at  182  and ventricular event V2 at  184 , by decreasing the time from an atrial pace to the next scheduled ventricular pace (PAV), but maintaining the escape time from the previous ventricular event until the current ventricular event (V2V). FIG. 8 thus illustrates how the time from an atrial pace to the next scheduled ventricular pace (PAV) is adapted to short V2V intervals.  
         [0051]    [0051]FIG. 9 illustrates a method  200  for adapting the A2V interval. In step  202 , the method waits for a ventricular event. Upon sensing a ventricular event, either a V sense or a V pace, but not a VSP, execution follows path  204  to step  206 . In decision step  206 , a determination is made as to whether the time from the previous ventricular event until the current ventricular event, i.e., the V2V interval, is less than DV2V. DV2V is defined as the minimum V2V interval which ends in a ventricular pace that is long enough such that it does not interfere with detection. If the V2V interval is less than the minimum, DV2V, then execution follows path  210  to step  212 . In step  212 , the dynamic PAV, DPAV, is decreased by the amount PAVD, the decrement amount used when long V2V intervals occur.  
         [0052]    Execution proceeds to decision step  214 . In decision step  214 , a determination is made as to whether the dynamic PAV (DPAV) is less than MINPAV, i.e., the minimum allowed DPAV value. If the dynamic PAV is not less than the minimum allowed PAV, then execution follows path  218  to step  219 . In step  219 , the dynamic PAV (DPAV) value is set equal to the minimum allowed PAV value (MINPAV). Execution then proceeds to step  220 .  
         [0053]    If decision step  214  determines that the dynamic PAV (DPAV) is less than the minimum PAV (MINPAV), then execution follows path  216 , to step  220 . In step  220 , the DPAV value is used as the current PAV in scheduling the next atrial and ventricular pace. After execution of step  220 , the method follows path  222  to step  202 , to await another ventricular event.  
         [0054]    Referring again to step  206 , if decision step  206  determines that the time from the previous ventricular event until the current ventricular event, the V2V interval, is not less than the minimum V2V interval, DV2V, then execution follows path  208  to step  224 . Decision step  224  determines whether the dynamic PAV (DPAV) is less than PAV, i.e., the time from the atrial pace to the next scheduled ventricular pace. If the dynamic PAV is not less than the PAV, then execution follows path  226  to step  202  to await a ventricular event. If decision step  224  determines that the DPAV value is less than the PAV value, then execution follows path  228  to step  230 .  
         [0055]    Decision step  230  determines whether all of the last N V2V intervals were greater than or equal to the minimum V2V interval, DV2V, where N can be 10 in some methods. If some of the last N V2V intervals were not greater than or equal to DV2V, then execution follows path  232  to execute step  220 , previously discussed. If decision step  230  determines that all of the last N V2V intervals had a value greater than or equal to DV2V, then execution follows path  234  to step  236 .  
         [0056]    In step  236 , the dynamic PAV (DPAV) value is increased by an amount PAVI, where PAVI is the amount used to increment the DPAV when an atrial pace occurs in a short V2V interval. Execution follows to step  238 . In decision step  238 , a determination is made as to whether the dynamic PAV value (DPAV) is greater than or equal to the time from the atrial pace to the next scheduled ventricular pace, PAV. If the DPAV value is not greater than or equal to the PAV value, then execution follows path  242  to execute step  220 , previously discussed. If decision step  238  determines that the DPAV value is greater than or equal to the PAV value, then execution follows path  240  to execute step  244 . In step  244 , the dynamic PAV value (DPAV) is set equal to the PAV value. Execution then follows path  246  to step  202 , to await the next ventricular event.  
         [0057]    [0057]FIG. 10 is a block diagram of a pacing system  440  in accordance with an exemplary embodiment of the present invention. As shown in FIG. 10, pacing system  440  comprises a pacemaker  442  including a controller  476 . Controller  476  may comprise, for example, a microprocessor.  
         [0058]    A ventricular pulse generator  478  of pacemaker  442  provides pacing pulses, generated under the control of controller  476 , for delivery through a ventricular pulse (VP) generator VP-lead  480  to one or more ventricular electrodes  446 . In the embodiment of FIG. 10, a ventricular electrode  446  is shown disposed in a right ventricle  420  of a heart  402 . It is to be appreciated that methods and apparatus in accordance with the present invention may be used with multiple chamber pacing. Thus, in some applications, one or more ventricular electrodes may also be located in or near a left ventricle  408  of heart  402 . An atrial pulse (AP) generator  486  of pacemaker  442  provides atrial pulses, also generated under the control of controller  476 , for delivery through an AP-lead  488  to one or more atrial electrodes  484 . Atrial pulse generator  486  and ventricular pulse generator  478  may each include one or more capacitors, and a switching circuit capable of charging the capacitor(s) by coupling the capacitor(s) to an energy source and discharging the capacitor(s) through the electrodes.  
         [0059]    Pacemaker  442  also includes a signal processor  490  which may be used to sense and process spontaneous signals from heart  402 . For example, signals may be sensed from right atrium  426  via atrial electrode  484 . By way of a second example, signals from right ventricle  420  may be sensed via ventricular electrode  446 . A method in accordance with the present invention may include the steps of sensing spontaneous signals from heart  402  and determining a desired ventricular pacing rate in response to the sensed ventricle signals. Signal processor  490  may comprise, for example, one or more amplifiers, and one or more filters.  
         [0060]    Pacemaker  442  also includes a memory  494 . Memory  494  may be used to store operating instructions for controller  476 . Memory  494  may also be used to store values in accordance with the present invention. Examples of values which may be stored include a chosen rate and a desired ventricular pacing rate. Pacemaker  442  also includes a telemetry antenna  496 . Telemetry antenna may be used, for example, to load instructions and values into memory  494  via controller  476 .  
         [0061]    Pacing system  440  may be used to implement the methods of the present invention using standard methods well known to those skilled in the art. The methods can be expressed as complete programs or other logical systems, for example, any combinations of Boolean logic, gates and timers. The logic and/or programs may reside within memory  494  and be executed by controller  476 .