Abstract:
The above object is achieved in accordance with the present invention by a cardiac stimulation device for stimulating a heart having a first ventricle and a second ventricle. The device includes a first ventricle sensing circuit that is configured to communicate with a first ventricle sensing electrode suited to be positioned in or at the first ventricle, to enable the first ventricle sensing circuit to sense the first ventricle. The device also includes a second ventricle pacing circuit, configured to communicate with a second ventricle pacing electrode suited to be positioned in or at the second ventricle of the heart, to enable the second ventricle pacing circuit to pace the second ventricle. The device includes a control circuit that operates with time cycles corresponding to normal heart cycles. The control circuit is configured to, within such a time cycle, to detect a cardiac event in the first ventricle with the first ventricle sensing circuit and, after a time duration that is greater than or equal to zero, to cause the second ventricle pacing circuit to deliver a pacing pulse. The control circuit is also configured to detect the aforementioned loop of events indicative of the pacemaker-mediated tachycardia by detecting one or both of (i) the regularity of one or more repetitious events related to operation of the first ventricle sensing circuit or the second ventricle pacing circuit, and (ii) a repetitive operation pattern of the heart stimulation device. The control circuit is configured to determine if the detected regularity satisfies a predetermined regularity criterion and to determine if the operation pattern satisfies a predetermined operation pattern criterion. Based on whether one or both of these criteria are satisfied, the control circuit determines whether the aforementioned loop of events, and thus the presence of pacemaker-mediated tachycardia, is likely to exist.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an implantable heart stimulation device with which it is possible to stimulate both the ventricles of a heart, i.e. a bi-ventricular pacer. The heart stimulation device can be a so-called pacemaker or an ICD (implantable cardioverter defibrillator) which also includes a pacing function. 
         [0003]    The invention also relates to a method of, in a patient who is treated with a bi-ventricular cardiac heart stimulation device, detecting a pacemaker-mediated tachycardia of a certain kind. 
         [0004]    2. Description of the Prior Art 
         [0005]    Several different implantable devices for stimulating a heart are known. The devices are normally able to sense the electrical activity of the heart. Some implantable devices are able to deliver stimulation pulses to and/or sense the right atrium (in some cases even the left atrium) and also to deliver stimulation pulses to and sense both the left and right ventricles. 
         [0006]    Devices that are able to deliver stimulation pulses to both the left and right ventricles can be called bi-ventricular pacers. Such devices can be used to treat patients who suffer from different severe cardiac problems, e.g. patients suffering from congestive heart failure (CHF). CHF is defined generally as the inability of the heart to deliver a sufficient amount of blood to the body. CHF can have different causes. It can for example be caused by a left bundle branch block (LBBB) or a right bundle branch block (RBBB). By using bi-ventricular pacing, the contraction of the ventricles can be controlled in order to improve the ability of the heart to pump blood. The stimulation pulses to the two ventricles can be delivered simultaneously but it is also known that the stimulation pulses to the two ventricles are delivered with a short time delay between them in order to optimize the pumping performance of the heart. 
         [0007]    U.S. Pat. No. 5,720,768 describes different possible electrode positions in order to stimulate or sense the different chambers of the heart. 
         [0008]    U.S. Pat. No. 6,070,100 describes that electrodes may be positioned in both the left and the right atrium as well as in the left and the right ventricles. 
         [0009]    A problem that occurs in connection with heart stimulation devices is pacemaker-mediated tachycardia (PMT). A PMT is a phenomenon that is known in connection with a pacemaker that has both an atrial channel and a ventricular channel. A PMT of this kind is a reentry arrhythmia in which the pulse generator acts as the anterograde limb of the tachycardia and the natural conduction path acts as the retrograde limb. An R-wave (QRS-complex) is thus conducted to the atrium, causing a retrograde P-wave which is sensed by the device. The device then emits a ventricular pacing pulse after a certain delay (PV-delay). This pacing pulse captures the ventricle and the evoked R-wave is then conducted back to the atrium, and the cycle repeats itself. Such an endless loop between an atrium and a ventricle can of course also occur in a heart stimulation device that has two ventricular channels in addition to an atrial channel. U.S. Pat. No. 6,611,714 discloses a heart stimulation device that deals with such endless loop problems. 
         [0010]    The articles “Pacemaker-Mediated Tachycardia in a Biventricular Pacing System” by van Gelder et al., PACE, vol. 24, December 2001, pp. 1819-1820 and “Cross-Ventricular Endless Loop Tachycardia During Biventricular Pacing” by Barold et al., PACE, vol. 24, December 2001, pp. 1821-1823 both describe that an endless loop tachycardia can occur if a dual chamber pacemaker (intended for sensing/pacing in an atrium and in a ventricle) is used as a bi-ventricular device. 
       SUMMARY OF THE INVENTION 
       [0011]    The invention is based on the recognition that a certain kind of pacemaker-mediated tachycardia can be a problem in a bi-ventricular cardiac heart stimulation device. This pacemaker-mediated tachycardia can be caused by the fact that a sensed event in a first ventricle causes the generation of a pacing pulse to the second ventricle, which pacing pulse causes a depolarization of the second ventricle with an associated electrical activity, which may also involve a repolarization, which electrical activity is transferred via the heart tissue to the first ventricle, and, after a certain time, is sensed in the first ventricle, which results in a new pacing pulse being applied to said second ventricle, which pacing pulse causes a depolarization of the second ventricle with an associated electrical activity, which may also involve a repolarization, which electrical activity is transferred via the heart tissue to the first ventricle, and, after a certain time, is sensed in the first ventricle, and so on. 
         [0012]    An object of the present invention is to provide a cardiac heart stimulation device with which it is possible to detect a certain loop of events that if the heart stimulation device were (or is) in operation in a patient could be (or can be) a pacemaker-mediated tachycardia of the above kind. 
         [0013]    The above object is achieved in accordance with the present invention by a cardiac stimulation device for stimulating a heart having a first ventricle and a second ventricle. The device includes a first ventricle sensing circuit that is configured to communicate with a first ventricle sensing electrode suited to be positioned in or at the first ventricle, to enable the first ventricle sensing circuit to sense the first ventricle. The device also includes a second ventricle pacing circuit, configured to communicate with a second ventricle pacing electrode suited to be positioned in or at the second ventricle of the heart, to enable the second ventricle pacing circuit to pace the second ventricle. The device includes a control circuit that operates with time cycles corresponding to normal heart cycles. The control circuit is configured to, within such a time cycle, to detect a cardiac event in the first ventricle with the first ventricle sensing circuit and, after a time duration that is greater than or equal to zero, to cause the second ventricle pacing circuit to deliver a pacing pulse. The control circuit is also configured to detect the aforementioned loop of events indicative of the pacemaker-mediated tachycardia by detecting one or both of (i) the regularity of one or more repetitious events related to operation of the first ventricle sensing circuit or the second ventricle pacing circuit, and (ii) a repetitive operation pattern of the heart stimulation device. The control circuit is configured to determine if the detected regularity satisfies a predetermined regularity criterion and to determine if the operation pattern satisfies a predetermined operation pattern criterion. Based on whether one or both of these criteria are satisfied, the control circuit determines whether the aforementioned loop of events, and thus the presence of pacemaker-mediated tachycardia, is likely to exist. 
         [0014]    By configuring the control circuit to perform such detection and determination, it is possible to detect whether a PMT of the specific kind defined above is likely to be the case. 
         [0015]    Preferably, the heart stimulation device according to the invention is an implantable heart stimulation device, i.e. a device that can be implanted in a human or animal being. 
         [0016]    It should be noted that when it is stated herein that for example a certain circuit is adapted or configured to enable sensing and pacing of an atrium or ventricle, this does not necessarily mean that the circuit actually is connected to an atrium or a ventricle. Instead it means that if the heart stimulation device, in which the circuit in question is included, is actually implanted in a body with suitably located electrodes, then the circuit in question would be able to sense and pace an atrium or a ventricle. Similarly, the expressions relating to atrial or ventricular pacing and sensing circuits or the like only mean that these circuits are adapted to be able to sense typical atrial or ventricular events and that they are able to deliver pulses which are of the kind that is typical for stimulating atria or ventricles. A “pacing pulse” or the like is thus a pulse with an energy and morphology which would make it suitable to pace the relevant heart chamber. 
         [0017]    It should also be noted that the mentioned “first ventricle” is not necessarily the ventricle that is paced/sensed first, if there is a normal time gap VV between the sense/pace in one ventricle and the pace (which may also be inhibited), during the same time cycle, in the other ventricle. The expressions “first ventricle” and “second ventricle” or the like are thus primarily used in order to distinguish between the two ventricles (or between the two ventricular channels of the heart stimulation device). This fact will be further explained below, when the label VV i  is explained. 
         [0018]    According to one embodiment of the heart stimulation device according to the invention, the control circuit is arranged such that the detection of the regularity involves detecting the regularity of one or more of the following repetitious events: 
         [0000]    a) the time between consecutive sensed events of the first ventricle sensing circuit,
 
b) the time between consecutive pacing pulses delivered with the second ventricle pacing circuit,
 
c) the time between a pacing pulse delivered with the second ventricle pacing circuit and the subsequent sensed event of the first ventricle sensing circuit.
 
         [0019]    Such repetition events can be used in order to determine whether the mentioned PMT is likely to be the case. 
         [0020]    According to a further embodiment of the heart stimulation device, the control circuit is configured such that the regularity criterion is that a measure of how much the respective time varies is below a predetermined value. If the respective time varies very little between different cycles, then it is likely that the mentioned PMT is the case. 
         [0021]    According to a further embodiment of the heart stimulation device, the control circuit is configured to change at least one pacing parameter and determine if this results in a change in the detected regularity and/or in the detected repetitive operation pattern. By changing a pacing parameter and by determining if this results in a corresponding, expected, change in the detected regularity or repetitive operation pattern, it can be determined whether the mentioned PMT is likely to be the case. 
         [0022]    The concept “pacing parameter” or the like is meant to encompass any setting that influences the operation of the heart stimulation device. Such settings include different programmed times and different sensitivities for detecting signals. 
         [0023]    According to a further embodiment of the heart stimulation device, the pacing parameter that is changed is the time between a sensed event of the first ventricle sensing circuit and the subsequent pacing pulse, during the same time cycle, delivered with the second ventricle pacing circuit. Such a change will result in a change in the operation of the heart stimulation device and the change in the operation can then be used to determine whether the mentioned PMT is the case. 
         [0024]    According to a further embodiment of the heart stimulation device, the control circuit is arranged to determine a measure related to the rate, or cycle length, with which the heart stimulation device operates before and after the change of the pacing parameter and to determine the time between a pacing pulse delivered with the second ventricle pacing circuit and the subsequent event sensed by the first ventricle sensing circuit before and after the change of the pacing parameter, and based on this information to determine whether said loop of events would be likely to be the case if the heart stimulation device were in operation in a patient. Such determinations can be used to decide whether the mentioned PMT is likely to be the case. How this can be done will be described in more detail below. 
         [0025]    According to a further embodiment of the heart stimulation device, the repetitive operation pattern is V 2 , R 1 , V 2 , R 1 , V 2 , R 1  . . . , where V 2  is a pacing pulse delivered with the second ventricle pacing circuit and R 1  is a sensed event of the first ventricle sensing circuit. The sensed event R 1  can be an R-wave or a T-wave. 
         [0026]    That the repetitive operation pattern is V 2 , R 1 , V 2 , R 1 , V 2 , R 1  . . . , preferably means that no other events (in an atrium or a ventricle) than the R 1  events are sensed, at least not outside the refractory periods of the heart stimulation device, as long as this pattern occurs. 
         [0027]    Another possible repetitive operation pattern is V 2 , P, R 1 , V 2 , P, R 1 , V 2 , P, R 1 , . . . . 
         [0028]    According to a further embodiment of the heart stimulation device, the operation pattern criterion is that the operation pattern lasts longer than a predetermined time or that the number of paced and sensed events in said operation pattern is higher than a predetermined number. If the operation pattern lasts a long time, then it is likely that the mentioned PMT is the case. 
         [0029]    Another aspect of the invention concerns providing a device that is actually able to break (terminate) the detected loop of events. In accordance with this aspect of the invention, the control circuit is arranged to break the operation pattern with which the heart stimulation device operates by changing the operation of the heart stimulation device, wherein the change in operation is done in a manner particularly adapted to be likely to stop the particular, above defined, loop of events that would be likely to be the case if the heart stimulation device were in operation in a patient. 
         [0030]    According to a further embodiment of the heart stimulation device, the change in operation involves preventing at least one pacing pulse of the second ventricle pacing circuit from being generated. 
         [0031]    According to a further embodiment of the heart stimulation device, the control circuit is configured to prevent the aforementioned at least one pacing pulse of the second ventricle pacing circuit from being generated by one or more of the following measures: 
         [0032]    a) simply inhibiting at least one such pacing pulse and thereafter return to the normal operation of the heart stimulation device, 
         [0033]    b) changing a refractory period of the operation of the heart stimulation device, such that the heart stimulation device does not react on at least one sensed event of the first ventricle sensing circuit, and thereafter return to the normal operation of the heart stimulation device, 
         [0034]    c) changing a blanking period of the operation of the heart stimulation device, such that the heart stimulation device does not sense at least one event that would otherwise be sensed by the first ventricle sensing circuit, and thereafter return to the normal operation of the heart stimulation device, 
         [0035]    d) changing a sensitivity setting of the operation of the heart stimulation device, such that the heart stimulation device does not sense at least one event that would otherwise be sensed by the first ventricle sensing circuit, and thereafter return to the normal operation of the heart stimulation device with the normal sensitivity setting, 
         [0036]    e) delivering a pacing pulse at least substantially simultaneously with a first ventricular pacing circuit, and the second ventricle pacing circuit. 
         [0037]    It will be explained further below how these different manners can be used to break the detected loop of events. 
         [0038]    According to a further embodiment of the heart stimulation device, the change in operation involves temporarily shortening the pacing parameter which is the time between an event sensed by the first ventricle sensing circuit and the subsequent pacing pulse, during the same time cycle, delivered with the second ventricle pacing circuit, and thereafter return to the normal operation of the heart stimulation device. By shortening the mentioned pacing parameter, it is less likely that the electrical activity caused by a pacing pulse to the second ventricle will be transferred and sensed in the first ventricle. Consequently, it can be possible to break the operation pattern in this manner. 
         [0039]    Another aspect of the invention concerns providing a device that is actually able to prevent future occurrences of the loop of events of the above defined kind. In accordance with this aspect of the invention, the control circuit is also configured to be able to prevent such future occurrences of the loop of events if the heart stimulation device is in operation in a patient, by changing one or more pacing parameters, wherein the change in pacing parameters is done in a manner particularly adapted to prevent the mentioned loop of events. 
         [0040]    According to a further embodiment of the heart stimulation device, the device has a memory and the control circuit is configured to store in the memory a sequence of heart stimulation device events, such that, based on an analysis of this sequence of heart stimulation device events, it would be possible to determine the of the said loop of events. By analyzing the sequence of events, it is thus possible to determine the likely cause of the endless loop with which the heart stimulation device operates. The sequence of heart stimulation device events can for example be analysed by a physician during a medical check-up. 
         [0041]    It should be noted that the expression “heart stimulation device events” includes both sensed events and delivered pacing pulses. 
         [0042]    According to a further embodiment of the heart stimulation device, the control circuit is also configured to carry out an analysis of the stored sequence of heart stimulation device events, and based on this analysis, to automatically change the one or more pacing parameters, such that a future loop of events of the above defined kind is prevented if the heart stimulation device is in operation in a patient. According to this alternative, the device itself thus automatically carries out the necessary change in pacing parameters in order to prevent future occurrences of the endless loop of events. 
         [0043]    According to a further embodiment of the heart stimulation device, the control circuit is configured to make the change in pacing parameters in order to prevent a future loop of events of the above defined kind by one or more of the following measures: 
         [0044]    a) changing at least one sensitivity setting of the operation of the heart stimulation device, such that the heart stimulation device is less sensitive, and thus less likely to sense at least one kind of event, the sensing of which could trigger the above defined loop of events if the heart stimulation device is in operation in a patient, 
         [0045]    b) changing at least one sensitivity setting of the operation of the heart stimulation device, such that the heart stimulation device is more sensitive, and thus more likely to sense at least one kind of event, the lack of sensing of which could trigger the above defined loop of events if the heart stimulation device is in operation in a patient, 
         [0046]    c) changing the energy of the pacing pulses delivered by the heart stimulation device, 
         [0047]    d) extending at least one blanking or refractory period of the operation of the heart stimulation device, such that the heart stimulation device is less likely to react on at least one kind of event, which, if the heart stimulation device reacted on it, could trigger the above defined loop of events if the heart stimulation device is in operation in a patient. 
         [0048]    It will be explained further below how these measures may prevent future occurrences of the loop of events. 
         [0049]    According to a further embodiment of the heart stimulation device, the device circuit also has a first atrial sensing and/or pacing circuit, adapted to communicate with a first atrial sensing and/or pacing electrode suited to be positioned in an atrium of a heart, wherein the first atrial sensing and/or pacing circuit is adapted to enable sensing and/or pacing of such an atrium. The present invention is advantageous to use in connection with a heart stimulation device that also has the ability to sense and/or pace at least one atrium. 
         [0050]    Another object of the invention is to provide a method of, in a patient who is treated with a bi-ventricular cardiac heart stimulation device, detecting a pacemaker-mediated tachycardia of the above defined kind. 
         [0051]    This object of the invention is achieved by a method that includes the following steps: 
         [0052]    detect one or both of the following: 
         [0053]    i) the regularity of one or more repetitious events related to the ventricles, 
         [0054]    ii) a repetitive operation pattern of the heart stimulation device, determine one or both of the following: 
         [0055]    iii) if the detected regularity fulfils a predetermined regularity criterion, 
         [0056]    iv) if the operation pattern fulfils a predetermined operation pattern criterion, and, 
         [0057]    based on the above steps, decide whether the above defined pacemaker-mediated tachycardia is likely to be the case. 
         [0058]    The method of the invention also concerns breaking a pacemaker-mediated tachycardia of the above defined kind and preventing future occurrences of such a pacemaker-mediated tachycardia. Different manners of carrying out the method of the invention are described below. 
         [0059]    The method of the invention has advantages corresponding to those described above in connection with the device according to the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0060]      FIG. 1  schematically illustrates a heart stimulation system with a heart stimulation device connected to leads with sensing and pacing electrodes positioned in a heart, in accordance with the present invention. 
           [0061]      FIG. 2  schematically illustrates an embodiment of a control circuit in the heart stimulation device of  FIG. 1 . 
           [0062]      FIG. 3  schematically illustrates a time scale for events that may occur in the heart stimulation device of  FIG. 1 . 
           [0063]      FIG. 4  is a flow chart for an embodiment of a method according to the present invention. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0064]      FIG. 1  shows schematically an embodiment of an implantable heart stimulation device  10  according to the invention. The device  10  has a housing  12 . The housing  12  includes a control circuit  14 . The device  10  has a connector portion  13 . Via the connector portion  13 , the device  10  can be connected to different leads. In  FIG. 1  the device  10  is connected to three leads  20 ,  30  and  40 . 
         [0065]    The lead  20  includes a pacing and sensing electrode  21 ,  22 . In the shown example, this electrode  21 ,  22  is a bipolar electrode with a tip portion  21  and a ring portion  22 . However, it is within of the scope of the invention that instead unipolar electrodes can be used, as is known to those skilled in the art. Similarly to the lead  20 , the lead  30  includes a pacing and sensing electrode  31 ,  32  and the lead  40  includes a pacing and sensing electrode  41 ,  42 . The device  10  together with the leads  20 ,  30 ,  40  and the electrodes  21 ,  22 ;  31 ,  32 ;  41   42  can be said to constitute an implantable heart stimulation system. 
         [0066]      FIG. 1  also schematically illustrates a heart with a right atrium RA, a left atrium LA, a right ventricle RV and a left ventricle LV. 
         [0067]    The electrode  21 ,  22  constitutes a first atrial sensing and/or pacing electrode  21 ,  22  which is positioned in a first atrium  1 A of the heart, according to this embodiment the right atrium RA, in order to enable sensing and/or pacing of this atrium RA. 
         [0068]    The electrode  31 ,  32  constitutes a first ventricular sensing and pacing electrode  31 ,  32 , which is positioned in a first ventricle  1 V of the heart, in this embodiment the right ventricle RV. The first ventricular sensing and pacing electrode  31 ,  32  is adapted to enable sensing and pacing of this first ventricle  1 V. 
         [0069]    The electrode  41 ,  42  constitutes a second ventricular sensing and pacing electrode  41 ,  42 , which is positioned at a second ventricle  2 V of the heart, in this embodiment the left ventricle LV. The second ventricular sensing and pacing electrode  41 ,  42  is adapted to enable sensing and pacing of this second ventricle  2 V. The lead  40  may for example be introduced via the right atrium RA and the coronary sinus such that the electrode  41 ,  42  is positioned in for example the middle or great cardiac vein of the heart. How to introduce the lead  40  in this manner is known to a person skilled in the art. 
         [0070]    Although not shown in  FIG. 1 , it is also possible that the system is connected to further leads and/or further electrodes, for example electrodes positioned in order to sense and/or pace the left atrium LA and electrodes designed to enable defibrillation. 
         [0071]      FIG. 2  shows schematically the control circuit  14  in more detail. The control circuit  14  includes a memory  15  connected to a control portion  18 . The control circuit  14  includes a first atrial sensing and/or pacing circuit  25 ,  27 . In this embodiment, this circuit  25 ,  27  includes a sensing circuit  25  and a pacing circuit  27 . The first atrial sensing and/or pacing circuit  25 ,  27  communicates with the first atrial sensing and/or pacing electrode  21 ,  22  via the lead  20 . The first atrial sensing and/or pacing circuit  25 ,  27  is thus adapted to sense and/or pace an atrium  1 A, in this case the right atrium RA. 
         [0072]    The control circuit  14  also includes a first ventricular sensing circuit  35  and a first ventricular pacing circuit  37 . These circuits  35 ,  37  communicate with the first ventricular sensing and pacing electrode  31 ,  32  via the lead  30 . The circuits  35 ,  37  are thus adapted to sense and pace a first ventricle  1 V, in this case the right ventricle RV. 
         [0073]    The control circuit  14  also includes a second ventricular sensing circuit  45  and a second ventricular pacing circuit  47 . These circuits  45 ,  47  communicate with the second ventricular sensing and pacing electrode  41 ,  42  via the lead  40 . These circuits  45 ,  47  are adapted to sense and pace a second ventricle  2 V, in this case the left ventricle LV. 
         [0074]    The control circuit  14  is arranged, or programmed, to include several operational features. 
         [0075]    As is normal in a heart stimulation device, the first ventricular sensing circuit  35  and the second ventricular sensing circuit  45  are able to sense events typical for an R-wave (QRS-complex) in the respective ventricle. The ventricular sensing mentioned in this application is thus primarily designed to sense R-waves. However, it should be noted that the ventricular sensing circuits  35 ,  45  may sometimes also sense a T-wave (ventricular repolarization). Consequently, the operation of the device  10  may depend not only on whether an R-wave is sensed but also on a sensed T-wave. 
         [0076]    As is also normal in a heart stimulation device, the first atrial sensing and/or pacing circuit  25 ,  27  is arranged to be able to detect events typical for a P-wave. 
         [0077]    The control circuit  14  is arranged to be able to operate with time cycles corresponding to normal heart cycles. Such an operation is normal for an implantable heart stimulation device. The time cycles are determined by preset timer intervals which also may depend on detected signals. 
         [0078]    The control circuit  14  is also arranged to, within a time cycle, be able to deliver pacing pules pulses with both said the first ventricular pacing circuit  37  and said the second ventricular pacing circuit  47  with a time gap VV, during the normal operation of the device  10 , between a pacing pulse delivered, or inhibited, by one of said the first ventricular pacing circuit  37  and the second  47  ventricular pacing circuits circuit and a pacing pulse delivered, or inhibited, by the other one of those said first  37  and second  47  ventricular pacing circuits  37  and  47 , wherein said the time gap VV is ≧0. A typical value of VV can be between 0 ms and 80 ms, for example 30 ms. For example, if the sense/pace channel  35 ,  37  is the channel that is normally first with regard to the VV time gap, then, if the first ventricular sensing circuit  35  senses an event, then a pacing pulse can be delivered with the second ventricular pacing circuit  47  after the time VV. The delivery of the pacing pulse with the second ventricular pacing circuit  47  can also be inhibited if the second ventricular sensing circuit  45  senses an event before the pacing pulse is delivered with the second ventricular pacing circuit  47 . 
         [0079]    It should also be noted that the control circuit  14  can be arranged to operate in the following manner: if an event is sensed by the ventricular sensing circuit (for example the second ventricular sensing circuit  45 ) that is associated with the ventricle that is normally paced last (i.e. after the time gap VV) during a time cycle when no event has been sensed by the other ventricular sensing circuit (in this example the first ventricular sensing circuit  35 ) and no pacing pulse has been delivered with the corresponding ventricular pacing circuit (in this example the first ventricular pacing circuit  37 ), then a pacing pulse is immediately, or at least almost immediately, delivered with this pacing circuit (in this example thus the first ventricular pacing circuit  37 ). In order to distinguish between the normal time gap W explained above and the short delay “immediately, or at least almost immediately” that has just been explained, herein the label VV i  is used for the latter. VV i  is ≧0, but normally quite short, for example 10 ms. The time gap that is ≧0 that is mentioned in, inter alia, claim  1  below can thus either be the time gap VV or the time gap VV i . 
         [0080]    As is normal for implantable heart stimulation devices, the device  10  is also normally set up to operate with PV and AV delays. PV can for example be defined as the time between the sensing with said first atrial sensing and/or pacing circuit  25 ,  27  and a subsequent pacing pulse, which may also be inhibited, of the first ventricular pacing circuit  37 . AV can for example be defined as the time between the pacing with said first atrial sensing and/or pacing circuit  25 ,  27  and a subsequent pacing pulse, which may also be inhibited, of the first ventricular pacing circuit  37 . It is well known to those skilled in the art how an implantable heart stimulation device is set up in order to operate with PV and AV delays. Furthermore, the device  10  is normally set up to operate with well known blanking and refractory periods. 
         [0081]    Although not described in any detail here, the control circuit  14  can be arranged to include several other operational features that are known in connection with heart stimulation devices. Such features include, for example, the ability to detect evoked responses to delivered pacing pulses (such detection is normally done with a detection logic that is different from that used for detecting R-waves or T-waves); the ability to deliver back-up pulses if a heart chamber is not captured when a pacing pulse has been delivered; the ability to perform capture threshold searches; the ability to sense the physiological activity of the patient in whom the device has been implanted; the ability to carry out defibrillation; the ability to communicate with the help of so-called telemetry, etc. 
         [0082]      FIG. 3  shows schematically on a common time scale heart stimulation device events related to an atrial channel A, a first ventricular channel  1 V and a second ventricular channel  2 V. The black markers (like the mark  52 ) indicate a blanking period for the respective channel. During the blanking period, the sense circuit in question is disabled and no sensing is thus possible during the blanking periods. The hatched lines (like the line  54 ) indicate refractory periods for the respective channels. The heart stimulation device will not react on possible events that are sensed during such refractory periods. A line pointing upwardly (for example the lines  55  and  58 ) indicates a sensed event in the respective channel and a line pointing downwardly (like the lines  56  and  57 ) indicates a pacing pulse delivered by the respective channel. 
         [0083]      FIG. 3  illustrates a chain of events that may occur in a bi-ventricular heart stimulation device.  55  indicates an event sensed by the first atrial sensing circuit  25 . After a certain delay (AV-delay) a pacing pulse  56  is delivered with the first ventricular pacing circuit  37 . After the time Vv a pacing pulse  57  is delivered with the second ventricular pacing circuit  47 . A certain sensed event (or the lack of sensing of a certain event that should have been sensed) may cause the heart stimulation device  10  to operate in an endless loop.  FIG. 3  illustrates that the first ventricular sensing circuit  35  senses an event  58 . This sensing can for example constitute oversensing, i.e. the sensing is not caused by any real cardiac event that should have been sensed. This sensed event  58  causes a pacing pulse  59  to be delivered with the second ventricular pacing circuit  47  after a time gap VV. The pacing pulse  59  causes depolarization of the second ventricle  2 V with an associated electrical activity. This electrical activity may also involve a repolarization (T-wave). The electrical activity is transferred via the heart tissue to the first ventricle  1 V. This transfer takes a certain time. Since the transfer takes a certain time it can “arrive” in the first ventricle  1 V after the refractory period  60 . This transferred electrical activity is sensed as an event  61  by the first ventricular sensing circuit  35 . The event  61  causes the generation of a pacing pulse  62  with the second ventricular pacing circuit  47  after the time gap VV. Because of the mentioned transfer time and the W time gap, the pacing pulse  62  is emitted to the second ventricle  2 V after the biological refractory period caused by the previous pacing pulse  59 . The second ventricle  2 V is therefore captured again by the pacing pulse  62 . Consequently, the second ventricle  2 V is depolarized and an associated electrical activity, which may also involve a repolarization, is transferred via the heart tissue to the first ventricle  1 V and is there sensed again as an event  63 . The sensed event  63  causes another pacing pulse  64  to be generated by the second ventricular pacing circuit  47 . An endless loop of events is thus the case, i.e. a PMT has been generated. 
         [0084]    It should be noted that the events  65  and  66  in  FIG. 3  illustrate retrograde P-waves occurring in the atrial channel. However, these events  65 ,  66  do not cause any change in the operation of the heart stimulation device  10 , since these events  65 ,  66  fall within the refractory period for the atrial channel (post ventricular atrial refractory period, PVARP). 
         [0085]    It should also be noted that although not shown in  FIG. 3 , it is possible that the first ventricle  1 V is in fact the ventricle that is normally associated with the ventricular channel that is the last channel with regard to the VV time gap. A sensing in such a channel can cause an almost immediate (after the time gap VV i ) generation of a pacing pulse with the other ventricular channel. This pacing pulse may cause a depolarization with an associated electrical activity in the ventricle in question. This electrical activity is transferred via the heart tissue to the other ventricle and a similar endless loop of events to that described in connection with  FIG. 3  may occur. 
         [0086]    According to the present invention, the control circuit  14  is arranged to be able to detect whether the above described loop of events is likely to be the case by performing the following steps: 
         [0087]    detect one or both of the following: 
         [0088]    i) the regularity of one or more repetitious events related to the first ventricular sensing circuit  35  or the second ventricular pacing circuit  47 , 
         [0089]    ii) a repetitive operation pattern of the heart stimulation device  10 , determine one or both of the following: 
         [0090]    iii) if the detected regularity fulfils a predetermined regularity criterion, 
         [0091]    iv) if the operation pattern fulfils a predetermined operation pattern criterion, and, 
         [0092]    based on the above steps, determine whether the loop of events would be likely to be the case if the heart stimulation device  10  were in operation in a patient. 
         [0093]    The mentioned regularity can be one or more of the following repetitious events: 
         [0094]    a) the time between consecutive sensed events of said first ventricular sensing circuit  35 , 
         [0095]    b) the time between consecutive pacing pulses delivered with said second ventricular pacing circuit  47 , 
         [0096]    c) the time between a pacing pulse delivered with said second ventricular pacing circuit  47  and the subsequent sensed event of said first ventricular sensing circuit  35 . 
         [0097]    The regularity criterion can be that a measure of how much the respective time varies is below a predetermined value. If a PMT of the described kind is the case, the time between the respective repetitious event will be very stable. The regularity criterion can thus be that the standard deviation or the coefficient of variance (standard deviation divided by the average) is small for the respective time. In order to determine the standard deviation or the coefficient of variance, the times between the respective consecutive events should be monitored over several time cycles, for example over at least 8 time cycles. It is not necessary to determine the regularity of all the mentioned repetitious events a), b) and c). However if more than one of these repetitious events are monitored, then the accuracy in the determination of whether a PMT of the above-mentioned kind is the case will increase. 
         [0098]    The mentioned repetitive operation pattern to be detected can be the repetitive operation pattern V 2 , R 1 , V 2 , R 1 , V 2 , R 1  . . . , where V 2  is a pacing pulse delivered with the second ventricular pacing circuit  47  and R 1  is a sensed event of the first ventricular sensing circuit  35 . The operation pattern criterion can thereby be that the operation pattern lasts longer than a predetermined time, for example longer than 8 s, or that the number of paced and sensed events in said operation pattern is higher than a predetermined number, for example higher than 10. 
         [0099]    In order to further improve the detection of whether the above described loop of events is likely to be the case, the control circuit  14  can be arranged to change at least one pacing parameter and determine if this results in a change in the detected regularity and/or in the detected repetitive operation pattern. The pacing parameter that is changed can be the time VV or VV i  between a sensed event of the first ventricular sensing circuit  35  and the subsequent pacing pulse, during the same time cycle, delivered with the second ventricular pacing circuit  47 . The control circuit  14  can be arranged to determine a measure related to the rate, or cycle length, with which the heart stimulation device  10  operates before and after the change of the pacing parameter and to determine the time between a pacing pulse delivered with the second ventricular pacing circuit  47  and the subsequent event sensed by the first ventricular sensing circuit  35  before and after the change of the pacing parameter, and based on this information to determine whether said loop of events would be likely to be the case if the heart stimulation device  10  were in operation in a patient. If for example the time W is increased with 20 ms, then, if the mentioned PMT is the case, also the time between consecutive sensed events (like  61  and  63 ), or the time between consecutive paced events (like  62  and  64 ) will increase with 20 ms. However, the time between a paced event (like  62 ) and the following sensed event (like the event  63 ) will not change. 
         [0100]    It should be noted that in order to increase the accuracy of the detection of the PMT, the different described manners of detecting such a PMT may be combined in one and the same heart stimulation device  10 . 
         [0101]    Once the above described loop of events has been detected, the control circuit  14  can also be arranged to break, or terminate, the operation pattern with which the heart stimulation device  10  operates by changing the operation of the heart stimulation device  10 . The change in operation is done in a manner particularly adapted to be likely to stop the particular, above defined, loop of events that would be likely to be the case if the heart stimulation device  10  were in operation in a patient. The change in operation can involve preventing at least one pacing pulse of said second ventricular pacing circuit  47  from being generated. This can be done by one or more of the following measures: 
         [0102]    a) inhibiting at least one such pacing pulse and thereafter return to the normal operation of the heart stimulation device  10 , 
         [0103]    b) changing a refractory period of the operation of the heart stimulation device  10 , such that the heart stimulation device  10  does not react on at least one sensed event of the first ventricular sensing circuit  35 , and thereafter return to the normal operation of the heart stimulation device  10 , 
         [0104]    c) changing a blanking period of the operation of the heart stimulation device  10 , such that the heart stimulation device  10  does not sense at least one event that would otherwise be sensed by the first ventricular sensing circuit  35 , and thereafter return to the normal operation of the heart stimulation device  10 , 
         [0105]    d) changing a sensitivity setting of the operation of the heart stimulation device  10 , such that the heart stimulation device  10  does not sense at least one event that would otherwise be sensed by the first ventricular sensing circuit  35 , and thereafter return to the normal operation of the heart stimulation device  10  with the normal sensitivity setting, 
         [0106]    e) delivering a pacing pulse at least substantially simultaneously with the first ventricular pacing circuit  37 , and the second ventricular pacing circuit  47 . 
         [0107]    The different points a) to e) will now be explained. 
         [0108]    a): If a pacing pulse is not delivered by the second ventricular pacing circuit  47 , then no depolarization will occur in the second ventricle, which means that no electrical activity will be transferred to the first ventricle  1 V. Consequently, the PMT is broken. 
         [0109]    b): The ventricular refractory period  60  of the first ventricular sensing circuit  35  may be increased such that the heart stimulation device  10  will not react on the sensed event (like  61 ) transferred from the second ventricle  2 V. 
         [0110]    c): Similarly, if a blanking period is extended for such a long time that a transferred event is not sensed, then the PMT will be broken. 
         [0111]    d): If the first ventricular sensing circuit  35  is made less sensitive, a transferred electrical activity (like the event  61 ) will not be sensed by the first ventricular sensing circuit  35 . Consequently, the PMT will be broken. 
         [0112]    e): If pacing pulses are delivered simultaneously by the first ventricular pacing circuit  37  and the second ventricular pacing circuit  47 , both the ventricles  2 V,  1 V will be biologically refractory after the delivery of such pacing pulses. Consequently, it is less likely that a transferred electrical activity will depolarize the first ventricle  1 V. If the first ventricle is not depolarized, no event (like the event  61 ) will occur and be sensed by the first ventricular sensing circuit  35 . This will lead to the fact that no pacing pulse (like the pacing pulse  62 ) will be delivered by the second ventricular pacing circuit  47 . The pacing pulse  62  will thus be inhibited, and the PMT will be broken. 
         [0113]    Another possible manner of changing the operation of the device  10  in order try to break the mentioned loop of events involves arranging the control circuit  14  to temporarily shorten the pacing parameter VV (or VV i ) which is the time between an event sensed by the first ventricular sensing circuit  35  and the subsequent pacing pulse, during the same time cycle, delivered with the second ventricular pacing circuit  47 , and thereafter return to the normal operation of the heart stimulation device  10 . If VV is made shorter, then it is more likely that the first ventricle  1 V is biologically refractory when the electrical activity is transferred from the second ventricle  2 V. Consequently, it is less likely that such a transferred electrical activity will depolarize the first ventricle  1 V. Therefore, the PMT can be broken. 
         [0114]    The control circuit  14  can also be configured to be able to prevent, or aid in preventing, future occurrences of the loop of events of the above defined kind if the heart stimulation device  10  is in operation in a patient. This can be done by changing one or more pacing parameters, wherein the change in pacing parameters is done in a manner particularly adapted to pre-vent the mentioned loop of events. The control circuit  14  can hereby be arranged to store in the memory  15  a sequence of heart stimulation device events, such that, based on an analysis of this sequence of heart stimulation device events, it would be possible to determine the cause said loop of events. This analysis can be performed by a physician at a medical check-up, when, for example, via telemetry the physician is informed of the sequence of heart stimulation device events that have been stored in the memory  15 . Based on this analysis, the physician can reprogram the device  10  such that future occurrences of the loop of events will be prevented. Alternatively, the control circuit  14  can be arranged to automatically carry out an analysis of the stored sequence of heart stimulation device events, and based on this analysis, to automatically change one or more pacing parameters, such that a future loop of events of the above defined kind is prevented if the heart stimulation device  10  is in operation in a patient. 
         [0115]    The control circuit  14  can be configured such that the change in pacing parameters in order to prevent a future loop of events of the above defined kind involves one or more of the following measures: 
         [0116]    a) changing at least one sensitivity setting of the operation of the heart stimulation device  10 , such that the heart stimulation device  10  is less sensitive, and thus less likely to sense at least one kind of event, the sensing of which could trigger the above defined loop of events if the heart stimulation device  10  is in operation in a patient, 
         [0117]    b) changing at least one sensitivity setting of the operation of the heart stimulation device  10 , such that the heart stimulation device  10  is more sensitive, and thus more likely to sense at least one kind of event, the lack of sensing of which could trigger the above defined loop of events if the heart stimulation device  10  is in operation in a patient, 
         [0118]    c) changing the energy of the pacing pulses delivered by the heart stimulation device  10 , 
         [0119]    d) extending at least one blanking or refractory period of the operation of the heart stimulation device  10 , such that the heart stimulation device  10  is less likely to react on at least one kind of event, which, if the heart stimulation device  10  reacted on it, could trigger the above defined loop of events if the heart stimulation device  10  is in operation in a patient. 
         [0120]    Below follow some explanations concerning these measures a) to d). 
         [0121]    a): The PMT can be initiated by oversensing. By making the heart stimulation device  10  (or at least the relevant atrial or ventricular channel of the device  10 ) less sensitive, it is less likely that oversensing will occur. Consequently, it is less likely that a PMT will be initiated. 
         [0122]    b): A PMT can also be initiated by undersensing. If this is the case, then future initiation of the PMT can be prevented by making the heart stimulation device  10  (or the relevant channel of the device  10 ) more sensitive. 
         [0123]    c): A PMT can also be caused by a loss of capture, i.e. by the fact that a certain heart chamber is not captured by a generated pacing pulse. If the energy of the pacing pulses is increased, the probability for a loss of capture decreases. It is, for example, possible for the control circuit  14  to perform a capture threshold test in order to determine an appropriate energy of the pacing pulses such that loss of capture is avoided. 
         [0124]    d): By extending blanking or refractory periods, it can be avoided that transferred electrical activity, of the kind explained above, will be sensed by the first ventricular sensing circuit  35 . Consequently, it can be avoided that a PMT is initiated. 
         [0125]    The invention also concerns a method of, in a patient who is treated with a bi-ventricular cardiac heart stimulation device, detecting a pacemaker-mediated tachycardia of the above defined kind. 
         [0126]    A schematic flow chart illustrating a method according to the invention is shown in  FIG. 4 . According to the method, a procedure for detecting a PMT of the above described particular kind is carried out. If a PMT is detected, then the sequence of events that characterises characterizes the PMT is stored in a memory. Furthermore, the PMT is terminated. The sequence of events stored in the memory can then be analysed. This can be done either automatically by the device  10  itself or by a physician at a medical check-up. Thereafter, pacing parameters concerning the operation of the heart stimulation device  10  are changed in order to prevent future occurrences of the PMT. The change of pacing parameters can either be performed by a physician or automatically by the device  10  itself. 
         [0127]    The method according to the invention will now be described in some more detail. However, since the method in may respects corresponds to the operation of the heart stimulation device  10  explained above, specific details of the method which correspond to the operation of the device are omitted below. The method according to the invention thus includes the following steps: 
         [0128]    detect one or both of the following: 
         [0129]    i) the regularity of one or more repetitious events related to the ventricles, 
         [0130]    ii) a repetitive operation pattern of the heart stimulation device, determine one or both of the following: 
         [0131]    iii) if the detected regularity fulfils a predetermined regularity criterion, 
         [0132]    iv) if the operation pattern fulfils a predetermined operation pattern criterion, and, 
         [0133]    based on the above steps, decide whether the above defined pacemaker-mediated tachycardia is likely to be the case. 
         [0134]    It should be mentioned that, preferably, the method also includes the step of providing means for sensing and/or pacing in at least a first atrium of the heart. 
         [0135]    The detection of said regularity can involve detecting the regularity of one or more of the following repetitious events: 
         [0136]    a) the time between consecutive sensed events of the first ventricle, 
         [0137]    b) the time between consecutive pacing pulses delivered to the second ventricle, 
         [0138]    c) the time between a pacing pulse to the second ventricle and the subsequent sensed event of the first ventricle. 
         [0139]    The regularity criterion can be that a measure of how much the respective time varies is below a predetermined value. 
         [0140]    Analogously to the above description of the device  10  according to the invention, the repetitive operation pattern can be V 2 , R 1 , V 2 , R 1 , V 2 , R 1  . . . , where V 2  is a pacing pulse to the second ventricle and R 1  is a sensed event of said first ventricle. The operation pattern criterion can be that the operation pattern lasts longer than a predetermined time or that the number of paced and sensed events in said operation pattern is higher than a predetermined number. 
         [0141]    Furthermore, the method may include the step of changing at least one pacing parameter and determine if this results in a change in the detected regularity and/or in the detected repetitive operation pattern. The pacing parameter that is changed can be the time VV or VV i  between a sensed event in the first ventricle and the subsequent pacing pulse, during the same time cycle, to the second ventricle. The method can thereby include the step of determining a measure related to the rate, or cycle length, with which the heart stimulation device operates before and after the change of the pacing parameter and also determining the time between a pacing pulse to the second ventricle and the subsequent sensed event in the first ventricle before and after the change of the pacing parameter. Based on this information, it can be determined whether the above defined pacemaker-mediated tachycardia is (likely to be) the case. 
         [0142]    The method can also include steps for breaking, or terminating, the detected, above defined, pacemaker-mediated tachycardia. This can be done by changing the operation of the heart stimulation device, wherein the change in operation is done in a manner particularly adapted to stop the particular, above defined, pacemaker-mediated tachycardia that has been detected. This change in operation can involve preventing at least one pacing pulse to the second ventricle from being generated. This can be done by one or more of the following measures: 
         [0143]    a) simply inhibiting at least one such pacing pulse and thereafter return to the normal operation of the heart stimulation device, 
         [0144]    b) changing a refractory period of the operation of the heart stimulation device, such that the heart stimulation device does not react on at least one sensed event in the first ventricle, and thereafter return to the normal operation of the heart stimulation device, 
         [0145]    c) changing a blanking period of the operation of the heart stimulation device, such that the heart stimulation device does not sense at least one event in the first ventricle, and thereafter return to the normal operation of the heart stimulation device, 
         [0146]    d) changing a sensitivity setting of the operation of the heart stimulation device, such that the heart stimulation device does not sense at least one event in the first ventricle, and thereafter return to the normal operation of the heart stimulation device with the normal sensitivity setting, 
         [0147]    e) deliver a pacing pulse at least substantially simultaneously to the first and the second ventricle. 
         [0148]    Another manner of breaking the loop of events can be to temporarily shorten the pacing parameter VV (or VV i ) which is the time between a sensed event in the first ventricle and the subsequent pacing pulse, during the same time cycle, to the second ventricle, and thereafter return to the normal operation of the heart stimulation device. 
         [0149]    The method of the invention can also include preventing future pacemaker-mediated tachycardias, of the above defined kind. This can be done by changing one or more pacing parameters, wherein the change in pacing parameters is done in a manner particularly adapted to prevent the particular pacemaker-mediated tachycardia that has been detected. This method can include recording and analyzing a sequence of heart stimulation device events, and, based on this analysis, determine the cause of the pacemaker-mediated tachycardia, and change said one or more pacing parameters, based on the analysis, such that future pacemaker-mediated tachycardia of the above defined kind is prevented. The change in pacing parameters in order to prevent future pacemaker-mediated tachycardia of the above defined kind, can involve one or more of the following measures: 
         [0150]    a) changing at least one sensitivity setting of the operation of the heart stimulation device, such that the heart stimulation device is less sensitive, and thus less likely to sense at least one kind of event in the heart, the sensing of which could trigger the above defined pacemaker-mediated tachycardia, 
         [0151]    b) changing at least one sensitivity setting of the operation of the heart stimulation device, such that the heart stimulation device is more sensitive, and thus more likely to sense at least one kind of event in the heart, the lack of sensing of which could trigger the above defined pacemaker-mediated tachycardia, 
         [0152]    c) changing the energy of the pacing pulses delivered by the heart stimulation device, 
         [0153]    d) extending at least one blanking or refractory period of the operation of the heart stimulation device, such that the heart stimulation device is less likely to react on at least one kind of event in the heart, which, if the heart stimulation device reacted on it, could trigger the above defined pacemaker-mediated tachycardia. 
         [0154]    The method according to the invention can, for example, be performed on a human or animal being suffering from congestive heart failure, for example on a on a human or animal being suffering from a bundle branch block. 
         [0155]    Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventor to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of his contribution to the art.