Abstract:
An implantable electrostimulation device having at least three input channels, (each forming a sensing channel), which are each connected to at least one electrode or to one terminal for an electrode, using which at least three different electrical potentials accompanying an excitation of cardiac tissue (myocardium) in a heart may be detected. Uses a signal processing unit which is connected to the input channels and is implemented to analyze the time curve of the potentials detected via the three sensing channels as three input signals in chronological relation to a periodically repeating trigger signal, which triggers a time window, and which is also implemented to detect predefined signal features for each of the three input signals within the time window triggered by the trigger signal, store them, and compare them to corresponding signal features of preceding time windows or of another input channel within the same time window.

Description:
[0001]     This application takes priority from German Patent Application DE 10 2005 049 009.3 filed Oct. 11, 2005 the specification of which is hereby incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an implantable electrostimulation device for a heart, in particular an implantable cardioverter/defibrillator.  
         [0004]     2. Description of the Related Art  
         [0005]     Implantable cardiac pacemakers and implantable cardioverters/defibrillators have reached a high level of development. The devices must operate reliably over years, since they are only accessible in the implanted state via an operation. This sets limits to the complexity of the devices. However, cardiac pacemakers and defibrillators are to be capable of detecting pathological states of the heart to be treated both with high sensitivity and also with high specificity, so that a state in need of treatment does not remain unrecognized and, in addition, unnecessary or incorrect treatment is avoided as much as possible. In connection with implantable cardioverters/defibrillators (ICDs), the difference between tachycardias which have their origin in the ventricle (ventricular tachycardias), and supraventricular tachycardias, which have their origin in the atrium, for example, is cited here.  
         [0006]     Furthermore, the problems still exists of being able to recognize successful treatment as reliably as possible, in order to otherwise be able to adapt the treatment. In the simplest case, this relates to setting the strength of a stimulation pulse. This is to be dimensioned precisely so that the cardiac tissue (myocardium) of the stimulated heart chamber—i.e., atrium or ventricle—responds to the stimulation pulse and contracts as a result. However, stimulation pulses which are too strong are to be avoided in order to avoid unnecessary energy consumption, because the energy must be taken from a battery whose exhaustion makes an operation and replacement of the stimulation device necessary. Precisely in biventricular pacemakers for resynchronization therapy (CRT: cardiac resynchronization therapy), detecting the success of stimulation (capture detection) still causes problems, particularly in the left ventricle.  
         [0007]     An array of individual solutions to the problems indicated here are known.  
       BRIEF SUMMARY OF THE INVENTION  
       [0008]     The object of the present invention is to provide a cardiac stimulation device which has means which may expediently supplement or even replace the known means for solving the problems indicated.  
         [0009]     This object is achieved according to the present invention by an implantable electrostimulation device which has at least three input channels, which are connected to at least one electrode or one terminal for an electrode, using which the at least three different electrical potentials accompanying excitation of the cardiac tissue (myocardium) in the heart may be detected. Each of the input channels forms a sensing channel. The electrostimulation device has a signal processing unit, which is connected to the three input or sensing channels and is implemented to analyze the time curve of the potentials detected via the three sensing channels as three input signals in chronological relation to a periodically repeating trigger signal. The trigger signal triggers a time window. Furthermore, the signal processing unit is implemented to detect signal features, which are predefined for each of the three input signals, within the time window triggered by the trigger signal, store them, and compare them to corresponding signal features of preceding time windows of the same sensing channel. Alternatively, the signal processing unit may also be implemented to compare signal features corresponding to one another of the input signals originating from different sensing channels within the same time window with one another.  
         [0010]     The input or sensing channels may be connected to an electrode to be situated intracardially or to a neutral electrode, such as a pacemaker housing, as a counter electrode. The signals detected from the sensing channels are then unipolar signals.  
         [0011]     However, it is preferable if each of the sensing channels is connected to two electrodes situated in the heart or in the periphery of the heart so that each of the sensing channels detects a bipolar input signal. For three bipolar sensing channels, at least three electrodes are required, each two of which are each connected to a sensing channel, without two sensing channels being connected to the same two electrodes. Two sensing channels thus always only share one of the two electrodes to which they are connected.  
         [0012]     The present invention is based on the recognition that a type of electrocardiogram may be recorded using each of the three sensing channels, the three electrocardiograms obtained in this way differing, in particular in regard to the occurrence in relation to a shared reference instant of signal features corresponding to one another. By analyzing time shifts of this type, valuable additional information may be obtained about a current heart status, which may not be inferred from a single electrocardiogram. In particular, the analysis of the chronological relationship between signal features corresponding to one another in the different input channels allows improved tachycardia recognition, improved setting of the stimulation threshold value of a biventricular cardiac pacemaker, efficiency control of a biventricular cardiac pacemaker, and above all the recognition of ventricular fibrillations. If the three sensing channels each detect a far field of the myocardial potentials and a fourth sensing channel is provided for detecting a near field of the myocardial potentials, a ventricular fibrillation may be recognized in that the deflections of the three far field signals are very slight, while an excitation having a high rate may be recognized in the near field channel.  
         [0013]     From the description above, it may be seen that the signal processing unit is preferably implemented to compare a detected current signal feature of an input signal in regard to the instant of the occurrence of a shape feature in the time window to the instant of the occurrence of a corresponding shape feature in another input signal or in a preceding time window. In regard to this aspect of the present invention and in regard to the entire present invention overall, a shape feature means a special signal feature which relates to the signal shape. In the simplest case, a shape feature is a signal peak. More complex shape features may be detected with the aid of a template approach. In general, a signal feature may be a shape feature, but it may also be a time feature in principle, for example.  
         [0014]     An embodiment variation in which the signal processing unit is implemented to detect the occurrence of an amplitude maximum as a signal feature and the instant of its occurrence in the time window is especially preferred.  
         [0015]     The trigger signal for triggering the time window may be an arbitrary, repeating trigger signal. For example, it may originate from a marker channel. More advantageously, however, the signal processing unit is implemented to derive the trigger signal from one of the input signals in such a way that the trigger signal is triggered by a signal feature of this input signal, such as the occurrence of an amplitude maximum. The occurrence of an amplitude maximum is a shape feature and thus also a signal feature of an input signal.  
         [0016]     Furthermore, it is possible with the aid of a loop recorder for the input signals also to process time windows whose beginning is prior in time to the occurrence of the particular trigger signal. Moreover, the time window may be defined in relation to the trigger signal by specifying absolute time values, or by specifying relative time values which relate to the duration of a particular cardiac cycle.  
         [0017]     The instant of the occurrence of the signal feature within the time window may also be detected in absolute time values in relation to the trigger signal, or by detecting the relative chronological position of the instant of the occurrence of the signal feature in the particular time window. The last case means that the instant of the occurrence of the signal feature is not detected in absolute time values, but rather in relation to the duration of the time window.  
         [0018]     In relation to the trigger signal, it is advantageous if the trigger signal is derived from an input signal which represents a near field potential.  
         [0019]     For the latter purpose, a fourth input channel is preferably provided as a fourth sensing channel, which is connected to at least one electrode or one terminal for an electrode, using which the near field of a fourth electrical potential accompanying the excitation of the cardiac tissue may be detected. The signal processing unit is implemented in these cases for the purpose of deriving the trigger signal from the input signal of the fourth sensing channel.  
         [0020]     The first three sensing channels are preferably far field channels and are each connected to an electrode or a terminal for an electrode, using which a far field of an electrical potential accompanying an excitation of the cardiac tissue may be detected in the operating state of the implantable electrostimulation device.  
         [0021]     In a preferred embodiment variation, the three input channels are connected to three electrode line terminals, namely a terminal for a proximal defibrillation electrode, such as an atrial shock electrode, a terminal for a distal defibrillation electrode, such as a ventricular shock electrode, and the housing of the implantable cardioverter/defibrillator, which is thus an active housing.  
         [0022]     Preferably, at least one bipolar, right-ventricular electrode line, which has a ventricular ring electrode and a ventricular tip electrode, via which the near field is to be detected, is provided for detecting the input signal representing a near field via a near field channel.  
         [0023]     Preferably, at least three input channels are far field channels, which are each connected to an electrode or to a terminal for an electrode, using which a far field of an electrical potential accompanying an excitation of the cardiac tissue (myocardium) is to be detected in the operating state of the implantable electrostimulation device, so that an input signal representing a particular far field is applied to the input channels. The implantable electrostimulation device has a signal processing unit which is connected to the input channels and is implemented to produce three differential signals differing from one another, which result from the difference of two signals applied to each input channel, and to calculate a particular maximum amplitude value for a particular cardiac cycle for each of the differential signals, which represents the maximum deflection of the particular differential signal within the particular cardiac cycle.  
         [0024]     Instead of calculating the maximum amplitude value, the signal processing unit may be implemented to determine and analyze the amplitude (deflection) of each of the differential signals at a fixed reference instant within a cardiac cycle. However, the maximum amplitude value is the most informative and the simplest to determine. If necessary, the reference instant may be predefined by a marker channel known per se, which generates marker signals upon detecting a P wave, a T wave, or an R wave, for example.  
         [0025]     In the special case, the differential signal is a signal which is to be represented as an electrocardiogram. In a special embodiment variation, the signal processing unit is therefore implemented to derive an electrogram from the input signals of each two input channels—i.e., preferably three electrocardiograms in total. Here, electrogram means an electrical signal which may be represented as an electrocardiogram. In this special case, the signal processing unit is implemented to determine one amplitude per cardiac cycle for each of the electrocardiograms. This amplitude is preferably the maximum deflection of the electrocardiogram, i.e., the maximum amplitude which occurs at the instant of a particular R wave. The R wave is a signal wave occurring as a result of the depolarization of the myocardium, which results in contraction of the particular cardiac muscle.  
         [0026]     The present invention is based on the recognition that three maximum amplitudes obtained in this way have an information content which allows, for example, both differentiation of ventricular and supraventricular tachycardias and also reliable recognition of respective ventricular contractions.  
         [0027]     In an embodiment variation which is especially preferred because of its simplicity, the signal processing unit is implemented to analyze the absolute value and sign of the maximum amplitude values in relation to one another. The three maximum amplitude values result in a 3-tuple, which is characteristic of the starting location of a ventricular excitation and thus allows right-side and also left-side ventricular contractions to be differentiated from one another and, in addition, a stimulated ventricular contraction to be differentiated from a natural ventricular contraction and finally a ventricular contraction of ventricular origin to be differentiated from a ventricular contraction of supraventricular origin. This is explained in greater detail in connection with the exemplary embodiments.  
         [0028]     In addition to analysis of the 3-tuple, a template approach may also be used for comparing the maximum amplitude values. For this purpose, every detected maximum amplitude value is assigned to one of three categories, for example, namely small amplitude, moderate amplitude, or large amplitude. In this way, very compact descriptions of the maximum amplitude values to be compared to one another may be produced, which are also simple to analyze.  
         [0029]     The present invention is based on the recognition that the particular amplitude of a particular differential signal may be understood as the absolute value of a vector, which may be added to the two other vectors to produce a summation vector, which particularly describes the direction of the excitation propagation in the myocardium at the particular instant. For this purpose, the angles between the three vectors forming the components of the summation vector are also to be considered. These angles are predefined by the spatial arrangement of the three electrodes connected to the three input channels in their implanted state. Since this spatial arrangement of the electrodes in implanted state remains essentially constant, the angles are constants, whose precise value is of subordinate significance if the information of interest may already be obtained by analyzing direction changes of the summation vector. The present invention makes use of this recognition, that essential information may be obtained solely by analyzing direction changes, and supplements it with the further recognition that in the simplest case only the absolute values of the amplitudes and their signs have to be analyzed in order to detect a direction change of the summation vector. For this purpose, it suffices to ascertain the particular amplitudes at a specific instant within the cardiac cycle, so that only three amplitude values are to be determined per cardiac cycle, namely one for each of the differential signals. A further recognition is that the most suitable reference instant is the instant of the occurrence of the R wave. This is suitable for two reasons: firstly, it is easy to identify, since it is inherent to every differential signal as the instant of the occurrence of the maximum amplitude. In addition, this instant identifies a particular ventricular contraction. Therefore, the origin location of the excitation may be concluded ideally from the direction of the summation vector at this instant. In turn, this allows—as already noted—stimulated ventricular excitations to be differentiated from natural ventricular excitations, left-ventricular excitations from right-ventricular excitations, and supraventricular excitations from ventricular excitations.  
         [0030]     A further, more special advantage of the present invention is that it may be implemented with relatively little effort as a supplement to an implantable cardioverter/defibrillator known per se, by providing input channels for receiving the input signals, which are connected to electrodes required in any case, namely, in a preferred embodiment variation to a shock electrode in proximity to the atrium, to a ventricular shock electrode, and to an electrode formed by the housing of the implantable cardioverter/defibrillator. In a preferred embodiment variation, the atrial shock electrode is a relatively large-area, oblong proximal defibrillation electrode on an electrode line and the ventricular shock electrode is also an oblong distal defibrillation electrode on the same electrode line. This embodiment variation is connected to one input channel each, having a terminal for the atrial shock electrode and the ventricular shock electrode, while the third input channel is connected directly to the electrode formed by the active housing of the implantable cardioverter/defibrillator.  
         [0031]     As will be explained in greater detail below, such an implantable cardioverter/defibrillator (ICD) is easy to design in such a way that it may differentiate ventricular tachycardias from supraventricular tachycardias, for example, with greater reliability than known devices.  
         [0032]     If the implantable stimulation device is additionally implemented as a triple-chamber or multichamber pacemaker, which is capable as a biventricular pacemaker of stimulating both the right and also the left ventricle, the further advantage results that a multichamber pacemaker may easily thus be implemented as defined in the present invention so that it may differentiate the ventricular contractions because of right-ventricular or left-ventricular stimulation from one another and from natural ventricular contractions. This is particularly very advantageous in connection with the stimulation success control (capture control) explained at the beginning.  
         [0033]     As already explained, to implement the present invention, it suffices to provide the implantable electrostimulation device—whether it is an implantable cardioverter/defibrillator or a multichamber cardiac pacemaker or a combination of the two—with a signal processing unit, which is to be connected in the above-mentioned way to electrodes and is implemented to analyze the absolute value and sign of the maximum amplitude values in relation to one another.  
         [0034]     In a preferred embodiment variation, the electrostimulation device has a memory, in which an angle value is preferably stored permanently for each of the differential signals and/or each of the electrocardiograms, which allows, together with a particular amplitude of a particular electrocardiogram or the maximum amplitude value of the differential signal, a vector to be determined, which, upon vector addition with corresponding vectors which are derived from angles and amplitudes of the particular two other electrocardiograms or differential signals, produces a summation vector, whose direction is then to be analyzed by the signal processing unit. In the present case, the direction of the summation vector means the signed orientation of the vector, i.e., direction and also orientation of the summation vector.  
         [0035]     The implantable electrostimulation device is preferably implemented for the purpose of ascertaining a possible direction change of the summation vector from cardiac cycle to cardiac cycle (beat to beat), i.e., to determine at least one amplitude value per cardiac cycle at the given reference instant for each of the electrocardiograms or differential signals. As already noted, it suffices to determine precisely one amplitude value for each electrocardiogram or differential signal per cardiac cycle, preferably at the instant of a ventricular contraction identified by a R wave, in order to implement the present invention in an especially simple and efficient way. In order to allow a change of the direction of the summation vector from cycle to cycle, the signal processing unit is preferably connected to a memory, in which the direction of the summation vector for the particular preceding cardiac cycle is to be stored. The signal processing unit is preferably implemented so that it only generates a signal identifying a direction change of the summation vector when the summation vector changes its direction by more than a predefined tolerance angle. It may be advantageous in this connection if the implantable stimulation device is implemented so that the directions of the summation vectors of multiple preceding cardiac cycles are to be stored.  
         [0036]     In an especially preferred embodiment variation, the signal processing unit is implemented so that it may access predefined angle ranges, which are characteristic for specific excitation locations. In contrast to the preceding case, the signal processing unit does not only check whether the direction of the summation vector has changed in relation to the direction of the summation vector from the preceding cardiac cycle or the preceding cardiac cycles. Rather, the signal processing unit checks whether the particular ascertained direction of the summation vector falls into one of multiple angle ranges, one of these angle ranges being assigned to a stimulated right-ventricular ventricular contraction, for example, while another angle range is assigned to a stimulated left-ventricular ventricular contraction and a third angle range is assigned to a ventricular contraction because of natural stimulation conduction from the atrium to the ventricle. Further angle ranges may, for example, be assigned to the typical ectopic excitation foci for the ventricular stimulation.  
         [0037]     In order to allow even more precise classification, particularly of tachycardias, or to determine a particular reference instant within a cardiac cycle as precisely as possible, the electrostimulation device preferably has a further input channel which is connected to an electrode or a terminal for an electrode, using which a near field of an electrical potential accompanying excitation of the cardiac tissue is to be detected in the operating state of the electrostimulation device, so that a signal representing the near field is applied to the further input channel. In this preferred case, the signal processing unit is also connected to the further input channel and is implemented to analyze the input signal representing the near field in each case in regard to signal peaks identifying a ventricular contraction.  
         [0038]     In a preferred variation, in order to be able to use the particular heart rate to classify tachycardias, the signal processing unit is implemented in any case to analyze the chronological interval of the ascertained signal peaks and to derive the duration of a particular RR interval or a heart rate therefrom.  
         [0039]     In addition, the signal processing unit may be implemented to derive a marker signal from a particular signal peak.  
         [0040]     With appropriate implementation of the signal processing unit, this marker signal may be used to determine the amplitude (or deflection) of the differential signal or the electrocardiogram for each cardiac cycle at the instant predefined by the particular marker signal and to analyze the amplitudes thus obtained as absolute values of the components of a summation vector to determine the direction of the summation vector.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0041]     The present invention will now be explained in greater detail on the basis of exemplary embodiments with reference to the figures. In the figures:  
         [0042]      FIG. 1 : shows the sketch of a single-chamber ICD according to the present invention having attached electrode lines and the position of the electrodes in the human heart,  
         [0043]      FIG. 2 : shows electrocardiograms as they are to be detected using the constellation shown in  FIG. 1 ,  
         [0044]      FIG. 3 : shows a sketch, similar to  FIG. 1 , but in this case having a dual-chamber ICD,  
         [0045]      FIG. 4 : shows electrocardiogram signals as they may be obtained using the dual-chamber ICD shown in  FIG. 3 ;  
         [0046]      FIG. 5 : shows a sketch of a triple-chamber ICD having attached, implanted electrodes;  
         [0047]      FIG. 6 : shows an embodiment variation in which the input signals of the three far field channels are produced by analyzing the differential amplitudes, which are detected in pairs between an active housing of the triple-chamber ICD and a ventricular shock electrode, the ventricular shock electrode and an atrial shock electrode, and finally the active housing of the ICD and the atrial shock electrode;  
         [0048]      FIG. 7 : shows an embodiment variation in which the input signals of the three far field channels are produced by analyzing the differential amplitudes, which are detected in pairs between an active housing of the triple-chamber ICD and a ventricular shock electrode, the ventricular shock electrode and an atrial ring or tip electrode, and finally the active housing of the ICD and the atrial ring or tip electrode;  
         [0049]      FIG. 8 : shows an embodiment variation in which the input signals of the three far field channels are produced by analyzing the differential amplitudes which are detected in pairs between a left-ventricular ring or tip electrode and a ventricular shock electrode, the ventricular shock electrode and an atrial ring or tip electrode, and finally the left-ventricular ring or tip electrode and the atrial ring or tip electrode;  
         [0050]      FIG. 9 : shows a sketch to explain the determination of the maximum amplitudes in the particular electrocardiogram;  
         [0051]      FIG. 10 : shows a sketch to explain how a direction of the excitation propagation may be ascertained from the amplitude values ascertained as shown in  FIG. 5 ;  
         [0052]      FIG. 11 : shows a compact illustration of the amplitude values ascertained as shown in  FIG. 5 ;  
         [0053]      FIG. 12 : shows a sketch, similar to  FIG. 6 , but for a case typical for a ventricular tachycardia;  
         [0054]      FIG. 13 : shows a graphical illustration of a classification algorithm, as it is implemented by the signal processing unit of an implantable stimulation device according to the present invention;  
         [0055]      FIG. 14 : shows a sketch, similar to  FIG. 1 , but in this case having a triple-chamber ICD;  
         [0056]      FIG. 15 : shows electrocardiogram signals as they may be obtained using the triple-chamber ICD shown in  FIG. 10 ;  
         [0057]      FIG. 16 : shows a sketch, similar to  FIG. 6 , but for the case of successful biventricular stimulation here;  
         [0058]      FIG. 17 : shows a compact illustration of the amplitudes as they result in the case of a successful biventricular stimulation, similar to  FIG. 7 ;  
         [0059]      FIG. 18 : shows a sketch, similar to  FIG. 6 , but for the case of successful right-ventricular stimulation but unsuccessful left-ventricular stimulation;  
         [0060]      FIG. 19 : shows a compact illustration of the amplitudes, similar to  FIG. 7 , for the case of a right-ventricular stimulation success and a left-ventricular capture loss, as shown in  FIG. 12 ;  
         [0061]      FIG. 20 : shows a sketch, similar to  FIG. 6 , but here for the case in which the stimulation is only successful in the left ventricle;  
         [0062]      FIG. 21 : shows the amplitudes of the electrocardiograms on which the sketch in  FIG. 14  is based in a compact illustration, similar to  FIG. 7 ;  
         [0063]      FIG. 22 : shows a report about right-side or left-side capture losses, to be prepared here with the aid of a triple-chamber ICD according to the present invention and transmitted telemetrically to a home-monitoring center;  
         [0064]      FIG. 23 : shows a schematic illustration of the input channels of an ICD according to the present invention;  
         [0065]      FIG. 24 : shows the input signals of a near field channel as the trigger channel and of three far field channels, a time window being determined by a trigger signal derived from the near field channel, which begins chronologically before the trigger signal and ends after the trigger signal; this embodiment variation requires a loop recorder;  
         [0066]      FIG. 25 : shows a compact illustration of the maximum amplitudes of the far field signals shown in  FIG. 24  and, in addition, a compact illustration of the time offset between the particular instant of the occurrence of a maximum amplitude in relation to the instant of the trigger signal;  
         [0067]      FIG. 26 : shows the input signals of a near field channel as a trigger signal and of three far field channels, a time window being determined by a trigger signal derived from the near field channel which begins chronologically after the trigger signal and ends after the trigger signal; and  
         [0068]      FIG. 27 : shows a compact illustration of the maximum amplitudes of the far field signals shown in  FIG. 26  and, in addition, a compact illustration of the time offset between the particular instant of the occurrence of a maximum amplitude in relation to the instant of the trigger signal.  
     
    
     DETAILED DESCRIPTION  
       [0069]      FIGS. 1 through 13  are used to explain the present invention in regard to implantable cardioverters/defibrillators (ICDs) in the form of single-chamber ICDs ( FIGS. 1 and 2 ) and dual-chamber ICDs ( FIGS. 3 through 8 ) and the analysis of the signal amplitudes obtained using these ICDs in regard to the classification of ventricular tachyarrhythmias ( FIGS. 9 through 13 ).  
         [0070]     The object of this analysis is to reduce a higher number of undesired ICD interventions, typical until now, because of supraventricular tachycardias (SVT).  
         [0071]     Known single-chamber ICDs analyze ventricular interval changes (RR intervals) and use known onset and RR stability criteria for treatment inhibition solely in the case of sinus tachycardias and atrial fibrillations. Other known differentiation criteria are morphology criteria for differentiating between supraventricular and ventricular tachycardias.  
         [0072]     In addition, atrial intervals are analyzed in known dual-chamber ICDs to improve the differentiation between ventricular tachycardias and supraventricular tachycardias.  
         [0073]     The way in which the differentiation of various ventricular tachycardias may be improved using an implantable electrostimulation device according to the present invention will be explained in the following.  
         [0074]      FIG. 1  shows a suitable electrostimulation device in the form of a single-chamber ICD  10 . The single-chamber ICD  10  has an active housing  12 , i.e., a housing having an electrically conductive surface which forms an electrode.  
         [0075]     An electrode line  16 , which carries an oblong proximal shock electrode  18  and a distal shock electrode  20 , also oblong, as well as a comparatively small-area ring electrode  22  and a tip electrode  24 , also small-area, is connected to a header  14  of the ICD  10 . The tip electrode  24  is located at the distal end of the electrode line  16 .  
         [0076]     The position of the electrodes in the implanted state is shown in  FIG. 1 . The proximal shock electrode  18  is located in the lower end of the vena cava superior in the area of the transition to the atrium in the implanted state. The distal shock electrode is located in the ventricle of a heart, the ventricular ring electrode  22  and the ventricular tip electrode  24  are situated in the area of the apex of the ventricle of the heart.  
         [0077]     Using the system shown in  FIG. 1 , the electrocardiograms shown in  FIG. 2  may be obtained, each of which represents the curve of the potential difference between two electrodes as a differential signal, namely in  FIG. 2   a  the differential signal which results as the difference of the potentials applied to the proximal shock electrode  18  and the active housing  12 .  FIG. 2   b  shows the differential signal which results from the curves of the electrical potentials applied to the two shock electrodes  18  and  20 .  FIG. 2   c  shows the differential signal which results from the potentials applied to the active housing  12  and the distal shock electrode  20 .  
         [0078]     The differential signals in  FIGS. 2   a  through  2   c  each represent far field signals.  
         [0079]     Finally,  FIG. 2   d  shows the curve of the potential difference between the ventricular ring electrode  22  and the ventricular tip electrode  24 . This differential signal is a near field signal.  
         [0080]     While the three shock electrodes, namely the active housing  12 , the proximal shock electrode  18 , and the distal shock electrode  22 , record differential signals which represent the far field of the ventricular excitation, a differential signal which represents the near field of the ventricular excitation is recorded via the electrode pair formed by the ventricular ring electrode  22  and the ventricular tip electrode  24 . These differential signals are thus spiked signals which are especially suitable for the interval classification (i.e., the determination of the RR interval or the heart rate) and therefore are also already analyzed in known implantable stimulation devices.  
         [0081]     The three differential signals recorded with the aid of the three shock electrodes, in contrast, represent the far field of the electrical potentials accompanying the ventricular contraction and are thus comparable to the signals which are obtained for a classical surface electrocardiogram with the aid of electrodes glued to the body. The three shock electrodes at least approximately form an equilateral triangle and thus correspond to the model presented according to Einthoven.  
         [0082]      FIG. 5  shows a triple-chamber ICD which, in addition to the electrodes already described with reference to  FIGS. 1 and 3 , has a third electrode line  60 , which is guided as a left-ventricular electrode line via the coronary sinus and a lateral vein branching from the coronary sinus into the periphery of the left ventricle and which has a left-ventricular ring electrode  62  and a left-ventricular tip electrode  64  on its end.  
         [0083]     A triple-chamber ICD of the type shown in  FIG. 5  allows the differential signals for the three far field channels to be ascertained in different ways, as shown in  FIGS. 6, 7 , and  8 . The configuration in  FIG. 6  has a result corresponding to those already explained with reference to  FIGS. 1 and 2 , while the configuration in  FIG. 7  corresponds to the configuration which was already explained with reference to  FIGS. 3 and 4 . The configuration in  FIG. 8  is similar to the configuration from  FIGS. 3, 4 , and  7 , with the exception that the active housing of the ICD is not used as the third electrode, but rather the left-ventricular ring or tip electrode. Such a configuration, as shown in  FIG. 8 , will be explained once again in greater detail below with reference to  FIGS. 14 and 15 .  
         [0084]     It will now be explained with reference to following  FIGS. 9 through 13  how information about the origin location of a tachycardia, for example, may be obtained solely by analyzing the maximum amplitudes of the three differential signals (see  FIGS. 2   a  through  2   c  and  4   a  through  4   c ). The known theory of Einthoven is used to explain this. The absolute values of the three maximum amplitudes may be understood as absolute values of vectors in the Einthoven triangle, which is known per se, which may be added vectorially and used to calculate a summation vector which reproduces the direction of the excitation propagation in the myocardium.  
         [0085]     First,  FIG. 9  shows the determination of the maximum amplitude of a particular differential signal. The maximum amplitude occurs precisely once per cardiac cycle, at the instant of the R wave, i.e., at the instant of the contraction of the cardiac tissue as a result of the depolarization of the cardiac muscle cells. Therefore, three amplitude values are obtained per cardiac cycle from the three differential signals, which, as shown in  FIG. 10 , represent the absolute values of three vectors  50 ,  52 , and  54 , which may be added by vector addition to produce a summation vector  56 . In principle, the consideration of the angles between the vector  50 ,  52 , and  54  is required for the vector addition. These angles are predefined by the position of the three shock electrodes  12 ,  18 , and  20  spanning a triangle. In an ideal equilateral triangle, each of the angles is 60°. As results from the following, knowing the precise sizes of the angles between the individual vectors  50 ,  52 , and  54  is not absolutely necessary to nonetheless derive a sufficiently precise qualitative statement about the direction of the excitation propagation in the myocardium from the signs and absolute values of the three maximum amplitudes, which are to be derived from the differential signals recorded using the three shock electrodes  12 ,  18 , and  20 .  
         [0086]     A compact illustration of the three maximum amplitudes is shown in  FIG. 11 . Expressed in numbers, the three absolute values and signs of the three maximum amplitudes may also be summarized in a 3-tuple, which represents the direction of the excitation propagation sufficiently precisely in each case.  
         [0087]      FIGS. 9 through 11  show typical amplitudes during a natural excitation propagation from the atrium to the ventricle via the AV node of a heart.  
         [0088]      FIG. 12  shows how strongly the three maximum amplitudes change when the ventricular excitation has its origin in an ectopic excitation focus in the right ventricle, as is the case during a ventricular tachycardia, for example. It may be seen how the direction of the summation vector (expressed by an angle a) has changed by 145°.  
         [0089]     In the meaning of the angle definition for the angle a introduced with  FIGS. 10 and 12 , the summation vector in the normal sinus rhythm extends in a direction characterized by an angle a between 30° and 90°.  
         [0090]     Since the propagation of the ventricular excitation also originates from the AV node in the case of supraventricular tachycardias (SVT), a is also between approximately 30° and 90° in the case of supraventricular tachycardias. This is particularly true for supraventricular tachycardias such as ventricular fibrillation, ventricular flutter, stable, atrial reentry tachycardia, AV node reentry tachycardia, or Wolf-Parkinson-White syndrome.  
         [0091]     In the case of ventricular tachycardias, the origin location of the tachycardia is in the ventricle. Originating from an arrhythmogenic origin location, the excitation front propagates in the myocardium of the ventricle in an altered direction. This altered excitation propagation direction is reflected in a direction change of the summation vector. In turn, this direction change is reflected in an altered angle a (see  FIG. 12 ). An angle change of this type occurs in a monomorphous ventricular tachycardia or in a polymorphous ventricular tachycardia independently of the frequency of the atrium.  
         [0092]     In the case of a paroxysmal ventricular tachycardia, the direction of the summation vector change suddenly with occurrence (onset) of the tachycardia. A ventricular extrasystole also results in a direction change of the summation vector, precisely for a cardiac cycle in which the ventricular extrasystole occurs. In contrast, a sustained ventricular tachycardia produces a permanent change of the direction of the summation vector over many cardiac cycles. A stable ventricular reentry tachycardia having a 1:1 retrograde line (from the ventricle to the atrium) results in a change of the direction of the summation vector by approximately 180°.  
         [0093]     However, a stimulated ventricular contraction (using a typical simulation electrode situated in the apex of the ventricle) also results in a similar direction of the summation vector. This is to be considered when determining a tachycardia and may simultaneously be used for stimulation success control (capture control), as will be described in the following.  
         [0094]     In order to be able to classify tachycardias automatically as reliably as possible through the signal processing unit, in a preferred embodiment variation, the signal processing unit is implemented so that it may not only execute an analysis of the direction of the summation vector by analyzing the 3-tuple described, but rather additionally also considers known differentiation criteria.  
         [0095]     For this purpose, it may be especially advantageous if the ICD is a dual-chamber ICD, as shown in  FIG. 3 . This dual-chamber ICD  10 ′ is connected to a second electrode line  30 , which is used for stimulating the atrium and which has an atrial ring electrode  32  and atrial tip electrode  34  on its distal end. In addition to the four differential signals already shown in  FIG. 2 , a fifth differential signal is to be obtained via these two atrial electrodes  32  and  34 , which results from the potentials to be detected in the atrium and reflects their near field.  
         [0096]      FIG. 13  illustrates the mode of operation of a signal processing unit which considers both the direction of the summation vector and also rate criteria, as are to be obtained using electrode pairs such as the ventricular ring electrode  22  and the ventricular tip electrode  24  or the atrial ring electrode  32  and atrial ring electrode  34 , to classify ventricular tachycardias.  
         [0097]     As may be seen from  FIG. 13 , the starting point for a classification of the ventricular tachycardia is a high ventricular rate (RR rate). In this case, the signal processing unit first ascertains, by analyzing the 3-tuple described above, whether the direction of the excitation propagation (the summation vector does not first have to be calculated for this purpose—as explained above) is located in a normal angle range (NW) or in an abnormal angle range (AW).  
         [0098]     If the signal processing unit ascertains that the direction of the excitation propagation in the ventricle is in an abnormal angle range (AB), the signal processing unit further checks whether the ventricular rate (RR rate) is stable or unstable. In both cases, a ventricular tachycardia is detected.  
         [0099]     If the signal processing unit ascertains that the direction of the excitation propagation is in the normal angle range (NW), the signal processing unit checks further whether the ventricular rate (RR rating) has changed gradually (no onset) or has changed suddenly (onset). In a first case (no onset), the signal processing unit detects a sinus tachycardia.  
         [0100]     In the second case (sudden occurrence of the tachycardia, onset), the signal processing unit checks whether the ventricular rate is unstable or stable. In both cases, the signal processing unit ascertains a supraventricular tachycardia (SVT).  
         [0101]     The following abbreviations are used in  FIG. 13 :  
         [0102]     NW normal angle range  
         [0103]     AW abnormal angle range  
         [0104]     ST sinus tachycardia  
         [0105]     SVT supraventricular tachycardia  
         [0106]     VT ventricular tachycardia  
         [0107]     As already indicated, the direction of the excitation propagation may also be expediently analyzed for stimulation success control. This is particularly advantageous in biventricular stimulation devices, i.e., for example, in triple-chamber ICDs.  
         [0108]     Such a triple-chamber ICD is shown in  FIG. 14 . In contrast to the single-chamber and dual-chamber ICDs shown in  FIGS. 1 and 2 , the triple-chamber ICD  10 ′″ from  FIG. 14  is equipped with a third electrode line  60 , which is used for stimulating the left ventricle and is typically inserted via the coronary sinus and a lateral vein branching from the coronary sinus. The left-ventricular electrode line  60  has a left-ventricular tip electrode  64  on its distal end.  
         [0109]     The differential signals shown in  FIG. 15 , which are to be recorded using the triple-chamber ICD shown in  FIG. 14 , correspond to those which were already explained in connection with  FIG. 2 .  
         [0110]     As already noted, it may be inferred from the direction of the excitation propagation whether a ventricular contraction may be attributed to a successful stimulation.  
         [0111]     This is explained in greater detail with reference to  FIG. 16 . In a stimulated heart, the electrical excitation in the heart begins at the point of the stimulation electrode. From there, the excitation front propagates via the ventricle. This means that the excitation propagation and thus the direction of the summation vector is different than in a non-stimulated heart action, in which the excitation originates from the AV node.  
         [0112]     In a biventricular stimulation of the heart, the excitation front will propagate in an altered direction, originating from the two stimulated origin locations. This changed excitation propagation direction is reflected in a different angle of the summation vector. During a successful biventricular stimulation, an angle results as shown in  FIG. 16  and maximum amplitudes of the differential signals result as are shown in compact form in  FIG. 17 .  
         [0113]     The illustration in  FIG. 17  shows the polarity of the individual maximum amplitudes and their amplitude ratio to one another. Therein, I identifies the maximum amplitude of the differential signal which was obtained between the active housing  12  and the proximal shock electrode  18 , II identifies the maximum amplitude of the differential signal which was recorded between the proximal shock electrode  18  and the distal shock electrode  20 , and III identifies the maximum amplitude of the differential signal which was detected between the distal shock electrode  22  and the active housing  12 .  
         [0114]     In  FIG. 16  and following  FIGS. 18 and 20 , a solid five-pointed star identifies the location of an effective stimulation, while an empty (white) five-pointed star identifies the location of an ineffective stimulation.  
         [0115]      FIG. 18  and the associated compact illustration of the maximum amplitudes of the differential signals show how an ineffective stimulation in the left ventricle acts on the direction of the excitation propagation (see  FIG. 18 ) and the absolute values and the signs of the maximum amplitudes (see  FIG. 19 ).  
         [0116]     Correspondingly,  FIGS. 20 and 21  represent the case of the ineffective right-ventricular stimulation.  
         [0117]     It is notable that the characteristic angle changes of the summation vector as a result of the altered propagation direction may already be inferred from the signs of the individual maximum amplitudes.  
         [0118]     A special advantage of the stimulation success control on the basis of the analysis of the maximum amplitudes of the differential signals presented here is that this analysis and thus the establishment of a stimulation success may occur without the strength of the stimulation pulses having to be changed for this purpose.  
         [0119]     In typical stimulation devices having automatic stimulation strength adaptation, the strength of the stimulation pulses is reduced step-by-step until a lack of stimulation success is established. In the stimulation success control on the basis of the analysis of the maximum amplitudes of the differential signals, the further advantage results that the shock electrodes and not the stimulation electrodes may be used for the stimulation success control. Moreover, the detection of the stimulation success on the basis of the analysis of the maximum amplitudes is independent of set pacemaker parameters.  
         [0120]     As may be seen from  FIGS. 18 and 20 , the signal processing unit may be implemented for stimulation success control so that it responds to a sudden change of the excitation propagation direction. Depending on whether this sudden direction change is an angle change to the left or right, it may be seen by the signal processing unit in which of the chambers the capture loss has occurred.  
         [0121]     If a capture loss is established, the stimulation device is preferably implemented to increase the stimulation strength for the particular chamber step-by-step (for example, in steps of 0.1 V) until a new excitation propagation direction which is characteristic for stimulation successful on both sides results again.  
         [0122]     As  FIG. 22  shows, the information about right-ventricular or left-ventricular capture losses obtained in the way described may be summarized into a report, which may be transmitted telemetrically from the implanted stimulation devices, for example, to a home monitoring center.  FIG. 18  shows the possible appearance of such a report. In order to produce such a report, the signal processing unit is implemented to count events of lacking right-ventricular or left-ventricular stimulation success day by day and to transmit these numbers telemetrically to a home monitoring center.  
         [0123]      FIGS. 24 through 27  show how the input signals of three far field channels within a time window, which is related to an instant t trigger , which is derived from the input signal of a near field channel, are analyzed. In  FIG. 24 , the time window begins at an instant t lower , which is before the instant t trigger , and ends with an instant t upper , which is after the instant t trigger . The embodiment variation shown in  FIG. 24  thus requires a loop recorder for recording the total of four input signals. In the embodiment variation shown in  FIGS. 26 and 27 , the time window begins at an instant t lower , which is after the instant t trigger , so that no loop recorder is required for the embodiment variation shown in  FIGS. 26 and 27 .  
         [0124]     It is to be noted that in the exemplary embodiment shown in  FIGS. 24 through 27 , only the maximum amplitudes (A FFmax ) of the particular far field signals within the time window, i.e., between the instants t lower  and t upper , are always detected.  FIG. 26  shows that these are not necessarily the absolute amplitude maxima of the particular far field signals within a particular cardiac cycle.  
         [0125]     In addition,  FIGS. 24 through 27  show that besides the maximum amplitude values A FFmax  within the time window, the instants (t FFmax ) of the occurrence of these maximum amplitudes are also detected and analyzed. These instants may also be represented as a 3-tuple—similar to the maximum amplitude values—which is graphically shown in compact form in  FIGS. 25 and 27 .