Abstract:
A heart stimulator has a stimulation energy delivering assembly, including at least one lead adapted for implantation in contact with cardiac tissue, and an atrial arrhythmia detector, and a control unit connected to the stimulation energy delivering assembly and to the detector. The control unit controls the stimulation energy delivering assembly to deliver at least one atrial arrhythmia abolishing therapy and, if continued atrial arrhythmia is detected, to deliver antithrombus stimulation energy pulses of lower energy than a defibrillation shock, but with different timing and with sufficient energy for producing atrial contraction for increasing hemodynamic blood transportation away from the atrium and for preventing thrombi formation in the atrium. For a patient suffering from a chronic or paroxysmal non-curable atrial arrhythmia, the control unit controls the stimulation energy delivering assembly to deliver the antithrombus energy without any preceding arrhythmia abolishing therapy.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a heart stimulator comprising stimulation energy delivering means intended to deliver stimulation energy through at least one implanted lead to the heart of a patient and an atrial arrhythmia detecting means. 
     2. Description of the Prior Art 
     There is a 2-4% prevalence of atrial fibrillation in humans around the age of 70 years. In case of chronic atrial fibrillation, paroxysmal atrial fibrillation, and other atrial arrhythmias there is a high risk of medical complications due to possible thrombi formations, such as embolism from the left side of the heart to cerebral arteries or to other arteries of the upper and lower part of the human body. In the case of thrombi formation in the right atrium or ventricle there is also a risk for accompanying embolism to the pulmonary vessels, with a risk of resulting pulmonary embolism. 
     Patients suffering from atrial arrythmia such as atrial fibrillation are often subject to medication with anti-thrombotic drugs, such as Dicumarol or Warfarine with the accompanying high risks of medical problems. Overdoses are associated with risks of bleeding, bruises and mental problems in relation to the elevated risk of bleeding etc., and undermedication is associated with a too low plasma concentration of Dicumarol and Warfarine with again an obvious risk of thrombi formation. 
     Moreover, there is a risk of low compliance to self-medication among such patients, who often are elderly. The medication also requires frequent medical check-ups of, e.g. protrombine values, to be able to adapt the level of medication to the needs of the drug metabolism in the individual patient. This means massively increasing medical care expenses. In addition thereto there are in quite a number of clinical cases contraindications to the use of drugs such as the ones mentioned above. 
     In spite of the frequent usage of the medical treatment described above there is no clear evidence as to the effectiveness of the treatment. 
     It is traditionally widely accepted that atrial thrombi form only after 2-3 days of atrial fibrillation, and that atrial thrombi in a fibrillating atrium form after the onset of the arrhythmia. Therefore, e.g. cardioversion of atrial fibrillation of less than 3 days duration without anticoagulation prophylaxis is believed to be safe and is commonly performed, see M. F. Stoddard, “Risk of Thromboembolism in New Onset or Transient Atrial Fibrillation”, Progress in Cardiovascular Diseases, Vol. XXXIX, No. 1, July/August, 1996, pp. 69-80. 
     An implantable atrial defibrillator specially designed for keeping the energy consumption on a comparatively low level is described in U.S. Pat. No. 5,433,729, and U.S. Pat. No. 5,464,429 discloses an apparatus for defibrillation of a heart, in which sequences of stimulation pulses and defibrillation shocks are delivered to the heart. The stimulation pulses used have an amplitude and/or pulse duration which greatly exceed the amplitude and/or pulse duration of ordinary pacemaker pulses, however, the high-energy stimulation pulses contain considerably less energy than an ordinary defibrillation shock, thereby reducing the total energy consumption. 
     In European Application 1 0 727 241 a device for electric heart stimulation is described, which device delivers a mild therapy to achieve an electrical influence on the heart which in a gentle manner reduces the occurrence of malfunctions in the muscles of the heart for correcting or restoring a collapsed conduction system. 
     In the following electrical thrombosis preventive stimulation or thromboembolic prophylactic stimulation is denoted by TPS and by thromboembolic prophylactic stimulation capture or TPS capture is meant a sufficient stimulation or energy level for achieving supporting hemodynamic blood transportation away from the atrium. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a heart stimulator for preventing or reducing thrombosis and embolism occurrence in case of atrial fibrillation or other atrial arrhythmia by administering an electrical therapy, this therapy not being intended for treatment of the atrial fibrillation or atrial arrhythmia in itself. 
     This object is achieved in accordance with the principles of the present invention in a heart stimulator which, in combination with a conventional pacing regimen, or as a stand-alone therapy, delivers pulses to cardiac tissue with an energy content lower than a conventional defibrillation shock, the energy content of the delivered pulses being sufficient to produce an atrial contraction that promotes the efficiency of hemodynamic blood transportation away from the atrium, thereby preventing thrombi formation in the atrium. 
     Thus, with the heart stimulator according to the invention an electrical prophylactic therapy is provided by electrical stimulation of heart tissue such that at least a partial contraction of the atrium is achieved with resulting improved hemodynamic blood transportation away from the atrium, where thrombi formation can be initialized. Thus the atrial contraction is not necessarily performed in an optimal physiological manner, but effective enough to protect the patient against thrombi formation and possible left or right sided embolic events. The heart stimulator according to the invention offers a comparatively cheap and easily performed therapy of the above discussed universally spread disease. Another advantage of the heart stimulator according to the invention is that it can be easily realized by modifying existing pacemaker and defibrillator apparatuses. 
     The heart stimulator according to the invention can suitably be used for providing thromboprophylactic electrical stimulation of the atrium when ordinary defibrillation therapy has not been successful within 2-3 days after the onset of the atrial arrhythmia, as discussed above. The heart stimulator according to the invention can also be used for delivering the thromboprophylactic therapy to patients having chhronic or paroxysmal, non-cureable atrial fibrillation. The therapy delivered by the heart stimulator according to the invention can also be a supplement to thromboprophylactic medication. 
     The antithrombus stimulation energy pulses delivered by the stimulator according to the invention are of lower energy than a defibrillation shock, but, according to an embodiment of the invention, of sufficient magnitude for causing a contraction in at least a part of atrial muscular tissue. Thus, the thromboprophylactic therapy delivered by the heart stimulator according to the invention is considerably less energy consuming than a regular defibrillation therapy. 
     According to another embodiment of the heart stimulator according to the invention said stimulator has a control unit which contains synchronizing circuitry which controls the energy delivery stage to deliver antithrombus stimulation energy in a phase of the cardiac cycle sufficiently separated from the vulnerable phase of the cycle. In this way the patient is protected against the triggering of dangerous cardiac events. 
     The heart stimulator according to the invention can be a pacemaker or a defibrillator having different modes of operation, one of these modes being a thrombo-prophylactic stimulation mode for delivery of antithrombus stimulation energy. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 schematically shows a first embodiment of the heart stimulator according to the invention in the form of a DDD-pacemaker modified to include a special antithrombus stimulation mode of operation. 
     FIG. 2 is a normal DDD therapy ECG. 
     FIGS. 3-5 show corresponding diagrams for different examples of thromboprophylactic stimulation by the stimulator in FIG.  1 . 
     FIG. 6 shows schematically a second embodiment of the heart stimulator according to the invention in the form of a VDD pacemaker provided with an antithrombus stimulation mode of operation. 
     FIG. 7 shows a typical example of an ECG illustrating the operation of the embodiment shown in FIG.  6 . 
     FIG. 8 shows schematically a third embodiment of the heart stimulator according to the invention in the form of a VVI pacemaker provided with an antithrombus stimulation mode of operation. 
     FIG. 9 shows an ECG for the heart stimulator in FIG. 8 operating in a normal pacing mode. 
     FIG. 10 shows an ECG for the heart stimulator in FIG. 8 operating in the antithrombus stimulation mode of operation. 
     FIG. 11 is an ECG for an antithrombus stimulation mode of operation integrated with ordinary pacing stimulation. 
     FIG. 12 shows an ECG in case of antithrombus stimulation in the form of bursts or strong crosstalk stimuli, 
     FIG. 13 shows schematically a fourth embodiment of the heart stimulator according to the invention in the form of an atrial defibrillator provided with an antithrombus stimulation mode of operation. 
     FIG. 14 is a flow diagram illustrating the ordinary operation of the heart stimulator shown in FIG.  13 . 
     FIG. 15 is a flow diagram illustrating the antithrombus stimulation mode of operation of the heart stimulator in FIG.  13 . 
     FIG. 16 shows schematically a fifth embodiment of the heart stimulator according to the invention in the form of a dual chamber defibrillator provided with a thromboembolic prophylactic stimulation circuit. 
     FIG. 17 shows schematically a sixth embodiment of the heart stimulator according to the invention including a device for delivering DC thromboembolic prophylactic current injection. 
     FIG. 18 shows an ECG recorded during operation of the heart stimulator in FIG.  17 . 
     FIG. 19 shows schematically a seventh embodimenet of the heart stimulator according to the invention including a multisite thromboembolic prophylactic stimulation device. 
     FIG. 20 shows schematically an eighth embodiment of the heart stimulator according to the invention provided with a multisite and/or multichamber thromboembolic prophylactic stimulation device. 
     FIG. 21 is a block diagram illustrating the operation of a conventional pacing system provided with a mode switch. 
     FIG. 22 is a block diagram illustrating the operation of a heart stimulator according to the invention including a conventional pacing system with TPS electronics. 
     FIG. 23 is a block diagram illustrating the operation of a heart stimulator according to the invention without a pacing system. 
     FIG. 24 is a flow chart exemplifying the therapy decision making in connection with a heart stimulator of the type shown in FIG.  23 . 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     As mentioned above the heart stimulator according to the invention can easily be realized by modifying currently existing heart stimulator designs and current electrode positioning of implantable systems can be used. Contractive stimulus for thromboembolic prophylactic therapy can thus be applied directly in the right atrium, epicardially on the right and/or left atrial wall, in the coronary vein system, in and on the right ventricular wall or epicardially on the left ventricular wall. The electrode placement could be atrial appendix, atrial lateral wall, atrial septum, coronary sinus or in the upper portion of the right ventricle, preferably in the outflow tract. The thromboembolic prophylactic stimulation can be provided in the form of a contractive electrical stimulus of an energy below the defibrillation threshold value. Such low energy defibrillation or electrical convertion can be used in a crosstalk manner in the heart stimulator according to the invention. Thus a low-energy electrical ventricular stimulus of an amplitude below defibrillation therapy level can cause atrial contraction without major disturbance or side effects on the heart rhythm. 
     An atrial shock stimulus with or without “defibrillation capture” can be applied for thromboembolic prophylactic stimulation by the heart stimulator according to the invention. Such atrial shock stimulus can be applied directly on the right or left atrium or as interatrial crosstalk. Thromboembolic prophylactic stimuli can also be applied in the right ventricle as ventriculo-atrial crosstalk. 
     The heart stimulator according to the invention can also deliver thromboembolic prophylactic therapy in the form of an electric DC stimulation of heart tissue of the general kind described, however for other purposes, in the above mentioned European Application 0 727 241. 
     As mentioned above the heart stimulator according to the invention can be an ordinary pacemaker or defibrillator modified to deliver thromboembolic prophylactic stimulation. The heart stimulator according to the invention can, however, as well be realized as an apparatus solely for cardiac related management of thromboembolic risks and their treatment. 
     The thromboembolic prophylactic therapy can be applied in an automatically programmed mode or induced by a physician and/or by the patient himself or herself. The thromboembolic prophylactic therapy can be applied in varying set-ups depending on the condition of the heart conduction system of the individual patient. 
     The thromboembolic prophylactic stimulation can consist of a combined DC and pulsed heart tissue stimulation too. FIG. 1 shows a first embodiment of the heart stimulator according to the invention in the form of a DDD pacemaker  2  connected through leads  4 ,  6  to the right atrium  8  and the right ventricle  10  respectively of the heart  12  of a patient. The stimulator  2  is devised for delivering thromboembolic prophylactic stimulation pulses in the atrium and in the ventricle. In the ventricle a crosstalk stimulation is used for the thromboembolic prophylactic therapy. The atrial electrode tip  14  can be positioned in the atrial appendage, in the lateral cardiac wall or in the cardiac septal wall as shown in FIG.  1 . 
     The heart stimulator  2  delivers in the thromboembolic prophylactic mode bursts of pulses or high voltage stimulation pulses in the atrium, c.f. FIGS. 3 and 4. In addition thereto pacing is performed in VVI mode or atrial pace inhibition mode with thromboembolic prophylactic crosstalk stimuli in the ventricle. 
     FIG. 2 shows the ECG for normal DDD pacing therapy disclosing an atrial pacing stimulation pulse A, the P-wave, a ventricular pacing stimulation pulse V, the QRS-complex and the T-wave. 
     FIG. 3 shows the ECG in a mode of operation in which the heart stimulator according to FIG. 1 delivers a burst of thromboembolic prophylactic stimulation pulses TPS followed by a ventricular stimulation pulse V. As appears from this diagram an atrial stimulation pulse A does not result in atrial capture, i.e. no P-wave is observed at  15  but a state of atrial fibrillation AF continues. 
     FIG. 4 shows the ECG for a mode of operation in which a strong high voltage thromboembolic prophylactic stimulation pulse TPS is delivered followed by a ventricular stimulation pacing pulse V. The thromboembolic prophylactic stimulation pulse TPS is a strong stimuli and has the character of an “extended” pacing pulse or atrial DC pulse. 
     The therapies illustrated in FIGS. 3 and 4 can also be combined, even in the same patient. 
     FIG. 5 shows an ECG for a mode of operation in which the application of thromboembolic prophylactic stimulation bursts TPS are separated by a plurality of RR-intervals, possibly including spontaneous heart activity with inhibited pacing and thromboembolic prophylactic treatment as indicated to the left of the figure. Bursts of thromboembolic prophylactic stimulation pulses TPS can typically be delivered a couple of times per 24 hours. 
     The result of the thromboembolic prophylactic therapy can be arrhythmia management or improvement as well, but this is not the main purpose, as discussed above. 
     FIG. 6 shows a second embodiment of the heart stimulator according to the invention comprising a VDD pacemaker designed for atrial activity detection and thromboembolic prophylactic crosstalk stimulation in the ventricle. 
     Thus a sensor  18  is located in the atrium  8  and  30  when an arrhythmia is sensed by this sensor  18 , at  20  in FIG. 7, without the appearance of a P-wave, thromboembolic prophylactic crosstalk stimulation TPS is delivered to the ventricle in the form of a high-voltage pulse or burst of pulses during the refractory period of the ventricle Ref in FIG.  7 . 
     For this embodiment special electrode positions, at  21  in FIG. 6, could be necessary such as the right ventricular outflow tract or close to the valvular area. 
     In FIG. 8 a third embodiment of the heart stimulator  22  according to the invention is shown, including a VVI pacemaker and means for atrial fibrillation detection. 
     Detection of atrial fibrillation can be made e.g. from R-R interval variations during the detection period. Thus atrial fibrillation can be indicated if e.g. the mean value of the RR intervals RR mean  is much less than a RR crit  denoting a critical value for atrial fibrillation. 
     In case of no detected atrial arrhythmia there is no need for thromboembolic prophylactic therapy and pacing in the VVI-mode is performed according to the ECG shown in FIG.  9 . In case of detection of atrial fibrillation, t 1 ≠t 2 ≠T, periods of atrial fibrillation being denoted by AF in FIG. 10, thromboembolic prophylactic therapy is delivered in the form of bursts of thromboembolic prophylactic stimulation pulses in the non-vulnerable phase of the cardiac cycle, shown at  24  in the ECG in FIG.  10 . In this case the thromboembolic prophylactic stimulation mode is applied according to a separate variability criteria. The thromboembolic prophylactic stimulation can also comprise delivery of high-voltage stimulation pulses. These pulses or bursts of pulses are delivered in the refractory period as mentioned above. 
     FIG. 11 shows an ECG illustrating a situation in which the thromboembolic prophylactic stimulation, in the form of a single high-voltage pulse  28 , is integrated with pacing stimulation delivered in the ventricle with a heart stimulator according to FIG. 6 or  8 . The pulse  28  thus constitutes an extended, supra threshold stimulation. An ordinary pacing pulse is shown at  26 . 
     FIG. 12 shows the ECG in a situation with bursts of thromboembolic prophylactic stimulation pulses, at  30 , the amplitude of the burst pulses being lower than the amplitude of an ordinary stimulation pulse  26 , or strong thromboembolic prophylactic crosstalk stimuli  32 , cf. FIG.  11 . 
     FIG. 13 shows a fourth embodiment of the heart stimulator according to the invention including an atrial defibrillator. This defibrillator is disposed such that, if atrial fibrillation or atrial tachycardia is detected, a shock of lower energy than an ordinary defibrillation shock is delivered periodically as thromboembolic prophylactic stimulation for improvement of blood circulation and replenishment. The thromboembolic prophylactic stimulation function is inhibited if the atrial rhythm is normalized, e.g. sinus rhythm is detected, and the thromboembolic prophylactic stimulation function is activated in case of unsuccessful atrial defibrillation therapy. The thromboembolic prophylactic stimulation therapy is controlled to be applied temporally separated from the ventricular vulnerable phase of the cardiac cycle, as mentioned above. The thromboembolic prophylactic stimulation function is synchronized to the cardiac cycle such that optimized left and right atrial blood drainage is accomplished. 
     The atrial defibrillator  34  thus contains a defibrillator unit DEF and a pacemaker P for stimulation in the ventricle. A control unit  36  is connected to the pacemaker P and the defibrillator DEF for controlling the delivery of thromboembolic prophylactic stimulation in response to sensed state of the heart. The defibrillator also comprises electronics  37 , containing e.g. memories, programs for thromboembolic prophylactic stimulation etc. The defibrillator DEF is connected through leads  38 ,  40 , to atrial defibrillation electrodes  44 ,  46  and the pacemaker P is connected through leads  42 ,  43 ,  47  to electrodes in the atrium  49  and the ventricle  48 ,  50  respectively. The defibrillator is communicating with an external programmer in the normal way by telemetri. 
     FIG. 14 shows a block diagram illustrating normal defibrillation. Thus when detecting atrial fibrillation, no P-wave, at block  52 , defibrillation therapy is applied, at block  54 . As a result of this therapy the heart resumes its normal atrial rhythm and P-waves appear, at block  56 . 
     If atrial defibrillation is detected in block  52  in FIG. 15, a defibrillation therapy is delivered, at block  54 . If a P-wave is detected as a result of the therapy atrial or ventricular Astim or Vstim is delivered in traditional therapies by the output block  55 .If no P-wave is detected as a result of the defibrillation, block  54 , this information is sent to a counter  57  and if “no P-wave” appears a predetermined number of times n a time delay, block  59 , is started and a warning, block  61 , is given. If no P-wave appears within a predetermined time delay, such that “no P-waves” appear n+1 times the output block  55  is controlled by a TPS control block  63  to deliver thromboembolic prophylactic therapy. The thromboembolic prophylactic therapy can be selectable via a program structure, in block  55 , or a physician can be able to use a real time controlled “open” loop to select e.g. the number and pattern of thromboembolic prophylactic stimuli and the time between each stimulation event. This new therapy can be combined with the traditional ones. 
     If a P-wave appears during the mentioned predetermined time delay the output block  55  is controlled to deliver traditional therapies. 
     FIG. 16 shows a fifth embodiment of the heart stimulator according to the invention including a dual chamber defibrillator. Thus this heart stimulator  62  comprises a defibrillator unit DEF, a pacing unit PACE and a thromboembolic prophylactic stimulation unit TPS. The pacing unit PACE is connected via leads  64 ,  66 ,  68  to electrodes  70  for atrial pacing in coronary sinus and ventricular electrode poles  72 ,  74 . The defibrillator unit DEF is connected through leads  76 ,  78 ,  80  to electrodes in the coronary sinus  82 , in the superior vena cava  84  and in the right ventricle  86  respectively. 
     The heart stimulator  62  contains a memory and telemetry unit m 1  which communicates with an external programming unit m 2 . 
     The thromboembolic prophylactic therapy can be applied on the atrium similar to anti-tachycardia therapy using pulses with voltages above the pacing stimulation pulse level. The thromboembolic prophylactic therapy can be applied simultaneously with AV-defibrillation, or together with pacing therapy of the patient by mode switching between the different modes of operation of the heart stimulator  62 . 
     In FIG. 17 a sixth embodiment of the heart stimulator according to the invention is shown. The stimulator  88  comprises a defibrillator unit DEF, a pacing unit PACE, a DC-voltage source  90  connected to a unit  92  for delivering thromboembolic prophylactic therapy. The stimulator  88  is connected through a lead  94  to the atrium  8  of the heart  12  for delivering thromboembolic prophylactic DC current injection therapy to the right atrium  8 . 
     The electrode system can be similar to the electrode system of a pacing device or an atrial defibrillator system. 
     The treatment of the heart tissue with DC as a thromboembolic prophylactic therapy can be performed with different characteristics. FIG. 18 shows an ECG with one example of DC energy delivery during a comparatively long time, at  96 , and during a shorter time, at  98 . 
     The term DC energy or DC therapy means that kind of mild therapy defined in the previously mentioned European Application 0 727 241. 
     The stimulator in FIG. 17 can also be of an atrio-ventricular setup with electrodes for the thromboembolic prophylactic therapy similar to the electrodes of a dual chamber defibrillator system, and the pacing unit PACE can be adapted for stimulation both in the atrium and the ventricle. The ECG in FIG. 18 shows the delivery of both atrial pacing pulses A and ventricular pacing pulses V. 
     The DC energy and other types of TPS therapies can be delivered between two selectable electrodes which can be implantable transvascular or extravascular electrodes, including epicardial electrodes. 
     FIGS. 19 and 20 show two more modifications of the stimulator shown in FIG. 17 for multi-site and/or multi-chamber thromboembolic prophylactic stimulation. These embodiments of the heart stimulator according to the invention comprise a defibrillator unit  100 , a pacing unit  102 , a DC voltage unit  104  and a thromboembolic prophylactic stimulation unit  106  as in the embodiment shown in FIG.  17 . The stimulator further comprises a multisite unit  110  for connecting the stimulator  108  to a plurality of electrode leads  112 ,  114 ,  116 ,  118 . In the embodiment shown in FIG. 19 the stimulator  108  is connected to the heart  12  through four electrode leads  112 ,  114 ,  116 ,  118  with epicardial electrodes  120 ,  122 ,  124 , and  126  respectively. Thus with this embodiment multisite thromboembolic prophylactic stimulation can be provided. 
     FIG. 20 shows an alternative embodiment for providing multi-site unichamber thromboembolic prophylactic stimulation. Thus thromboembolic prophylactic stimulation of the atrium  8  can be applied through the leads  128 ,  130 ,  132 ,. In addition thereto the stimulator  108  is connected to the ventricle  10  through a lead  134  for ventricular stimulation (and possibly sensing). 
     The shown multi-site atrial electrode array can be realized as one multi-polar atrial electrode. 
     In these embodiments different kinds of therapy (pacing, defibrillation, thromboembolic prophylactic therapy) can be performed simultaneously or sequentially. 
     The described embodiments of the heart stimulator according to the invention can deliver thromboembolic prophylactic therapy of different kinds. Thus the thromboembolic prophylactic stimulation can comprise ordinary “standard” pacing pulses with amplitudes and/or energies below pacing threshold values, equal to the pacing threshold value or above this threshold value. The thromboembolic prophylactic stimulation can consist in burst therapy similar to the therapy delivered by known antitachy pacing systems. The thromboembolic prophylactic stimulation can comprise pacing with DC voltage and/or “defibrillator therapy”. The defibrillation therapy is then performed with energy levels below those of ordinary defibrillation, but mostly above pacing stimulation energy levels in atrium and ventricle. The thromboembolic prophylactic stimulation capture is obtained when the stimulation or energy level is sufficient for achieving or supporting haemodynamic blood transportation away from the atrium. Relevant TPS capture is verified by the physician during implantation or follow-up procedures. Such evaluation can be made on the basis of e.g. fluoroscopy, echocardiography, ultrasound Doppler measurements, and atrial related ECG pattern changes. 
     FIG. 21 shows a block diagram of a conventional pacing system with mode switch comprising pacing electronics  138  and a sensing unit  140 . By atrial and ventricular detectors  142  and  144  respectively the status of the atrium—atrial arrhythmia  146 , atrial stimulation  148  and sensed P-wave  150 —and of the ventricle—QRS complex  152  or ventricular stimulation  154 —are detected. 
     In case of P-wave detection, block  150 , and QRS complex detection, block  152 , no stimulation is given to the heart, block  156 . In case of atrial and ventricular stimulation the pacing system is operating in a DDD mode, block  158 , and its operation is fed back through the block pacing therapy  160  to the sensing unit  140 . 
     An event memory  162  is also connected to the sensing unit  140  for storing sensed cardiac events. 
     In case of detection of atrial arrhythmia the mode of operation is switched to a VVI mode, blocks  164 ,  166 . 
     FIG. 22 shows a heart stimulator according to the invention including a pacing system of the type shown in FIG.  21  and provided with means for thromboembolic prophylactic therapy. Thus block  168  represents the pacing system in FIG.  21 . Atrial arrhythmias are detected by an atrial detector  170 . Atrial arrhythmias can, however, also be jointly detected in the ventricle from R-R interval variations, blocks  172 ,  174 . t 1  denotes a time constant for therapy interval programming, block  176 . This time constant t 1  can vary from once every second R-R interval to twice daily. 
     The information from block  170  about possible detected atrial arrhythmias together with information from block  174  about the detected R R variability and synchronizing information from block  178  and the time constant t 1 , block  176 , are supplied to the summing unit  180 , which in response to these information controls the thromboembolic prophylactic stimulation means  182 . The resulting operation of the thromboembolic prophylactic stimulation means  182  is stored in a memory  184  and this information is fed back to the pacing system  168 . The memory  184  is also connected to the cardiac event memory  186  which in its turn is connected to the pacing system  168 , too. 
     The designation t 2 , block  188 , denotes a time constant for switching from active thromboembolic prophylactic therapy mode into a supervising mode in case of detected atrial arrhythmia inhibition, block  190 . 
     An external programmer  192  is by telemetric means  194  communicating with the pacing system  168  in a conventional manner. Programmable parameters are magnitudes of the time constants t 1  and t 2 , pulse morphology, pulse shape (slew rate, decay time), pulse width, pulse amplitude, pulse sequence, pulse burst or no pulse burst, pulse burst morphology and different therapeutic intervals. 
     Patients previously not equipped with a pacemaker system or a defibrillator will receive a heart stimulator according to the invention just including a special thromboembolic prophylactic stimulation device as illustrated in FIG.  23 . This embodiment is similar to the embodiment shown in FIG. 22 with the exception that there is no pacing system  168 . For controlling the operation of this heart stimulator sensing electronics  196  and controlling electronics  198  are provided for controlling the operation in response to received information. For the rest the same reference numerals are used as in FIG.  22 . 
     FIG. 24 is a flow chart illustrating the therapy decision making according to the invention for an atrial arrhythmia patient. 
     In case of atrial arrhythmia diagnosis  198  defibrillating therapy  200  and/or an anti-blood coagulation therapy  202  are applied. If the patient is not positively responding to the defibrillating therapy, block  204 , and/or if there are complications or undesirable side effects from the anti-coagulation therapy, thromboembolic prophylactic stimulation therapy delivered by the heart stimulator according to the invention is applied after suitable time delays, block  208 . The result of this therapy is followed up, block  210 , and fed back to the thromboembolic prophylactic stimulation means, block  208 , for possibly modifying the therapy. Depending on the results of the thromboembolic prophylactic therapy also alternatives, including combinations of different therapies, can be considered, block  212 . 
     Thus by using e.g. a pacemaker lead of conventional uni- or bipolar type, placed in the right atrium of the heart or epicardially at the left atrium it is possible with a heart stimulator according to the invention to stimulate the left atrium to contract periodically in order to prevent thrombous formation in an atrium of regular size or in a dilated atrium. With the heart stimulator according to the invention the need for anticoagulent or related antithrombotic medication is reduced. It also reduces the need for electroconvertion with its risks, costs and ineffectiveness. 
     The therapy applied by the heart stimulator according to the invention is a more “powerful stimulation” than ordinary pacing stimulation in order to achieve an intermittent antithrombotic contractive therapy. 
     The decision for implementing the thromboembolic prophylactic therapy by the heart stimulator according to the invention is in the individual case to be taken by the physician. The therapy can be implemented by an automatic adaptive algorithm or by a custom defined therapy program with selectable semi-automatic limitations (stimulation period during day or night, number and morphology of consecutive pulses during the stimulation period, etc.), safeguarding escape mode in case of ventricular arrhythmia detection, and by program selectable time limiting supervising mode at detection of recurrent normal atrial sinus rhythm, e.g. in paroxysmal atrial fibrillation. Usually the thromboembolic prophylactic therapy is applied after about a 24 hour period of atrial arrhythmia, e.g. atrial fibrillation. 
     The heart stimulator according to the invention comprises at least one thromboembolic prophylactic stimulation electrode and possibly at least one sensing electrode. The sensing electrode is not necessarily implanted in the heart and/or on the heart, however, it is to be positioned for best P-wave and QRS detection and monitoring, which also includes multisite setup electrode applications as described above. 
     The risk of thrombous formation can be determined by pressure, flow and Doppler measurements. By forming an average value of the ECG signal, especially in relation to the P-wave, during a plurality of cardiac cycles before the thromboembolic prophylactic therapy, and after this therapy the difference between these average values gives an indication of the effect of the therapy. Sometimes the trembling motion of a fibrillating atrium seem to promote thrombous formation. 
     Although modifications and changes may be suggested by those skilled in the art, it is the invention of the inventors to embody within the patent warranted heron all changes and modifications as reasonably and properly come within the scope of their contribution to the art.