A CARDIAC STIMULATING DEVICE

Abstract: An implantable cardiac stimulating device with means (16, 18, 20, 22) arranged to deliver stimulating pulses to and to sense events in an atrium and a ventricle. The device includes means (24) for sensing an evoked response of the ventricle and means (20) to deliver a back-up pulse if no evoked response is sensed. The device also includes a control system (26) arranged to operate the device in at least a first and a second manner. In the first manner, no stimulating pulse is delivered to the atrium and in the second manner, a stimulating pulse is delivered to the atrium. The control system (26) is arranged to prevent the delivery of a back-up pulse during at least the first heart cycle when the device changes from the first to the second manner of operation.

Description: A cardiac stimulating device BACKGROUND OF THE INVENTION 1. Field of the invention The present invention relates to an implantable cardiac stimulating device. More precisely, the invention concerns a dual chamber pacemaker, i.e. a pacemaker which is adapted to stimulate and/or sense both an atrium and a ventricle of the heart. In particular, the invention concerns a pacemaker which includes means to deliver stimulating pulses to the atrium and to the ventricle and means for sensing events in the atrium and in the ventricle. The means arranged for sensing events in the ventricle includes means for sensing an evoked response of the ventricle to a delivered stimulating pulse. Furthermore, the cardiac stimulating device includes means which may be arranged to deliver a back-up pulse to the ventricle if the means arranged to sense an evoked response does not sense any evoked response to a delivered stimulating pulse. Moreover, the cardiac stimulating device includes a control system which is arranged to operate the device in at least a first and a second manner, wherein according to the first manner no stimulating pulse is delivered to the atrium and according to the second manner, stimulating pulses are delivered to the atrium. Such a control system may work in response to the means for sensing events in the atrium, such that a pulse is delivered to the atrium if and only if no event is sensed in the atrium within the atrial escape interval. 2. Description of the prior art For a general overview of the function of an implantable cardiac stimulating device, see, for example, US-A-5 873 895, columns 2-5. US-A-5 476 487 describes the function of an implantable cardiac stimulating device which emits a back-up pulse to the ventricle when no evoked response is detected to an applied ventricular pulse; see in particular columns 1 -2. A cardiac stimulating device which works with such back-up pulses is hereinafter also called an autocapture pacemaker (Autocapture ™ pacing system). Such a pacemaker may operate in the following manner. Shortly after a ventricular pulse has been delivered, for example 5-20 ms after the delivery, the pacemaker begins sensing whether a response occurs in the ventricle. This sensing may, for example, take place during 40-60 ms. If a response is sensed during this time interval, then no back-up pulse is emitted. However, if no evoked response is sensed, then a back- up pulse is emitted. The back-up pulse is preferably emitted immediately at the elapse of this time interval. The back-up pulse is usually emitted with increased output power in order to secure capture of the heart. For example, the normal ventricular pulse may be emitted with an amplitude of 1.5 V and the back-up pulses may be emitted with an amplitude of 4.5 V. In order to facilitate the understanding of the present invention, some basic working principles of a normal dual chamber pacemaker according to the prior art will now be described with reference to Fig 1 . Fig 1 schematically depicts a timing/waveform diagram indicating cardiac and pacemaker events. A represents a stimulating pulse delivered to the atrium. V represents a stimulating pulse delivered to the ventricle. P indicates the atrial depolarisation, in this case a paced depolarisation. R indicates ventricular depolarisation. R is thus in this case the evoked response to a delivered ventricular pulse V. T represents the ventricular repolarisation. A heart cycle HC, as this concept is used in the 99P2023SE present description and in the following claims, starts with a paced or sensed atrial event and ends at the subsequent paced or sensed atrial event. AVI is the so-called AV-interval. This is a pre-set, normally programmable, time interval beginning with a sensed or paced atrial event. If no ventricular event is sensed during the AV-interval, then a ventricular pulse is delivered at the end of this interval. A normal AV-interval may have a length of, for example, 150-210ms. AEI indicates the atrial escape interval. This is a predetermined, normally programmable, time interval which starts with an atrial or a ventricular sensed or paced event. If no atrial event has been sensed at the end of this pre-set atrial escape interval, then a stimulating pulse is delivered to the atrium at the end of this interval. BP is the ventricular blanking period. This is a short time interval after the delivery of an atrial pulse during which interval the ventricular sense amplifier is disabled and therefore cannot detect any signals. If there were no ventricular blanking period, then the ventricular sense amplifier could sense a paced event in the atrium and interpret this as a ventricular event. The pacemaker normally also operates with a similar atrial blanking period (which is not shown in Fig 1 ). Additionally, the following concepts, which are not shown in Fig 1 , will be explained. The "ventricular biological refractory period" is a period following a ventricular event (an R-wave) during which the heart will not respond to a ventricular stimulation. The "vulnerable period" is a part of the heart cycle, normally coincident with a part of the T-wave, during which a stimulation pulse may possibly cause repetitive rhythms such as, for example, tachycardia or ventrical fibrillation. The "ventricular complex" includes the ventricular depolarisation and a short time before and after this depolarisation. The above explanations of the mentioned concepts should not be taken as definite definitions, but rather as examples of how these concepts are normally used. Furthermore, it should be noted that in Fig 1 no back-up pulses are shown. The function of an autocapture 99P2023SE pacemaker will thus involve additional time intervals, as described above in connection with the explanation of the function of a pacemaker working with back-up pulses. SUMMARY OF THE INVENTION Although the above-described dual chamber pacemakers, which also emit back-up pulses, have been found to function well, the inventor of the present invention has invented further improvements concerning such pacemakers. In particular, the inventor has found that in such pacemakers a stimulating back-up pulse to the ventricle may sometimes occur during a phase of the heart cycle when such a stimulating pulse is not wanted. It has been found that such unwanted pulses may be initiated for example after the occurrence of a PVC (PVC = Premature Ventricular Contraction, i.e. a ventricular complex which occurs early) or in case an actual atrial event is not sensed by the pacemaker. Furthermore, the inventor has found that such an unwanted pulse is more likely to occur when the pacemaker changes its manner of operation from a period in which no stimulating pulse has been delivered to the atrium to a period in which one or more stimulating pulses are delivered to the atrium. The inventor has found that such a situation may occur in connection with, inter alia, the following circumstances: when a normal sinus node activity is not sensed (undersensing); in case of an ectopic focus (for example when the AV-node is the stimulating source); in case of atrial flutter or fibrillation; in case of atrial noise mode operation of the pacemaker; in case a sensor operated pacemaker (i.e. a pacemaker in which the stimulation rate changes with physical activity) changes the pacing rate. As an example of a series of events which may lead to an unwanted back-up pulse, the inventor has found that the following may happen. An atrial stimulating pulse may sometimes in fact be delivered just before or during a following ventricular complex. This may for example happen if there is a premature ventricular 99P2023SE contraction or if an atrial event which occurs is not sensed by the pacemaker. After an atrial stimulating pulse is delivered by the pacemaker, this pulse is followed by the above-mentioned ventricular blanking period. Since no event can be detected in the ventricle during this blanking period, the pacemaker will not sense a spontaneous ventricular depolarisation and will therefore emit a ventricular pulse at the end of the AV-interval. This ventricular pulse may occur during the ventricular biological refractory period. Therefore, this pulse will not initiate a ventricular depolarisation. Since no depolarisation is sensed in response to the delivered pulse, the pacemaker will deliver a back-up pulse in accordance with the autocapture function. Since in this case a ventricular event has in fact occurred, this back-up pulse is not needed. Furthermore, this back-up pulse may be emitted during the above-mentioned vulnerable phase of the heart cycle. An object of the present invention is to obtain an improved dual chamber autocapture pacemaker. A further object is to avoid the delivery of unnecessary back-up pulses at a point in time at which such pulses are not wanted. In particular, the invention aims at avoiding the delivery of such unwanted back-up pulses when the pacemaker changes its manner of operation to a manner in which one or more atrial pulses are delivered. According to the present invention, these objects are obtained by an implantable cardiac stimulating device adapted to be connected to a first electrode means to be positioned to stimulate and sense an atrium of the heart and to a second electrode means to be positioned to stimulate and sense a ventricle of the heart, the cardiac stimulating device comprising: means arranged to deliver stimulating pulses to said atrium, means arranged for sensing events in said atrium, means arranged to deliver stimulating pulses to said ventricle, means arranged for sensing events in said ventricle, said means arranged for sensing events in said ventricle including means arranged to sense an evoked response of the ventricle to a delivered stimulating pulse to the ventricle, 99P2023SE said means arranged to deliver stimulating pulses to the ventricle being arranged to deliver a back-up pulse to the ventricle if said means arranged to sense an evoked response in said ventricle does not sense any evoked response to a delivered stimulating pulse, a control system arranged to automatically operate the cardiac stimulating device, in response to said means arranged for sensing events in said atrium, in at least a first and a second manner such that in the first manner no stimulating pulse is delivered to the atrium and in the second manner stimulating pulses are delivered to the atrium, wherein the control system is arranged to prevent the delivery of said back-up pulses to the ventricle during at least the first heart cycle from the point in time when the cardiac stimulating device changes from the first to the second manner of operation. Since no back-up pulse is delivered during said first heart cycle, the above mentioned occurrence of an unwanted back-up pulse is avoided during, at least, the heart cycle when the pacemaker changes from the first to the second manner of operation. According to a preferred embodiment of the invention, the control system is arranged such that the delivery of said back-up pulses is prevented during a predetermined number of consecutive heart cycles starting with said first heart cycle, wherein said predetermined number is larger than 1 . It has been found to be beneficial to prevent the delivery of back-up pulses during a plurality of heart cycles after said change in the manner of operation. According to a further embodiment of the invention, said predetermined number is 2-5, preferably 2 or 3. It has thus been found to be advantageous to limit the number of consecutive heart cycles during which no back-up pulses are delivered. The probability of unwanted back-up pulses has been found to be higher during the first heart cycles after said change in the manner of operation. Furthermore, it is beneficial to allow the delivery of back- 99P2023SE up pulses after the elapse of some heart cycles, since such back-up pulses, during normal operation, aid in securing the capture of the heart. The inventor has thus found that to prevent the delivery of the back-up pulses during the first two or three heart cycles after said change in the manner operation is particularly beneficial. According to a further embodiment of the invention, said means arranged to deliver stimulating pulses to said ventricle is arranged such that the stimulating pulses to the ventricle are delivered with increased output power during said heart cycles in which said control system prevents the delivery of said back-up pulses. Since the output power of the stimulating pulses is increased, the likelihood that the heart actually will respond to the delivered stimulating pulses increases. According to a still further embodiment of the invention, said increased output power is of essentially the same magnitude as that at which said back-up pulses are delivered. BRIEF DESCRIPTION OF THE DRAWINGS Fig 1 is a schematic representation of a timing/waveform diagram which depicts the cardiac pacemaker events in a dual chamber implantable cardiac stimulating device according to the prior art. Fig 2 is a schematic block diagram of a cardiac stimulating device according to the invention. Fig 3 is a simplified flow chart which functionally illustrates an example of the operation of the implantable cardiac stimulating device according to the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig 2 schematically shows an implantable cardiac stimulating device according to the present invention. The implantable cardiac stimulating device is hereinafter also called a pacemaker. The invention concerns dual chamber pacemakers. The pacemaker is 99P2023SE thus adapted to be connected to a first electrode means 10 to be positioned to stimulate and sense an atrium of the heart. The heart is schematically depicted by 8. The pacemaker is also adapted to be connected to a second electrode means 12 to be positioned to stimulate and sense a ventricle of the heart. The pacemaker also includes pulse-generating means for producing stimulating pulses. In particular the pacemaker includes means 16 arranged to deliver stimulating pulses to the atrium and means 18 arranged for sensing events in the atrium. Furthermore, it includes means 20 arranged to deliver stimulating pulses to the ventricle and means 22 arranged for sensing events in the ventricle. The means 22 arranged for sensing events in the ventricle includes means 24 arranged to sense an evoked response of the ventricle to a delivered stimulating pulse. Such means 24 arranged to sense an evoked response may function as has been described above, i.e. to sense the evoked response during a certain time interval after the delivery of a stimulating pulse. The means 20 arranged to deliver stimulating pulses to the ventricle is arranged, during normal operation conditions, to deliver a backup pulse to the ventricle if said means 24 arranged to sense an evoked response in the ventricle does not sense any evoked response to a delivered stimulating pulse. The pacemaker also includes a control system 26 arranged to automatically control the operation of the pacemaker. In particular, the control system 26 controls that a stimulating pulse is delivered to the atrium if no relevant event has been sensed in the atrium during a predetermined period of time. It can thus be said that the control system 26 operates the pacemaker in a first and a second manner, wherein in the first manner no stimulating pulse is delivered to the atrium and in the second manner stimulating pulses are delivered to the atrium. According to the second manner of operation a stimulating pulse is thus delivered to the atrium at the elapse of a predetermined time interval, for example at the elapse of the atrial escape interval. Furthermore, according to the present 99P2023SE invention, the control system 26 is arranged such that no back-up pulse is delivered to the ventricle during at least the first heart cycle from the point in time when the pacemaker starts an episode of operating in the second manner. Preferably, the delivery of back-up pulses is prevented during a predetermined number of consecutive heart cycles starting from said point in time. This predetermined number of heart cycles may for example be 1 -9, preferably 2-5, in particular 2 or 3. During these heart cycles, the pacemaker may be arranged to deliver the ventricular stimulating pulses with an increased output power. The ventricular pulses may normally for example have an amplitude of between 1 and 2 V. The amplitude of the ventricular pulses with increased output power may be, for example, between 4 and 5 V. The ventricular pulses may thereby be delivered at a maximum output power, which may substantially correspond to the output power at which the back-up pulses are normally delivered. It should be noted that it may, of course, happen that an episode of pacing in the second manner only comprises one heart cycle. In this case it is sufficient to prevent the delivery of the back-up pulse during this heart cycle. It is, however, also possible to arrange the control system 26 such that back-up pulses are prevented during a predetermined number of heart cycles (e.g. 3) even if the episode of pacing according to the second manner only comprises one (or two) heart cycles. As an alternative to counting the number of heart cycles, the control system 26 can be arranged such that the pacemaker goes back to the normal operation (i.e. with back-up pulses) after a predetermined time. However, independently of in which way the pacemaker is arranged in order to inhibit the back-up pulses, it is preferred that a possible back-up pulse is inhibited for at least one heart cycle as soon as a pacing pulse is delivered to the atrium in the case when no pacing pulse was delivered to the atrium during the preceding heart cycle. Fig 3 shows a simplified flow chart of the operation of the pacemaker according to the invention. In Fig. 3 Y stands for yes 99P2023SE and N for no. At 32, a cycle count is set to 0. The purpose of the cycle count is to count the number of consecutive heart cycles during which a stimulating pulse is delivered to the atrium. At 34, the timer counting the atrial escape interval is started. If no P-wave is sensed during the atrial escape interval (blocks 36 and 38), then a stimulating pulse is delivered to the atrium (block 40). Furthermore, the cycle count is increased by 1 (block 42). On the other hand, if a P-wave is sensed during the atrial escape interval, then the cycle count is set to be 0 (block 44). At block 46, the timing of the AV-interval starts. If an R-wave is sensed during the AV-interval (blocks 48 and 50), then a new atrial escape interval starts according to block 34. However, if no R-wave is sensed during the AV-interval, then a new decision is taken at block 52. If the cycle count is either 0 or higher than 3, then a normal ventricular stimulating pulse is delivered at block 54. If an evoked response is sensed, then the process goes back to block 34 to start the timing of a new atrial escape interval. If no evoked response is sensed, then a back-up pulse is delivered according to block 58. After the delivery of this back-up pulse, the process goes back to block 34. On the other hand, if the cycle count in block 52 is equal to 1 , 2 or 3, then a ventricular pulse with increased output power is delivered at the end of the AV-interval (block 60). After the delivery of this ventricular pulse, the process goes back to block 34. In this case, therefore, no back-up pulse will be delivered. It should be pointed out that Fig 3 just depicts a very simplified example of the operation of the pacemaker according to the invention. Additional or alternative time counters may be used. Furthermore, the operation of the pacemaker normally involves several other parameters which are not indicated in the simplified flow charts, since the operation of a pacemaker is well known to the person skilled in the art. In Fig 3 the critical number of heart cycles is set to be 3. This number is just given as an example, and the number of heart cycles can be set at other values as has been explained above. 99P2023SE The present invention is not limited to the above-described preferred embodiments. Many alternatives, modifications and equivalencies may be used. Therefore, the above embodiments should not be taken as limiting the scope of the invention, which is defined by the appendant claims.

Claims: Claims 1 . An implantable cardiac stimulating device adapted to be connected to a first electrode means (10) to be positioned to stimulate and sense an atrium of the heart and to a second electrode means (12) to be positioned to stimulate and sense a ventricle of the heart, the cardiac stimulating device comprising: means (16) arranged to deliver stimulating pulses to said atrium, means (18) arranged for sensing events in said atrium, means (20) arranged to deliver stimulating pulses to said ventricle, means (22) arranged for sensing events in said ventricle, said means (22) arranged for sensing events in said ventricle including means (24) arranged to sense an evoked response of the ventricle to a delivered stimulating pulse to the ventricle, said means (20) arranged to deliver stimulating pulses to the ventricle being arranged to deliver a back-up pulse to the ventricle if said means (24) arranged to sense an evoked response in said ventricle does not sense any evoked response to a delivered stimulating pulse, a control system (26) arranged to automatically operate the cardiac stimulating device, in response to said means (18) arranged for sensing events in said atrium, in at least a first and a second manner such that in the first manner no stimulating pulse is delivered to the atrium and in the second manner stimulating pulses are delivered to the atrium, wherein the control system (26) is arranged to prevent the delivery of said back-up pulses to the ventricle during at least the first heart cycle from the point in time when the cardiac stimulating device changes from the first to the second manner of operation. 2. An implantable cardiac stimulating device according to claim 1 , wherein said control system (26) is arranged such that the delivery of said back-up pulses is prevented during a predetermined number of consecutive heart cycles starting with said first heart cycle, wherein said predetermined number is larger than 1 . 3. An implantable cardiac stimulating device according to claim 2, wherein said predetermined number is 2-5. 4. An implantable cardiac stimulating device according to claim 3, wherein said predetermined number is 2 or 3. 5. An implantable cardiac stimulating device according to any of claims 1 -4, wherein said means (20) arranged to deliver stimulating pulses to said ventricle is arranged such that the stimulating pulses to the ventricle are delivered with increased output power during said heart cycles in which said control system (26) prevents the delivery of said back-up pulses. 6. An implantable cardiac stimulating device according to claim 5, wherein said increased output power is of essentially the same magnitude as that at which said back-up pulses are delivered.

Inventors: LINDGREN ANDERS, LINDGREN, ANDERS
Applicants: LINDGREN ANDERS, ST JUDE MEDICAL, LINDGREN, ANDERS, ST. JUDE MEDICAL AB
Application year: 2000
Main classification: A61N1
IPC codes: A61N1