A CARDIAC STIMULATING DEVICE

Abstract: The invention concerns an implantable cardiac stimulating device (10). The device (10) comprises means (22) for varying the rate of the stimulating pulses up to a maximum pacing rate (M). Means (24) are arranged for sensing at least one evoked response parameter to the delivered stimulating pulses (21). The device (10) comprises means (26) for measuring a time gap (G) between a stimulating pulse (21) and the associated evoked response parameter. According to the invention, the maximum pacing rate (M) is lowered if said time gap (G) does not decrease with increasing pacing rate.

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 such a stimulating device comprising a housing, a control circuit enclosed in the housing, said control circuit being adapted to be connected to a first electrode to be positioned to stimulate a first ventricle of the heart. The control circuit comprises means for delivering stimulating pulses to the first electrode. Furthermore the device comprises means for varying the rate of stimulating pulses up to a maximum pacing rate. Moreover, the device comprises means arranged for sensing at least one evoked response parameter of the mentioned first ventricle. 2. Description of the prior art Cardiac stimulating devices of the above kind are known in the art. The rate of stimulating pulses may thereby be varied either in response to the sensing of an intrinsic atrial depolarisation or by detecting the body' s need for cardiac output by means of a sensor (a so-called sensor controlled/rate-responsive pacemaker) . Most pacers are arranged to stimulate the right ventricle of the heart, but it is also known to stimulate the left ventricle. In particular for the treatment of congestive heart failure or other severe cardiac failures it is known to stimulate the left ventricle, or both ventricles, in order to optimize the hemodynamic performance of the heart. US-A-5 728 140 describes a method and an apparatus for pacing the left ventricle of the heart . The pacing electrode is positioned within the interventricular septum proximate the left ventricular wall thereof . US-A-5 720 768 describes different possible electrical positions in order to stimulate or sense the different chambers of the heart . Also the article "A Method for Permanent Transvenous Left Ventricular Pacing" by Blanc et al, PACE, Vol. 21, 1998, pp. 2021-2024, describes a method for positioning leads for left ventricular pacing. US-A-4 928 688 describes a method and an apparatus for treating patients suffering from congestive heart failure by stimulating both the ventricles. The document discusses the problem involved when the left and right ventricles contract in asynchrony. In order to effect substantially simultaneous contraction of both ventricles, the document suggests means for separately processing sensed cardiac signals from each of the right and left ventricles. If ventricular contractions are not sensed in both the ventricles within a period of coincidence defined by a time delay, the pacing pulse will be emitted at the end of this time delay, but only to the ventricle for which a QRS-complex has not been sensed. The time delay is suggested to be in the order of 5- 10 ms. SUMMARY OF THE INVENTION Pacemakers are becoming more and more automatic in their functions. One such automatic function is that the pacemaker includes means for varying the rate of stimulating pulses, i.e. the pacing rate. Thereby, the pacemaker normally has a preset maximum pacing rate. From literature it is known that a progressive heart disease may alter the compliance patterns due to geometric remodelling of the myocardium. Such a remodelling may lead to different problems, it may for example result in a dissynchronization of the ventricles, in particular at higher pacing rates. The inventors of the present invention have found that the time gap between a stimulating pulse and the associated evoked response parameter may be monitored in order to detect heart problems, such as dissynchronization. Normally, when the pacing rate increases, the time gap between a stimulating pulse and the associated evoked response parameter becomes shorter. However, at a certain pacing rate, said time gap may stop decreasing although the pacing rate increases . The present inventors have found that such a situation is an indication of heart problems, such as a dissynchronization between the ventricles. It is an object of the present invention to provide an implantable cardiac stimulating device which is such that safety for the patient carrying the device will be increased, should any impending dissynchronization situation as described above occur. The object of the invention is obtained by an implantable cardiac stimulating device, comprising: a housing, a control circuit enclosed in said housing, said control circuit being adapted to be connected to a first electrode to be positioned to stimulate a first ventricle of the heart , said control circuit comprising means for delivering stimulating pulses to said first electrode, means for varying the rate of stimulating pulses up to a maximum pacing rate, means arranged for sensing at least one evoked response parameter of said first ventricle to the delivered stimulating pulses, wherein the control circuit comprises means arranged to measure a first time gap between a stimulating pulse and the associated evoked response parameter sensed by said means for sensing, means for monitoring said first time gap at the varying pacing rates with which the stimulating pulses are delivered, and wherein the control circuit is arranged such that said maximum pacing rate is lowered if said first time gap does not decrease with increasing pacing rate. Since, according to the invention, the maximum pacing rate is lowered if the mentioned time gap does not decrease with increasing pacing rate, the risks to which the patient is exposed are reduced. According to a further embodiment of the invention, said control circuit comprises means for storing said measured first time gap for one or more pacing rates, wherein the control circuit comprises means arranged for comparing the present measured first time gap with a previously stored first time gap for the corresponding pacing rate, and wherein the control circuit is arranged such that the maximum pacing rate is lowered also in case the difference between the present measured first time gap and the corresponding stored first time gap exceeds a predetermined value. According to this embodiment a comparison is thus made between a present time gap and a corresponding stored time gap. The stored time gap may be, for example, a time gap measured one or more days before the present time gap is measured. The stored time gap may represent a normal time gap for the patient in question. The fact that the present time gap exceeds the stored time gap with a predetermined value is an indication of a heart problem; a previous heart problem may, for example, have become worse. In response to the detected problem, the maximum pacing rate is thus lowered in order not to expose the patient to high risks . According to a further embodiment of the invention, the control circuit comprises means for monitoring the change in time gap ΔG when the pacing rate increases, wherein the control circuit is arranged such that said maximum pacing rate is lowered if said change in time gap ΔG is below a predetermined value. According to this embodiment the maximum pacing rate may thus be lowered before the mentioned time gap starts decreasing with increasing pacing rate. According to still another embodiment of the invention, said maximum pacing rate includes at least one of the maximum sensor rate and the maximum track rate. The maximum sensor rate is a programmable value in rate-modulated pacing systems. When the sensor is controlling the pacing rate, the pacing rate will not exceed the programmed maximum sensor rate. The maximum tracking rate is a programmable value in dual-chamber sensing and tracking modes. ' The maximum tracking rate determines the highest ventricular pacing rate that can be achieved in response to atrial sensed events. The maximum tracking rate is also called the ventricular tracking limit at the highest synchronous rate. According to a further embodiment of the invention, said control circuit is adapted to be connected also to a second electrode to be positioned to stimulate a second ventricle of the heart, wherein said control circuit comprises means for delivering stimulating pulses also to said second electrode, wherein the device also comprises means arranged for sensing at least one evoked response parameter to the stimulation of the second ventricle, wherein said means arranged to measure the first time gap between a stimulating pulse and the associated evoked response parameter is also arranged to measure a corresponding second time gap between a stimulating pulse and the associated evoked response parameter of said second ventric1e, and wherein the control circuit is arranged such that said maximum pacing rate is lowered if at least one of said first and second time gaps does not decrease with increasing pacing rate. This embodiment thus allows for bi-ventricular pacing. According to a further embodiment of the invention, said control circuit is arranged such that said maximum pacing rate is lowered also if the difference between said first and second time gaps exceeds a predetermined value. If the difference between said first and second time gaps is too large, this is also an indication that the heart does not respond properly to the pacing. In order to reduce the risks to which the patient is exposed, the maximum pacing rate is thus lowered. According to still another embodiment of the invention, said control circuit comprises means arranged to enable the delivery of the stimulating pulses to said first and second electrodes within the same cycle of the heart such that there may be a time interval between them, wherein the control circuit is arranged such that said time interval is variable, wherein the control circuit comprises means arranged to compare the occurrence in time of the sensed evoked response parameter to stimulation of the first ventricle with the sensed evoked response parameter to stimulation of the second ventricle, and wherein the control circuit comprises means arranged to control the delivery of stimulating pulses to said first and second electrodes such that the difference in occurrence in time between said sensed evoked response parameter to stimulation of the first ventricle and said sensed evoked response parameter to stimulation of the second ventricle is minimized. Since, according to this embodiment, the stimulating pulses to the ventricles may be delivered such that there is a time interval between them, it is possible to synchronize the sensed evoked response parameters for the left and right ventricle. Such synchronisation is important for patients with severe congestive heart failure. According to still a further embodiment of the invention, said means arranged for sensing at least one evoked response parameter to stimulation for the first and/or second ventricles are arranged for sensing an electrical evoked response parameter. Such an electrical evoked response parameter may be sensed by, for example, the electrode or electrodes used for stimulating the ventricles. According to another embodiment of the invention, said means arranged for sensing at least one evoked response parameter to stimulation for the first and/or second ventricle are arranged for sensing a mechanical evoked response parameter. The mechanical evoked response parameter may constitute the actual contraction of the ventricle or ventricles . Such a mechanical response parameter may be sensed by, for example, an accelerometer, means for sensing pressure or means for sensing the impedance. An advantage with the sensing of a mechanical evoked response parameter is that this parameter is directly indicative of the contraction of the ventricles. BRIEF DESCRIPTION OF THE DRAWINGS Fig 1 is a schematic representation of a device according to the invention connected to a heart ; Fig 2 is a block diagram of a control circuit of a device according to the invention; Fig 3 is a schematic representation of an electro- cardiographic response signal to a stimulating pulse; Fig 4 is a schematic graphical representation of the relationship between a time gap and the pacing rate; Fig 5a,b,c are schematic representations of typical electro-cardiographic response signals to stimulation of the left and right ventricles . DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig 1 shows an implantable cardiac stimulating device 10, hereinafter also called a pacemaker, according to the invention. The pacemaker 10 comprises a housing 12. A control circuit 14 (see Fig 2) is enclosed in the housing 12. The control circuit 14, and thereby the pacemaker 10, is adapted to be connected to a first electrode 16. Fig 1 shows such an electrode 16 which is connected to the pacemaker 10 via a lead 13. The first electrode 16 is adapted to be positioned to stimulate a first ventricle 18 of the heart 19. The first ventricle 18 is in this case the right ventricle. According to an embodiment of the invention, the pacemaker 10 is also adapted to be connected to a second electrode 36. Fig 1 shows such a second electrode 36 connected to the housing 12 via a lead 15. The second electrode 36 is positioned to stimulate a second ventricle 38 of the heart 19. The second ventricle 38 is in this case the left ventricle. The electrodes 16, 36 may include more than one electrical conductor in order to allow for bipolar pacing and sensing. Fig 2 shows a schematic representation of a block diagram of a control circuit 14 which is enclosed in the housing 12 of the pacemaker 10. The control circuit 14 comprises means 20 for delivering stimulating pulses 21 to the first electrode 16. The control circuit also comprises means 22 for varying the rate of stimulating pulses up to a maximum pacing rate M. The maximum pacing rate M may be the maximum sensor rate and/or the maximum track rate. The control circuit 14 also comprises means 24 arranged for sensing at least one evoked response parameter of said first ventricle 18 to the stimulating pulses delivered via said first electrode 16. The evoked response parameter may either be a parameter which indicates a mechanical contraction of the ventricle 18 or a parameter indicating an electrical response. The mechanical contraction may, for example, be sensed by an accelerometer, a pressure sensor or an impedance sensor. The impedance may, for example be sensed by an electrode 16, 36 connected to the pacemaker 10. The evoked response parameter may also be an electrical evoked response parameter which is sensed, for example, by the electrode 16, 36 positioned in the ventricle. Such an electrical evoked response parameter may be, for example, the T-wave or the R-wave in the electrical evoked response. The control circuit 14 also comprises means 26 arranged to measure a first time gap G between a stimulating pulse and the associated evoked response parameter sensed by said means for sensing 24. Fig 3 shows a schematic representation of a typical electrical sensed response signal . A stimulating pulse is represented by the reference number 21. In the electrical response to such a stimulating pulse 21 an R-wave (also called QRS-complex) and a T-wave may be detected. In the example according to Fig 3 the sensed evoked response parameter is the T-wave. G represents the mentioned time gap between the stimulating pulse 21 and the associated evoked response parameter sensed by the means for sensing 24. Fig 2 also shows that the control circuit 14 comprises means 28 for monitoring the first time gap G at the varying pacing rates with which the stimulating pulses 21 are delivered. The control circuit 14 is arranged such that said maximum pacing rate M is lowered if said first time gap G does not decrease with increasing pacing rate. Fig 4 shows a schematic representation of the relationship between the time gap G and the pacing rate. The pacemaker 10 normally has a preset, programmable maximum pacing rate M. The maximum pacing rate is represented with M in Fig 4. The time gap between a stimulating pulse 21 and the associated evoked response parameter normally decreases when the pacing rate increases. However, for some patients, for example those with a progressive heart disease which may alter the compliance patterns due to geometric remodelling of the myocardium, the heart disease may be such that the previously set maximum pacing rate M is in fact too high for the patient. According to the present invention, the maximum pacing rate M is lowered if the mentioned first time gap G does not decrease with increasing pacing rate. In Fig 4 the point 29 on the curve is a point where the mentioned time gap G does not decrease with increasing pacing rate. When this point 29 is reached, the maximum pacing rate M is thus lowered according to the present invention. According to a preferred embodiment of the invention the control circuit 14 comprises means 34 for monitoring the change in time gap ΔG when the pacing rate increases . The control circuit 14 is hereby arranged such that the maximum pacing rate M is lowered if the change in time gap ΔG is below a predetermined value. In Fig 4 two examples of ΔG are indicated. ΔGi is relatively large and ΔG 2 is smaller. When ΔG is below a predetermined value the maximum pacing rate M is thus lowered. Thereby the maximum pacing rate M may be lowered before the point 29 is reached. Thereby the risks to which the heart is exposed are reduced even further. To return to Fig 2 , the control circuit 14 also comprises means 30 for storing the measured first time gap G for one or more pacing rates . The control circuit 14 further comprises means 32 arranged for comparing the present measured first time gap with a previously stored first time gap for the corresponding pacing rate. The control circuit 14 is arranged such that the maximum pacing rate M is lowered also in case of difference between the present measured first time gap and the corresponding stored first time gap exceeds a predetermined value. Thereby a further measure is taken in order to reduce the risk for the patient . As explained above in connection with Fig 1, the pacemaker 10 may be adapted to be connected to a second electrode 36. According to this embodiment, the control circuit 14 comprises means 40 (see Fig 2) for delivering stimulating pulses also to the second electrode 36. The pacemaker 10 also comprises means 44 arranged for sensing at least one evoked response parameter to the stimulation of the second ventricle 38. The means 26 arranged to measure the first time gap G is thereby also arranged to measure a corresponding second time gap between a stimulating pulse and the associated evoked response parameter of the second ventricle 38. The control circuit 14 is hereby arranged such that the maximum pacing rate M is lowered if at least one of said first and second time gaps does not decrease with increasing pacing rate. This embodiment thus has the advantage that both ventricles are monitored. According to still another embodiment, the control circuit 14 is arranged such that the maximum pacing rate M is lowered also if the difference between the first and second time gaps exceeds a predetermined value. The inventors have found that also a relatively large difference in time gap G between the left and right ventricles is an indication that the heart does not respond properly to the pacing. Thus the maximum pacing rate M is lowered also in this case, in order to reduce the risks to which the patient is exposed. According to the embodiment shown in Fig 2 , the control circuit 14 also comprises means 46 arranged to enable the delivery of the stimulating pulses to the first 16 and the second 36 electrodes within the same cycle of the heart such that there may be a time interval dT between them. Furthermore, the control circuit 14 is arranged such that said time interval dT is variable. The control circuit 14 comprises means 48 arranged to compare the occurrence in time of the sensed evoked response parameter to the stimulation of the first ventricle 18 with the sensed evoked response parameter to the stimulation of the second ventricle 38. The control circuit 14 also comprises means 50 arranged to control the delivery of the stimulating pulses to the first 16 and second 36 electrodes such that the difference in occurrence in time ΔT between the sensed evoked response parameter to the stimulation of the first ventricle 18 and said sensed evoked response parameter to the stimulation of the second ventricle 38 is minimized. This embodiment is further illustrated in Fig 5a, 5b, 5c. The evoked response parameter may be a mechanical or an electrical sensed evoked response parameter as explained above. Fig 5a, 5b, 5c show typical electrical evoked responses . The electrical evoked response parameter may be related to either the R-wave or the T-wave. Moreover, different alternatives exist for detecting the evoked response. The evoked response parameter may for example be a peak or a maximum 52 or a certain predetermined slope 54, 55 of the wave which is detected. Also other possible points on the curve in the electrical evoked response may be detected, e.g. a zero-crossing. Instead of directly comparing the occurrence in time of a slope or peak or other point on the respective wave, it is possible to measure an integral of the difference between the wave in question in the evoked response to the stimulation of the first ventricle 18 and the wave in the evoked response to the stimulation of the second ventricle 38. The time interval dT is thereby set such that said integral is minimized. The sensing of the evoked response may be done either with a unipolar or with a bipolar arrangement . When the R-wave is sensed it may be advantageous to use a unipolar sensing. When the T-wave is sensed it may be advantageous to use a bipolar sensing. In Fig 5b an example is shown where the peak of the R-wave is detected. The curve 56 represents the electrical evoked response to a stimulation pulse 21 for the first ventricle 18. The curve 57 represents the corresponding evoked response for the second ventricle 38. According to this example, the stimulating pulses 21 to the first 16 and the second 36 electrodes are delivered simultaneously. In the example shown, the peak of the curve 56 occurs before the peak of the curve 57. The difference in occurrence in time between the peaks of the curves 56 and 57 is represented by ΔT. According to this embodiment of the invention, the control circuit 14 thus comprises means 50 which delivers the stimulating pulses to the first 16 and second 36 electrodes at different times such that the difference in occurrence in time ΔT of the peaks of the curves 56 and 57 is minimized. Fig 5c illustrates that the stimulating pulse to the second ventricle 58 is delivered before the stimulating pulse to the first ventricle 18. Hereby the two peaks of the curves 56 and 57 occur substantially simultaneously. In order to make the peaks occur simultaneously it is possible to either deliver the stimulating pulse to the electrode 36 (corresponding to the curve 57) earlier in time or to deliver the pulse to the electrode 16 (corresponding to the curve 56) later in time. A physician may determine which of the two possibilities is most suitable for a particular patient . The present invention is not limited to the above-described preferred embodiments. Various 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 (10), comprising: a housing (12) , a control circuit (14) enclosed in said housing (12) , said control circuit (14) being adapted to be connected to a first electrode (16) to be positioned to stimulate a first ventricle (18) of the heart (19) , said control circuit (14) comprising means (20) for delivering stimulating pulses (21) to said first electrode (16), means (22) for varying the rate of stimulating pulses up to a maximum pacing rate (M) , means (24) arranged for sensing at least one evoked response parameter of said first ventricle (18) to the delivered stimulating pulses (21) , wherein the control circuit (14) comprises means (26) arranged to measure a first time gap (G) between a stimulating pulse (21) and the associated evoked response parameter sensed by said means (24) for sensing, means (28) for monitoring said first time gap (G) at the varying pacing rates with which the stimulating pulses (21) are delivered, and wherein the control circuit (14) is arranged such that said maximum pacing rate (M) is lowered if said first time gap (G) does not decrease with increasing pacing rate. 2. An implantable cardiac stimulating device (10) according to claim 1, wherein said control circuit (14) comprises means (30) for storing said measured first time gap (G) for one or more pacing rates, and wherein the control circuit (14) comprises means (32) arranged for comparing the present measured first time gap (G) with a previously stored first time gap (G) for the corresponding pacing rate, and wherein the control circuit (14) is arranged such that the maximum pacing rate (M) is lowered also in case the difference between the present measured first time gap (G) and the corresponding stored first time gap (G) exceeds a predetermined value. 3. An implantable cardiac stimulating device (10) according to claim 1, wherein said control circuit (14) comprises means (34) for monitoring the change in time gap (ΔG) when the pacing rate increases, and wherein the control circuit (14) is arranged such that said maximum pacing rate (M) is lowered if said change in time gap (ΔG) is below a predetermined value. 4. An implantable cardiac stimulating ' device (10) according to anyone of preceding claims, wherein said maximum pacing rate (M) includes at least one of the maximum sensor rate and the maximum track rate. 5. An implantable cardiac stimulating device (10) according to anyone of the preceding claims, wherein said control circuit (14) is adapted to be connected also to a second electrode (36) to be positioned to stimulate a second ventricle (38) of the heart, wherein said control circuit (14) comprises means (40) for delivering stimulating pulses also to said second electrode (36), wherein the device (10) also comprises means (44) arranged for sensing at least one evoked response parameter to the stimulation of the second ventricle (38) , wherein said means (26) arranged to measure the first time gap (G) between a stimulating pulse (21) and the associated evoked response parameter is also arranged to measure a corresponding second time gap between a stimulating pulse and the associated evoked response parameter of said second ventricle (38) , and wherein the control circuit (14) is arranged such that said maximum pacing rate (M) is lowered if at least one of said first and second time gaps (G) does not decrease with increasing pacing rate. 6. An implantable cardiac stimulating device (10) according to claim 5, wherein said control circuit (14) is arranged such that said maximum pacing rate (M) is lowered also if the difference between said first and second time gaps (G) exceeds a predetermined value. 7. An implantable cardiac stimulating device (10) according to anyone of the claims 5 and 6, wherein the control circuit (14) comprises means (46) arranged to enable the delivery of the stimulating pulses (21) to said first (16) and second (36) electrodes within the same cycle of the heart such that there may be a time interval (dT) between them, wherein the control circuit (14) is arranged such that said time interval (dT) is variable, wherein the control circuit (14) comprises means (48) arranged to compare the occurrence in time of the sensed evoked response parameter to stimulation of the first ventricle (18) with the sensed evoked response parameter to stimulation of the second ventricle (38) , and wherein the control circuit (14) comprises means (50) arranged to control the delivery of stimulating pulses (21) to said first (16) and second (36) electrodes such that the difference in occurrence in time (ΔT) between said sensed evoked response parameter to stimulation of the first ventricle (18) and said sensed evoked response parameter to stimulation of the second ventricle (38) is minimized. 8. An implantable cardiac stimulating device (10) according to any one of claims 1-7, wherein said means (24, 44) arranged for sensing at least one evoked response parameter to stimulation for the first (18) and/or second (38) ventricles are arranged for sensing an electrical evoked response parameter. 9. An implantable cardiac stimulating device (10) according to anyone of claims 1-7, wherein said means (24, 44) arranged for sensing at least one evoked response parameter to stimulation for the first (18) and/or second (38) ventricles are arranged for sensing a mechanical evoked response parameter.

Inventors: OBEL MARTIN, SKANSEN JAN, OBEL, MARTIN, SKANSEN, JAN
Applicants: OBEL MARTIN, SKANSEN JAN, ST JUDE MEDICAL, OBEL, MARTIN, SKANSEN, JAN, ST. JUDE MEDICAL AB
Application year: 2000
Main classification: A61N1
IPC codes: A61N1