An Implantable Medical Device Configured to Provide an Intracardiac Function

Abstract

An implantable medical device configured to provide for an intracardiac function comprises a body, a sensor arrangement arranged on the body and configured to receive cardiac sense signals, and a processing circuitry operatively connected to the sensor arrangement. The processing circuitry is configured to process cardiac sense signals received using the sensor arrangement to detect atrial events caused by atrial activity based on a comparison of the cardiac sense signals to a sense threshold for a number of cardiac cycles, to evaluate whether a reduction criterion is fulfilled, wherein the reduction criterion is fulfilled if in X1 out of Y1 cardiac cycles no atrial events have been detected, X1 being a natural number equal to or larger than 1 and Y1 being a natural number equal to or larger than X1, and to reduce the sense threshold if said reduction criterion is fulfilled.

Claims

1. An implantable medical device configured to provide for an intracardiac function, the implantable medical device comprising: a body; a sensor arrangement arranged on the body and configured to receive cardiac sense signals; and a processing circuitry operatively connected to the sensor arrangement, wherein the processing circuitry is configured to process cardiac sense signals received using the sensor arrangement to detect atrial events caused by atrial activity based on a comparison of the cardiac sense signals to a sense threshold for a number of cardiac cycles, to evaluate whether a reduction criterion is fulfilled, wherein the reduction criterion is fulfilled if in X1 out of Y1 cardiac cycles no atrial events ,have been detected, X1 being a natural number equal to or larger than 1 and Y1 being a natural number equal to or larger than X1, and to reduce the sense threshold if said reduction criterion is fulfilled.

2. The implantable medical device according to claim 1, wherein said sensor arrangement is implemented by an electrode arrangement configured to receive electrical signals as cardiac sense signals.

3. The implantable medical device according to claim 1, wherein the body is formed by a lead which is connectable to a generator of the implantable medical device.

4. The implantable medical device to claim 1, wherein the body is formed by a housing of a leadless pacemaker device.

5. The implantable medical device according to wherein the processing circuitry configured to reduce the sense threshold for a detection of an atrial event (As) in a current cycle if in the current cycle it is found that said reduction criterion is fulfilled, wherein the reduction criterion is fulfilled if in X1 out of Y1 cardiac cycles prior to the current cycle no atrial events have been detected.

6. The implantable medical device according to claim 1, wherein the processing circuitry is configured to reduce the sense threshold by a reduction factor said reduction criterion is fulfilled.

7. The implantable medical device according to claim 6, wherein the processing circuitry is configured to reduce the sense threshold additionally by a step factor if said reduction criterion is fulfilled for at least two consecutive cycles.

8. The implantable medical device according to claim 1, wherein the processing circuitry is configured to evaluate whether a further, second reduction criterion is fulfilled, wherein the further, second reduction criterion is fulfilled if in X2 out of Y2 cardiac cycles no atrial events have been detected, X2 being a natural number larger than X1 and Y2 being a natural number larger than Y1.

9. The implantable medical device according to claim 1, wherein the processing circuitry configured to reduce the sense threshold by a second reduction factor associated with the further, second reduction criterion.

10. The implantable medical device according to claim 1, wherein the processing circuitry is configured not to reduce the sense threshold beyond a lower absolute threshold.

11. The implantable medical device according to claim 1, wherein the processing circuitry is configured to determine a peak amplitude (PA) associated with a detected atrial event, wherein the processing circuitry is configured to update the sense threshold using an average threshold reference based on the equation
ST(t)=PC.Math.ATR(t), where ST(t) is the current sense threshold, PC is a percentage ratio, and ATR(t) is the current average threshold reference.

12. The implantable medical device according to claim 11, wherein the current average threshold reference is determined by the equation
ATR(t)=W.Math.PA(t1)+(1W).Math.ATR(t1), where W indicates the update weight which determines how much the average threshold reference should change based on the previous peak amplitude . . . , PA(t1) is the peak amplitude as determined for the previous cycle t1, and ATR(t1) is the previous average threshold reference.

13. The implantable medical device according to claim 1, wherein the processing circuitry comprises a first processing channel having a first gain for processing a first processing signal derived from cardiac sense signals received via the sensor arrangement and a second processing channel having a second gain for processing a second processing signal derived from cardiac sense signals received via the sensor arrangement, the second gain being higher than the first gain.

14. The implantable medical device according to claim 13, wherein the processing circuitry is configured to process the first processing signal to detect a ventricular activity and the second processing signal to detect an atrial activity.

15. Method for operating an implantable medical device for providing for an intracardiac function, comprising: receiving, using a sensor arrangement arranged on a body of the implantable medical device, cardiac sense signals; and processing, using a processing circuitry operatively connected to the sensor arrangement, cardiac sense signals received using the sensor arrangement to detect atrial events (As) caused by atrial activity based on a comparison of the cardiac sense signals to a sense threshold for a number of cardiac cycles, to evaluate whether a reduction criterion is fulfilled, wherein the reduction criterion is fulfilled if in X1 out of Y1 cardiac cycles no atrial events have been detected, X1 being a natural number equal to or larger than 1 and Y1 being a natural number equal to or larger than X1, and to reduce the sense threshold if said reduction criterion is fulfilled.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0070] The various features and advantages of the present invention may be more readily understood with reference to the following detailed description and the embodiments shown in the drawings. Herein,

[0071] FIG. 1 shows a schematic view of the human heart, with an implantable medical device in the shape of a leadless pacemaker device implanted therein;

[0072] FIG. 2 shows a schematic view of an implantable medical device;

[0073] FIG. 3 shows a schematic view of an implantable medical device, indicating signal vectors between different electrodes of the implantable medical device;

[0074] FIG. 4 shows a schematic view of a processing circuitry of an embodiment of an implantable medical device;

[0075] FIG. 5A shows a first processing signal in the shape of an intracardiac electrogram (IEGM) processed by a first processing channel of the processing circuitry;

[0076] FIG. 5B shows a second processing signal processed by a second processing channel of the processing circuitry;

[0077] FIG. 6 shows an example of a sense threshold adapted over a multiplicity of cardiac cycles based on a peak amplitude and reduction criteria;

[0078] FIG. 7 shows a schematic view of multiple cardiac cycles and an evaluation of one reduction criterion;

[0079] FIG. 8 shows an example, in a schematic view, of multiple cardiac cycles and an evaluation of two reduction criteria; and

[0080] FIG. 9 shows a schematic view of the human heart, with an implantable medical device in the shape of cardiac stimulation device having a lead implanted in the right ventricle.

DETAILED DESCRIPTION

[0081] Subsequently, embodiments of the present invention shall be described in detail with reference to the drawings. In the drawings, like reference numerals designate like structural elements.

[0082] It is to be noted that the embodiments are not limiting for the present invention, but merely represent illustrative examples.

[0083] In the instant invention it is proposed to provide an implantable medical device providing for an intracardiac function, in particular a ventricular pacing, specifically a so-called VDD pacing.

[0084] FIG. 1 shows, in a schematic drawing, the human heart comprising the right atrium RA, the right ventricle RV, the left atrium LA and the left ventricle LV, the so-called sinoatrial node

[0085] SAN being located in the wall of the right atrium RA, the sinoatrial node SAN being formed by a group of cells having the ability to spontaneously produce an electrical impulse that travels through the heart's electrical conduction system, thus causing the heart to contract in order to pump blood through the heart. The atrioventricular node AVN serves to coordinate electrical conduction in between the atria and the ventricles and is located at the lower back section of the intra-atrial septum near the opening of the coronary sinus. From the atrioventricular node AVN the so-called HIS bundle H is extending, the HIS bundle H being comprised of heart muscle cells specialized for electrical conduction and forming part of the electrical conduction system for transmitting electrical impulses from the atrioventricular node AVN via the so-called right bundle branch RBB around the right ventricle RV and via the left bundle branch LBB around the left ventricle LV.

[0086] In case of a block at the atrioventricular node AVN, the intrinsic electrical conduction system of the heart H may be disrupted, causing a potentially insufficient intrinsic stimulation of ventricular activity, i.e., insufficient or irregular contractions of the right and/or left ventricle RV, LV. In such a case, a pacing of ventricular activity by means of a pacemaker device may be indicated, such pacemaker device stimulating ventricular activity by injecting stimulation energy into intracardiac tissue, specifically myocardium M.

[0087] In one embodiment, an implantable medical device 1 in the shape of a leadless cardiac pacemaker device, as schematically indicated in FIG. 1, is provided for a ventricular pacing action, the leadless pacemaker device having a body 10 formed by the housing of the leadless pacemaker device.

[0088] In another embodiment, as shown in FIG. 9, the implantable medical device 1 may be a stimulation device having a generator 18 and at least one lead forming a body 10 of the implantable medical device 1 and extending transvenously from the generator 18 into the patient's heart.

[0089] Whereas common implantable medical devices are designed to sense a ventricular activity by receiving electrical signals from the ventricle RV, LV they are placed in, it may be desirable to provide for a pacing action which achieves atrioventricular (AV) synchrony by providing a pacing in the ventricle in synchrony with an intrinsic atrial activity. For such pacing mode, also denoted as VDD pacing mode, it is required to sense atrial activity and identify atrial events relating to atrial contractions in order to base a ventricular pacing on such atrial events.

[0090] Referring now to FIGS. 2 and 3, in one embodiment an implantable medical device 1 in the shape of a leadless pacemaker device configured to provide for an intracardiac pacing, in particular in a VDD pacing mode, comprises a housing 10 enclosing electrical and electronic components for operating the implantable medical device 1. In particular, enclosed within the housing 10 is a processing circuitry 15, comprising for example also a communication interface for communicating with an external device, such as a programmer wand. In addition, electrical and electronic components such as an energy storage in the shape of a battery are confined in the housing 10. The housing 10 provides for an encapsulation of components received therein, the housing 10 having the shape of, e.g., a cylindrical shaft having a length of for example a few centimeters.

[0091] The implantable medical device 1 is to be implanted immediately on intracardiac tissue M. For this, the implantable medical device 1 comprises, in the region of a tip 100, a fixation device 14 for example in the shape of nitinol wires to engage with intracardiac tissue M for fixedly holding the implantable medical device 1 on the tissue in an implanted state.

[0092] The implantable medical device 1 in the embodiment of FIGS. 2 and 3 does not comprise leads, but receives signals relating to a cardiac activity by means of an electrode arrangement arranged on the housing 10 and also emits stimulation signals by means of such electrode arrangement. In the embodiment of FIGS. 2 and 3, the implantable medical device 1 comprises different electrodes 11, 12, 13 making up the electrode arrangement and serving to emit pacing signals towards intracardiac tissue M for providing a pacing and to sense electrical signals indicative of a cardiac activity, in particular indicative of atrial and ventricular contractions.

[0093] A first electrode 11 herein is denoted as pacing electrode. The first electrode 11 is placed at a tip 100 of the housing 10 and is configured to engage with cardiac tissue M.

[0094] A second electrode 12 herein is denoted as pacing ring. The second electrode 12 serves as a counter-electrode for the first electrode 11, a signal vector P arising between the first electrode 11 and the second electrode 12 providing for a pacing vector P for emitting pacing signals towards the intracardiac tissue M.

[0095] In addition, the second electrode 12 serves as a sensing electrode for sensing signals, in particular relating to ventricular contractions, a signal vector V arising between the second electrode 12 and the first electrode 11, the signal vector V being denoted as near-field vector.

[0096] The second electrode 12 is placed at a distance from the first electrode 11 and for example has the shape of a ring extending circumferentially about the housing 10. The second electrode 12 is for example placed at a distance of about 1 cm from the tip 100 of the housing at which the first electrode 11 is placed.

[0097] The implantable medical device 1, in the embodiment of FIGS. 2 and 3, in addition comprises a third electrode 13 placed at a far end 101 of the housing 10, the third electrode 13 serving as a sensing electrode for sensing signals indicative of cardiac activity in the far-field. In particular, a signal vector A arises between the third electrode 13 and the first electrode 11, the signal vector A picking up signals being indicative for example of atrial contractions and being denoted as far-field vector.

[0098] The electrodes 11, 12, 13 are in operative connection with the processing circuitry 15, the processing circuitry 15 being configured to cause the first electrode 11 and the second electrode 12 to emit a pacing signal for providing a stimulation at the ventricle. The processing circuitry 15 furthermore is configured to process signals received via the electrodes 11, 12, 13 to provide for a sensing of cardiac activity, in particular atrial and ventricular contractions.

[0099] If the implantable medical device 1 has the shape of a stimulation device comprising a generator 18 and a lead extending from the generator 18, as shown in the embodiment of FIG. 9, a similar electrode arrangement comprising for example three electrodes 11, 12, 13 may be arranged on a lead implanted in and extending into the right ventricle RV, as shown in FIG. 9, such that the above also applies to an embodiment of the implantable medical device 1 having a lead extending into the patient's heart. In this case, the processing circuitry may be part of the generator 18 and may be in operative connection with an electrode arrangement arranged on the lead.

[0100] In order to provide for a pacing in the ventricle in which the implantable medical device 1 is placed, in particular to enable a pacing in the VDD mode, a sensing of atrial activity is required to provide for detected atrial sense markers in order to time a pacing in the ventricle to obtain atrioventricular (AV) synchrony. For this, a far-field signal from in particular the right atrium RA (see FIGS. 1 and 9) shall be sensed in order to allow for a synchronous pacing in the right ventricle RV by means of the implantable medical device 1 being implanted on intracardiac tissue M in the right ventricle RV.

[0101] Referring now to FIG. 4, the processing circuitry 15 comprises, in one embodiment, two processing channels 16, 17 for processing different processing signals relating to ventricular activity and atrial activity. Herein, typically, an intracardiac electrogram (IEGM) contains a signal portions relating to ventricular activity (in particular a QRS wave) and atrial activity (in particular a P wave), signal portions relating to atrial activity however resulting from a far-field signal source and hence being far less pronounced and having a far smaller amplitude then signal portions relating to a ventricular activity in the near-field, i.e., arising in close proximity to the implanted implantable medical device 1. For this reason, the two processing channels 16, 17 are associated with different gains G1, G2, a first processing channel 16 serving to process a first processing signal to identify ventricular events at a rather low gain G1 and a second processing channel 17 being configured to process a second processing signal to identify atrial events at a significantly higher gain G2.

[0102] In particular, the first processing channel 16 is connected to the electrode arrangement comprised of the electrodes 11, 12, 13, the first processing channel 16 being configured in particular to sense and process a signal received via the electrodes 11, 12 (near-field vector V in FIGS. 2 and 3). The first processing channel 16 comprises a first amplification stage 161 having a gain G1 and, following the amplification stage 161, a detection stage 162 which is configured to identify ventricular sense markers Vx from the first processing signal processed within the first processing channel 16.

[0103] The second processing channel 17 is likewise connected to the electrode arrangement comprised of electrodes 11, 12, 13, wherein the second processing channel 17 may in particular be configured to process a signal sensed via the far-field vector A, that is in between the electrodes 11, 13 placed at the tip 100 and the far end 101 of the housing 10 as illustrated in FIGS. 2 and 3. The second processing channel 17 comprises a second amplification stage 171 having a second gain G2, the second amplification stage 171 being followed by a processing stage 172 and a second detection stage 173.

[0104] The processing stage 172 serves to pre-process the second processing signal after amplification. The detection stage 173 in turn serves to evaluate and analyze the processed signal in order to identify atrial events within the second processing signal, the second processing channel 17 then outputting atrial sense markers As indicative of atrial events detected in the processed signal.

[0105] In addition, the processing circuitry 15 comprises a timing stage 174 which uses timing information received from the first processing channel 16 and the second processing channel 17 to provide for a pacing timing, in particular a VDD timing for achieving an atrial-ventricular synchronous pacing.

[0106] In order to identify and analyze atrial events, the gain G2 of the second processing channel 17 is (significantly) higher than the gain G1 of the first processing channel 16. This generally allows to analyze signal portions relating to atrial events, but makes it necessary to discern such signal portions relating to atrial events from other signal portions, in particular signal portions relating to ventricular events in the near-field and hence being far stronger than signal portions originating from atrial events in the far-field.

[0107] Within the processing stage 172, for example a bandpass filtering, a windowing (e.g., partial blanking), a smoothing by means of a moving average filtering and a rectification may take place. A first or second order difference may be applied to remove a non-zero baseline while enhancing P wave defections.

[0108] FIGS. 5A and 5B show examples of signals S1, S2 as processed in the different processing channels 16, 17, FIG. 5A at the top showing a signal S1 as processed by the first processing channel 16 and FIG. 5B at the bottom showing a signal S2 as processed by the second processing channel 17. As a result of the processing, ventricular events Vx and atrial events As are identified and corresponding markers are output.

[0109] As apparent from FIG. 5B, the sensing of atrial events As uses a windowing scheme, employing in particular a blanking window T.sub.blank for blanking out signal portions of the signal S2 potentially relating to ventricular activity.

[0110] In particular, by means of the detection of ventricular events Vx in the first processing channel 16 a timing in between atrial events As and ventricular events Vx may be determined. According to such timing a start point and an end point of the blanking window T.sub.blank may be set, hence excluding signal portions from the processing which do not relate to atrial activity. Strong ventricular signals in this way may be suppressed such that signal portions relating to a ventricular activity may not interfere with a detection of atrial events.

[0111] During the blanking window T.sub.blank, the second processing channel 17 may be turned off. In particular, the amplification stage 171 of the second processing channel 17 may be switched of in order to save power.

[0112] Generally, a detection for atrial events takes place outside of the blanking window T.sub.blank. Herein, a detection window T.sub.sense for detecting atrial events may start at the end of a prior blanking window T.sub.blank. Alternatively, a detection window T.sub.sense mayas shown in the embodiment of FIG. 5Bhave a delay with respect to the end of a prior blanking window T.sub.blank, such that a signal processing within the second processing channel 17 starts at the end of a prior blanking window T.sub.blank, a detection for atrial events however starting only after a certain delay.

[0113] Generally, an atrial event As is assumed to be present if, in the sense window T.sub.sense, the signal S2 crosses a sense threshold ST, as it is shown in FIG. 5B. The comparison may take place based on a rectification of the sense signal S2. Alternatively, a positive and negative sense threshold ST may be used, which may have the same value or may differ in their values. A threshold crossing herein may be assumed if one signal value is larger than the sense threshold ST. Alternatively, a crossing of the sense threshold ST is assumed if a predefined number of signal values are larger than the sense threshold ST, for example two or more consecutive sample values.

[0114] Generally, if an atrial event As is detected, as it is the case for the second cardiac cycle in FIG. 5B, the atrial event As is used for a further processing, in particular to update the sense threshold ST and to achieve an atrial-ventricular synchronous pacing.

[0115] In particular, the atrial event As is taken as that point in time at which a crossing of the sense threshold ST is identified. At the time of the atrial event As a peak detection window PDW starts, and based on data recorded during that peak detection window PDW a peak amplitude PA is determined as the maximum signal value within the peak detection window PDW. This is indicated in FIG. 5B for the second cycle at the right.

[0116] Also, in case of a detection of an atrial event As, an atrial-ventricular delay AVD may be determined and used for subsequent processing. If no ventricular event Vx is detected after lapse of the atrial-ventricular delay AVD, a pacing signal may be injected to cause a ventricular stimulation.

[0117] The peak amplitude PA, in one embodiment, may be used to update the sense threshold ST. In particular, the processing circuitry 15 may be configured to update the sense threshold ST using an average threshold reference and a percentage ratio according to the formula

ST=PC.Math.ATR(t),

where ST is the current sense threshold, PC is the percentage ratio, and ATR(t) is the average threshold reference for the current cycle t. The percentage ratio may lie for example in the range between 0% and 100%.

[0118] The average threshold reference may be determined based on a mean value for a number of previous cardiac cycles in which atrial events have been identified and correspondingly peak amplitude values have been obtained. The average threshold reference in this case for example may be determined as the average of the peak amplitude values in the previous cardiac cycles.

[0119] In another embodiment, the average threshold reference may be computed based on the peak amplitude PA according to the following equation:

ATR(t)=W.Math.PA(t1)+(1W).Math.ATR(t1),

where W indicates the update weight which determines how much the average threshold reference should change based on the previous peak amplitude . . . , PA(t1) is the peak amplitude as determined for the previous cycle t1, and ATR(t1) is the previous average threshold reference.

[0120] For the actual cycle t the average threshold reference hence is determined based on the peak amplitude PA determined for that cycle t and based on the previously valid average threshold reference at cycle t1. For each cycle for which an atrial event As is detected, hence, the average threshold reference is updated and computed anew, such that the average threshold reference is dynamically adjusted on a cycle-by-cycle basis.

[0121] If no (valid) atrial event As is detected, no peak amplitude PA is determined and the average threshold reference ATR is not updated. In this way it is avoided that a false detection of an atrial event As may cause a false increase of the sense threshold ST and a subsequent capture loss of atrial activity. This is the case for the first cardiac cycle as shown in FIGS. 5A and 5B, in which no crossing of the sense threshold ST is detected and correspondingly no atrial event As is identified.

[0122] The sense threshold ST generally is set based on the peak amplitude value PA of detected atrial events As in previous cardiac cycles. Correspondingly, dependent on the peak amplitude values in the previous cardiac cycles the sense threshold ST may be dynamically raised or lowered.

[0123] This however only applies if atrial events As are detected. If no atrial event As is detected in a cardiac cycle, no peak amplitude PA is determined, and no dynamic adjustment of the sense threshold based on the above-noted scheme takes place. However, as there is a general desire to reliably detect atrial events As and to obtain a steady, reliable capture of atrial events As to obtain atrial events As in as many cycles as possible, in case of a missing of a significant number of atrial signals measures should be taken to recapture signals associated with atrial activity. A missing of one or multiple atrial events As in cardiac cycles may indicate that the sense threshold ST is too high and hence should be lowered in order to recapture atrial activity.

[0124] For this reason it is proposed to dynamically adjust the sense threshold ST in a step decay procedure if, based on one or multiple reduction criteria, it is found that a significant number of atrial events As are missed for cardiac cycles. These reduction criteria are formulated as X out of Y criteria. A reduction criterion is fulfilled, correspondingly, if in X out of Y cardiac cycles no atrial events As are detected.

[0125] The reduction criteria herein may include a short-term criterion and a long-term criterion.

[0126] In a short-term criterion, for example, it may be evaluated whether in X1 out of Y1 cardiac cycles atrial events As have not been detected. If this is the case, the short-term criterion is assumed to be true and the sense threshold ST is adjusted in a suitable manner. X1 and Y1 herein are natural numbers, where X1 is larger than then 0 and Y1 is equal to or larger than X1. Within the short-term criterion herein it shall be checked whether in very recent cardiac cycles a significant number of atrial events As are missed. Y1 may correspondingly take values for example in between 1 to 4, and X1 may assume a value between 1 and Y1.

[0127] For example, Y1 may be equal to 2, and X1 may be equal to 1 or 2. It hence is checked whether in one out of two previous cardiac cycles atrial events are missed, or whether in 2 out of 2 previous cardiac cycles atrial events are missed.

[0128] In a long-term criterion it may for example be evaluated whether in X2 out of Y2 cardiac cycles atrial events As have not been detected. If this is the case, the long-term criterion is assumed to be true and the sense threshold ST is adjusted in a suitable manner. X2 and Y2 are natural numbers, where Y2 is larger than Y1 and may for example have a value between 4 and 16, and X2 is larger than X1 and smaller or equal to Y2 and may for example have a value between 3 to 15.

[0129] For example, Y2 may be equal to 8, and X2 may be equal to 5. It hence is checked whether in 5 out of 8 previous cardiac cycles atrial events As have been missed.

[0130] If the first reduction criterion, corresponding to the short-term reduction criterion, is fulfilled, a first reduction factor may be applied. If alternatively or in addition the second reduction criterion is fulfilled, corresponding to the long-term reduction criterion, a second reduction factor may be applied, wherein the second reduction factor may cause a stronger reduction than the first reduction factor.

[0131] Herein, in one embodiment, if the second reduction criterion is fulfilled only the second reduction factor is applied.

[0132] The first reduction factor and the second reduction factor may both be percentage values. The reduction factors may be programmable, for example in steps between 5 to 10%, for example 6.25%. Each reduction factor may assume a value between 0 to 100%, wherein the second reduction factor generally may cause a stronger reduction than the first reduction factor.

[0133] The respective reduction factor is applied if the respective reduction criterion is fulfilled for a current cardiac cycle. In addition, if for multiple consecutive cardiac cycles the respective reduction criterion is repeatedly fulfilled, in addition to the reduction factor an additional step factor may be applied to further reduce the sense threshold.

[0134] This is illustrated in FIG. 6.

[0135] Generally, if no reduction criterion is fulfilled, the sense threshold ST is computed based on the average threshold reference ATR (which is computed based on peak amplitude values PA of previous cardiac cycles) by applying a predefined, normal percentage ratio PC, the percentage ratio having for example a value in between 40% and 100%, for example 80%. The percentage ratio PC is applied as a factor, the sense threshold ST hence taking a value of PC times ATR.

[0136] If in a cardiac cycle i it is found that a first reduction criterion, corresponding for example to a short-term reduction criterion, is fulfilled, the sense threshold ST is reduced by a first reduction factor L1. The first reduction factor L1 for example is applied to the valid average threshold reference ATR, such that the sense threshold ST is computed for example as L1 times the current value of the average threshold reference ATR.

[0137] If in a predefined number of multiple consecutive cycles the first reduction criterion is fulfilled, an additional step decay may be applied by multiplying the currently valid sense threshold ST with a step factor L11, wherein the step factor L11 may be repeatedly applied, as shown in FIG. 6. The step factor L11 may be applied if in two consecutive cycles the reduction criterion is fulfilled. The step factor L11 may alternatively be applied only if in more than two consecutive cycles the reduction criterion is fulfilled, as this is shown in FIG. 6 at the cardiac cycle i+a.

[0138] If in a cardiac cycle i+b an atrial event is detected and the first reduction criterion is no longer fulfilled, the average threshold reference ATR is adjusted according to the now determined peak amplitude PA, and the sense threshold ST is determined by reverting the step factors L11 and by only applying the reduction factor L1 to the average threshold reference ATR.

[0139] If at cycle i+c again it is found that the first reduction criterion has been fulfilled for the predefined number of consecutive cycles, the step factor L11 is again applied to reduce the sense threshold ST.

[0140] At cardiac cycle i+d it is found that (potentially in addition to the first reduction criterion) also the second reduction criterion is fulfilled, in which case the reduction factor L2 associated with the second reduction criterion is applied to compute the sense threshold ST.

[0141] The second reduction factor L2 causes an emphasized decrease of the sense threshold ST, as apparent from FIG. 6.

[0142] At cardiac cycle i+e it is found that the second reduction criterion has been fulfilled for a predefined number of consecutive cardiac cycles, such that additional step factors L21 associated with the second reduction criterion are applied to further reduce the sense threshold ST.

[0143] Herein, as visible from FIG. 6, the sense threshold ST cannot be reduced beyond a lower absolute threshold LAT, which serves as a lower bound and hence represents the absolute minimum of the sense threshold ST.

[0144] At cardiac cycle i+f an atrial event is detected, and correspondingly the average threshold reference ATR is adjusted and the step factors L21 are reverted, such that the sense threshold ST is determined by applying only the reduction factor L2 associated with the second reduction criterion to the absolute threshold reference ATR.

[0145] At cardiac cycle i+g the second reduction criterion is no longer true and an atrial event is detected, and correspondingly the average threshold reference ATR is increased and the sense threshold ST is computed by applying only the reduction factor L1 associated with the first reduction criterion.

[0146] At cardiac cycle i+h also the first reduction criterion is no longer fulfilled and an atrial event is detected, and correspondingly the average threshold reference ATR is adjusted and the sense threshold ST is computed by applying only the percentage ratio PC indicative of normal operation.

[0147] FIG. 7 shows an example of an adjustment of the sense threshold ST based on a single reduction criterion, formulated as an X out of Y criterion in which X and Y are equal to 2. It hence is evaluated whether in 2 out of 2 cardiac cycles no atrial events As have been detected.

[0148] In the example shown in FIG. 7 (and likewise in the example if FIG. 8) in the first row the cardiac cycles are numbered. In the second row detected atrial events As and ventricular events Vx are shown. If no atrial events As are detected, this is indicated by crosses. Also, if a detection occurred, a peak amplitude value (PA) for the measured P-wave in the current cycle is indicated.

[0149] In the shown example, the reduction criterion is not fulfilled in cardiac cycles i to i+3. However, in cardiac cycle i+4 it is found that in the current and previous cardiac cycles no atrial events have been detected, and correspondingly the 2 out of 2 reduction criterion is fulfilled. Hence, the reduction factor L1 associated with the first reduction criterion is applied, and the sense threshold ST is computed by multiplying the currently valid average threshold reference value ATR (7.5) by the reduction factor L1 (0.45). The threshold ST for cycle i+5 hence assumes a value of 3.4.

[0150] In the following cardiac cycles i+5, i+6 the reduction criterion is not fulfilled, and hence the sense threshold ST is again computed based on the regular percentage ratio PC (0.8). In cardiac cycle i+7, however, the reduction criterion again is fulfilled, and the reduction factor L1 is applied to compute the sense threshold ST for the following cycle, i+8.

[0151] In the shown example the average threshold reference ATR is computed as ATR(i+1)=W*PA(i)+(1W)*ATR(i), where the update weight W is 0.5.

[0152] In another example, shown in FIG. 8, two reduction criteria are applied. The first criterion is a short-term criterion and is fulfilled if in 1 out of 2 cardiac cycles no atrial events As are detected (sixth row in FIG. 8). The second reduction criterion is a long-term criterion and is fulfilled if in 5 out of 8 cardiac cycles no atrial events As have been detected (seventh row in FIG. 8).

[0153] In the example of FIG. 8, no reduction criterion is fulfilled in cardiac cycles i to i+2, and correspondingly the sense threshold ST is computed by applying the normal percentage ratio PC (0.8). The first reduction criterion is fulfilled in cardiac cycles i+3 and i+4, and correspondingly the reduction factor L1 (0.6) associated with the first reduction criterion is applied to compute the sense threshold ST. In cardiac cycle i+5 again no reduction criterion is fulfilled, and hence the regular percentage ratio PC is applied. In cardiac cycles i+6 to i+8 the first reduction criterion is fulfilled and correspondingly the first reduction factor L1 is applied to compute the sense threshold ST. In cardiac cycle i+9, then, also the second reduction criterion is fulfilled, and the reduction factor L2 (0.4) corresponding to the second reduction criterion is applied to compute the sense threshold ST. The second reduction criterion is also fulfilled in cardiac cycle i+10, whereas in cardiac cycle 1+11 again no reduction criterion is fulfilled.

[0154] By adjusting the sense threshold ST in a step-decay fashion to reduce the sense threshold ST in a step-wise manner in case of missed atrial events As, a reliable atrial capture can be obtained, in particular by suitably lowering the sense threshold ST if in a significant number of cardiac cycles no atrial events have been detected.

[0155] Using atrial sense markers As output by the second processing channel 17, ventricular synchronous pacing may be achieved. For this, it can be detected whether, following a detected atrial sense marker As, an intrinsic ventricular sense marker Vx occurs (output by the first processing channel 16) within a predefined time delay window after the atrial sense marker As, in which case no stimulation is required. If no ventricular sense marker Vx is detected, a stimulation pulse may be emitted, causing synchronous pacing in the ventricle.

[0156] Conversely, also asynchronous pacing can be performed.

[0157] Utilizing a far-field electrical signal received by means of an implantable medical device can offer a superior detection of far-field events, in particular atrial events in case the implantable medical device is implanted into the ventricle. A tracking of far-field events by using and evaluating electrical signals may allow for an increased consistency and reliability in particular with respect to external factors such as posture and patient activity.

[0158] It will be apparent to those skilled in the art that numerous modifications and variations of the described examples and embodiments are possible in light of the above teachings of the disclosure. The disclosed examples and embodiments are presented for purposes of illustration only. Other alternate embodiments may include some or all of the features disclosed herein. Therefore, it is the intent to cover all such modifications and alternate embodiments as may come within the true scope of this invention, which is to be given the full breadth thereof. Additionally, the disclosure of a range of values is a disclosure of every numerical value within that range, including the end points.

LIST OF REFERENCE NUMERALS

[0159] 1 Implantable medical device (leadless pacemaker device) [0160] 10 Body (housing) [0161] 100 Tip [0162] 101 Far end [0163] 11 First electrode (pacing electrode) [0164] 12 Second electrode (pacing ring) [0165] 13 Third electrode [0166] 14 Fixation device [0167] 15 Processing circuitry [0168] 16 First processing channel [0169] 161 Amplification stage [0170] 162 Detection stage [0171] 17 Second processing channel [0172] 171 Amplification stage [0173] 172 Processing stage [0174] 173 Detection stage [0175] 174 Timing stage [0176] 18 Generator [0177] A Atrial vector [0178] As Atrial event [0179] AVD Atrial-ventricular delay [0180] AVN Atrioventricular node [0181] G1, G2 Gain [0182] H HIS bundle [0183] i Cardiac cycle [0184] L1, L2 Reduction factor (in %) [0185] L11, L21 Step factor (in %) [0186] LA Left atrium [0187] LAT Lower absolute threshold [0188] LBB Left bundle branch [0189] LV Left ventricle [0190] M Intracardiac tissue (myocardium) [0191] P Pacing vector [0192] PA Peak amplitude [0193] PDW Peak detection window [0194] RA Right atrium [0195] RBB Right bundle branch [0196] RV Right ventricle [0197] 51, S2 Signal [0198] SAN Sinoatrial node [0199] ST Sense threshold [0200] T.sub.blank Blanking window [0201] T.sub.sense Detection window [0202] V Ventricular vector [0203] VX Ventricular event