IMPLANTABLE HEART STIMULATOR

Abstract: Implantable heart stimulator comprising a pulse generating means (4) for generating stimulation pulses adapted to be applied to a heart via at least one electrode lead (6), a control means (8) with threshold search means (10) for initiating a stimulation threshold search in order to determine a stimulation threshold of heart tissue. The control means further comprises threshold search timing means (12) that generates a timing signal at same time(s) every day in order to initiate threshold searches provided that a stable condition index fulfils predetermined criteria. The stable condition index is related to the condition of the heart and is based on result from at least one heart stability test.

Description: Tit le Implantable heart stimulator Technical field of the invention The present invention relates to an implantable heart stimulator according to the preamble of the independent claim. Background of the invention Immediately after implantation of a heart stimulator and insertion of an heart electrode lead into the heart and attaching a stimulation electrode to heart tissue, the stimulation threshold, i.e. the least energy (pulse amplitude if the pulse width is constant) required to achieve heart contraction, is relatively high. After the first months following implantation of a heart stimulator the stimulation threshold eventually stabilizes at a more or less constant value in the order of some Volts (1-5 Volts) . Natural fluctuations of the stimulation threshold occur due to e.g. the activity of the patient (awake or asleep), the intake of drugs, etc. A threshold search algorithm is implemented in many pacemakers of today that performs threshold searches either in response of one or many losses of capture or at timely intervals, e.g. 8 hours since last threshold search was performed. Figure 1 discloses an IEGM illustrating the principles of a threshold search algorithm according to established standard prior art, see e.g. US-5 , 476 , 487 , and applicable in relation with the present invention. A and V designate atrial and ventricular stimulation pulses, respectively. BU is a high output backup pulse delivered if loss of capture (LOC) occurs. As can be seen (complex 3) the pre-programmed AV- interval is prolonged with Δ when a LOC occurs (complex 2) . The reason for that is to await any intrinsic event if the first LOC was the result of a fusion beat. A fusion beat occurs when an intrinsic event more or less coincides with a generated stimulation pulse. In this case there is no intrinsic activity and the LOC was not a result of a fusion beat but was due to a changed stimulation threshold of the heart tissue, and a stimulation threshold search is initiated. In figure 1 during the threshold search the pre-programmed AV-interval is shortened to "AV-short" to override any intrinsic heart activity. The ventricular stimulation amplitude is successively stepped up by a predetermined amplitude step of e.g. 0,1-0,3 V and each unsuccessful ventricular stimulation pulse is followed by a back-up pulse. This is performed until the stimulation threshold is detected, i.e. capture is detected from the ventricular stimulation pulse, and the stimulation pulse amplitude is then set to a value that equals the stimulation threshold plus a working margin, e.g. 0,3 V. As an alternative the ventricular stimulation amplitude may start at an amplitude above the stimulation threshold and then successively be stepped down until non-capture occurs. It should be noted that the threshold search according to the established technique disclosed in the above-mentioned US-5, 476, 487 is performed by using a pre-programmed AVI shortened to "AVI-short" as indicated above. US-5, 766, 229 is related to a capture verification method and apparatus for an implantable pacemaker utilizing heart rhythm stability measurements to minimized the likelihood of fusion. The heart rhythm stability is measured by determining a mean cardiac interval and a standard deviation of the measured intervals. If those values are acceptable (stable heart rhythm) the capture verification is performed. In "User's manual for Microny SR+", pp 15-20, (Pacesetter AB, ordering no. 6324565 E500E, published in 1995) is an "Autocapture Pacing system" described where the threshold search is conducted every eight hour. A threshold search is also automatically conducted when two consecutive stimuli have resulted in no capture . The algorithm is disabled when noise is detected and for as long as the noise is sensed. Every threshold search requires the stimulation rate to be below the basic rate plus 20 min "1 . As indicated above a stimulation threshold search might be performed either in response of loss of capture or at predetermined times. The present invention is primarily directed to those threshold searches performed at a timely basis and not in response of detected loss of capture. The object of the present invention is to achieve an improved implantable heart stimulator adapted to perform stimulation threshold searches that obtains a reliable result with a high degree of safety. Short description of the inventive concept The above-mentioned objects is achieved by an implantable heart stimulator provided with the characterizing features set forth in the characterizing portion of the independent claim. Preferred embodiments are set forth in the dependent claims. According to one important feature of the present invention is a threshold search performed at the same time(s) every day which then takes into account the natural variations of the stimulation threshold. According to a preferred embodiment of the invention is a stable condition index determined related to the condition of the heart and based on result from a heart stability test that belongs to a first group of tests related to detection of parameters indicating emerging fusion beats. According to another preferred embodiment of the invention is a stable condition index determined related to the condition of the heart and based on result from a heart stability test that belongs to a second group of tests related to the spontaneous activity of the heart . Short description of the appended drawings Figure 1 discloses an IEGM illustrating the principles of a threshold search algorithm according to established standard prior art . Figure 2 discloses a schematic block diagram of an implantable heart stimulator 2 according to the invention. Detailed description of preferred embodiments of the invention. Figure 2 discloses a schematic block diagram of an implantable heart stimulator 2 according to the invention. The heart stimulator comprises a pulse generating means 4 for generating stimulation pulses adapted to be applied to a heart via at least one electrode lead 6 and a control means 8. The control means comprises a threshold search means 10 for initiating a stimulation threshold search in order to determine a stimulation threshold of heart tissue and a threshold search timing means 12. The threshold search timing means 12 generates a timing signal 14 at the same time every day to the threshold search means 10 in order to initiate threshold searches. According to a preferred embodiment of the invention is the timing signal generated every 8th hours, e.g. 00, 08, 16, 24 hours. The timing signal need not be generated at a regular interval but instead at predetermined times, e.g. at 05 and 23 o'clock. The generation of the timing signal is not affected by threshold searches performed due to loss of capture. When a timing signal is generated and applied to the threshold search means a stable condition index is determined that is related to the condition of the heart and is based on the result from one or many heart stability tests . According to a preferred embodiment of the invention the heart stability test belongs to a first group of tests related to detection of parameters indicating emerging fusion beats. A first heart stability test that belongs to the first group of tests fulfils its criterion for an acceptable result if a value of a detected evoked response signal following an applied stimulation pulse does not indicate a fusion beat. This value may represent e.g. the integral of the evoked response signal measured during an interval, e.g. 15-55 ms , following the stimulation pulse as for instance as disclosed in W099/65569. A second heart stability test that also belongs to the first group of tests fulfils its criterion for an acceptable result if an impedance signal representing the conditions inside the heart in combination with the timing of the stimulation pulse does not indicate a fusion beat. According to another preferred embodiment of the invention the stability test belongs to a second group of tests related to the spontaneous activity of the heart. Almost all implantable heart stimulators of today are equipped with a sensing means adapted to sense evoked and spontaneous electrical activity of the heart. To perform the tests of the second group such sensing means arranged in the control means is utilised. A third heart stability test that belongs to the second group fulfils its criterion for an acceptable result if the number of inhibited stimulation pulses is lower than a predetermined number during a time interval immediately before a threshold search is to be performed. If, for example, the predetermined number is set to 1 and the time interval is set to 1 minute the result of the test is not acceptable if one stimulation pulse is inhibited less than one minute before the threshold search is to be performed. A fourth heart stability test that belongs to the second group of tests fulfils its criterion for an acceptable result if the stimulation rate during the threshold search is at least 10 bpm higher than the spontaneous rate immediately before the threshold search (V-V interval shorter than R-R interval) . In a heart stimulator working according to the Wl-mode the spontaneous rate may be tested by temporarily slow down stimulation rate before the search shall be performed (increase V-V interval) . If the spontaneous rate is only 0-10 bpm less than the intended stimulation rate during threshold search the intended stimulation rate is increased by 5-20 bpm (decrease V-V interval) . The reason for stimulating with a higher rate is to override any spontaneous activity. In a heart stimulator working according to a DDD mode of operation the AV/PV delay is temporarily prolonged to seek spontaneous conduction. The conducted PR/AR interval must be at least 10-70 ms longer than the intended AV/PV intervals to be used during the threshold search. A fifth heart stability test that also belongs to the second group of tests fulfils its criterion for an acceptable result if no PVC is detected. A sixth heart stability test that also belongs to the second group of tests fulfils its criterion for an acceptable result if no spontaneous intervals is shorter than the intended threshold search interval during the e.g. 50 cardiac cycles preceding a threshold search. In a heart stimulator working according to the DDD/VDD-mode a tracked spontaneous rate must not exceed the base rate with more than a programmable value. The absolute rate shall not exceed 120 bpm. A seventh heart stability test that not belongs to any of the above-mentioned groups fulfils its criterion for an acceptable result if a sensor indicated stimulation frequency (rate responsive frequency) not exceeds a basic stimulation frequency with more than a predetermined value, preferably 10 or 20 beats per minute. The stable condition index is determined by the control means in dependence of the result (s) from at least one heart stability test. This may be performed in many different ways. According to a straightforward way of determining the index is a logical signal generated as a result of a heart stability test, whereas logical 1 indicates that the result of the stability test is acceptable and logical 0 indicates that the result not is acceptable. The logical signals are then applied at an AND-gate that generates a logical 1 as an output signal if all logical input signals are logical 1. According to an alternative way of determining the stable condition index is an analog value generated as a result of each test, the analog value, e.g. between 0-10, where values in the interval 7-10 is acceptable. All values from the different tests are then added together and an interval of the sum defining an acceptable overall result may be determined. The invention is equally applicable for single chamber heart stimulators as for double chamber heart stimulators. The tests used is dependent of the presently used stimulation mode (e.g. WI , VDD, DDI , DDD) . If the threshold search not is performed due to. a non- acceptable value from any of the tests all the tests are repeated within a predetermined period of time, e.g. within 5 minutes. The time for generation of timing signals by the threshold search timing means is not altered in such cases. The present invention is not limited to the above-described preferred embodiments. Various alternatives, modifications and equivalents 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. Implantable heart stimulator (2) comprising a pulse generating means (4) for generating stimulation pulses adapted to be applied to a heart via at least one electrode lead (6), a threshold search timing means (12) that generates a timing signal at the same time(s) every day in order to initiate threshold searches at regular intervals, a control means (8) with threshold search means (10) for initiating a stimulation threshold search provided that a stable condition index fulfils predetermined criteria, wherein the stable condition index is related to the condition of the heart and is based on the result from at least one stability test, characterized in that said stability test is at least one of the following: A value of a detected evoked response signal following an applied stimulation pulse does not indicate an emerging fusion beat . - A value of a detected evoked response signal following an applied stimulation pulse does not indicate a fusion beat. An impedance signal representing the conditions inside the heart in combination with the timing of the stimulation pulse does not indicate a fusion beat. The number of inhibited stimulation pulses is lower than a predetermined number during a time interval imme- diately before a threshold search is to be performed. 2. Heart stimulator according to claim 1 characterized i n that said stability test is fulfilled if the integrated evoked response signal does not indicate emerging fusion. 3. Heart stimulator according to claim 1 characterized l n that said stability test is fulfilled if the integrated evoked response signal does not indicate fusion. 4. Heart stimulator according to claim 1, characterized l n that said timing signal is generated at a regular interval, e.g. every 8th hours. 5. Heart stimulator according to claim 1 or 2, charac- terized in that said heart stability test belongs to a second group of tests related to the spontaneous activity of the heart . 6. Heart stimulator according to claim 3, characterized in that a heart stability test fulfils its criterion for an acceptable result if the stimulation rate during the threshold search is at least 10 bpm higher than the spontaneous rate immediately before the threshold search (V- V interval shorter than R-R interval) . 7. Heart stimulator according to claim 3, characterized in that the heart stability test fulfils its criterion for an acceptable result if no PVC is detected. 8. Heart stimulator according to claim 1, characterized l n that a heart stability test fulfils its criterion for an acceptable result if a sensor indicated stimulation frequency not exceeds a basic stimulation frequency with more than a predetermined value, preferably 10-30 beats per minute.

Inventors: BUDGIFVARS GOERAN, OBEL MARTIN, UHRENIUS AASA, BUDGIFVARS, GOERAN, OBEL, MARTIN, UHRENIUS, AASA
Applicants: BUDGIFVARS GOERAN, OBEL MARTIN, ST JUDE MEDICAL, UHRENIUS AASA, BUDGIFVARS, GOERAN, OBEL, MARTIN, ST. JUDE MEDICAL AB, UHRENIUS, AASA
Application year: 2001
Main classification: A61N1
IPC codes: A61N1