Heart stimulator with verification of atrial capture

Abstract: The present invention relates to a heart stimulator, with verification of capture in the atrium of a heart, containing an atrial pulse generator (6) which, via an atrial electrode (2), emits stimulation pulses in the atrium and a ventricular detector (10) which senses depolarization signals in the ventricle via a ventricular electrode (8). According to the invention, the time elapsing between two consecutive stimulation pulses emitted in the atrium can be varied, and variation in ventricular events associated with the stimulation pulses is studied. If the ventricular events vary in time in the same way stimulation pulses emitted in the atrium vary, AV conduction is functional, and capture is present in the atrium.

Description: The present invention relates to a heart stimulator according to the preamble to patent claim 1. In heart stimulation, an electrical pulse, i.e. a heart stimulation pulse, is delivered to heart tissue in the atrium and/or ventricle for the purpose of inducing contraction of the atrium and/or the ventricle, thereby supporting the heart's pumping capacity. "Capture" refers to the depolarization of heart cells produced by a stimulation pulse. Depolarization (usually) results in the contraction of heart tissue. In a stimulation pulse, the lowest energy content needed to achieve depolarization of heart cells is usually referred to as the "stimulation threshold". The response of heart tissue to a stimulation pulse is referred to as "evoked response" (ER) which can be capture or non-capture. AUTOCAPTURE™ refers to automatic control and setting of the stimulation threshold level. When this function is used for capture in the ventricle, the "stimulation threshold" can be defined as the lowest stimulation level at which at least two heart stimulation pulses result in capture. Checks on the stimulation threshold are performed at certain intervals. From US-5 165 404 a heart stimulator is known which contains means for determining the stimulation threshold of stimulation pulses. The description of the prior art in US-5 165 404 indicates the way the stimulation threshold can be determined by successive reduction in the energy content of stimulation pulses and study of the heart's response to each stimulation pulse. When the heart stops responding, the stimulation threshold is defined as the lowest stimulation pulse amplitude which evoked a heart response. From US-4 686 988 a heart stimulator is known which comprises means to determine atrial capture or not. Two sensing amplifiers are employed, a first amplifier for sensing intrinsic P-waves and a second amplifier for sensing P-waves in reponse to a stimulation pulses. Output signals from these two amplifiers are selectively telemetered to an external receiver, where the occurrence of atrial stimulation pulses and P-waves can be monitored. In identification of the stimulation threshold, the way in which the lowest level at which capture occurs by reducing the amplitude of the stimulation pulse was described above. Since the energy content of the stimulation pulse is decisive to the success of stimulation, the duration of the stimulation pulse is also important to successful stimulation. Identification of the stimulation threshold is performed using the prevailing programmed duration. Typical values for the duration of the stimulation pulse range from 0.03 to 1.0 ms. The AUTOCAPTURE™ systems currently in use are for AUTOCAPTURE™ in the ventricle in which signal levels for evoked response are so high that they are easily detected and identified. However, signals in the atrium are much weaker, and emitted stimulation pulses interfere with measurements. This means that detection of evoked response must be performed in some other manner, so the autocapture used in the ventricle is not directly applicable in the atrium. AUTOCAPTURE™ means that the level of stimulation voltage is periodically checked, making pacemaker operation more reliable. Another effect of AUTOCAPTURE™ is to minimize current consumption, thereby increasing the life of the pacemaker battery. Particularly in dual chamber pacemakers, i.e. DDD pacemakers, which stimulate and sense in both the ventricle and atrium, there is also a need for an AUTOCAPTURE™ function in the atrium in order to achieve more reliable stimulation and minimize current consumption. DDD pacemakers can be used in patients with widely varying heart conditions. Some patients have intact conduction between the atrium and ventricle (AV conduction), and some do not. The mechanical pumping capability of the atrium also varies from patient to patient. The atrium in some patients does beat electrically, i.e. there is depolarization of the heart cells, but this depolarization fails to evoke any mechanical contraction of the atrium, i.e. the atrium lack a pumping capability. US-4 905 696 shows that an impedance signal measured in the ventricle undergoes a rapid, easily detected change in atrial contraction. Measurement is made between the tip of the electrode in the ventricle and the pacemaker's metal enclosure, or an indifferent electrode closer to the stimulation electrode, by means of a high-frequency signal applied to the electrode tip. When the atrium contracts, impedance in the ventricle drops as the ventricle fills with blood. This causes a rapid, detectable change in the impedance signal. A review of the prior art in US-4 905 696 also sets forth other ways of detecting atrial contraction which utilize an electrode in the atrium. The present invention sets forth a solution to the problems of verifying capture in the atrium. This is achieved with a heart stimulator of the above-described type and with the features set forth in the characterizing part of patent claim 1. Thus, in the heart stimulator according to the invention the time elapsing between two consecutively stimulation pulses emitted in the atrium is varied, and variations in ventricular events corresponding thereto are studied. This makes it possible to verify whether the patient's AV conduction is intact. If ventricular events vary in time in the same way the stimulator pulses emitted in the atrium vary, AV conduction and capture are present in the atrium. According to an advantageous embodiment of the heart stimulator according to the invention, the stimulator also contains an impedance meter for measuring impedance between the atrium and the ventricle, the impedance meter being arranged to measure impedance if the interval between consecutive ventricular depolarizations fails to vary in the same way as the interval between stimulation pulses in the atrium, capture being deemed to be present if impedance measurement shows the presence of an atrial contraction and no concomitant ventricular activity is detected in the ventricle's depolarization signal. If, according to the invention, the absence of functional AV conduction has been verified in the patient, an examination must therefore be performed to ascertain whether stimulation pulses delivered to the atrium cause atrial contraction, i.e. whether the patient has a functional atrium. This is investigated by studying the change in impedance between the atrial electrode and the ventricular electrode during atrial stimulation. To ensure that the change in impedance is caused by an atrial contraction and not by concomitant ventricular activity, the depolarization signal in the ventricle is studied to make sure no concomitant ventricular activity is present. Capture is present in the atrium if atrial contraction is verified in the aforementioned manner and no concomitant ventricular activity is found. If the measured impedance value indicates that no atrial contraction is present, despite the use of maximal stimulation energy, the atrium is non-functional, and the heart stimulator, according to another advantageous embodiment of the heart stimulator according to the invention, automatically switches to a stimulation mode which only stimulates the ventricle, e.g. VVI or VVI-R. Verification of capture in the atrium is a function which can be constantly activated or which is only activated at a specific interval, e.g. once an hour. The heart stimulator according to the invention will now be described in greater detail, referring to the FIGURES in the attached drawings in which FIG. 1 is a block diagram of a heart stimulator according to the invention; FIG. 2 is a block diagram of an indication means according to the invention. The same or similar components in the FIGURES have been assigned the same designations. FIG. 1 shows a block diagram of a heart stimulator comprising an atrial electrode 2, an atrial detector 4, an atrial pulse generator 6, a ventricular electrode 8, a ventricular detector 10 and a ventricular pulse generator 12. The atrial detector 4 is connected, via a counter 14, to a programmable control device 16 to which the atrial pulse generator 6, the ventricular detector 10 and the ventricular pulse generator 12 are connected. The ventricular detector 10 is also connected to an impedance meter 18, a comparison unit 20 and a measurement unit 22. An indication means 24 is connected to the impedance meter 18, the comparison unit 20 and the measurement unit 22. The indication means 24 is also connected to the programmable control device 16. The atrial detector 4 is able to sense depolarization signals in the atrium, and the atrial pulse generator 6 is able, via the atrial electrode 2, to deliver stimulation pulses to the atrium. The ventricular detector 10 is able to sense depolarization signals in the ventricle, and the ventricular pulse generator 12 is able, via the ventricular electrode 8, to deliver stimulation pulses to the ventricle. The programmable control device 16 can communicate with a programmer (not shown) outside the body by means of telemetry (radio signals). Various parameters for the heart stimulator's operation, such as the heart stimulator's operating mode (e.g. VVI, DDD), the frequency at which capture is to be checked, etc., can be set with the programmer. The reprogramming of the heart stimulator with the aid of a programmer is familiar to anyone well-versed in the art and will, therefore, not be described here. This is also beyond the scope of this patent application. In stimulation of the atrium, the control device 16 causes the atrial pulse generator 6 to emit a stimulation pulse with a preset energy content to the heart. This control is exercised via the counter 14, which can vary a first interval between consecutively emitted stimulation pulses by shortening or lengthening the interval by a predetermined period of time, said time being brief in relation to the interval between consecutively emitted stimulation pulses in the atrium. This predetermined period of time, with which the first interval is shortened or lengthened, is on the order of 10-100 ms. The counter 14 sends information on the duration of the first interval to the comparison unit 20. The ventricular detector 10 senses the heart's depolarization signal in the ventricle. This signal is sent to e.g. the comparison unit 20 which measures the time between consecutive QRS complexes, associated with the atrial stimulation pulses, in the depolarization signal, thus constituting a second interval. The comparison unit 20 also compares the first interval with the second interval. A high degree of measurement reliability is attained when a predetermined number of consecutive first intervals is varied and compared to a predetermined number of second intervals associated with the first time intervals. Comparison between the first interval and the second interval, or between the first interval and second intervals, can be performed e.g. by determining the difference between the time intervals. If the difference is less than a predetermined minimum value, the intervals are deemed to be equal. If the time between QRS complexes varies in the same way as the time between stimulation pulses emitted in the atrium, electrical conduction between the atrium and ventricle (AV conduction) is intact, and capture is then present in the atrium. If, in this manner, capture is found to be present in the atrium, a first signal 26 will go "high". This signal is sent to the indication means 24. The first signal 26 is "low" in the normal state. If the time between QRS complexes does not vary in the same way the time between atrial stimulation pulses varies, i.e. if the difference between intervals exceeds the predetermined minimum value, a second signal 28 will go "high", said signal being sent to the impedance meter 18. The second signal 28 is normally "low". The impedance meter 18, which is connected to the ventricular detector 10, measures, if the second signal 28 received from the comparison unit 20 is "high", impedance between the electrode in the atrium and the electrode in the ventricle during atrial stimulation. In atrial contraction, there is an easily detectable change in impedance which is utilized by the impedance meter 18 for detecting atrial contraction. Impedance measurement can be performed in a plurality of ways familiar to anyone well-versed in the art, e.g. as shown in above-cited US-4 905 696. If impedance measurement has shown that atrial contraction is present, a third signal 30 goes "high". This signal is emitted by the impedance meter 18 and sent to the indication means 24 and the measurement unit 22. The normal state of the third signal 30 is "low", but the signal goes "high" for a given period of time when atrial contraction is detected. Thus, the third signal 30 is sent to the measurement unit 22 to which a detected depolarization signal from the ventricular detector 10 is also sent. If the third signal 30 is "high" and the depolarization signal does not indicate simultaneous ventricular activity, a fourth signal 32 goes "high". This fourth signal 32 is sent to the indication means 24. So the fourth signal 32 goes "high" if impedance measurement shows that atrial contraction is present and no simultaneous ventricular activity is detected. Generation of the fourth signal 32 verifies the presence of capture in the atrium. The first 26, third 30 and fourth signal 32 are therefore sent to the indication means 24. FIG. 2 shows a block diagram of the indication means 24 comprising an OR gate 34 and a NOT gate 36. The first signal 26 and the third signal 30 are sent to the OR gate 34. If either the first signal 26 or the third signal 30 is "high", the OR gate emits a first indication signal 38 which is "high", indicating the presence of capture in the atrium. The fourth signal 32 is sent to the NOT gate 36. If the third signal is "low" for a predetermined period of time with no atrial contraction at a predetermined point in time, and the NOT gate 36 emits an output signal which is the inverse of the input signal, a second indication signal 40, which is "high" at the predetermined point in time, indicates that no atrial contraction is occurring and no capture is accordingly present in the atrium. The predetermined point in time, which determines when the second indication signal 40 is relevant, depends on when impedance measurement is performed and this depends, in turn, on the time during which the atrium is stimulated. When the programmable control device 16 creates a time window after each emitted atrial stimulation pulse, if the second signal 28 is "high", and the second indication signal 40 is studied during this time window, a "high" state for the second indication signal 40 can easily be identified. If the second indication signal is "high", this means that no atrial contraction is occurring. So the atrium lacks any pumping capability, and the heart stimulator can accordingly switch to a stimulation mode in which the atrium is not stimulated, e.g. VVI or VVI-R. The check on the presence of atrial capture can be performed after every atrial stimulation or at every second or third atrial stimulation. Another option is to perform the check at a given interval, e.g. once an hour. The stimulation threshold can be checked when capture is detected in the atrium. Such a check can be made at given intervals, e.g. 1, 2, 4 or 8 hours. By definition, the stimulation threshold is the lowest level of stimulation at which at least two consecutive stimulation pulses result in capture. When the stimulation threshold is checked, the prevaling level of stimulation is the starting point, and this level is gradually reduced by a predefined amount. If capture is still present (at least two consecutive captures), the level is again reduced by the predetermined amount etc. until no capture is detected. The new stimulation threshold is identified as the lowest level at which capture is present, and the new stimulation level is obtained when a safety margin is added to an identified stimulation threshold. Means (not shown) for performing a check on the level of stimulation are arranged inside the programmable control device 16. The above-described method for checking the level of stimulation in the atrium works the same way as the check on the level of stimulation in the ventricle, a well-known technique for anyone well-versed in the art, and will not be described here. The atrial electrode 2 and the ventricular electrode 8 can be arranged on individual electrode leads placed in the atrium and ventricle respectively or arranged on the same electrode lead which then serves as a single lead. Reference designations 2 Atrial electrode 4 Atrial detector 6 Atrial pulse generator 8 Ventricular electrode 10 Ventricular detector 12 Ventricular pulse generator 14 Counter 16 Programmable control device 18 Impedance meter 20 Comparison unit 22 Measurement unit 24 Indication means 26 First signal 28 Second signal 30 Third signal 32 Fourth signal 34 OR gate 36 NOT gate 38 First indication signal 40 Second indication signal

Claims: A heart stimulator with verification of capture in the atrium of a heart, comprising an atrial pulse generator (6) which, via at least one atrial electrode (2) arranged in the atrium, emits stimulation pulses in the atrium, a ventricular detector (10) which senses depolarization signals in the ventricle via a ventricular electrode (8), arranged in the ventricle, and a counter (14), which is connected to the atrial pulse generator (6), to vary and set a first interval between consecutive stimulation pulses emitted in the atrium, characterized in that there is a comparison unit (20), connected to the ventricular detector (10), which is arranged to measure a second interval between consecutive, sensed depolarization signals associated with the atrial stimulation pulses, either to compare the variation in the first interval with variation in the second interval or to compare variations in a predetermined number of first intervals with variations in a predetermined number of second intervals in order to determine the presence of capture in the atrium when the comparison yields a value less than a predetermined minimum value. A heart stimulator according to claim 1, characterized in that the heart stimulator also contains an atrial detector (4) which senses depolarization signals in the atrium, via the atrial electrode, a ventricular pulse generator (12) which, via the ventricular electrode (8), emits stimulation pulses in the ventricle and an impedance meter (18) for measuring impedance between the atrium and the ventricle, the impedance meter (18) being arranged to measure impedance if comparison of variations between the first and second interval, or between the first and second intervals, yields a value less than the predetermined minimum value, capture being present in the atrium if impedance measurement shows the presence of a contraction and no ventricular activity at the same time as the atrial contraction is detected in the ventricle's depolarization signal. A heart stimulator according to claim 1 or 2, characterized in that the comparison unit (20) sets a first signal (26) "high" if the interval between depolarization signals sensed in the ventricle varies in the same way as the interval between stimulation pulses emitted in the atrium, and the comparison unit (20) sets a second signal (28)) "high", sending it to the impedance meter (18), if this is not the case. A heart stimulator according to claim 3, characterized in that the heart stimulator also contains a measurement unit (22), connected to the ventricular detector (10) and the impedance meter (10), the impedance meter (18) setting a third signal (30) "high" if the measured impedance value indicates the presence of an atrial contraction, the measurement unit (22) setting a fourth signal (32) "high" when the third signal is "high" if the measured depolarization signal shows that no depolarization is present in the ventricle at the same time as an atrial contraction. A heart stimulator according to claim 4, characterized in that the first (26), third (30) and fourth (32) signals are sent to an indication means (24), whereupon the indication means (24 sets a first indication signal (38), sent to the control device (16) to indicate the presence of capture in the atrium, "high" if the first (26) or fourth (32) signal is "high". A heart stimulator according to claim 5, characterized in that the indication means (24) sets a second indication signal (40), sent to the control device (16) to indicate that no atrial contraction is present, "high" if the measured impedance value indicates that no atrial contraction is present, whereupon the heart stimulator switches to a WI mode. A heart stimulator according to any of claims 1-5, characterized in that stimulation energy in the atrium is automatically reduced successively, when capture is present in the atrium, to determine the atrium's stimulation threshold so as to obtain a value for the lowest stimulation energy required to produce capture.

Inventors: FRANBERG PER, HOEGNELID KURT, FRANBERG, PER, HOEGNELID, KURT, Högnelid, Kurt
Applicants: ST JUDE MEDICAL, ST. JUDE MEDICAL AB
Application year: 1997
Main classification: A61N1
IPC codes: A61N1