HEART STIMULATING DEVICE

Abstract: A heart stimulating device for detecting fusion and pseudofusion beats (27, 31) by measuring electrical impedance encountered by a delivered stimulation pulse (24, 28, 32). The device includes a pulse generator (7) for delivering a stimulation pulse and impedance measuring means (6, 42, 45, 46) for measuring the impedance. The device further includes logic means (6) for determining a response to the delivered stimulation pulse to involve a fusion or pseudofusion heart beat if the measured electrical impedance belongs to a predetermined impedance range, and adapted to determine the response to the delivered stimulation pulse not to involve a fusion or pseudofusion heart beat if the measured electrical impedance does not belong to said predetermined impedance range.

Description: HEART STIMULATING DEVICE FIELD OF THE INVENTION The present invention relates to a heart stimulating device naving the capability to detect the occurrence of fusion and pseudofusion heart beats, i.e. delivery of electrical stimulation pulses simultaneously or with a small separation in time from spontaneous heart beats. Fusion beats generally refers to a situation where atrial or ventricular depolarization starts from two different positions in the heart. Such a situation arises when a natural heart beat occurs simultaneously with an electrical stimulation pulse from a heart stimulating αevice and they both contπcute to the depoiarisation of a heart chamber. Pseudofusion beats generally refers to a situation where atrial or ventricular depolarization starts spontaneously in the heart and a stimulation pulse is delivered subsequently and does not contribute to the depolarization. In a demand pacemakers or other heart stimulating devices that aim to electrically stimulate a patient's heart only in the absence of normal intrinsic activity, fusion beats or pseudofusion beats present a particular problem, since intrinsic events are to be favoured and stimulation energy is to be saved until really needed. In that way the longevity of a battery powered heart stimulating device is improved. Some terminology used in this disclosure is explained below. Impedance: The total resistance to the flow of electrical current through a conductor, including resistance produced by tissue and electronic components including resistors, capacitors and coils. In pacing, impedance and resistance are sometimes used interchangeably, but strictly speaking, impedance involves a time-dependent relationship between voltage and current, whereas resistance is a fixed number. Resistance and impedance is usually stated n ohms. IEGM: An abbreviation for mtracardiac electrogram. IEGM signals are emitted by active cardiac tissue and sensed through electrodes placed on or within the heart. QRS or QRS complex: The ventricular depolarization as seen on tne electro cardiogram or m the IEGM signals. Intrinsic: Inherent or belonging to the heart itself. An intrinsic beat is a naturally occurring heart beat. R wave: an intrinsic ventricular event. R refers to the entire intrinsic QRS complex. voked response: The electrical activation of the myocardium by a pacemaker output pulse. The ability of cardiac tissue to respond depends on its activity state. Ventricular fusion beat: the pacemaker impulse appears close to a spontaneous QRS complex and partly contributes to the ventricular depolarization. Ventricular pseudofusion beat: the pacemaker impulse appears within a spontaneous QRS complex and does not contribute to the ventricular depolarization. Atrial fusion beat: the pacemaker impulse appears close to a spontaneous P wave and partly contributes to the atrial depolarization. Atrial pseudofusion beat: the pacemaker impulse appears within a spontaneous P wave and does not contribute to the atrial depolarization. A fusion or pseudofusion beat, either in a ventricle or an atrium, may be referred to as simply a fusion beat in contexts where problems related thereto are similar to those of regular fusion beats. DESCRIPTION OF PRIOR ART In today's pacemakers, pulses may be delivered although intrinsic heart activity is present at a rate close to that of the pacemaker. Moreover, backup pulses may by delivered by the pacemaker for safety reasons when an evoked response is rendered undetectable by a fusion or pseudofusion beat. In these and other cases, the detection of evoked response is disturbed by intrinsic heart activity. Further, in order to detect lead breakage or other anomalies, such a pacemaker may be provided with means to determining a measure of the impedance of the lead arrangement by measuring voltage at leading and trailing edges of a delivered stimulation pulse. However, prior art pacemakers still need improvement to effectively overcome the problem of consuming the battery energy prematurely due to failure to favour present intrinsic activity in the heart. Detection of fusion and pseudofusion beats present a general problem in the art. SUMMARY OF THE INVENTION It has surprisingly been found in our extensive experimental studies that an impedance encountered by an electrical stimulation pulse delivered to the heart systematically takes _ower values if the heart is intrinsically stimulated at the same time or within a small separation in time, i.e., in case of fusion/pseudofusion heart beats. The invention aims at overcoming the prior art shortcomings relateα to fusion or pseudofusion beats. Consequently, it is a main object of the present invention to attain a reliable detection of fusion/pseudofusion beats by means of impedance measurements. This objects is attained by a device according to claim 1. Further objects are to detect fusion/pseudofusion beats to enable as well as to detect fusion/pseudofusion beats to enable reduction of the energy used for stimulating the heart. Even farther objects and advantages of the present invention will be evident to a person skilled in the art from the appended claims and the embodiment description below. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a block diagram of a heart stimulating device according the invention including, pulse output means having the ability to measure impedance and logic means having the ability to detect fusion and pseudofusion beats based on the measured impedance; Fig. 2 shows examples of ICG signals that indicate the occurrence of a spontaneous beat, a paced beat, a fusion beat, and a pseudofusion beat; Fig. 3 shows schematically one embodiment of pulse output means in the device of fig. 1 including voltage measuring circuitry; Fig. 4 shows schematically one embodiment of pulse output means in the device of fig. 1 including voltage measuring circuitry; Fig. 5 shows a shape that a stimulation pulse would have a given impeαance and, by dashed lines, alternate shapes corresponding to different impedances. A PREFERRED EMBODIMENT OF THE INVENTION To describe an embodiment of a heart stimulating device according the invention, references are made to Fig. 1. The described type of device is operable to inhibit stimulation to favour intrinsic events. The device has sensing inputs 1, 2 that feed IEGM signals to a sense amplifier 3. The sensing inputs 1, 2 are to be connected to a cable (not shown) and a cardiac electrode (not shown) which provide the IEGM signals to the heart stimulating device. The sense amplifier provides amplified IEGM signals to two detection means 4, 5. A filtered first R wave detection means 4 is operable to detect intrinsic QRS complexes. An evoked response (ER) detection means 5 is operable to detect stimulation-evoked QRS complexes. Each of the detection means 4, 5 is connected to a logic means 6 to which it indicates a detected QRS complex. The logic means controls a pace pulse generator 7 and a backup pulse generator 8 which are operable to transmit stimulation pulses to a patient's heart. A charge means 9 is connected to both pulse generators 7, 8 to provide them with the necessary stimulation energy. The pulse generators 7, 8 are to be connected via the same cable (not shown) to the same cardiac electrode (not shown) as the sense amplifier 3. However, it is preferred that the sense amplifier 3 has a bipolar mode of operation and the pace pulse generator 7 has a unipolar mode of operation. In the case of unipolarity, a conductive casing (not shown) of the heart stimulating device works as one pole and a cardiac electrode as the other. Further, the pace pulse generator 7 includes a measuring means for measuring an impedance encountered by a stimulation pulse when delivered. In this embodiment, such the result of such a measurement is fed to the logic means for determination whether or not the delivered stimulation pulse gave rise to a fusion beat or a pseudofusion heart beat. The operation of the measuring means may be controlled by the logic means 6, which also includes timing means in order to stimulate the patient's heart at an appropriate rate that can be based on detections made in the detection means 4, 5. Additionally or alternatively, the inventive heart stimulation αevice could include means for detecting P waves and atrial evoked response in a manner similar to what is indicated above for ventricular operation. With reference to fig. 2 an explanation will be given to different events taking place in a heart as seen on an ECG plot. Curve 20 shows an spontaneous or intrinsic beat without any pacemaker pulse being delivered. A QRS complex is seen as a peak at 21 and repolaπzation occurs at 22. Curve 23 shows a oeat evoked entirely by a stimulation pulse delivered by the heart stimulating device without any intrinsic heart activity leading to ventricular contraction. A pulse is delivered at 24 (width and hight not drawn to scale) which results in a QRS complex at 25 followed by repolarization at 26. In the case of curve 27, a pulse 28 is delivered at the start of a spontaneous ventricular depolarization, i.e., somewhat earlier. A QRS complex 29 displays increased width and a change in the appearance of the T wave 30. This suggests that part of the ventricle was depolarized by the stimulation pulse 28, but that the heart also was depolarized by spontaneous conduction. The result is a fusion beat. Despite the appearance of the stimulation pulse 32 in curve 31, the following QRS complex 33 and the repolarization 34 has a configuration typical of spontaneous depolarization. The stimulation pulse 32 obviously occurs too late to affect the ongoing depolarization. This results in a pseudofusion beat. It has been surprisingly found that the impedance encountered by a delivered stimulation pulse differs considerably in the situation of curve 23, wherein the heart activity is controlled only by the stimulation, and the situations of curves 27 and 31, showing fusion and pseudofusion beats, respectively. Impedance decreases of at least 30 ohms, and seemingly about 200 ohms, have been measured in the fusion and pseuαofusion heart beats compared to the purely stimulated oeats. Fig. 3 shows circuitry known per se for measuring impedance in cardiac leads (cable and electrode) . The measuring circuitry is incorporated in the pulse generator 7 and includes a capacitor 40 connected to terminal 10 through a switch 41 and to terminal 11. A voltage measuring means for measuring the voltage over terminals 10 and 11 is indicated by 42. Fig. 4 shows alternative circuitry known per se for measuring impedance in cardiac leads. The circuitry includes a capacitor 43 connected to terminal 10 through a switch 44 and a low ohm resistor 45 and to terminal 11. A voltage measuring means for measuring the voltage over resistor 45 is indicated by 46. Naturally, combinations of the circuitry of figs. 3 and 4 could be used for the impedance measuring means in the invention. References will now be made to fig. 5. Curve 60 shows details of a reference stimulation pulse as it could be sensed as IEGM signals m a heart stimulating device according to the invention. A delivery of a stimulation pulse begins at 61. The signal level changes momentarily to 62 from which point a discharge of the stimulation energy proceeds to point 63 along a curve segment 64 that can be approximated by an RC discharge curve. The length 65 of the pulse is in the order of 0.5 ms . After the pulse, the signal resumes its voltage level at 66. Once an implanted cardiac electrode assumes a stable position in the heart, its impedance will be rather constant under normal stimulating conditions undisturbed by intrinsic activity. A typical value is 500 ohms. In fig. 5, pulse delivery under such a normal impedance circumstances is represented by dashed curve 67 oriented below curve segments between points 62 and 66. Dashed curve 68 represents a lower impedance of about 300 ohms which indicates the occurrence of a fusion or pseudofusion beat. In dashed curve 70, no αischarge of the pulse energy occurs during the pulse. Thus, there is an indication of lead break or similar. The signal magnitude 69 of the stimulation pulse in fig. 5 is about 5 V. The circuitry of figs. 4 and/or 5 is used to determine whether the impedance encountered by the stimulation pulse belongs to a range below or above a reference value (corresponding to curve 64, fig. 5). If the impedance is found to be below the reference, an indication of a fusion/pseudofusion beat is presented in the logic means of the device. If the impedance is found to be equal to or above the reference, an indication of the absence of a fusion/pseudofusion beat is presented in the logic means of the device. The presently preferred arrangement for measuring the impedance that a stimulating pulse encounters when delivered to the heart includes a resistor connected m series in the electrical circuit for effecting the pulse. Such a resistor has a low resistance not to dissipate energy unnecessarily but enough to provide a signal for measurement. The impedance is calculated from the relationship between resistor current and pulse voltage in a well-known manner. It is further preferred that the measurement be made during a short interval only immediately following point 62 in fig. 5. However, the current (voltage across known resistor) and/or voltage could be measured in one or several points along the pulse curve to obtain an impedance measure. One specific alternative is to measure the voltage in the points 62 and 63 of fig. 5 so as to obtain an impedance measure. It should be noted that the figures may not be drawn to scale but are rather intended to illustrate functions and qualities of the invention and electrical signals relating to its operation. Further, it is to be understood that this embodiment description includes merely illustrative examples of the application of the invention. For instance, the invention is applicable in atrial as well as ventricular fusion and pseudofusion beat detection. Many variations and modifications may be made without departing from the scope of the invention as defined by the appended claims.

Claims: CLAIMS 1. Heart stimulating device comprising pulse generator means (7) adapted to deliver a stimulation pulse (24,28,32) to a heart, and impeαance measuring means (6,42,45,46) adapted to measure an electrical impedance encountered by said delivereα stimulation pulses, c h a r a c t e r i z e d b y logic means (6) adapted to determine a response to the delivered stimulation pulse to involve a fusion or pseudofusion heart oeat (27,31) if the measured electrical impedance belongs to a predetermined impedance range, and adapted to determine the response to the delivered stimulation pulse not to involve a fusion or pseudofusion heart beat if the measured electrical impedance does not belong to said predetermined impeαance range. 2. The heart stimulating device of claim 1, wherein the impedance measuring means is adapted to determine the impedance by measuring a stimulation pulse voltage delivered by the device. 3. The heart stimulating device of claim 2, wherein the impedance measuring means is adapted to measure the voltage at least twice during a stimulation pulse. 4. The heart stimulating device of any one of claims 1-3, wherein the impedance measuring means is adapted to determine the impedance by measuring a stimulation pulse current delivered by the device. 5. The heart stimulating device of claim 4, wherein the impedance measuring means is adapted to measure the current at least twice during a stimulation pulse.

Inventors: LINDGREN ANDERS, LINDGREN, ANDERS
Applicants: LINDGREN ANDERS, PACESETTER AB, LINDGREN, ANDERS
Application year: 1998
Main classification: A61N1
IPC codes: A61N1