Heart stimulator

Abstract: A heart stimulator which delivers stimulation pulses to a heart when the heart's spontaneous rate (1) drops below a defined basic rate (4) is described. In order to reduce the period (STIM) during which the heart stimulator emits stimulation pulses, the defined basic rate (4) is slow. When the spontaneous heart rate (1) drops below the defined basic rate (4), stimulation pulses are initially emitted at a faster stimulation rate (5), in relation to the basic rate, which then drops towards the defined basic rate (4).

Description: This invention relates to a heart stimulator comprising at least one pulse generator for generating and delivering stimulation pulses to a heart, at least one detector for sensing events in the heart and a control device for controlling the pulse generator's emission of stimulation pulses according to events sensed by the detector, the pulse generator emitting stimulation pulses if a spontaneous heart rate among events sensed by the detector drops below a defined basic rate. A heart stimulator of this kind is referred to as inhibiting, i.e. it inhibits emission of stimulation pulses as long as the heart operates at an adequate pulse rate. The heart stimulator can be single chamber or dual chamber. The operating mode is designated with a three-letter alphabetic code in which the first letter designates the site of stimulation, A therefore designating an atrial site, V designating a ventricular site and D designating both atrial and ventricular sites, the second letter designates the site of sensing and the third letter designates the response to signals, I designating inhibition. Thus, AAI is a heart stimulation mode in which the atrium is stimulated and sensed and in which stimulation pulses are inhibited when spontaneous events are detected. VVI is the same mode although with a ventricular site, and DDI applies to both the atrium and the ventricle. DDI requires a dual chamber stimulator, whereas AAI and DDI can be programmed in both single chamber and dual chamber stimulators. Stimulation pulses are emitted at the basic rate when the spontaneous heart rate drops below the basic rate. The basic rate is normally 60 to 80 beats/min. The heart stimulator can be programmed with hysteresis to prevent competitive stimulation when the spontaneous heart rate oscillates around the defined basic rate. The hysteresis consists of a rate slower than the basic rate. The spontaneous heart rate must then drop below this slower rate before stimulation pulses are emitted. Such a stimulator is known from US-A-50 16 630 One type of pathological condition in the heart, i.e. the vasovagal syndrome, manifests itself in a sudden drop in the spontaneous heart rate, causing the patient to faint, followed by restoration of the normal rate in a few seconds or minutes. The speed with which the spontaneous heart rate drops varies from one patient to another. Patients with this condition are generally provided with a heart stimulator which stimulates the heart when the spontaneous rate plummets to keep the patient from fainting. For known heart stimulators of today, the spontaneous heart rate, once the heart stimulator has begun stimulation, must exceed the stimulation rate before the inhibiting function is reactivated so as to "silence" the stimulator. So stimulation of the heart could still continue, even after the spontaneous heart rate has normalized. Since people with this type of heart condition do not normally have any other heart defects, limiting stimulation to the briefest periods possible would be advantageous. The object of the invention is to achieve a heart stimulator in which stimulation periods are limited to durations which are no longer than necessary. This object is achieved with a heart stimulator in accordance with the invention in that the above-described heart stimulator is devised so the control device induces the pulse generator, when the determined spontaneous heart rate drops below the defined basic rate, to emit stimulation pulses at a faster stimulation rate, in relation to the defined basic rate, which subsequently slows down to the defined basic rate. A slower basic rate can more appropriately be used here than in prior art heart stimulators. Stimulation pulses are then emitted at a faster rate which can correspond to the basic rate in the prior art, said faster rate then slowing until it reaches the defined basic rate or until the spontaneous heart rate exceeds the stimulation rate. For patients with the above-described type of heart defect in which the heart rapidly recovers and then operates normally, the heart stimulator according to the present invention institutes the inhibiting function much earlier than in a prior art heart stimulator. The speed at which the increased stimulation rate drops to the defined basic rate can be fast or slow and is appropriately programmable so it can be adapted to different patients. It is advantageous if the defined basic rate is slower than a spontaneous resting rate for the heart, preferably between 30 and 60 pulses/minute. A basic rate slower than the patient's resting pulse ensures that no unnecessary stimulations are supplied. Alternately, the basic rate can be established by having the control device comprise an averager which calculates the average value for the spontaneous heart rate over a defined number of preceding heart cycles, the control device then setting the defined basic rate at the calculated average value less a defined value. This directly adapts the basic rate to the spontaneous heart rate. The spontaneous heart rate is e.g. slower at night than in daytime, and the basic rate would be adapted to this circumstance. The use of a mean value could keep the basic rate from changing too rapidly. Defined intervals for the basic rate can also be established to prevent the rate from e.g. becoming too fast when the patient is very active for a long period of time. A refinement of the heart stimulator is achieved in accordance with the invention in that the increased stimulation rate is programmable in the control device. The faster stimulation rate can be established for each patient and programmed into the heart stimulator by a physician. Alternately, the faster stimulation rate can be established when the control device determines the spontaneous heart rate and sets the faster heart rate equal to the established spontaneous heart rate. When the spontaneous heart rate drops suddenly, the heart is initially stimulated at the same rate, providing a more natural transition from spontaneous activity to stimulation. Here, it would be advantageous if the control device sets the faster stimulation rate equal to the established spontaneous heart rate plus/minus a defined rate margin, the defined rate margin then consisting of either a fixed value or a percentage of the spontaneous heart rate determined. A third way of establishing the faster stimulation rate is achieved when an activity sensor senses a level of physical activity in the heart stimulator patient, and the control device sets the faster stimulation rate according to the sensed level of activity. The sensor-controlled stimulation rate then corresponds to the spontaneous rate in the physical activity. A refinement of the heart stimulator is achieved in accordance with the invention in that the control device controls the pulse generator in such a way that it emits a defined number of stimulation pulses at the defined basic rate, when the spontaneous heart rate drops below the defined basic rate, before the pulse generator emits stimulation pulses at the faster stimulation rate. If the patient's heart recovers quickly, one or two stimulation pulses may suffice until an adequate spontaneous heart rate reappears. Inhibition of the heart stimulator would then occur more rapidly than if the faster stimulation rate were emitted immediately. Like prior art heart stimulators, having the control device programmed with a defined hysteresis rate which is slower than the defined basic rate would be advantageous for the heart stimulator according to the invention, whereby the spontaneous heart rate must drop below the hysteresis rate for the pulse generator to emit any stimulation pulses. For example, the hysteresis rate, in combination with a defined basic rate corresponding to a spontaneous resting heart rate, ensures that the spontaneous heart rate, e.g. at night, can be allowed to drop below the basic rate without emission of any stimulation pulses. Referring to three Figures, the invention will be described in greater detail below., figure 1 illustrates in diagram form the function of a heart stimulator according to the prior art, figure 2 illustrates in diagram form the function of the heart stimulator according to the invention, and figure 3 illustrates in a block diagram one embodiment of the heart stimulator according to the invention. The heart stimulator 6 in fig. 3 comprises an atrial pulse generator 7 which generates atrial stimulation pulses and delivers same to the atrium in a heart 10 via a first electrode conductor 8 and a first tip electrode 9. The atrial stimulation pulses are then conducted through body tissue to the heart stimulator's 6 enclosure 11, which serves as an indifferent electrode, and back to the atrial pulse generator 7. An atrial detector 12 is connected to the first electrode conductor 8 and the enclosure 11 for sensing events in the atrium. In the corresponding manner, a ventricular pulse generator 13 generates ventricular stimulation pulses and delivers same to the ventricle in a heart 10 via a second electrode conductor 14 and a second tip electrode 15. The ventricular stimulation pulses are conducted back to the ventricular pulse generator 13 via body tissue and the heart stimulator's 6 enclosure 11. A ventricular detector 16 is connected to the second electrode conductor 14 and the enclosure 11 for sensing events in the ventricle. The pulse generators 7, 13 and detectors 12, 16 are controlled by a control device 17 which receives information on the events sensed by the detectors 12, 16 in the heart 10. The control device 17 can be programmed by a physician with different functions by means of a telemetry unit 18 in the heart stimulator 6 and an extracorporeal programming unit 19. The physician can e.g. program the basic rate and operating mode, e.g. AAI, VVI or DDI. A motion sensor 20 senses the heart stimulator patient level of physical activity, and this signal is sent to the control device 17 which e.g. can determine a faster stimulation rate at which the heart stimulator 6 is to stimulate the heart 10 when spontaneous activity ceases. In fig. 1 is shown a spontaneous heart rate signal 1 in a frequency-time (f-t) diagram. The levels for a basic rate 2, e.g. 80 pulses/minute, and a hysteresis rate 3, e.g. 50 pulses/minute, programmed in a prior art heart stimulator are also shown in the diagram. The heart stimulator inhibits emission of stimulation pulses as long as the spontaneous heart rate 1 is faster than the hysteresis rate 3. In one pathological condition in a heart, the spontaneous heart rate 1 can drop suddenly and return to a normal rate and normal function a few seconds to a few minutes later. When this happens to the spontaneous heart rate 1 in fig. 1, the heart stimulator according to the prior art stimulates the heart at the basic rate 2, as marked in the diagram with a bolder basic frequency line. The spontaneous heart rate 1, marked during the period of stimulation, illustrates restoration of normal cardiac function. Since stimulation occurs at a faster rate, and each stimulation resets, so to speak, biological spontaneous function, the spontaneous heart rate 1 during stimulation cannot usually be visualized. But in e.g. the heart stimulator's VVI mode, the spontaneous heart rate signal 1 would show the atrium's recovery. Spontaneous activity must exceed the basic rate 2 for termination of stimulation pulses, the heart stimulator again being inhibited until the next time the spontaneous heart rate 1 drops below the hysteresis rate 3. Periods of inhibition (INHIBIT) and stimulation (STIM) are shown on the time axis t. In fig. 2 is shown the same spontaneous heart rate signal 1 in an f-t diagram to illustrate how the heart stimulator 6 according to the present invention operates under the same circumstances. Another basic rate, e.g. 50 beats/min, is drawn in the diagram. In contrast to the basic rate 2 in fig. 1, the basic rate 4 is, according to the invention, much slower. In principle, no hysteresis rate is necessary, but one could be used to avoid unnecessary stimulation, e.g. during sleep when the spontaneous heart rate 1 can drop below the basic rate 4. When the spontaneous heart rate 1 drops below the basic rate 4, the heart stimulator 6 initially emits a few stimulation pulses at the basic rate. This is to give the heart time to terminate the stimulation function if it recovers rapidly. The heart stimulator 6 thereafter continues stimulation at a faster rate 5 which subsequently slows. The faster rate 5 can be initially programmed at e.g. 80 beats/minute or, if the heart stimulator 6 senses the spontaneous rate 1, be set on the basis of the latest spontaneous heart rate 1 sensed. The faster stimulation rate can also be determined by the control device 17 according to the level of sensed activity. In this instance, the faster stimulation rate 5 is based on the latest spontaneous heart rate 1 sensed plus a defined rate margin. The faster stimulation rate 5 slows until it reaches the basic rate 4 or until the spontaneous heart rate 1 is again fast enough to activate the heart stimulator's 6 inhibiting function. As the period designation under the time axis t shows, the stimulation period (STIM) is shorter than in a prior art stimulator.

Claims: A heart stimulator (6) comprising at least one pulse generator (7, 13) for generating and delivering stimulation pulses to a heart (10), at least one detector (12, 16) for sensing events in the heart (10) and a control device (17) for controlling the pulse generator's (7, 13) emission of stimulation pulses according to events sensed by the detector (12, 16), the pulse generator (7, 13) emitting stimulation pulses if a spontaneous heart rate among events sensed by the detector (12, 16) drops below a defined basic rate (1), characterized in that the control device (17) induces the pulse generator (7, 13), when the determined spontaneous heart rate (1) drops below the defined basic rate (4), to emit stimulation pulses at a faster stimulation rate (5), in relation to the defined basic rate (4), which subsequently slows down to the defined basic rate (4). A heart stimulator of claim 1, wherein the defined basic rate (4) is slower than a spontaneous resting rate for the heart, preferably between 30 and 60 pulses/minute. A heart stimulator of claim 1, wherein the control device (17) comprises an averager which calculates the average value for the spontaneous heart rate (1) over a defined number of preceding heart cycles, the control device (17) then setting the defined basic rate (4) at the calculated average value less a defined value. A heart stimulator of any of the above claims, wherein the faster stimulation rate (5) is programmable in the control device (17). A heart stimulator of any of claims 1 - 3, wherein the control device (17) determines the spontaneous heart rate (1) and sets the faster heart rate (5) equal to the established spontaneous heart rate (1). A heart stimulator of claim 5, wherein the control device (17) sets the faster stimulation rate (5) equal to the established spontaneous heart rate (1) plus/minus a defined rate margin, the defined rate margin then consisting of either a fixed value or a percentage of the spontaneous heart rate (1) determined. A heart stimulator of any of claims 1 - 3, wherein an activity sensor (20) senses a level of physical activity in the heart stimulator patient, and the control device (17) sets the faster stimulation rate (5) according to the sensed level of activity. A heart stimulator of any of the above claims, wherein the control device (17) controls the pulse generator (7, 13) in such a way that it emits a defined number of stimulation pulses at the defined basic rate (4), when the spontaneous heart rate drops below the defined basic rate (4), before the pulse generator emits stimulation pulses at the faster stimulation rate (5). A heart stimulator of any of the above claims, wherein the control device (17) is programmed with a defined hysteresis rate which is slower than the established basic rate (4), whereby the spontaneous heart rate (1) must drop below the hysteresis rate for the pulse generator (7, 13) to emit any stimulation pulses.

Inventors: FRANBERG PER, LINDGREN ANDERS, MOBERG LENNART, FRANBERG, PER, LINDGREN, ANDERS, MOBERG, LENNART
Applicants: PACESETTER AB
Application year: 1993
Main classification: A61N1
IPC codes: A61N1