A tachycardia eliminating apparatus, stimulating the respiratory system

Abstract: A tachycardia eliminating apparatus (1), comprising heart beat detecting means (2) for detecting heart beats in an input heart activity signal (HAS) and being devised to deliver an output heart beat signal indicating each detected heart beat; tachycardia detecting means (4) coupled to the output of the heart beat detecting means (2) and being devised to generate a tachycardia indication signal in response to a detected tachycardia status of a heart as reflected in the heart beat signal; and respiratory system stimulating means (6) coupled to the output of the tachycardia detecting means (4) and being devised to generate an output respiratory system stimulation signal (RSS) for stimulating a respiratory system to achieve a stimulated breathing rate, where the ratio of the detected heart beat rate to the stimulated breathing rate corresponds to a predetermined value.

Description: Technical Field of the Invention The present invention relates to an apparatus for stimulating living tissue. Background of the Invention Normal rhythms of the heart are essentially regular with frequencies between 50 and 100 beats/minute depending mainly on the current bodily activity. Any deviation from this regular rhythm is called an arrhythmia, thus also including fast or slow regular rhythms. A fast rhythm is generally called a tachycardia or a tachyarrhythmia, and a slow rhythm is called a bradycardia or a bradyarrhythmia. The rhythm of the heart is normally initiated in the sinus node, but there is a capability of spontaneous activity in different foci of the whole cardiac conduction system, i.e. the atriums, AV-node, His bundle, right and left bundle branches and Purkinje system. The reason that the sinus node normally controls the contraction of the heart is that this is the focus with the highest natural frequency, i.e. with a normal discharge frequency in the range of 50-100 pulses/minute. The natural frequency of electrically pulsing foci outside the sinus node, also referred to as ectopic foci, decreases with increasing distance from the sinus node. For example, the natural frequency of the atriums is normally 45-50 pulses/minute, of the AV-node 40-45 pulses/minute and of the ventricular structures < 35 pulses/minute. However, the frequencies of ectopic foci may for different reasons change and interfere with the normal sinus rhythm, thereby causing ectopic rhythms or beats of the heart. Ectopic beats may in turn initiate a more serious arrhythmia, such as a re-entry tachycardia, atrial or ventricular fibrillation and atrial or ventricular flutter. It is therefore an important task in cardiac pacing and defibrillation technology to detect and eliminate ectopic beats. Description of Prior Art In prior art it is widely known to control the rhythm of the heart by means of a pacemaker, such as the one described in for example EP-B1-0 387 363. It is also known to correct a disturbed heart rhythm by means of a defibrillator. In apparatuses of this kind the current rhythm of the heart is sensed, and when needed stimulation signals are transferred to the heart muscle via a stimulation electrode in order to achieve a desired rhythm. It is also known that there is a mutual influence between breathing and cardiac rhythms. This was discussed at the Cardiostim Congress 1995 in St. Petersburg, Russia, where Professor W.M. Pokrowski presented experimental studies that showed how fast breathing rate synchronised with heart rate could reduce or eliminate a tachycardia. In particular previously frequent ectopic extra beats were avoided with this method. A method and an apparatus for diaphragmic stimulation for the purpose of effecting respiration in order to maintain an adequate concentration of oxygen in the blood of a human being are known from for example WO 86/00234. Problems in prior art Prior art apparatuses for correcting heart arrhythmias are in principle devised to generate and deliver an electric shock to the heart muscle in order to force the heart out of a pathological rhythm. Such chocks can be quite brutal and the whole cardiac conductive system is thereby blocked for some time until a desired rhythm is induced and established. Apparatuses of this kind are primarily suitable for stopping bradycardias, which are comparatively easy to treat with a low number of single heart stimulating pulses. Tachycardia of different kinds are, however, not so easily treated and in particular it has turned out that prior art apparatuses are not really adequate for preventing an entry into a pathological heart rhythm in response to detected tendencies or early signs of a tachycardia. Therefore, there is a need for an apparatus with the capability of providing a more gentle and prophylactic heart stimulation. Summary The problem to be solved by the present invention is to provide an apparatus for eliminating a detected established tachycardia. An aspect of the this problem is to eliminate a detected approaching tachycardia. In accordance with the invention and as defined in the independent claim 1 , the problem is solved by an apparatus devised for an indirect stimulation of the heart by stimulating the respiratory system such that the stimulated breathing rate is synchronised with the detected cardiac rate in order to eliminate a detected tachycardia. The invention utilises the anatomical fact that there is a mutual interdependence between breathing rate and heart rate. In particular it is possible to induce a predetermined desired heart rate by stimulating the respiratory system to a breathing rate corresponding to the desired heart rate. In particular it is possible to induce a predetermined desired heart rate by stimulating the respiratory system to a breathing rate corresponding to the desired heart rate. An inventive apparatus for eliminating a heart tachycardia comprises cardiac status detecting means, e.g. in the form of heart beat detecting means, tachycardia detecting means and respiratory system stimulating means. The cardiac status detecting means is devised to detect the status of the cardiac system in an input cardiac activity signal. In the embodiment where the cardiac status means is a heart beat detecting means, said means is in a per se known manner devised for detecting heart beats in an input heart activity signal, such as an ECG signal, and to deliver an output heart beat signal indicating each detected heart beat. The tachycardia detecting means is coupled to the output of the heart beat detecting means and is preferably devised to detect the period between consecutive heart beats and to deliver an output tachycardia indication signal in response to a tachycardia status detected in the heart beat signal. This may for example be achieved by comparing the detected heart beat rate with a predetermined rate or a predetermined limit value. The respiratory system stimulating means is coupled to the output of the tachycardia detecting means and is devised to generate an output respiratory system stimulation signal suitable for stimulating a respiratory system in order to achieve a breathing rate, where the ratio of the detected heart beat rate to the stimulated breathing rate corresponds to a predetermined value, or where the breathing corresponds to a predetermined desired heart beat rate. In an embodiment of the invention suitable for external use, i.e. placed outside a patients body, the inventive apparatus is devised to deliver a stimulation signal suitable for controlling an arrangement for mechanical induction of a certain breathing rate. Currently it is considered that the in practice most useful mechanical breathing arrangement is a ventilator capable of providing the lungs of a patient with air or another suitable gas or mixture of gases. An embodiment for this arrangement is preferably provided with an integrated or separate ventilator controlling means which is couplable to the output of the respiratory system stimulating means. The ventilator controlling means is further couplable to a ventilator and is devised to control the ventilator in response to a respiratory system stimulating signal. An external embodiment of this kind may for example take the heart activity signal from a surface ECG arrangement. Another embodiment of the inventive apparatus is also provided with heart stimulating means devised to stimulate a heart directly in a per se known manner. The heart stimulating means is coupled to an output of the tachycardia detecting means and/or to an output of the heart beat detecting means, and is devised to generate an output heart stimulation signal, which in a per se known manner is transferable to the heart muscle via a heart stimulation electrode system. A variety of the latter embodiment comprises synchronisation means devised to synchronise respiratory system signals with direct heart stimulation signals. The synchronisation means is coupled to an output of the tachycardia detecting means and to an input of the heart stimulating means. Another input of the synchronisation means may optionally also be feed-back coupled to the output of the heart stimulating means. The synchronisation means is further coupled to an input of the respiratory system stimulating means. In order to be able to synchronise the output stimulation signals it is devised to trigger the heart stimulating means and the respiratory system stimulating means to deliver stimulation signals in response to the tachycardia indication signal and such that the respiratory system stimulation signal has a predetermined time relation to the heart stimulation signal. The synchronisation may for example be carried out with a suitable time delay. Different organs of the respiratory system may be stimulated in order to induce breathing as long as they are capable of triggering a co-ordinated activity in the respiratory system. In one embodiment of the invention the respiratory system stimulating means is devised to deliver a stimulation signal suitable for stimulating a respiratory system muscle, which for example may be the diaphragm. Another attractive embodiment is devised to deliver a stimulation signal suitable for stimulating a respiratory system nerve, for example nervus pherenicus. A further embodiment of the invention is implantable and in this embodiment each signal input and output is couplable to implantable stimulation electrodes of suitable design. This embodiment may be realised as a specialised apparatus or may be incorporated in an implantable heart stimulator, such as a pacemaker or a defibrillator. Further embodiments of the invention may be devised to stimulate only selected parts of the heart, e.g. an atrium, to beat in synchronisation with breathing muscles by means of the inventive apparatus coupled to a specifically situated cardiac stimulation electrode and a breathing muscle or nerve stimulation electrode. Advantages With the inventive apparatus the heart rate may be controlled in a mild manner which is suitable for preventing or eliminating pathological states of the heart in response to early signs of a approaching or an established tachycardia. A heart rhythm deviating from a normal rhythm shows an increased inclination to fall into an arrhythmia, and with the inventive apparatus it is possible to interfere at an early stage and thereby avoiding the cruder heart stimulation methods employed by means of prior art heart stimulators. Using the inventive apparatus the heart is, as it were, massaged into taking up or maintaining a certain heart rate. Further advantages and properties of the invention are apparent from the following description in connection with the accompanying drawings and the dependent claims. Brief Description of the Drawings Fig 1A shows schematically a block diagram of an embodiment of the inventive tachycardia eliminating apparatus coupled to a human body; Fig 1B is a block diagram showing functional components of an embodiment of the invention; Fig 1C shows another embodiment of the invention according to Fig 1B; Fig 2 shows an embodiment of the invention suitable for use together with a ventilator; Fig 3 shows an implantable embodiment of the invention devised for stimulating a respiratory system muscle or a respiratory system nerve and possibly also for stimulating the heart; and Figs 4-7 are diagrams showing the general form of signals involved with the inventive apparatus. Detailed Description Fig 1A shows schematically an embodiment of an apparatus 1 for eliminating a tachycardia according to the present invention. Apparatuses of this kind are normally realised with logical circuits but may also be designed in more traditional circuitry. However, the figures depicts functional features of the invention by means of block diagrams. In Fig 1A, a tachycardia eliminating apparatus 1 is coupled to a human body 7 and obtains via a detection lead 3 an input cardiac activity signal HAS that represents the activity of the heart 9. Different cardiac activity signals reflecting for example heart beats, blood pressure, blood flow, measured impedance or chemical composition of the blood are possible within the inventive concept. The cardiac activity signal HAS is input in a cardiac status detecting means 2 which is devised to detect the status of the cardiac system and to deliver an output cardiac status signal. An output of the cardiac status detecting means 2 is coupled to an input of a tachycardia detecting means 4, which is devised to detect, in the cardiac status signal, indications of an approaching or an established heart tachycardia and to deliver an output tachycardia indication signal in response to a deviation from a predetermined cardiac status. An output of the tachycardia detecting means is coupled to an input of a respiratory system stimulating means 6, to which a tachycardia indicating signal is delivered. The respiratory system stimulating means 6 is devised to generate, in response to a received tachycardia indication signal, an output respiratory system stimulation signal RSS suitable for stimulating a respiratory system organ to achieve a breathing rate corresponding to a predetermined desired heart beat rate. In Fig 1A the respiratory stimulating signal RSS is delivered by means of a respiratory system stimulation lead 5 to the diaphragm 22. In an embodiment of the invention as shown in Fig 1A a heart activity signal HAS is obtained from the heart 9 and is delivered to a heart beat detecting means 2 for detecting heart beats in a per se known manner. The heart activity signal HAS is for example an ECG signal sensed by means of a surface electrode or an implantable sensing electrode. The heart beat detecting means 2 is devised to deliver an output heart beat signal indicating each detected heart beat, e.g. in the form of a square wave signal were each positive edge indicates a heart beat. The tachycardia detecting means 4 is coupled to the output of the heart beat detecting means 2 and is devised to detect a tachycardia and/or single ectopic beats of the heart as reflected in the heart beat signal. For this purpose the tachycardia detecting means 4 may for example be devised to compare the detected heart beat rate with a predetermined limit value, e.g. 100 beats/minute, and detect whether the limit value is exceeded. The tachycardia detecting means 4 is further devised to generate and deliver an output tachycardia indication signal in response to a detected tachycardia status of the heart via the heart beat signal. An output of the tachycardia detecting means 4 is coupled to an input of a respiratory system stimulating means 6. The respiratory system stimulating means 6 is devised to generate an output respiratory system stimulation signal RSS in response to a tachycardia indication signal. The respiratory system stimulation signal RSS may have different forms but shall be reasonable values. In another embodiment the respiratory system stimulating means (6) is devised to generate an output respiratory system stimulation signal (RSS) suitable for stimulating a respiratory system to a breathing rate close to a predetermined desired heart beat rate, with a possible deviation in the breathing rate from the predetermined desired heart beat rate up to 1/2 of the value of said desired heart beat rate. The breathing may thus be stimulated to a breathing rate e.g. in the range: desired heart beat rate x (1-1/2) to desired heart beat rate x (1+ 1/2); or in a more narrow range. Fig 1B, which has reference numerals in common with Fig 1A for common components, shows an embodiment of the invention comprising a heart stimulating means 8 for direct electrical stimulation of the heart in parallel with respiratory system stimulation. An input of the heart stimulating means 8 is coupled to an output of the tachycardia detecting means 4 in order to obtain a tachycardia indication signal. The heart stimulating means 8 comprises a per se known pulse generator (not shown) and is devised to generate an output heart stimulation signal HSS suitable for direct heart stimulation via a heart stimulating electrode. As shown in Fig 1B with an intermittently drawn line, the heart stimulating means 8 may optionally also be coupled to the output of the heart beat detecting means 2 in order to obtain a heart beat signal. The heart stimulating means 8 of this embodiment is devised to generate a heart stimulation signal in a per se known pacing or defibrillating manner, in parallel with or fully independently of generating a heart stimulation signal in response to a tachycardia indication signal. Fig 1C shows a further embodiment of the invention according to Fig 1B provided in order to achieve a more finely tuned direct and indirect heart stimulation. For this purpose the tachycardia eliminating apparatus comprises a synchronisation means 10 for synchronising a respiratory system stimulation signal RSS with a direct heart stimulation signal HSS. An input of the synchronisation means 10 is coupled to an output of the tachycardia detecting means 4, whereas trigger signal outputs of said synchronisation means 10 are coupled to inputs of the heart stimulating means 8 and the respiratory system stimulating means 6, respectively. Functionally, the synchronisation means 10 is devised to trigger the heart stimulating means 8 and the respiratory system stimulating means 6 to deliver stimulation signals HSS,RSS in response to a tachycardia indication signal. The respiratory system stimulation means 6 and the heart stimulation means are naturally correspondingly devised to be triggered by trigger signals. The synchronisation is carried out such that the respiratory system stimulation signal RSS has a predetermined time relation to the heart stimulation signal HSS. In a further heart stimulating means 8 and the respiratory system stimulating means 6, respectively. Functionally, the synchronisation means 10 is devised to trigger the heart stimulating means 8 and the respiratory system stimulating means 6 to deliver stimulation signals HSS,RSS in response to a tachycardia indication signal. The respiratory system stimulation means 6 and the heart stimulation means are naturally correspondingly devised to be triggered by trigger signals. The synchronisation is carried out such that the respiratory system stimulation signal RSS has a predetermined time relation to the heart stimulation signal HSS. In a further development of this embodiment there may also be an optional feed-back coupling from an output of the heart stimulating means 8 to an input of the synchronisation means 10, in order to enable a refinement of the synchronisation in response to a feed-back heart stimulation signal HSS. In a further embodiment as depicted in Fig 1C, an input of the heart stimulating means 8 may be optionally coupled to the output of the heart beat detecting means in order to generate and deliver a heart stimulation signal in a per se known manner. Fig 2 shows schematically an inventive apparatus 1 in an embodiment devised for external use. The apparatus 1 receives a heart activity signal HAS from a heart 16 via an electrode lead 14. A generated respiratory system stimulation signal RSS suitable for controlling a ventilator is delivered to a ventilator controlling means 15, which is coupled to an output of the apparatus 1 and to an input of a ventilator 12. The ventilator controlling means 15 is devised to control the ventilator 12 in response to a respiratory system stimulating signal RSS delivered by the apparatus 1. Fig 3 shows an implantable tachycardia eliminating apparatus 1 according to the invention and schematically depicted when implanted in a patients body. The apparatus 1 is coupled to a sensing and possibly also stimulating electrode 24 which in its distal end is coupled to heart tissue. The apparatus 1 of the shown embodiment is also coupled on one hand to nervus pherenicus 25 via a nerve stimulation lead 24 and on the other hand to the diaphragm 22 via a muscle stimulation lead 18. This configuration of a system set up to stimulate the respiratory system is merely shown as an example, and other muscles or nerves of the respiratory system may be stimulated together or separately. In the embodiment for stimulating a respiratory muscle, the respiratory system stimulating means of the tachycardia eliminating apparatus 1 is devised to deliver a stimulation signal RSS suitable for stimulating the selected respiratory system muscle. Similarly, in the embodiment for stimulating a nerve, the respiratory system stimulating means 6 is devised to deliver a stimulation signal RSS suitable for stimulating a selected respiratory system nerve. The form of the signals needed in the different cases depends on the anatomic qualities of the specific, selected organ. Examples of signals involved with and generated by the inventive apparatus are shown in diagrams in Figs 4-7. In Fig 4 the signal S 1 is a typical ECG signal plotted as a voltage U over a time scale t. Fig 5 represents respiration signals S2,S3 with air pressure P plotted over the same time scale t as in Fig 4. Signal S2 depicts a normal breathing rate, whereas signal S3 shows an accelerated breathing rate stimulated by means of an apparatus according to the invention. Fig 6 shows an example of a stimulation signal S4 for stimulation of respiratory muscles or nerves with a voltage plotted again over the common time scale t. Stimulation of respiratory muscles and nerves in order to achieve a contraction phase is preferably effected by means of a pulse train with a duration T1 of e.g. 2-5 milliseconds. Such pulse trains may be repeatedly delivered with a frequency for example in the range 5-100 Hz, i.e. a pulse train period T2 of 0.01-0.2 seconds. Stimulation pulses with this kind of pulse trains are produced for a period T3 which may have a predetermined length in order to achieve a desired heart rhythm, or for a period T3 that is continuously determined in response to a sensed heart activity signal. The stimulation pulses may be positive, negative or biphasic. Fig 7 shows such a biphasic pulse with an amplitude U, pulse width T4 and pulse period T5. The values as well as the timing of the different signal parameters are selected with respect to the specifically selected organ of the respiratory system that is to be stimulated. However, examples of typical values would for the amplitude U be 1-12 volts, for the pulse width 0.1-2 milliseconds and for the pulse period T5 about 10-200 milliseconds. In the embodiment of an external apparatus controlling a ventilator the respiratory system stimulation signal is not a pulse train but rather a signal suitable for controlling ventilator beats. While preferred embodiments of the invention have been disclosed in detail, it should be understood by those skilled in the art that various other modifications may be made to the illustrated embodiments without departing from the scope of the invention as described in the specification and defined in the appended claims.

Claims: A tachycardia eliminating apparatus (1) for eliminating an approaching or an established tachycardia, comprising cardiac status detecting means (2) for detecting the status of the cardiac system in an input cardiac activity signal (HAS) and devised to deliver an output cardiac status signal; and tachycardia detecting means (4) coupled to the output of the cardiac status detecting means (2) and being devised to detect, in the cardiac status signal, indications of an approaching or an established heart tachycardia and to deliver an output tachycardia indication signal in response to a deviation from a predetermined cardiac status; characterized in respiratory system stimulating means (6) coupled to the output of the tachycardia detecting means (4) and being devised to generate, in response to a received tachycardia indication signal, an output respiratory system stimulation signal (RSS) suitable for stimulating a respiratory system organ to achieve a breathing rate corresponding to a predetermined desired heart beat rate. A tachycardia eliminating apparatus (1) according to claim 1, in which said cardiac status detecting means are heart beat detecting means (2) being devised to deliver an output heart beat signal indicating each detected heart beat; and said tachycardia detecting means (4) being coupled to the output of the heart beat detecting means (2), said predetermined value corresponding to the ratio of the detected heart beat rate to the stimulated breathing rate. A tachycardia eliminating apparatus (1) as recited in claim 1 or 2, characterized in that the tachycardia detecting means (4) is devised to generate a tachycardia indication signal if the heart beat rate of the heart beat signal exceeds a predetermined limit value. A tachycardia eliminating apparatus (1) as recited in any of the preceding claims, characterized in that the quotient of the detected heart beat rate by the stimulated breathing rate is a positive integer (1,2,3....). A tachycardia eliminating apparatus (1) as recited in any of the preceding claims, characterized in that the respiratory system stimulating means (6) is devised to generate an output respiratory system stimulation signal (RSS) suitable for stimulating a respiratory system to a breathing rate close to a predetermined desired heart beat rate, with a deviation in the breathing rate from the predetermined desired heart beat rate up to 1/2 of the value of said desired heart beat rate. A tachycardia eliminating apparatus (1) as recited in any of the preceding claims, characterized in that the respiratory system stimulating means (6) is devised to deliver a stimulation signal (RSS) suitable for stimulating a respiratory system muscle (22), e.g. a diaphragm (22). A tachycardia eliminating apparatus (1) as recited in any of the preceding claims, characterized in that the respiratory system stimulating means (6) is devised to deliver a stimulation signal (RSS) suitable for stimulating a respiratory system nerve (24), e.g. a nervus pherenicus (24). A tachycardia eliminating apparatus (1) as recited in any of the preceding claims, c haracterized in that the respiratory system stimulating means (6) is devised to deliver a stimulation signal (RSS) suitable for controlling a ventilator (12); and in that it further comprises a ventilator controlling means (15) which is couplable to the output of the respiratory system stimulating means (6) and which is couplable to a ventilator (12), said ventilator controlling means (15) being devised to control the ventilator (12) in response to a respiratory system stimulating signal (RSS). A tachycardia eliminating apparatus (1) as recited in any of the preceding claims, characterized in heart stimulating means (8) coupled to an output of the tachycardia detecting means (4) and being devised to generate an output heart stimulation signal (HSS) in response to a tachycardia indication signal. A tachycardia eliminating apparatus (1) as recited in claim 9, characterized in synchronisation means (10) coupled to an output of the tachycardia detecting means (2), to an input of the heart stimulating means (8) and to an input of the respiratory system stimulating means (6), said synchronisation means being devised to trigger the heart stimulating means (8) and the respiratory system stimulating means (6) to deliver stimulation signals (HSS,RSS) in response to the tachycardia indication signal and such that the respiratory system stimulation signal (RSS) has a predetermined time relation to the heart stimulation signal (HSS). A tachycardia eliminating apparatus (1) as recited in claim 10, characterized in that the synchronisation means (10) is coupled to the heart stimulating signal (HSS) output of the heart stimulating means (8) and is devised to utilise a feed-back heart stimulating signal (HSS) to synchronise the heart stimulating means (8) and the respiratory system stimulating means (6). A tachycardia eliminating apparatus (1) as recited in any of the preceding claims, characterized in that it is implantable and in that signal inputs and outputs are couplable to implantable stimulation electrodes. An implantable heart stimulator, characterized in that it comprises a tachycardia eliminating apparatus in accordance with any of the claims 1-12.

Inventors: HIRSCHBERG JAKUB, STRANDBERG HANS, HIRSCHBERG, JAKUB, STRANDBERG, HANS
Applicants: ST JUDE MEDICAL, ST. JUDE MEDICAL AB
Application year: 1997
Main classification: A61N1
IPC codes: A61N1