A device for cardiac stimulation with electrotonic inhibition

Abstract: Stimulator means (21, 22; 31, 32; 71, 72) can generate electrical stimulation pulses at at least one first intensity level (21, 22; 31, 32) and at least one second intensity level (71, 72). The first and second intensity levels are above and below a given stimulation threshold, respectively. The synchronous or asynchronous delivery of second-level pulses enables the conduction of the atrioventricular node to be modulated by electrotonic effect, for example, to reduce ventricular frequency in the event of atrial fibrillation.

Description: The present invention relates in general to electrical heart-stimulation techniques. Experimental studies on animals and on man (such as those reported, for example, in the work "Electronic inhibition and active facilitation of excitability in ventricular muscle" by J.M. Davidenko, M. Delmar, and J. Beaumont in J. Cardiov. Electrophysiol. Vol. 5, No. 11 November 1994, pages 945-960), have shown the effects of subthreshold electrical stimuli on response to subsequent stimulations. These effects are referred to briefly by the term electrotonic inhibition. According to this research, the introduction of a subthreshold conditioning stimulus between two above-threshold stimulations is followed by a transitory decay in the excitability of the muscle stimulated. In particular, a subthreshold stimulus triggered a certain period of time before the subsequent above-threshold stimulus may delay the response to the subsequent stimulus in question, increasing so-called latency, that is, the interval between the pulse and the respective rapid front of the action potential, and even having an actual inhibition effect. The degree of inhibition is directly proportional to the amplitude and to the duration of the subthreshold conditioning stimulus and is inversely proportional to the post-conditioning period between the electrotonic inhibition pulse and the next stimulation pulse. Electrotonic inhibition can explain some phenomena which occur during atrial fibrillation in man. This applies in particular to the paradoxical effect of a reduction in ventricular frequency due to the action of a vagotonic drug such as digitalis (which reduces atrial refractory periods and therefore considerably increases the frequency of local atrial fibrillation) and to the effect observed, also during atrial fibrillation, of a reduction in the shortest R-R periods of the electrocardiograph signal, to the point of disappearance, when the ventricle is stimulated at longer intervals. These phenomena can be explained only by electrotonic inhibition which would act on the atrioventricular node, actually reducing its conduction rate as a result of the increased atrial fibrillation frequency and because of the reverse conduction of ventricular stimuli. In this connection, the following works may usefully be consulted: "On the mechanism(s) of atrioventricular nodal transmission in atrial fibrillation" by F.L. Meijler and J. Jalife in Cardiology 1997; 42(4); pages 375-384, and "AV nodal function during atrial fibrillation; the role of electrotonic modulation of propagation" by F.L. Meijler, J. Jalife, J. Beaumont, and D. Vaidya in J. Cardiov. Electrophysiol. Vol. 7, No. 9 September 1996, pages 843-861. Recent tests in which subthreshold stimuli were used to terminate re-entrant ventricular tachycardia, both at local level and by involving the atrioventricular node, with a view to evaluating the optimal site for performing a radio frequency ablation, should also be considered as further support for the hypothesis of the effect of a reduction in the propagation rate and of an extension of the refractory period. The following works should also be referred to in this connection: "Subthreshold conditioning stimuli prolong human ventricular refractoriness" by J.R. Windle, W.M. Miles, D.P. Zipes, and E.N. Prystowsky, in Am. J. Cardiol. 1986; 57; pages 381-6 and "Subthreshold stimulation in the region of the slow pathway during atrioventricular node re-entrant tachycardia: correlation with effect of radio-frequency catheter ablation" by S. Willems, C. Weiis, T. Hofmann, C. Rickers, and T. Meinertz in JACC Vol. 29, No. 2, February 1997, pages 408-15. Specifically, the invention relates to a heart-stimulation device having the features set forth in the preamble of claim 1, which is known, per se, from US-A- 5 083 564. The object of the present invention is to provide improved solutions which can take advantage of the above-described phenomenon, known precisely as electrotonic inhibition, to achieve an improvement in the course of the electrical heart-stimulation effect. According to the present invention, this object is achieved by means of an electrical stimulator including the features of the preamble of claim 1 and having the specific characteristics recited in the characterising portion of claim 1. In this connection, it should be noted that, in the current state of the art, only sporadic use of antitachycardia electrical stimulators temporarily programmed to deliver trains of subthreshold pulses in order to terminate re-entrant atrioventricular tachycardia are known; reference should be made in particular to the work "Ultra rapid subthreshold stimulation for termination of atrioventricular node re-entrant tachycardia" by M. Fromer and M. Shenasa, in JACC Vol. 20, No. 4, October 1992, pages 879-83. Specific applications for the control of ventricular frequency during atrial fibrillation in particular are not known. Basically, the solution according to the invention can be configured, in the currently-preferred embodiment, as a dual chamber electrical stimulator which applies the methods bringing about electrotonic inhibition. This stimulator is particularly effective for the treatment of chronic or paroxysmal atrial fibrillation in patients in whom pharmacological control of ventricular frequency is difficult and in whom it is not possible or is not intended to perform permanent ablation. The dual-chamber stimulator is of an advanced type and any currently-known solution, particularly multi-pole stimulation of the right atrium or biatrial stimulation, may be used for preventing atrial fibrillation. In a preferred embodiment, the stimulator has an associated stimulation/sensing channel which can be dedicated independently to bringing about the electrotonic inhibition effect, preferably applied to the atrioventricular node. For this purpose, an electrode located chronically in the vicinity of this node can deliver stimuli of programmable amplitude, duration and frequency when a situation of atrial fibrillation is detected. The stimuli may be asynchronous or synchronous with local electrophysiological activity, with provision for capture between two successive physiological activations. The stimulation frequency may be adjusted by means of an algorithm (known per se ) which can take account of the ventricular period. The electrotonic effect may be controlled by subthreshold pulses or pulses which are effective with reference to the arrangement of the electrode dedicated to this function. The electrode used has characteristics of low polarization and is associated with amplifier circuits which can operate with the local electrograms originated during atrial fibrillation, in which electrogram frequencies of 300-600 per minute are common. The invention will now be described, purely by way of nonlimiting example, with reference to the appended drawings, in which: Figure 1 shows, in the form of a block diagram, an electrical heart-stimulator device formed in accordance with the invention, and Figure 2 shows the structure of a portion of the device of Figure 1 in greater detail. In Figure 1, a device, generally indicated 1, is substantially comparable to a normal electrical atrioventricular stimulator and, according to the invention, has a supplementary module which can perform an electrotonic inhibition function. In particular, the set of circuit components - which can be considered generally known - for bringing about the stimulation effect at the atrial level and at the ventricular level are generally indicated 2 and 3, respectively. In particular, respective stimulation electrodes, indicated 21 and 31, are driven, in order to perform the stimulation, by respective power stages 22 and 32. These in turn are controlled by a processing and timing unit 4, by means of respective sets of lines, indicated 23 and 33, (each set usually being constituted by a synchronization or trigger line, an amplitude-control line and a duration-control line). The unit 4 usually has a telemetering module 5 with associated respective antenna means 6, to permit bidirectional remote exchange of data. In this connection, it will be appreciated that the whole device indicated 1 is configured in general in a manner such that it can be implanted in the patient's body. Each of the electrodes 21 and 31 is also connected to the unit 4 by means of a so-called "sensing" line comprising, in series, a filter 24, 34, for rejecting interference and spurious signals, and an amplifier 25, 35 the gain of which is determined selectively by the unit 4 by means of an atrial or a ventricular gain-control line 250 or 350, respectively. The sensing signal coming from each of the amplifiers 25 and 35 is compared, in a respective threshold comparator, indicated 26 or 36, with a threshold level set by the unit 4, by means of a respective line 27, 37. Finally, respective detectors 28 and 38 - of known type - can detect the value of the so-called "A" period (atrial period) and of the so-called "V" period (ventricular period) in dependence on the output signal of the threshold comparators 26 and 36, generating corresponding - usually digital - signals for transmission to the unit 4 for processing and for bringing about a feedback effect on the driving of the stimulation. The solution according to the invention is characterized by the presence of a further stimulation and sensing channel, generally indicated 7. This channel is substantially similar to the channels 2 and 3 described above. It thus comprises an electrode 71 to be implanted in the heart muscle in the vicinity of the atrioventricular sinus in order to bring about an electrotonic stimulation effect at that site, in dependence on a driving signal generated by a power stage 72. The power stage 72 is driven by the unit 4 in substantially the same manner as the stages 22 and 32, by means of a set of respective synchronizing (trigger), amplitude-control and duration-control lines 73. In particular, the unit 4 can impart to the power stage 72, by means of the lines 73, control and stimulation signals such as to give rise, by means of the electrode 71, either to isolated subthreshold signals with a frequency comparable to the heart frequency, or to very rapid and repeated pulse trains. By way of example, one of these pulse trains may be constituted, for example, by 20 pulses at a frequency corresponding to 400 pulses per minute, these trains possibly even being repeated tens of times. In one possible embodiment of the invention, the above-mentioned pulse trains have a duration of the order of about 150 ms with a pulse-train repetition frequency of about 120 bpm (beats per minute). An individual pulse typically has a duration of the order of about 1 ms, with a subthreshold amplitude, for example, of around 0.1 volt. Naturally, this amplitude value may be rendered variable selectively in dependence on specific requirements of use. The frequency of repetition of the pulses within an individual train is preferably of the order of about 120 hertz. With regard to the structure of the sensing line, the channel 7 is also structurally similar to the channels 2 and 3 described above. In particular, a filter for rejecting interference and spurious signals and an amplifier, the gain of which is controlled by the unit 4 by means of a line 750, are indicated 74 and 75. A threshold comparator in which the output signal of the amplifier 75 is compared with a threshold level transmitted by the unit 4 on a line 77 is indicated 76. The result of the comparison is transmitted to a detector 78 which identifies the duration of the so-called "AVJ" period, that is, the period of the signal detected by the electrode 71 at its implantation site, and which transmits a corresponding signal to the unit 4. The diagram of Figure 2 shows the channel 7 of the device shown in Figure 1 with even more circuit detail. In particular, in Figure 2 the same reference numerals which have already appeared in Figure 1 are used to represent elements already mentioned above. It can be seen from Figure 2 that the power stage 72 is preferably in the form of two components 721 and 722 acting as a voltage regulator and as an output switch, that is, an actual power stage, respectively. An impedance separator stage (of known type) indicated 79, has the function of permitting bidirectional connection of the electrode 71 in the direction of output of the stimulation signal from the power stage 72, and in the input direction towards the sensing channel. The detailed diagram of Figure 2 also shows that the detector 78 is preferably in the form of a counter which is enabled to count by the output signal of the comparator 76 and the count frequency of which is determined by a clock signal present on an input line 781. The same drawing also shows that the unit 4 (also with a view to possible incorporation in a device which can be implanted) is preferably in the form of a microprocessor 400 with associated read-only and random-access memories 401 and 402, respectively. Since it has been demonstrated in the past that the variability of heart frequency during atrial fibrillation is connected with the vagal tonus, studies have recently been performed (see in particular the work "Post-ganglionic vagal stimulation of the atrioventricular node reduces ventricular rate during atrial fibrillation" by S. Garrigue et al, in Pace, April 1998, Vol. 21, No. 4, Part II, page 878) to evaluate the effect in reducing ventricular frequency during atrial fibrillation (induced in animals - rabbits) of a post-ganglionic vagal stimulation directly in contact with the atrioventricular node. It has been found that post-ganglionic vagal stimulation performed, in particular, with pulse trains in the manner described above, in contact with the atrioventricular node, can achieve an effective reduction in ventricular frequency during atrial fibrillation. Naturally, the principle of the invention remaining the same, the details of construction and forms of embodiment may be varied widely with respect to those described and illustrated, without thereby departing from the scope of the present invention as defined by the following claims.

Claims: An electrical heart-stimulation device comprising: stimulator means (21, 22, 31, 32; 71, 72) for applying electrical pulses for stimulating the heart muscle, and control means (4) for selectively controlling the generation of the pulses in the stimulator means, wherein: the stimulator means comprise first stimulator means (21, 22; 31, 32) for applying electrical stimulation pulses at at least one first intensity level, as well as second stimulator means (71, 72) for applying electrical stimulation pulses at at least one second intensity level, the first and second intensity levels being above and below a given stimulation threshold, respectively, and the control means (4) are configured in a manner such as to be able to control selectively both the generation of stimulation pulses at the first intensity level and the generation of stimulation pulses at the second intensity level, characterized in that at least the second stimulator means (71, 72) have associated means (74 to 78) for sensing the activity of the heart muscle and are configured to deliver respective stimuli when a situation of atrial fibrillation is detected. A device according to Claim 1, characterized in that the first stimulator means (21, 22; 31, 32) and the second stimulator means (71, 72) are configured for bringing about stimulation of distinct regions of the heart. A device according to Claim 2, characterized in that the first stimulator means (21, 22; 31, 32) are configured for bringing about the stimulation of at least one of an atrial region and a ventricular region. A device according to any one of the preceding claims, characterized in that the second stimulator means (71, 72) are configured for bringing about the stimulation of the heart muscle in the region of the atrioventricular node. A device according to any one of the preceding claims, characterized in that the control means (4) can vary selectively, at least in the second-level stimulation pulses, at least one intensity parameter selected from the group constituted by pulse amplitude, duration and frequency. A device according to Claim 1 or Claim 5, characterized in that the second stimulator means (71, 72) are configured for generating second-level stimulation pulses in at least one of the following forms: individual pulses, and repeated pulse trains. A device according to any one of Claims 1, 5 or 6, characterized in that the second stimulator means (71, 72) are configured for generating the second-level stimulation pulses either synchronously or asynchronously with the spontaneous heart activity.

Inventors: GAGGINI GUIDO, GARBEROGLIO BRUNO, MARCELLI EMANUELA, PLICCHI GIANNI, GAGGINI, GUIDO, GARBEROGLIO, BRUNO, MARCELLI, EMANUELA, PLICCHI, GIANNI
Applicants: SORIN BIOMEDICA CRM S R L, SORIN BIOMEDICA CRM S.R.L.
Application year: 1999
Main classification: A61N1
IPC codes: A61N1