Abstract:
In a first embodiment, hysteresis is provided in a rate-responsive pacemaker to allow for natural AV synchrony when possible. In the absence of natural SA node signals, the heart is stimulated at a rate determined by the sensing of physiological need. When a natural heart signal is detected, the hysteresis is activated to extend the escape interval by a predetermined amount which is related to the sensed physiological need. The stimulating pulses are inhibited as long as normal heart activity is sensed. The extension of the escape interval under such conditions eliminates possible competition between normal activity and the paced stimulation. In a second embodiment automatic mode switching is provided in a dual chamber pacemaker to allow for more efficient operation at higher heart rates. When the heart rate (natural or paced) exceeds a prescribed level, such as 90 beats per minute, the pacemaker operates in a single chamber mode, such as VVI.

Description:
This invention relates to cardiac pacemakers and, more particularly, to an implantable, programmable, rate-responsive cardiac pacemaker having the capability of automatically switching the pacemaker&#39;s mode of operation and/or varying the hysteresis rate of the pacemaker in response to sensed patient conditions. 
     BACKGROUND OF THE INVENTION 
     The technology of cardiac pacemakers has developed to a high level of sophistication of system performance. The current generation of cardiac pacemakers incorporates microprocessors and related circuitry to sense and stimulate heart activity under a variety of physiological conditions. These pacemakers may be programmed to control the heart in correcting or compensating for various heart abnormalities which may be encountered in individual patients. A detailed description of modern cardiac pacemaker technology is set forth in International Application Number PCT/US85/02010, entitled STIMULATED HEART INTERVAL MEASUREMENT, ADAPTIVE PACER AND METHOD OF OPERATION, or U.S. Patent Application Ser. No. 887,297, filed July 18, 1986, entitled PACEMAKER HAVING PVC RESPONSE AND PMT TERMINATING FEATURES assigned to the assignee hereof. The disclosures of those applications are incorporated herein by reference. 
     It has always been thought that in order to efficiently perform its function as a pump, the heart must maintain a natural AV synchrony. The term &#34;AV synchrony 38  relates to the sequential timing relationship that exists between the contractions of the atria and the ventricles. In a given heart cycle or beat, these contractions are typically manifest or measured by sensing electrical signals or waves that are attendant with the depolarization of heart tissue, which depolarization immediately precedes (and for most purposes can be considered concurrent with) the contraction of the cardiac tissue. These signals or waves can be viewed on an electrocardiogram and include a P-wave, representing the depolarization of the atria; the QRS wave (sometimes referred to as an R-wave, the predominant wave of the group), representing the depolarization of the ventricles; and the T-wave, representing the repolarization of the ventricles. (It is noted that the atria also are repolarized, but this atrial repolarization occurs at approximately the same time as the depolarization of the ventricles; and any electrical signal generated by atrial repolarization is generally minute and is masked out by the much larger QRS-wave on the electrocardiogram.) 
     Thus it is the P-QRS-T-cycle of waves that represents the natural AV synchrony of the heart. These waves, including the time relationships that exist therebetween, are carefully studied and monitored through conventional ECG techniques whenever the operation of the heart is being examined. 
     Initiation of the cardiac cycle normally begins with depolarization of the sinoatrial (SA) node. This specialized structure is located in the upper portion of the right atrium wall. For most adults, the SA node depolarizes spontaneously at an intrinsic rate of a little better than once each second (typically about 65-70 beats per minute). The rate of depolarization and, therefore, the heart rate are influenced by various physical factors, which may produce tachycardia or bradycardia depending upon the particular patient condition. 
     Optimally, in a normal cardiac cycle and in response to the initiating SA depolarization, the atrium contracts and forces the blood that has accumulated therein into the ventricle. A short time later (a time sufficient to allow the bulk of the blood in the atrium to flow through the one-way valve into the ventricle), the ventricle contracts, forcing the blood out of the ventricle to body tissue. A typical time interval between contraction of the atrium and contraction of the ventricle might be 60 ms; a typical time interval between contraction of the ventricle and the next contraction of the atrium might be 800 ms. Thus, it is an atrial contraction (A), followed a relatively short time thereafter by a ventricle contraction (V), followed a relatively long time thereafter by the next atrial contraction, that produces the desired AV synchrony. Where AV synchrony exists, the heart functions very efficiently as a pump in delivering life-sustaining blood to body tissue; where AV synchrony is absent, the heart functions as an inefficient pump (largely because the ventricle is contracting when it is not filled with blood). 
     Multiple-mode, demand-type, cardiac pacemakers are designed, insofar as is possible, to maintain an AV synchrony for damaged or diseased hearts that are unable to do so on their own. A demand-type pacemaker is one that provides a stimulation pulse only when the heart fails to produce a natural depolarization on its own within a prescribed escape interval. In a dual chamber pacemaker, this is realized by placing electrodes in both the right atrium and right ventricle of the heart. These electrodes are coupled through intravenous and/or epicardial leads to sense amplifiers housed in an implanted pacemaker. Electrical activity occurring in these chambers can thus be sensed. When electrical activity is sensed, the pacemaker assumes that a depolarization or contraction of the indicated chamber has occurred. If no electrical activity is sensed within a prescribed time interval, typically referred to as an atrial or ventricular escape interval, then a pulse generator, also housed within the pacemaker housing, generates a stimulation pulse that is delivered to the indicated chamber, usually via the same lead or electrode as is used for sensing. This stimulation pulse causes or forces the desired depolarization and contraction of the indicated chamber to occur. Hence, by first sensing whether a natural depolarization occurs in each chamber, and second, by stimulating at controlled time intervals each chamber with an external stimulation pulse in the absence of a natural depolarization, the AV synchrony of the heart can be maintained. Thus, with a demand pacer, the heart will either beat on its own (without stimulation from the pacemaker at a rate that is at least just slightly faster than the stimulation rate defined by the escape interval), or the heart will be stimulated by the pacer at a rate controlled by the escape interval. The stimulation rate provided by the pacemaker is typically referred to as the &#34;programmed rate.&#34; 
     Unfortunately, there are many operating constraints and conditions of the heart that complicate the operation of a demand-type pacemaker. For example, there are certain time periods following a depolarization of cardiac tissue (prior to repolarization) when the application of an external electrical impulse is ineffective -- that is, it serves no useful purpose, and thus represents an unneeded expenditure of the pacemaker&#39;s limited energy. Therefore the application of stimulation pulses during these time periods is to be avoided. 
     Further, as demonstrated below, artificially maintaining AV synchrony at high heart rates (e.g., greater than 90 beats per minute) by stimulating both the atrium and the ventricle may not be an efficient way to maintain cardiac output. That is, stimulating the atrium at these high rates may also represent an unneeded expenditure of the pacemaker&#39;s limited energy. 
     Rate responsive pacemakers employ some type of physiological sensor for sensing a change in the metabolic needs of a patient. This sensed change, in turn, is used to adjust the rate at which stimulation pulses are delivered to the heart of the patient by the pacemaker. Thus, as the metabolic needs of the patient increase -- indicating a need for the heart to beat faster -- the rate at which the pacemaker stimulates the heart is increased as a function of this sensed increase in metabolic need. As the metabolic needs of the patient decrease -- indicating a need for the heart to beat slower -- the rate at which the pacemaker stimulates the heart is correspondingly decreased. 
     In a demand pacer, the physiological sensor (which may be one of numerous types) adjusts the pacing rate by adjusting the escape interval of the pacer. As the escape interval is thus adjusted as a function of sensed physiological need, the rate at which stimulation pulses are provided to the heart -- and hence the heart rate -- is correspondingly varied as a function of sensed physiological need. 
     Rate-responsive demand pacers may be either single chamber pacers that sense and stimulate in the ventricle (e.g., a VVI mode of operation) at a rate determined by the particular physiological sensor used, or dual chamber pacers that sense and stimulate in both the atrium and the ventricle (e.g., a DDD mode of operation) at a rate determined by the physiological sensor. Patients who are candidates for single chamber rate-responsive pacing usually include patients exhibiting partial or complete heart block. When heart block exists, the ventricle does not consistently follow the atrium, and the needed and desired AV synchrony is lost. Patients who are candidates for dual chamber rate-responsive pacing include this same group of patients (who are candidates for single chamber pacing) plus patients whose atrial contractions are irregular or intermittent. 
     Heart block, for purposes of this disclosure, means that the stimulus from the SA node -- the heart&#39;s natural pacemaker -- is unable to travel to the ventricle to stimulate the ventricle at the appropriate time, i.e., the heart&#39;s anteograde conduction path is somehow blocked at least some of the time. 
     A dual chamber rate-responsive pacer advantageously allows both the atrium and/or the ventricle to be stimulated at a rate commensurate with the sensed physiological need despite an irregular, intermittent, or non-functioning S-A node. Disadvantageously, operation of a dual chamber pacemaker, when providing stimulation pulses to both the atrium and the ventricle, expends significantly more power than a single chamber pacemaker, thereby shortening the useful life of the pacemaker&#39;s batteries. As indicated more fully below, while dual chamber rate-responsive pacing may be very beneficial at lower heart rates, single chamber rate-responsive pacing may be more than adequate to maintain cardiac output at higher heart rates. Thus, what is needed is a dual chamber rate-responsive pacer that conserves power by automatically switching to a single chamber mode of operation at higher heart rates. 
     A single chamber rate-responsive pacer (or a dual chamber pacer operating in a single chamber mode) advantageously allows the ventricle to be stimulated at a rate commensurate with the sensed physiological need despite a completely, intermittently, or partially blocked anteograde conduction path. 
     It has recently been discovered that many patients who exhibit partial, intermittent, or complete heart block at normal heart rates, e.g., 70 beats per minute (bpm), will exhibit normal anteograde conduction at higher rates, e.g., 110-120 bpm. Thus, if these patients are fitted with a conventional VVI pacer, or a dual chamber pacer programmed to operate in a VVI mode, such pacer provides ventricular stimulation as required at normal heart rates as defined by the pacemaker&#39;s programmed rate. Disadvantageously, however, natural AV synchrony is lost whenever the pacer provides a stimulation pulse to the ventricle. If this patient (with a conventional VVI pacer and exhibiting partial, complete or intermittent heart block only at the lower normal heart rates) exercises, and assuming the patient&#39;s SA node functions normally, the SA node attempts to make the heart beat faster as the physiological needs resulting from the exercise increase. As long as heart block exists, however, such attempts are ineffective, and the pacer will continue to provide ventricular stimulation pulses at the programmed rate. At some point (which will vary from patient to patient), as the patient continues to exercise, the natural conduction path is restored, and the ventricle is stimulated from the SA node (i.e., heart block no longer is present), and natural AV synchrony is advantageously restored. The result is that the patient, having his or her natural AV synchrony restored, feels great. 
     After exercise, when the heart beat rate returns to normal levels, heart block returns, and the VVI pacer again takes over stimulating the ventricle at the programmed rate. Natural AV synchrony is lost. The patient typically feels all right, but not as good as when natural AV synchrony was present. 
     If a rate-responsive VVI pacemaker is employed, or a rate-responsive dual chamber pacemaker is programmed, or otherwise switched, to operate in the VVI mode, the physiological sensor used therewith senses the increased physiological need brought on by the patient&#39;s exercise. This causes the pacing interval (referred to herein as the escape interval) of the rate-responsive pacemaker to be adjusted accordingly. As long as heart block exists, this presents no problem (and, in fact, the rate-responsive pacemaker continues to perform its intended function). However, should heart block cease, then the ventricle is stimulated from the SA node through the natural anteograde conduction path and AV synchrony should, in theory, return. Unfortunately, because the basic pacing or escape interval of the rate-responsive pacer is also changing (being adjusted in accordance with the sensed physiological need), it is possible and quite probable that competition will exist between the SA node and the rate-responsive pacemaker. Such competition may result when the programmed rate of change of the VVI pacemaker does not match the rate of change of the heart&#39;s SA node. Thus, an R-wave may not be sensed because it does not fall within a shortened escape interval of the rate-responsive pacemaker. Conversely, an R-wave may not be sensed because it occurs prior to the termination of a pacemaker-defined refractory period. In either event, AV synchrony can be lost. 
     What is needed, therefore, is a rate-responsive pacemaker that prevents competition between the rate-responsive pacemaker and the heart&#39;s SA node should the anteograde conduction path be restored. Such a rate-responsive pacemaker is realized, according to the teachings of the present invention by providing hysteresis. A programmable cardiac pacemaker with hysteresis is disclosed in U.S. Pat. No. 4,263,915 (McDonald et al). As indicated in that patent, the concept of hysteresis as a technique of cardiac pacing is well-known in the prior art. According to the patent disclosure, the hysteresis concept is introduced into a pacemaker which is generating artificial stimulating pulses at a constant rate. However, the disclosure of that patent does not extend to the provision in the present invention of utilizing the hysteresis concept in a pacemaker of the rate-responsive type. 
     SUMMARY OF THE INVENTION 
     The present invention relates to two features of a rate-responsive pacemaker: (1) automatic mode switching of a dual chamber pacer to a single chamber pacer at heart rates exceeding a prescribed heart rate threshold; and (2) the inclusion of hysteresis with rate-responsive pacing. Advantageously, the first feature conserves the limited energy of the pacemaker&#39;s battery. The second feature allows the physiological sensor of the rate-responsive pacemaker to vary the pacing interval as a function of sensed physiological need, and also allows the heart&#39;s SA node to take over should a heart block condition cease. As a variation of the second feature, the present invention also provides for varying the hysteresis rate as a function of the sensor rate. The two features of the present invention will be described independent of each other; but it is to be understood that the two features could be combined within the same rate-responsive pacer. 
     Before proceeding with a more detailed explanation of the present invention, it will be helpful to understand the following definitions: 
     Intrinsic Rhythm: The &#34;intrinsic rhythm&#34; or &#34;intrinsic rate&#34; of the heart is that rate at which the heart naturally beats on its own, without being stimulated by a pacemaker-provided stimulus. 
     Sensor Rate: As used herein, &#34;sensor rate&#34; refers to the rate at which the physiological sensor, whatever that sensor might be, indicates the heart should beat. The sensor rate may be considered equivalent to the programmed rate of a non-rate-responsive pacemaker. For example, the sensor rate might be 70 bpm when the patient is at rest. If increased physiological activity is sensed by the sensor, the sensor rate increases appropriately. 
     Hysteresis: As used herein &#34;hysteresis&#34; means extension of the range of rates at which inhibition of pacer pulses will occur below the sensor rate by an amount equivalent to the basic pacing interval (as defined by the sensor rate) plus the amount of the hysteresis interval. For example, if the basic sensor rate is 70 bpm (pacing interval of 857 ms) and a hysteresis interval of 300 ms is added, the total pacing interval increases to 1157 ms., which is equal to a rate of 52 bpm. When set this way, once the pacemaker is inhibited by intrinsic rhythm above a rate of 70 bpm it will remain inhibited until the intrinsic rhythm drops below 52 bpm. When this happens, pacing will begin at 70 bpm. Thus, hysteresis provides a longer escape interval, thereby giving the heart more of an opportunity to beat on its own before the pacer will &#34;step in&#34; and provide stimulation pulses. 
     Hysteresis Rate: As used herein, &#34;hysteresis rate&#34; is that rate below the &#34;sensor rate&#34; to which the intrinsic rhythm must drop before the pacer will provide a pacer stimulation pulse. For example, if the sensor rate is 90 bpm and the hysteresis rate is 20 bpm, the intrinsic heart rate would have to drop to 70 bpm before the pacer would start providing stimulation pulses (which pulses would be provided at the sensor rate). Keep in mind that although hysteresis rate is expressed in &#34;beats per minute,&#34; hysteresis is realized by changing the pacing interval of the pacemaker, as explained above. 
     The first feature of the present invention is operable whenever the rate-responsive pacemaker is set to operate in a dual chamber mode (e.g., DDD). This feature incorporates a rate threshold detector to determine when the heart&#39;s intrinsic rhythm (or the sensor rate, if the heart is unable to maintain a rhythm on its own) exceeds a prescribed heart rate threshold. If such a threshold is exceeded, the pacemaker automatically reverts to a single chamber mode of operation, such as VVI, for as long as the heart rate exceeds the prescribed threshold. 
     Arrangements in accordance with the second feature of the present invention incorporate hysteresis in a rate-responsive pacemaker by varying the escape interval of the system to a predetermined level upon sensing of a natural heart contraction during the escape interval. Classical rate-responsive theory teaches away from using hysteresis because such theory teaches that the physiological sensor must be maintained as the key rate-determining element of the rate-responsive pacer. Otherwise, the pacemaker ceases to be rate-responsive. In contrast, employing hysteresis in a rate-responsive pacer, as in accordance with the present invention, in effect removes control from the physiological sensor for longer periods of time. As a beneficial result of this feature, the patient enjoys the improved physiological condition associated with the restoration of natural AV synchrony. Furthermore, since stimulation pulses are inhibited under these conditions, the energy drain on the pacer power source is reduced and a longer life of the power source is realized. The concept of providing hysteresis as disclosed in the above-mentioned &#39;915 McDonald patent is entirely foreign to the present invention. Since the principal feature of the present invention is applicable to a rate-responsive pacemaker in which the sensor rate varies depending upon the need determined by the physiological sensor, arrangements in accordance with a further aspect of the present invention provide for varying the escape interval by differing amounts within a range between a minimum and maximum hysteresis which is related to the range of variation of the sensor rate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will be more apparent from the following more particular description thereof presented in conjunction with the accompanying drawings, wherein: 
     FIG. 1A is a schematic diagram of a dual chamber programmable system as conventionally implanted in a patient and having a physiological sensor associated therewith; 
     FIG. 1B is a schematic block diagram representing one particular arrangement in accordance with the first feature of the invention for developing automatic mode switching; 
     FIG. 1C is a flow chart depicting the main program steps included in the programmable pacemaker of FIG. lA and FIG. 1B; 
     FIG. 2 is a schematic diagram of a single chamber pacemaker system as conventionally implanted in a patient and having a physiological sensor associated therewith; 
     FIG. 3A is a graph illustrating the variation of paced rate with changes in output from the physiological sensor in the system of FIG. 2; 
     FIG. 3B is a graph illustrating the addition of hysteresis to paced rate as a function of physiological sensor output in accordance with one particular aspect of the invention; 
     FIG. 4 is a graph illustrating the addition of hysteresis to paced rate as a function of physiological sensor output in accordance with an alternative aspect of the invention; 
     FIG. 5 is a schematic block diagram representing one particular arrangement in accordance with the invention for developing the operation depicted in FIGS. 3 and 4; and 
     FIG. 6 is a schematic block diagram similar to FIG. 5, but further includes elements for developing a dual chamber operation of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best presently contemplated mode of carrying out the invention. This description is not to be taken in a limiting sense but is made for the purpose of describing the general principles of the invention. The scope of the invention should be determined with reference to the appended claims. 
     As reference is made to the drawings, like numerals will be used to refer to like parts or elements throughout. 
     As indicated previously, the present invention is directed to two features that may be included in a rate-responsive pacemaker. These features are independent of each other, and for the sake of simplicity and the following explanation, will be described independent of each other. Nevertheless, it is to be understood, that those skilled in the art could readily combine these two features, as independently described, within the same rate-responsive pacemaker. These two features are referred to herein as (1) automatic mode switching; and (2) variable hysteresis. 
     1. Automatic Mode Switching 
     The automatic mode switching feature of the present invention is intended for use with a dual chamber rate-responsive pacemaker. According to this feature of the invention, the pacemaker automatically reverts to a single chamber mode of operation whenever the heart rate exceeds a prescribed threshold. That is, while it is well recognized in the art that dual chamber rate-responsive pacing is an optimum mode of operation, such a mode of operation results in high battery current drain at fast pacing rates. Recent studies performed by at least one of the inventors of this application have shown that dual chamber rate-responsive pacing at high &#34;exercise&#34; heart rates (e.g., rates greater than 90 bpm) may be unnecessary. Thus, the first feature of the present invention provides a dual chamber rate-responsive pacemaker at low heart rates that automatically switches to a single chamber rate-responsive pacemaker at high &#34;exercise&#34; rates. 
     Referring to FIG. 1A, there is shown a schematic diagram of a dual chamber programmable pacemaker 70 as conventionally implanted in a patient. Included in FIG. 1A is the patient&#39;s heart 16, having a right atrium 19 and a right ventricle 14. An atrial pacing lead 72 passes into the right atrium 19 where an electrode tip 74 makes contact with the heart tissue. Similarly, a ventricular lead 76 passes through the right atrium 19 into the right ventricle 14, whereat an electrode tip 78 makes contact with the apex of the right ventricle 14. As shown in FIG. 1A, the leads 72 and 76 are unipolar leads, however, it is to be understood that bipolar leads could also be used. In addition, a physiological sensor 24 of any appropriate type known in the art is shown with an indicated output electrically connected to the dual chamber pacemaker 70. The manner in which the leads 72 and 76 are inserted into the heart, as well as the manner in which the pacemaker 70 and physiological sensor 24 are implanted in the patient, are well known in the art. 
     Referring next to FIG. 1B, a block diagram representing one particular arrangement in accordance with the automatic mode switching feature of the present invention is shown. A programmable pacemaker typically includes a programmable memory 80 in which various operating control words are programmably loaded. Such a memory is shown, for example, in U.S. Pat. No. 4,232,679, entitled PROGRAMMABLE HUMAN TISSUE STIMULATOR which patent is incorporated herein by reference. Of relevance to the present feature of the invention herein disclosed are two such control parameters: An operating mode control word 82, and a rate threshold control word 84. Using convention methods and circuits known in the art, the operating mode word 82 and the rate threshold word 84 are initially programmed into the memory 80 of the pacemaker 70. 
     Still referring to FIG. 1B, a sensed R-Wave (or other signal indicating a contraction of the heart) is directed to a heart rate detector 86. The heart rate detector 86 is of conventional design and includes a desired smoothing or averaging algorithm or other process steps in order to produce an output signal, on signal line 88, representative of the heart rate over the past few heart cycles. For example, a common type of heart rate detector 86 employs a moving average algorithm that determines the average heart rate over the past n heart cycles, where n is a relatively small integer, such as 5. The output of the heart rate detector 86, on signal line 88, is then compared with the rate threshold word 84 previously programmed in the memory 80. This comparison is done by conventional rate compare circuitry or software 90. Thus using conventional methods known in the art, a decision is made by rate compare circuitry 90 as to whether the detected heart rate, obtained from heart rate detector 86, is greater than the rate threshold word 84. If so, mode control logic 92 is enabled and causes a new operating mode word 82&#39;  to be written into the programmable memory 80. In accordance with the teachings of the present invention, this new operating mode word 82&#39; causes the pacemaker 70 to operate in a single chamber mode of operation, as opposed to the dual chamber mode of operation in which it was previously operating. Should the detected heart rate subsequently go lower than the rate threshold word 84, then the mode control logic 92 causes the prior operating mode word 82 to be rewritten into the memory 80. This causes the pacemaker to revert to the prior dual chamber mode of operation. 
     Typically, the rate-responsive pacemaker 70 will include a microprocessor or equivalent circuitry in order to carry out its desired functions. As such, the implementation of the automatic mode switching feature of the present invention is readily carried out by incorporating a simple routine within the controlling programs of the pacemaker as summarized in the flow diagram of FIG. 1C. Referring to FIG. 1C, for example, it is seen that if a dual chamber mode of operation has been programmed, such as the DDD mode of operation, block 94, then the heart rate is determined at block 96. Based on the determined heart rate, a moving average is computed over the last n heart cycles, as indicated at block 98. Based on this moving average, the rate threshold word 84 stored in the memory 80, is compared to the moving average at block 100. A determination is then made at decision block 102 as to whether the rate threshold exceeds the average heart rate. If so, then a single chamber operating mode control word 82&#39; is written in memory 80 at the location where the dual chamber operating mode control word 82 was stored. For the example shown in FIG. 1C, this single chamber mode of operation is indicated as the VVI mode. If the average heart rate is not greater than the threshold, as determined at decision block 102, then the dual chamber control word 82 is maintained in the memory 80 at the designated operating mode control word location. 
     Thus, in the manner described, the pacemaker can be programmed to operate in a dual chamber mode of operation at low heart rates, but the mode operation automatically switches to a single chamber mode of operation at higher paced rates. Typically, the rate threshold above which the heart rate must reach before the single chamber mode of operation is enabled is on the order of 90 bpm. Advantageously, however, this rate threshold value can be selected and pre-programmed as desired using conventional pacemaker programming techniques. 
     To confirm that dual chamber rate-responsive pacing may be unnecessary at high heart rates, a study was conducted of eight patients, ages 24-64 years, each having a programmable dual chamber pacemaker implanted, and each exhibiting heart block in sinus rhythm, were maximally exercised in three modes: DDD, VVT/RR (rate-responsive), and DDD (low rate)/VVT-RR (rates greater than 89 bpm). Exercise duration, work, blood pressure, pulse, oxygen uptake, anaerobic threshold (AT), and oxygen pulse were measured. There was no difference in any mode in symptoms or physiological indices. That is, the results indicated that ventricular rate-responsive pacing at high rates produced similar benefits as did dual chamber rate-responsive DDD pacing. Hence, the study suggests that dual chamber rate-responsive pacing may be unnecessary at high heart rates. 
     2. Variable Hysteresis 
     Referring next to FIG. 2, there is shown a simplified representation of one way that an implanted single chamber pacemaker 10 may make electrical contact with the heart. As shown in FIG. 2, the pacemaker 10 is a typical rate-responsive pacemaker having a bipolar lead 12 extending through the right atrium 19 into the right ventricle 14 of a heart 16. The SA node 17 is shown in the atrium 19. The bipolar lead 12 includes two electrically insulated conductors 18, 20. The first, inner conductor 18 is electrically connected to a distal tip 21 of the lead. This distal tip is typically placed in the apex of the right ventricle 14. A known distance from the distal tip 21 an electrode ring 22 is electrically connected to the other conductor 20 of the bipolar lead 14. (Although a bipolar lead is illustrated in FIG. 2, it is understood that a unipolar lead could likewise be used.) In addition, a physiological sensor 24 of any appropriate type known in the art is shown with an indicated output to the pacemaker 10. The manner in which the bipolar lead 12 is inserted into the heart, as well as the manner in which the pacemaker 10 and physiological sensor 24 are implanted in the body of a patient, is well known in the art. 
     The graph of FIG. 3A shows the classical transfer curve or characteristics of a typical rate-responsive pacer, such as the pacemaker 10 of FIG. 1. The physiological sensor output is the horizontal axis and the paced rate is the vertical axis. If the physiological sensor output indicates low-level physiological activity below a level P0, the paced rate is maintained at a minimum rate (which, for the example shown in FIG. 2 is 70 bpm). Similarly, if the physiological sensor output indicates high-level physiological activity above a level P2, the paced rate is maintained at a maximum rate (e.g., 150 bpm). If, however, the physiological sensor output indicates intermediate physiological activity, between the points P0 and P2, the paced rate varies as a function of the sensor output. For example, for the relationship shown in FIG. 3A, a sensor output of P1 causes a paced rate of 110 bpm. 
     The &#34;paced rate&#34; is the rate at which the stimulation pulses are provided to the heart from the pacemaker. This is typically the same as the &#34;sensor rate&#34;, but may not always be the same, especially if hysteresis is used. 
     It is noted that in FIG. 3A, the relationship between sensor output and paced rate is shown as being linear. This relationship could, of course, be other than linear if desired. 
     Referring next to FIG. 3B, a transfer curve is shown for a rate-responsive pacer that includes hysteresis, in accordance with the present invention. Curve A shows the same classical rate-responsive relationship between sensor output and paced rate as discussed above in connection with FIG. 3A. This curve is modified, however, in accordance with the teachings of the present invention, by the inclusion of a hysteresis curve B. 
     From FIG. 3B, it is seen that at the low-level physiological activity range, i.e., where the sensor output is below P0, the hysteresis rate is established at a minimum value of 10 bpm (meaning that, as explained above in the definitions, the intrinsic rate would have to drop to 60 bpm -- 10 bpm below the minimum paced rate of 70 bpm -- before the pacer steps in to provide a stimulation pulse). The corresponding escape interval is 500 ms. 
     In the intermediate physiological activity range, where the sensor output is between P0 and P2, the hysteresis rate is established at a value that varies between the minimum and maximum hysteresis rates as defined by curve B of FIG. 3B. For example, at a sensor output of Pl, an intermediate hysteresis rate of 20 bpm is shown. This means that at a physiological activity level producing a sensor output of P1, the intrinsic rate would have to drop to 90 bpm (escape interval of 667 ms.) -- 20 bpm below the sensor-defined paced rate of 110 bpm -- before the pacer is allowed to step in and provide stimulation pulses. 
     The relationship between the hysteresis rate and the sensor output within the intermediate range of sensor outputs, i.e., between points P0 and P2, of FIG. 3 is depicted as being linear. However, this is only exemplary, and it will be understood that any desired relationship could be established. 
     Of course, it is understood that FIG. 3B is only representative of one of an almost limitless number of possible arrangements that could be employed to combine hysteresis with rate-responsive pacing. For example, the minimum hysteresis rate could be set (programmed) to be zero at 70 bpm (escape interval=857 ms.), and the maximum hysteresis rate could be set to be 40 bpm at 150 bpm (escape interval=500 ms), in which case FIG. 4 would depict the relationship between the various parameters. 
     More significant than the mere implementation of hysteresis with rate-responsive pacing, however, is the result that such implementation provides. For the patient with intermittent heart block, whose natural conduction returns at higher intrinsic rates, a rate-responsive pacer with hysteresis as herein described allows the SA node to take total control of the heart at the higher intrinsic rates without any possible competition from the rate-responsive pacer. The increased hysteresis rate provided by adding hysteresis as above described inhibits any pacing pulses for a longer proportion of the pacing interval, and thus provides the SA node a longer portion of the normal heart cycle at elevated heart rates within which to control the beating of the heart. Furthermore, this relatively longer interval allows for any mismatch that may exist between the paced rate (as defined by the physiological sensor) and the intrinsic rate (as defined by the SA node), thereby significantly reducing the possibility of any competition between these rates. In addition, by varying the hysteresis rate as a function of the physiological sensor output as described above, an extra margin of operating range for the SA node is obtained at the higher heart rates. 
     Referring next to FIG. 5, there is shown a schematic block diagram representing one particular arrangement in accordance with the variable hysteresis feature of the invention for developing the operation depicted in FIGS. 3 and 4. More particularly, FIG. 5 is a block diagram of a single chamber rate-responsive pacemaker that includes a variable hysteresis feature. Shown in FIG. 5 is a pulse amplifier 40 that delivers a stimulation pulse to the ventricle of the heart over the lead 12. Electrically coupled to the lead 12 is a sense amplifier 50. The output of the sense amplifier 50 is connected to a pulse generator 52. The pulse amplifier 40, sense amplifier 50, and pulse generator 52 are of conventional design. 
     The pulse generator 52 generates output pulses at a rate controlled by a rate control signal appearing on signal line 54. This rate control signal is developed by rate/hysteresis control logic 56. In practice, and in accordance with conventional demand pacemaker operation, it is understood that the rate control signal 54 is essentially an escape interval or period of time which must time out before the pulse generator 52 is allowed to generate a pulse that is amplified by the amplifier 40 for delivery to the heart. However, the escape interval generated by the rate/hysteresis control logic 56 varies as a function of one of two inputs. A first input is received from sensor logic 58. It is the function of sensor logic 58 to generate the transfer function or characteristic of the type shown in FIGS. 3 and 4. That is, the sensor logic 58 develops an output signal (the vertical axis of FIG. 3A), as a function of the sensor input signal (the horizontal axis of FIG. 3A) received from the physiological sensor 24. 
     The other input to the rate/hysteresis control logic 56 is a logic signal derived from a flip-flop 60, or equivalent device, that indicates whether the regular, or the hysteresis, escape interval is to be used. If a stimulation pulse has been generated by the pulse generator 52 (indicating that the prior escape interval has timed out), then flip-flop 60 is RESET, thereby signaling the control logic 56 that the standard or regular interval is to be used. Escape interval is to be added to the regular escape interval, thereby extending the total escape interval the desired amount. If, however, intrinsic activity of the heart is sensed by sense amplifier 50, then flip-flop 60 is SET, indicating that the hysteresis escape interval is to be added to the regular escape interval, thereby extending the total escape interval the desired amount. Reset, thereby signaling the control logic 56 that the standard or regular escape interval is to be used. Further, the sensing of intrinsic activity by the sense amplifier 50 causes the rate/hysteresis control logic 56 to reset the escape interval. 
     The sensor logic 58 is realized using conventional logic circuits and/or software so as to realize the desired transfer function between the sensor 24 and the desired paced rate, as indicated in FIGS. 3 and 4. In a preferred embodiment, the sensor logic 58 is simply a look-up table wherein the desired transfer relationship (FIGS. 3 or 4) is pre-programmed into appropriate memory locations and the occurrence of one value as the sensor input causes the desired rate output signal to be generated based on conventional table-lookup techniques. 
     The rate/hysteresis control logic 56 is also of conventional design. An appropriate look-up table, and/or algorithm, allows the appropriate escape interval to be readily generated as a function of the two inputs above described. 
     Those skilled in the art will readily recognize that FIG. 5 is a simplified diagram of a programmable pacemaker. Numerous other details, such as generating refractory periods, blanking intervals, and the like, must be included within the design of a conventional pacemaker. However, such details are known in the art, and are not relevant to the present invention, and will therefore not be repeated herein. Reference can be made to the prior-cited applications or patents for such details, as well as to U.S. Pat. No. 4,590,944, which patent is also incorporated herein by reference. 
     Referring next to FIG. 6, the block diagram of a dual chamber pacemaker is shown that incorporates the variable hysteresis feature of the present invention. The pulse amplifier 40, sense amplifier 50, pulse generator 52, rate/hysteresis control logic 56, sensor logic 58, flip-flop 60, and sensor 24 function as above-described in connection with FIG. 5. To these elements an additional pulse amplifier 42 is connected to the pulse generator 52 for generating and delivering appropriate stimulation pulses to the atrium of the heart over an atrial lead 72. Similarly, the pulse amplifier 40 delivers stimulation pulses, or senses activity, from or to the ventricle of the heart over ventricular lead 76. Also, in FIG. 6, the output of the pulse generator 52 is connected to an A-V delay circuit 62. It is the function of the A-V delay circuit 62 to generate an input pulse for the ventricular pulse amplifier 40 immediately after the timing out of a prescribed atrialventricular (A-V) delay interval, which interval begins with the termination of the escape interval 54. If, during the A-V interval ventricular activity is sensed by the sense amplifier 50, the A-V delay circuit is reset by the output of the sense amplifier 50 in conventional manner. 
     As with FIG. 5, those skilled in the art will appreciate that FIG. 6 is a simplified diagram of a dual chamber pacemaker. Many elements are not shown in FIG. 6, such as an atrial sense amplifier, blanking circuitry, and the like, form an important part of a dual chamber pacemaker. However, these features and elements are known in the art, see previously referenced patents or applications and are not believed necessary to understand the present invention. 
     Although there have been described above specific arrangements of a rate-responsive pacemaker with automatic mode switching and/or variable hysteresis rate in accordance with the invention for the purpose of illustrating the manner in which the invention may be used to advantage, it will be appreciated that the invention is not limited thereto. Accordingly, any and all modifications, variations or equivalent arrangements which may occur to those skilled in the art should be considered to be within the scope of the invention as defined in the annexed claims.