Pacer with a hysteresis function

Abstract: A pacer including a hysteresis function. The extended escape interval generated after a spontaneous beat consists of the basic escape interval and the hysteresis interval. In the prior art, the hysteresis interval is of a fixed length. This is inconvinient in a rate responsive pacer where the basic rate varies between 30 to 150 bpm. The invention proposes a hysteresis interval which is a function of the variable basic escape interval. The invention is also useful in non rate responsive pacers.

Description: This invention relates to a cardiac pacer or a defibrillator with a pacing function for pacing the human heart. More particularly the invention relates to a pacer including a hysteresis function. Many pacer patients exhibit periods of normal heart sinus rhythm, where natural atrial (A) - ventricular (V) synchrony, sequential AV pumping and good hemodynamics are maintained. Should the natural heart rate drop for some reason, the depolarization of the ventricle is taken over by the pacer (single chamber or dual chamber in e.g. VVI-mode). In this situation the AV-synchrony is no longer maintained and the atrial contribution (15 - 20%) to the cardiac output is lost, viz. the pumping capacity of the heart is reduced. By creating a difference between the programmed pacer basic escape interval, viz. the interval following paced heartbeats, and the escape interval following a pacer inhibiting, sensed spontaneous (natural) heartbeat, in the sense that the last interval is extended, it is more likely that the patients with periods of normal sinus rhythm are more likely to remain in their own rhythm and thus benefit from the advantages of AV-synchrony. This difference between sensed and paced intervals is known as hysteresis. Also for other reasons than that one just described, hysteresis can be important. As the pacer is inhibited for longer periods of time, pacer longevity is increased. Further, when sinus rhythm is favoured, competition between natural and paced rhythm is reduced. Also in other modes than VVI, WT the hysteresis function can be employed for favouring the patient's own rhythm. A patient with an atrial synchronized paced as well as a dual chamber pacer operating in the AAI, AAT or DDD modes can benefit from the above advantages. In these modes, the hysteresis function is applied to the atrial escape interval. The conventional hysteresis function extends the escape interval following a sensed heart signal by introducing, as in Siemens-Elema's Pulse Generator 704, a fixed hysteresis interval. This interval could be programmable, and technically realized by programming a second (higher) trigger level into the escape interval counter. For a more detailed explanation see Siemens brochure A91003-M3372-L772-02-7600, published March 1985. With the advent of the rate responsive (RR) pacers, the fixed interval hysteresis could generate rather strange effects. In an RR-pacer the sensor signal sets the basic stimulation rate (basic escape interval) within a wide rate range, typically 30-150 beats/min (bpm), depending on the workload. At 60 bpm a 250 ms hysteresis time extends the escape interval so that the corresponding rate is 48 bpm. At 150 bpm, when the rate is increased due to increased workload, the same hysteresis time extends the escape interval so that the corresponding rate is 92 bpm. This large difference at 150 bpm between the interval after pacing and the interval after sensing is inconvient to the patient as it corresponds to a large drop in heart rate, which does not take the high work load into account. Consequently, the blood pressure goes down and the patient feels dizzy and may faint. It is an object of the present invention to provide a rate responsive pacer with an improved hysteresis function which avoids the prior art problem with a fixed hysteresis interval. The present invention is defined in claim 1. To resolve the prior art problem the present invention proposes a hysteresis interval which is a function of the basic escape interval. In this way the hysteresis is related to the actual pacing rate. As a percentage hysteresis, the hysteresis interval is the same for the whole range of rates in terms of percentage of the actual basis escape interval. For example, referring to the example given above, 250 ms at 60 bpm (1000 ms) is an extension of 25%. At 150 bpm (400 ms) the same percentage extension (25% x 400 ms = 100 ms), would result in an extended escape interval of 500 ms corresponding to a rate of 120 bpm. EP-A-313 881 which was published after the filing date of the present patent but which has an earlier effective filing date, and therefore constitutes prior art in the sense of Art. 54(3) EPC, describes a rate responsive pacemaker with hysteresis to allow natural heartbeat when possible. This hysteresis is automatically added and is related to sensed physiological need. Only basic escape interval setting is required as the hysteresis interval follows therefrom, and the invention therefore provides a simplification of the initial hysteresis interval setting. The initial setting could, of course, if necessary, be replaced by a setting selected by the physician with regard to the individual patient's needs. The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred empodiment of the invention, as illustrated in the accompanying drawings. FIG. 1 shows the invention in a schematic block diagram; FIG. 2 shows a block diagram of the time base unit in a pacer according to the invention; FIG. 3 illustrates the operation of the escape interval counter for sensed and paced heart events. In FIG. 1 a human heart which has to be paced from a pacer 1 is generally designated with 2. A pacing/sensing electrode 3 is inserted in the human heart 2 in a manner and position that the heart can most efficiently be paced. The pacing/sensing electrode 3 is connected through a pacing/sensing lead 4 with a pacing pulse generator 5. A time base unit 6 controls the pacing rate of the pacing pulse generator 5 through line 7. Heart events are sensed by a detector 8 and a body activity related variable, such as activity, temperature, blood oxygen saturation, PH-value and QT-interval, is sensed by a sensor 9. It should be understood that pacer 1 is capable of pacing/sensing in either/both atrium or/and ventricle. The time base unit 6 (FIG. 2) creates the different basic escape intervals essentially by means of a BRM (Binary Rate Multiplier) 13. Examplifying with an 8-bit BRM 13, where the highest clock rate is 512 Hz (1.953 ms) and the lowest clock rate is 2 Hz (500ms), the different basic escape intervals are calculated with these rates as a basis. The time base unit has a pacer clock which delivers pulses at 4096 Hz on line 10. A prescaler 11 reduces the pacer clock frequency to 512 Hz, which is the frequency supplied to the BRM 13 on line 12. The BRM 13 produces pulses at a rate depending on the value of a control data word (CDW) on line 18. The pulses produced by the BRM 13 are supplied on line 14 to a post-scaler 15, which counts the pulses. When the count equals 96, a stimulation pulse is produced on line 16, as see formula (1) and (2). Escape interval time = 256/X x 1.953 ms x 96 X = BRM control data word 0 < X < 256 Stimulation pulse rate = 60 x X/48 impulses/min Basic rate (imp/min) and basic escape interval (time) in milliseconds for different CDW values X are given in table 1. X Time (ms) imp/min 24 2000 30 28 1714 35 32 1500 40 36 1333 45 40 1200 50 44 1091 55 48 1000 60 52 923 65 56 857 70 60 800 75 64 750 80 68 706 85 72 667 90 76 632 95 80 600 100 84 571 105 88 545 110 92 522 115 96 500 120 100 480 125 104 462 130 108 444 135 112 429 140 116 414 145 120 400 150 124 387 155 128 375 160 132 364 165 136 353 170 140 343 175 144 333 180 148 324 185 Different hysteresis intervals can now be added to these (basic) escape intervals in that more pulses are counted by the post-scaler 15. The escape interval can be extended e.g. 10% by counting 10% more pulses, viz. 106 pulses instead of 96. The hysteresis function is activated each time a natural heart event is sensed outside the pulse generator refractory time, i.e. the reset line 17 becomes active and the post-scaler 15 takes the percentage set in hysteresis unit 19 into account. Different programmable hysteresis intervals are illustrated in table 2 and resulting rates are listed in table 3. Hysteresis (% of basic escape interval time) Number of pulses counted by the post-scaler 0 96 10 106 15 110 25 120 Rate(imp/min) Time(ms) Hysteresis (%) Time(ms) Rate (hysteresis) 30 2000 10 2200 27 15 2300 26 25 2500 24 70 850 10 942 64 15 986 61 25 1071 56 100 600 10 660 91 15 690 87 25 750 80 150 400 10 440 136 15 460 130 25 500 120 The hysteresis effect is illustrated in FIG. 3. The figure shows, for different heart events, viz. sensed natural heart events S and paced heart events P, the operation of the escape interval counter programmable at two different triggering levels, and the resulting ECG diagram (middle part of the figure) including the pacing pulses. The basic rate is 60 imp/min and the percentage hysteresis 25%, viz. the escape interval counter is triggered at two levels, the first one being 1000 ms occurring after a paced heart event P, and the second one being 125% x 1000 ms = 1250 ms occurring after a sensed natural heart event S.

Claims: A pacer (1) with a hysteresis function and comprising a pulse generator (5) controlled by a time base unit (6); said unit producing a variable basic escape interval (P-P) between pacing pulses so that the pulses are delivered at a basic rate in the absence of a sensed, pacer inhibiting heart event, said unit further including means (15, 19) for setting and adding a hysteresis interval to said basic escape interval to form an extended escape interval (S-P) in response to a sensed, pacer inhibiting, heart event, the hysteresis interval being varied automatically in direct relation to the basic escape interval length, said hysteresis interval being a fixed percentage of said basic escape interval length which is in turn variable as a function of a body activity related sensor (9) signal, wherein said basic escape interval length is generated as a pulse count of pulses outputted from a Binary Rate Multiplier (13) in response to a control data word (18), said hysteresis interval is calculated as said fixed percentage of said pulse count.

Inventors: MOBERG LENNART, MOBERG, LENNART
Applicants: ST JUDE MEDICAL, ST. JUDE MEDICAL AB
Application year: 1988
Main classification: A61N1
IPC codes: A61N1