CA1330361C - Demand pacemaker using an artificial baroreceptor reflex - Google Patents
Demand pacemaker using an artificial baroreceptor reflexInfo
- Publication number
- CA1330361C CA1330361C CA000571785A CA571785A CA1330361C CA 1330361 C CA1330361 C CA 1330361C CA 000571785 A CA000571785 A CA 000571785A CA 571785 A CA571785 A CA 571785A CA 1330361 C CA1330361 C CA 1330361C
- Authority
- CA
- Canada
- Prior art keywords
- rate
- cardiac pacemaker
- heart
- response
- pulse generator
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Fee Related
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Classifications
-
- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61N—ELECTROTHERAPY; MAGNETOTHERAPY; RADIATION THERAPY; ULTRASOUND THERAPY
- A61N1/00—Electrotherapy; Circuits therefor
- A61N1/18—Applying electric currents by contact electrodes
- A61N1/32—Applying electric currents by contact electrodes alternating or intermittent currents
- A61N1/36—Applying electric currents by contact electrodes alternating or intermittent currents for stimulation
- A61N1/362—Heart stimulators
- A61N1/365—Heart stimulators controlled by a physiological parameter, e.g. heart potential
- A61N1/36514—Heart stimulators controlled by a physiological parameter, e.g. heart potential controlled by a physiological quantity other than heart potential, e.g. blood pressure
- A61N1/36564—Heart stimulators controlled by a physiological parameter, e.g. heart potential controlled by a physiological quantity other than heart potential, e.g. blood pressure controlled by blood pressure
Abstract
DEMAND PACEMAKER USING AN ARTIFICIAL BARORECEPTOR RESPONSE
ABSTRACT OF THE DISCLOSURE
A device for use in conjunction with a pulse generator is disclosed which provides a variable pulsing rate in response short term to variations in arterial blood pressure, like the baroreceptor system of the healthy body's cardiovascular system.
The system utilizes a pressure transducer implanted together with the pacemaker, which transducer is preferably located on the proximal axillary artery. The system features quick response based on physiological need, and also includes a reset feature which maintains a normal resting heart rate despite long term changes in blood pressure.
ABSTRACT OF THE DISCLOSURE
A device for use in conjunction with a pulse generator is disclosed which provides a variable pulsing rate in response short term to variations in arterial blood pressure, like the baroreceptor system of the healthy body's cardiovascular system.
The system utilizes a pressure transducer implanted together with the pacemaker, which transducer is preferably located on the proximal axillary artery. The system features quick response based on physiological need, and also includes a reset feature which maintains a normal resting heart rate despite long term changes in blood pressure.
Description
DEMAND PACEMAKER USING AN ARTIFICIAL BARORECEPTOR RESPONSE
BACKGROUND OF THE INVENTION
Field of the Invention - The present invention relate~
generally to a rate responsive cardiac pacemaker, and more particularly to a pacemake~ having a variable rate which is responsive to arterial pressure, thereby causing the cardiac rate to closely mimic the natural baroreceptor reflex response pattern of the heart to changing physiological need.
The cardiac pacemaker is perhaps one of the best known electronic marvels of modern medicine, and the implantation o~ a pacemaker in a patient has become almost a routine operàtion.
The small, electronic device pulses the heart of the patient contlnuously over an extended period of time, or, in the case of demand pacemakers, monitors the heart's natural operation and provides stimulating pulses only when the the heart skips a beat.
Pacemakers allow patients with heart problems which would have been either fatal or incapacitating without a pacemaker to resume relatively normal lives.
It will be realized by those skilled in the art that the modern pacemaker i9 a highly complex deviceJ capable of event sensing, two-way telemetry, and sensing and paoing in either or both of the atrium and the ventricle o~ the heart. Such pacemakers may be ~inely tuned by the physician subsequent to implant, and the parameters tweaked to result in optimum pacing performance.
Despite the impressive sophistication of such pacemakers, they represent a compromise due to a single ma~or difference bstt~een the he~lthy heart and a pacsd heart- namely the response to activity, exercis`e, or ~tre6s. A healthy heart is rate responsive to a number o~ ~actors including physical aativity or exercise. Variations in the cardiaa stroke volume and ~ystemic vascular resiatance occur inl the cardiovascular ~ystem due to ~ '''.
-1. .
physiological stresses such as exercise, temperature changes, postural changes, emotlon, hypoglycemia, Valsalva maneuvers, eta.
To maintain adequate perfusion pressure and cardiac output under the~e ~tresses, it iB neces6ary to ad~ust heart rate. The healthy heart may beat at ~0 or fewer beats per minute during repose or sleep, and at 120 or more bea~s per minute dur~ng strenuous exercise, for example. The heart paced by a pacemaker which i8 non-rate responsive will typically beat at a constant rate of approximately 70 beats per minute.
It wlll be appreciated that the paced heart will supply more blood than is needed during sleep, and may even prevent the patient from sleeping restfully. Even more seriously, patients paced at 70 beats per minute experience substantial difficulty in engaging in strenuous activity. A moderate level of activity lS such as walking will cause difficulty in some patients. It is apparent that a pacemaker whlch varies in response to physiological need xepresents a highly desirable device which wlll enable a normal active life ~or patients xequiring a pacemaker~
Physiological re~ponsive cardia¢ pacing must optimize cardiac rate to the level o~ metabolic need in the absence of normal variable cardiac rate. The slmplest answer to this problem i8 atrial tracking pacing, wherQ the patient has a full or partial AV block and a dual chamber pacemaker pulses the ventricle in re~ponsQ to normal cardiac activ~ty sensed in the atrium. However, thi~ techni~ue i8 not possiblQ in many patients with sinus bradycardia or atrial fibrillation, and so rate responsivQ pacing ~s nece~sary to mimic the normal variable cardiac rate.
A varlety o~ physiological respon~lve pacing systems have been proposed, with the sy6tems using a variety o~ physiological parameters as the basi~ for varying cardiac rate. These parameters include blood temperature, various sensed timing 1 ~3036 1 signals from the heart, blood chemistry, respiratory rate, nervous system activity, physical activity, and pressure measured wi~hin the heart at various location~. These systems will be discussed briefly below, and the problems inherent in each of the systems will become evident.
Venous blood temperature is measured in the right ventricle by Cook et al. in U.S. Patent No. 4,436,092. Sin~e blood temperature has been found to ri~e duriny exercise and the corresponding body core temperature increase~ blood tsmperature indicates greater physiological need for blood supply. However, the response of ~uch a ~ystem in qu$te slow. In addition, the ~ystem i8 lnexact due to the coarsene~ at which measuremant~ may be taken, the ingestion of cold liquids, and the effeat caused by presence of a fever.
~oth the QT interval and the P wave have been used to vary heart rate. The use of the QT interval is discussed in U.S.
Patent No. 4,228,803, to Rickards, and involve~ detection o~ ~he repolarization ~ wave subsequent to pacemaker ~timulation (indicating the Q wave). A shorter QT interval is used to produce a higher paced cardiac rate. This system is slow in response, and not highly specific due to variations caused both by drugs ingested and by the used of pacemaker stimulation rather than using ~ensed contractions.
The use of the P wave i5 taught in U.S. Patent No.
~,313,442, to Knudson et al. By responding to average atrial rate through detection of the P wave, the ~ystem varie~ cardiac rate. Thls i8 little more than a dual chamber system, and, as mentioned above, this technlqua is not possible in many patients with sinus brady~ardia or atrial Pibrillation. It i~ al80 slow due to timQ averaging, and pos~ibly ~ub~ect to errors due to faulty signal detection which could drive the heart at a greater that desired rate.
BACKGROUND OF THE INVENTION
Field of the Invention - The present invention relate~
generally to a rate responsive cardiac pacemaker, and more particularly to a pacemake~ having a variable rate which is responsive to arterial pressure, thereby causing the cardiac rate to closely mimic the natural baroreceptor reflex response pattern of the heart to changing physiological need.
The cardiac pacemaker is perhaps one of the best known electronic marvels of modern medicine, and the implantation o~ a pacemaker in a patient has become almost a routine operàtion.
The small, electronic device pulses the heart of the patient contlnuously over an extended period of time, or, in the case of demand pacemakers, monitors the heart's natural operation and provides stimulating pulses only when the the heart skips a beat.
Pacemakers allow patients with heart problems which would have been either fatal or incapacitating without a pacemaker to resume relatively normal lives.
It will be realized by those skilled in the art that the modern pacemaker i9 a highly complex deviceJ capable of event sensing, two-way telemetry, and sensing and paoing in either or both of the atrium and the ventricle o~ the heart. Such pacemakers may be ~inely tuned by the physician subsequent to implant, and the parameters tweaked to result in optimum pacing performance.
Despite the impressive sophistication of such pacemakers, they represent a compromise due to a single ma~or difference bstt~een the he~lthy heart and a pacsd heart- namely the response to activity, exercis`e, or ~tre6s. A healthy heart is rate responsive to a number o~ ~actors including physical aativity or exercise. Variations in the cardiaa stroke volume and ~ystemic vascular resiatance occur inl the cardiovascular ~ystem due to ~ '''.
-1. .
physiological stresses such as exercise, temperature changes, postural changes, emotlon, hypoglycemia, Valsalva maneuvers, eta.
To maintain adequate perfusion pressure and cardiac output under the~e ~tresses, it iB neces6ary to ad~ust heart rate. The healthy heart may beat at ~0 or fewer beats per minute during repose or sleep, and at 120 or more bea~s per minute dur~ng strenuous exercise, for example. The heart paced by a pacemaker which i8 non-rate responsive will typically beat at a constant rate of approximately 70 beats per minute.
It wlll be appreciated that the paced heart will supply more blood than is needed during sleep, and may even prevent the patient from sleeping restfully. Even more seriously, patients paced at 70 beats per minute experience substantial difficulty in engaging in strenuous activity. A moderate level of activity lS such as walking will cause difficulty in some patients. It is apparent that a pacemaker whlch varies in response to physiological need xepresents a highly desirable device which wlll enable a normal active life ~or patients xequiring a pacemaker~
Physiological re~ponsive cardia¢ pacing must optimize cardiac rate to the level o~ metabolic need in the absence of normal variable cardiac rate. The slmplest answer to this problem i8 atrial tracking pacing, wherQ the patient has a full or partial AV block and a dual chamber pacemaker pulses the ventricle in re~ponsQ to normal cardiac activ~ty sensed in the atrium. However, thi~ techni~ue i8 not possiblQ in many patients with sinus bradycardia or atrial fibrillation, and so rate responsivQ pacing ~s nece~sary to mimic the normal variable cardiac rate.
A varlety o~ physiological respon~lve pacing systems have been proposed, with the sy6tems using a variety o~ physiological parameters as the basi~ for varying cardiac rate. These parameters include blood temperature, various sensed timing 1 ~3036 1 signals from the heart, blood chemistry, respiratory rate, nervous system activity, physical activity, and pressure measured wi~hin the heart at various location~. These systems will be discussed briefly below, and the problems inherent in each of the systems will become evident.
Venous blood temperature is measured in the right ventricle by Cook et al. in U.S. Patent No. 4,436,092. Sin~e blood temperature has been found to ri~e duriny exercise and the corresponding body core temperature increase~ blood tsmperature indicates greater physiological need for blood supply. However, the response of ~uch a ~ystem in qu$te slow. In addition, the ~ystem i8 lnexact due to the coarsene~ at which measuremant~ may be taken, the ingestion of cold liquids, and the effeat caused by presence of a fever.
~oth the QT interval and the P wave have been used to vary heart rate. The use of the QT interval is discussed in U.S.
Patent No. 4,228,803, to Rickards, and involve~ detection o~ ~he repolarization ~ wave subsequent to pacemaker ~timulation (indicating the Q wave). A shorter QT interval is used to produce a higher paced cardiac rate. This system is slow in response, and not highly specific due to variations caused both by drugs ingested and by the used of pacemaker stimulation rather than using ~ensed contractions.
The use of the P wave i5 taught in U.S. Patent No.
~,313,442, to Knudson et al. By responding to average atrial rate through detection of the P wave, the ~ystem varie~ cardiac rate. Thls i8 little more than a dual chamber system, and, as mentioned above, this technlqua is not possible in many patients with sinus brady~ardia or atrial Pibrillation. It i~ al80 slow due to timQ averaging, and pos~ibly ~ub~ect to errors due to faulty signal detection which could drive the heart at a greater that desired rate.
~ 330361 Blood chemistry sensors may detect oxygen saturatlon or blood pH. The use o~ oxygen saturation i5 shown in U.S~ Patent No. 4, 202,339, to Wirtzfeld et al., and in U.S. Patent No.
4,467,807, to Bornz~n. An optical detector i~ used to measure the mixed venous oxygen saturation, typically in the right ventrlclP. A diminution inrthe mixed venouR oxygen 6aturation ls used to produce a higher paced cardiac rate~ The speed of this system i8 also quite 810W, and sensor reliability and life are not yet great enough to produce a very reliable product.
The use of pH sensing i~ taught in U.S. Patent No.
4,009,721, to Alcidi, and in U.S. Patent No. 4,252,124, to Mauer et al. A membrane pH sensor electrode i5 typlcally placed in the right ventricle, and ~enses pH, which is proportional to the ~lood concentration o~ carbon dioxide, which is generated in increasing amounts by exercise. A diminution in the p~ level is used to produce a higher paced cardiac rate. The speed of this system iB 810w, and sensor reliability over an extended lifetime is not yet great enough to produce a reliable product.
A respiratory rate sensor is shown in U.S. Patent No.
3,593,718, to Krasner. An increase in resplratory rata ¢au~es a the system to produce a higher paced cardiac rate. Cardlac rate does not exactly track respiratory rate in the normal heart, and the problem with the Krasner device i8 that it is either too slow if resplratory rate i~ time-averaged, or it may be too fast i~
instantaneous respiratory rate i~ used. In addition, the system uses variations in chest impedanca to prsduce a signal, maklng it both sub~ect to false ~ignals due to a variety of causes including Ioo~e ~ensor~, and highly ~ubject to damage ~rom defibrillation.
Activitles of the aentral nervous system are highly relevant to modification of cardiac rate. One use of nPrve impulses is detailed ln U. S. Patent No. 4,201,219, to Bozal Gonzale~, in which a neurodetector~ device is used to generate electrical signals indicative of nerve impulses. The frequency o~ the impulses is utilized to modify the paced cardiac rate. The implementation of this is considerably difficult, in that a stable, predictable coupling to the Hering nerv~ iB required. In addition, it is difficult to discriminate between the signals detected to obtain the single signal desired, in that the technology involved is still in its infancy. This approach, while probably having a fast re~ponse, thus has neither the sensor reliability nor the system ~pecificity necessary for a reliable product.
The approach whlch has found its way into the first generation o~ commercially available pacemaker~ 1~ the actlvity sensing varlable rate device, which varies rate in response to body movement. As body movement inoreaBe~ ~ 80 does the output from the sensor, typically a piezoelectric device producing an electrical output in response to vibratory movement induced by body movement. IncreaRing output from the eensor causes the system to produce a hlgher paced cardiac rate. Examples of such devlces are illustrated in U.S. Patent No. 4,140,132, to Dahl, and ~n U.S. Patent No. 4,428,378, to Anderson et al.
Activity sensing variable rate pacemakers have a fast response and good sen~or reliability. However, they are less than ideal in system speciflcity. For example, if a person with such a pacemaker was restfully riding in a car on a very bumpy road, his heart rate would increase dramatically at a time when such an lncrease was not warranted, and, indeed, would not be initiated by the normal healthy heart. Similarly, if the person was pedaling at a furiou3 rate on an exercise bicycle while his upper body were relatively motionless, he would likely run out of oxygen and pa6s out. De~pite the commercial implementation o~
such devices, lt will therefore be appreoiated that they are ~ar from perfect.
~5-1 3303~1 The last approach which has been taken is to use khe pres~ure of blood to determine an appropriate heart rata. Using blood pressure within the heart to regulate heart rate has been the basis for ~Pveral proposed systems, beginning with the system shown in U.S. Patent No. 3,358,690, to Cohen. Cohen uses a pressure sensor ln the atrium to detect a high pressure condition, and, after a short delay, provides a pacing pulse to the ventricle. This system also as~umes that the atrium is operating completely normally, ànd thus it ls nok possible to use this system in many patients with s1nus bradycardia or atrial fibrillation.
U.S. Patent No. 3,857,399, to Zacouto, teaches a ~ystèm that measures elther left ventricle pre~sure or intramyocardial pre~sure using a sensor located in the left ventricle. This 1 absolutely unacceptabla, since to lntroduce a sensor through th~
interventricular septum would be dangerous to ~ay tha least Likewise, a cutdown or percutaneou~ introduction of ~uch a sensor into the heart through an artery would result i~ nearosis of the artery.
U.S. Patent No. 4,566,456, to ~oning et al., uses a preOEsure sensor in the right ventricle, and, in responsQ to either khe pressure sensed or the time derivative o~ pressure sensed, provides a pacing pulse to the right ventricle. This system al~o assumes that the atrium i~ operating completely normally, and ~o it is not possible to use thi~ system in many patients with ~inuq bradycardia or atrial fibrillation.
Finally, U.S. Patent No. 4,600,017, to Schroeppel, teaches the use of a pressure ~ensor in the right ventricle to sense khe closing of the trlcuspid valve, and provides a pacing pulse thereafter. once again, if the atrium i8 not operating completely normally it is nok possible to usa thi~ ~ystem.
It may there~ore be appreciated khat there exists a substantial need for a physiological response variable rate 1 3303~ 1 pacemaker which has the deslrable features of fast re~ponse, long term reliability, and high specificity. The fast response of the system insurès that the heart rate will be varied according to current demand, not the demand of some previous time averaged period. Long term reliability is of course needed in order to maXe the device suitable for human implant. Finally the system must respond at times when a response is appropriate, and not respond when a response ia not appropriate. This combination of ob~ectives must of course be achieved with no ralative 10 disadvantage.
- SUMMARY OF THE INVENTION
The disadvantages and limitations of the background art discus~ed above are overcome by the present invention. With this invention, blood pre9~ure i~ used to regulate heart rate. It has been known for some tlme that an lnverse relationship exi~ts between blood pressure and heart rate, and that tha baroreceptor reflex, which is the blood pressure control system o~ the body, causes changes in cardiac rate to provide short term aontrol o~
blood pre6sure.
The invention is an artificial pacemaker system which in the preferred embodlment ad~ust~ heart rate using a mlcroprocessor-based method that is similar to the human body's natural baroreceptor reflex. (It wlll be apparent to those skllled in the art that other circuits could be utillzed instead of a microprocessor-based cir~uit without departing from the spirit of the present invention.) In the aystam of the present invention, arterial blood pressure is measured by a transducer which is preferably extravascular and located at an easily accessible artery such as the proximal axillary artery. A first embodiment o~ the invention uses a ~eedbacX type 6y8tem having a relatively fa6t re6pon6e ~eedback loop which ad~usts heart rate, within minimum and maximum limits, quickly, with a re~ponse time of a few seconds. The preferred ambodiment of the present invention also has this first fast feedback loop to adjust heart rate quickly, and also has a second relatively 810w feedback loop which tends to maintain resting heart rat~ at a nominal value over an extended period of time.
The fast loop is deslgned based on physiological investigations of the baroreceptor reflex that has shown a linear relationship between heart interval (the reciprocal of heart rate) and arterial pressure. The feedback signal used in the fast loop of the preferred embodiment is the mean arterial pressure, due to lts ease of measurement, low noise, lower frequencies, ease of calibration, and insensitivity to systolic pulse amplification due to pressure wave reflections in the arterial network. Using mean arterial pressure also provides the system with a fast response time, and a relatively high degree of system speciflcity.
This s~cond or slow loop is an electrical analog o~ the baroreceptor reset mechanism, and it accounts for long term changes in ~ensed blood pres~ure levels of the patient, such as those caused by hyperten~lon or by drift ln the output of the pressure transducar. The 810w loop of the pre~erred embodiment has a response time of hours or days, whereas the ~ast loop has a response time of seconds.
Implementation of the preferred embodiment utilize~ an extravascular arterial blood pres~ure tran~ducer on the pro~imal axillary artery. The pressure signal may be low pass filtered, and then sampled using an A/D converter. ~he microproaessor then .: :
computes the desired heart interval or heart rate based on the sampled blood pressure and other parameters.
The microprocessor would then output thi~ heart rate to a stimulator circui~, which i8 entirely conventional. Parameters sst by the phy~ician using telemetry would include ~ensitivity (gain of the fast ~eedback loop), intercept (a constant which may ' ~.
~8- ~
be used together with the sensitivity value in the control equation to determine the nominal resting hear~ rate), minimum and maximum heart rate, calibration constant~, as well as the usual pacemaker parameters. Telemetry may be used for calibratlon and for monitoring blood pressure, heart rate, and set parameters.
It may thus be appreciated that the invention enables a system having di~tinct advantage~ over previously exi~ting variable rate pacing systems. The system has the most important characteristic of a fast response to physiological need, making it closely follow the operation of a normal healthy heart. The system features a high degree of speci~icity, unlike most of the previously known systems. In addition, the present invention is highly reliable, and will operate over an extended li~etime. The system of the present invention achieves these advantages without lncurring any relative disadvantage, making it a highly desirable and marketable device.
DESCRIPTION OF THE DRAWINGS
~0 These and other advantages of the present invention are best understood with reference to the drawings, in which:
Figure 1 is a diagramatic illu~tration of the natural stretch barorecept~rs of a human being, showing the innervation of the carotid sinu~ and the aortic arch~
Figure 2 is a diagramakic illustration of the installation o~ the sy~tem o~ the present invention in the abdominal region of a human being;
Figure 3 is a graph approximating the linear portion of heart interval as a ~unction of arterial pres~ure;
Fi~ure 4 is a graph approximating the linear portion of heart rate as a function of arterial pres~ure;
Figura 5 i~ a block diagram o~ the ba6ic operation of the system of the present invention: and --Figure 6 is a block diagram of the operation of the preferred embodiment of the present invention, in which the sys~em shown in FigurP 3 is modified to maintain resting heart ~`~
rata at a nominal value.
--DETAILED DESCRIPTIO~ OF THE PREFERRED EMBODIMENT
~efor~ discussing the preferred embodiment of the present invention, it iB helpful to briefly discuss the natural baroreceptor heart rate control system, which iB shown in Figure 1. The heart 10 pumps oxygenated blood out through the aortic arch 12, which leads to the right subclavian artery 14, the rlght common carotid 16, the left common carotid 18, the left subclavian artery 20, and ~he thoractic aorta 22. The body's system utilizes stretch receptors located in arterial walls in tha aortic arch 12 and at the bifurcation of the carotid arteries 16, 18 in the carotid sinus portion of the neck. The bifurcation of the carotid arteries 16, 18 leads to exterior carotid arteries 24, 26, respectively, and to interior carotid arteries 28, 30, respectively.
Nerve fibers extending from ~tretch receptors in the aortic arch 12 join the left and right vagus nerves 32, 34, respectively, with these fibers being referred to as cardiac depressor nerves 36, 38. A number of nerves extend ~rom the stretch receptors at the bifurcation of th~ carotid arteries 16;
18 in the carotid ~inu~, with the area immediately above the btfurcations being referred to as the aarotid bodies 40, 42.
Nerve branches 44, 4~ extending from the carotid bodies 40, 42, respectively~ ~oin the ganglions of vagus 48, 50, respectively.
Other nerve fiber~ compri~ing the sinus nerve branches 52, 54 (ganerally r~ferred to a~ ~'Hering's nerves) o~ th~
glos~opharyngeal nerve~ 56, 58, respectively, also extend from the carotid bodies 40, 42, respectively, to the medulla ~not shown).
1 3303~ 1 Although the exact mechanism by which the body controls the heart rate in responsa to blood pressure is not well understood, it is known that nerve signals are generated in response to dlstortion, which varies in direct response to varying arterial blood pressure. Nerve pulses are generated at pressures typically above 50 mmHg, a~d occur at ever-increa~ing frequency until blood pressure reache~ approximately 170 mmHg. Heart rate varies inversely with the frèquPncy of the nerve impulses. The slope of the relationship between nerve impulse freguency a~ a function of carotid sinu~ pressure is greateet at the normal level of mean arterial pressure, which means that the body's system responds most effectively when blood pressure ls within a normal range.
The system of the present invention mimics the body's natural response by controlling heart rate in response to arterial blood pressure. As shown ln Figure 2, the present invention ha~ threa components- an electronic pulse generator 60, a pacing lead 62 implanted in a vein leading to the heart, and a pressure sensor 64 connected to the pulse generator 60 by a lead 66. In Figure 2, the pulse generator is ~hown implanted in ths right upper chest cavity. As is the case with a conventional pacemaker, the pulse ~enera~or 60 could be implanted in either side of the body.
The lead 62 illustrated in Figure 2 is a bipolar ventricular lead, although the ~ystem could also utilize a unipolar lead, or even an atrial lead in some instances. Likewise, in the case of a few prospective recipients, it may be even desirable to use a dual chamber pacemaker ~y~tem. It will be appreciated by those skilled in the art that the pacemaker technology used in the present invention i8 entirely ~tandard with the exception of the component3 utilized to provide a variable rata command to the paclng circuitry.
The pressure sensor 64 is used to monitor the pre~sure in an easily accessible artery such as the proxlmal axillary artery 68.
Any artery which i~ relatively close to the heart may be used, with the proximal axillary artery 68 being the preferred artery due to its location. Since the pre~erred location to implant the pulse generator 60 is the location shown in Figure 2 (although on either side of the chest), it is desirable to use an artery which is easily accessible through the incision used to implant the lead 62 and the pulse generator 60. The proximal axillary artery meets these requirements. Only a short distance away and even closer to the heart, the subclavian artery 70 may also be used, although it is less convenient to use the subclavian artery 70.
The pressure sensor 64 used must be located external to the artery, since placing a transducer within the artery would likely lead to necrosis of the artery. The transducar may sense the stretch in the arterial wall caused by pressure changa of blood within the artery and thereby produce a variable output indicative of or proportional to arterial pressure, much the same as the body's natural method o~ response. The pre~sure sensor 64 may operate by surrounding the artery and detecting pressure change with a strain detector device. Such pressure ~ensors are described in "ImplantablQ Sensors for Closed-Loop Prosthetic Systems,~ Edited by Wen H. Ko, Futura Publishing Co., Inr., (New York, 1985), on page~ 35-88. Alternatlvely, the pre~sure ~ensor 64 may measure pulse transi~ time, w~ich i6 indicative of arterial pressure.
Figure 3 illustrate~ a linear approximation of the relationship between heart interval (the reciproaal of heart rate) and arterial pressure. Between the minimum and maximum limitg, the relationship may ba expressed as a linear regresslon:
HI = a-P + b (1) where HI is heart interval, P is arterial pressure, a i8 slope, and b is the HI axis intercept. See also "Comparison of the Reflex Heart Response to Rising and Falling Arterial Pressure in Man,~' T. G. Pickering, ~. Grlbbin, and P. Sleight, Cardiovascular Research, Vol. 6, pp. 277-283 (1972~. It will of course be realized by those skilled in the art that the parameters a and b may vary from individual to individual, and will also be dependent on the measuremen~ of arterial pressure. In the graph, HI typically has a range of values between .35 seconds and 1 second.
~ikewise, Figure 4 depicts the same approximated linear relationship (using a ~aylor serie6 approximation), but with heart rate graphed as a function of arterial pressure. Between the minimum and maximum limits, the relationship may be expre~sed as a linear regression:
HR = c-P + d (2) where HR i8 heart rate, P i~ arterial pressure, c is slope, and d is the HR axis intercept. In the graph, HR typically has a range ; ~;
of values between 60 beats per second and 170 beats per second.
The linear relationshlp describing Figure 3 and expressed in equation 1 may be used to set up a proportional control loop, a6 shown ln Figure 5. The ~ystem ~hown in Figure 5 illustrates in simplified fashion the operation of the system o~ the present invention used to control the fre~uency of the ~timulus supplied by the pulse generator 60 (Figure 2) to the heart. A pulse generator 72 paces the heart tnot 6hown) to bump blood throughout the cardiovascular system 74. An output of the cardiovasaular system 74 is arterial blood pressure, which ~is monitored by a transducer 76 which produces an electrical output proportional to arterial blood pre~sure.
An error which could ~e introduced into the system at this point i8 peak amplification o~ the arterial wave oocurring as a result of rQflections in the arterial network (su~h a~ those occurring o~ bifurcations in the arteries). Another closely related error i5 high noise content of systolic arterial pressure -- . . . .... .... . . . .
-` 1 330361 due to the peak detection method of measurement, and aleo the inherent variability in the signal from one heartbeat to the next. It i8 therefore advantagaous to u3e mean pressure, which is smoother, has less noise, is not subject to peak amplification, and is ln fact easier to measure.
Measurement of mean p~essure may be made by filtering the output of the transducer 76 through a low pass ~ilter 78. A
suitable low pass filter i8, fox example, a second or third order filter (a ~utterworth filter would work well) with a time constant of approximately 0.3 ~econds to 1.6 6econds. Such a filter would have a cutoff frequency between approximately 0.1 Hz to 0.5 Hz. The tradeoff involved in selecting a cutoff frequency is that while lower cutoff frequencies minimize harmonics due to the pulsatile nature of the arterial pressure waveform, such lower frequencies al60 introduce more phase shift into the feedback loop and make the sy~tem less stable, The mQan pressure ~gnal output from ~he low pass filter 78 i8 ~upplied to an amplifier 80, in which a gain of a i~ provided.
The output of the amplifier a is then added to the constant input b and the eummed 5ignal ~s provided to a limiting device 82. Tha limiting device 82 will output the summed signal input to it, except when the summed signal i8 below a minimum value (for example, 0.35 seconds as æhown in the graph of Flgure 3) or above a maximum value (for example, l.o second a~ ~hown in the graph of ~5 Figure 3). In such cases where the summed ~ignal exceeds these limits, the limiting device 82 will output the llmit of the value.
The output of the limiting device 82 is HI, and it i8 supplied to the pulse generator 72. The pulse generator 72 will then pace the heart at a rate which is the rec~procal of HI. of course, as is well known in the art, the pulse generator 72 may operat4 as a demand pacer, pacing the heart only when the natural rate does not meet the calculated rate. Also, the pulse generator will have other inputs, all of which are well known in the art. The variables a and b are set by the physician to provide the desired response, and the minimum and maximum values of the limiting device 82 may also be set by the phy~ician in the preferred embodiment. All such settings may be made by two way telemetry, as is known in t~e art.
It will also be appreciated by those skilled in the art that the system shown in Figure 5 may be modified to utilize equation 2 above and reflect the control shown in Figure 4, by lo substituting c and d for a and b, respectively, and by using the limits shown in Figure 4 rather than those of Figure 3. Such limits on HR are typically between 60 and 170 beats per second.
In this case, the limiting device supplies HR to the pulse generator rather than HI.
It will be realized by those skilled in the art that since a microprocessor is used in the preferred embodiment to implement the control schema, the relationship between blood pressure and HR (or HI) need not be a linear approximation, but rather could be a nonlinear transfer function. By utilizing this approach, the system may be made to ~imulate the normal healthy response even more closely.
The system discusssd to this point in con;unction with Figure 5 is a fa~t acting system which varies heart rate as a function of arterial pressure. This system has a disadvantage in that it has no means for keeping the resting heart rate at a preset level. For example, if an individuals blood pressure changes over a relatively long period of time, the ~ystem of Figure 5 would also change the individual' 8 resting heart rate.
The practical effect of this is that with elevated blood pres~ure, the heart rate would rema~n low even when the physiological demands of the body were relatively high. In any case, the system would no longer closely mimic the normal functions of the body.
The system of Figure 5 may be modifled to overcome thi~
problem, a6 shown in Figure 6. Figure 6 operate~ the same as Figurs 5 with a single exception- the ~ixed value b i5 replaced with a variable b' which functions to maintain resting heart rate at a consistent value over an extended period of time~ The desired resting heart raté RHR is 6upplied to a reciprocal function device 84, which has as its output the desired heart interval RHI. The output of the l~miting device 82, which i8 heart interval HI, i~ ~ubtracted from RHI, tha desired resting heart interval, to produce an instantaneous error signal which i6 supplied to a reset controller 86. ~Note that if the system models equation 2 and Figure 4, the output of the limiting device 82 would be HR, and the reciprocal function device 84 would not not be needed to obtain RHI.) The reset controller would function over an extended time period, on a scale of days to weeks. It functions to assure that in the long term, resting heart rate remains constant. The response i8 closed loop, and preferably includes nonllnearities to ensure safety. The reset controller 86 may be, ~or example/ a proportional or proportional-plus-integral controller with nonlinearities, or it may alternatively utilize lead-lag or pole-placement to accompllsh the re~et ~unction. This system reset~
the intercept b' of the linear ~unction, while maintaining the slope a, and has the effect of moving the linear transfer function shown in Figure 3 up or down.
It may al80 be desirabla to ad~ust the ~lope a, and this i~
within the contemplation of the present invention. It would require only an additional control line from the ~eset controller 86 to the amplifier 80 to control gain a'. It also may be dasirable to limit eithar or both of a' and b~ within a range, but 6uch limit8 constituta fin~ tuning of a degree which does not need to be 6peci~1cally addre6sed herein, but which will be readily apparent to tho~e skilled in the art.
It will be apparent to those skilled in the art that the electronics of the system described above are easily attainable uslng available technology. The electronics may be contained in the same case as the pulse generator and a power source, and therefore may use the same telemetry, power, and control systems.
It wlll thus be appreciated that the present inventlon as descrlbed above defines a system having distinct advantages over prevlously existing variable rate pacing systems. The system has a fast response to physiologlcal need, and lt closely follow the operation of a nor~al he~lthy heart, while maintaining a desired resting heart rate. The system ~eatures a high degree of specificity, unlike previously known systems. In addition, it is highly reliable while remaining relatively simple to implant, and will operate over an extended lifetime. The system of the present invention achieves all of these advantages without incurring any relative disadvantage, there~ore making it a highly desirable improvement in the state of the art.
Although an exemplary embodiment of the present invention has been shown and descrlbed, it will be apparent to those having ordinary skill in the art that a number of changes, modifications, or alterations to the invention as described herein may be made, none of which depart ~rom the splrit of the present invention. All such changes, modifications, and alterations should therefore be seen as withln the scope of the present invention.
. . .
The use of pH sensing i~ taught in U.S. Patent No.
4,009,721, to Alcidi, and in U.S. Patent No. 4,252,124, to Mauer et al. A membrane pH sensor electrode i5 typlcally placed in the right ventricle, and ~enses pH, which is proportional to the ~lood concentration o~ carbon dioxide, which is generated in increasing amounts by exercise. A diminution in the p~ level is used to produce a higher paced cardiac rate. The speed of this system iB 810w, and sensor reliability over an extended lifetime is not yet great enough to produce a reliable product.
A respiratory rate sensor is shown in U.S. Patent No.
3,593,718, to Krasner. An increase in resplratory rata ¢au~es a the system to produce a higher paced cardiac rate. Cardlac rate does not exactly track respiratory rate in the normal heart, and the problem with the Krasner device i8 that it is either too slow if resplratory rate i~ time-averaged, or it may be too fast i~
instantaneous respiratory rate i~ used. In addition, the system uses variations in chest impedanca to prsduce a signal, maklng it both sub~ect to false ~ignals due to a variety of causes including Ioo~e ~ensor~, and highly ~ubject to damage ~rom defibrillation.
Activitles of the aentral nervous system are highly relevant to modification of cardiac rate. One use of nPrve impulses is detailed ln U. S. Patent No. 4,201,219, to Bozal Gonzale~, in which a neurodetector~ device is used to generate electrical signals indicative of nerve impulses. The frequency o~ the impulses is utilized to modify the paced cardiac rate. The implementation of this is considerably difficult, in that a stable, predictable coupling to the Hering nerv~ iB required. In addition, it is difficult to discriminate between the signals detected to obtain the single signal desired, in that the technology involved is still in its infancy. This approach, while probably having a fast re~ponse, thus has neither the sensor reliability nor the system ~pecificity necessary for a reliable product.
The approach whlch has found its way into the first generation o~ commercially available pacemaker~ 1~ the actlvity sensing varlable rate device, which varies rate in response to body movement. As body movement inoreaBe~ ~ 80 does the output from the sensor, typically a piezoelectric device producing an electrical output in response to vibratory movement induced by body movement. IncreaRing output from the eensor causes the system to produce a hlgher paced cardiac rate. Examples of such devlces are illustrated in U.S. Patent No. 4,140,132, to Dahl, and ~n U.S. Patent No. 4,428,378, to Anderson et al.
Activity sensing variable rate pacemakers have a fast response and good sen~or reliability. However, they are less than ideal in system speciflcity. For example, if a person with such a pacemaker was restfully riding in a car on a very bumpy road, his heart rate would increase dramatically at a time when such an lncrease was not warranted, and, indeed, would not be initiated by the normal healthy heart. Similarly, if the person was pedaling at a furiou3 rate on an exercise bicycle while his upper body were relatively motionless, he would likely run out of oxygen and pa6s out. De~pite the commercial implementation o~
such devices, lt will therefore be appreoiated that they are ~ar from perfect.
~5-1 3303~1 The last approach which has been taken is to use khe pres~ure of blood to determine an appropriate heart rata. Using blood pressure within the heart to regulate heart rate has been the basis for ~Pveral proposed systems, beginning with the system shown in U.S. Patent No. 3,358,690, to Cohen. Cohen uses a pressure sensor ln the atrium to detect a high pressure condition, and, after a short delay, provides a pacing pulse to the ventricle. This system also as~umes that the atrium is operating completely normally, ànd thus it ls nok possible to use this system in many patients with s1nus bradycardia or atrial fibrillation.
U.S. Patent No. 3,857,399, to Zacouto, teaches a ~ystèm that measures elther left ventricle pre~sure or intramyocardial pre~sure using a sensor located in the left ventricle. This 1 absolutely unacceptabla, since to lntroduce a sensor through th~
interventricular septum would be dangerous to ~ay tha least Likewise, a cutdown or percutaneou~ introduction of ~uch a sensor into the heart through an artery would result i~ nearosis of the artery.
U.S. Patent No. 4,566,456, to ~oning et al., uses a preOEsure sensor in the right ventricle, and, in responsQ to either khe pressure sensed or the time derivative o~ pressure sensed, provides a pacing pulse to the right ventricle. This system al~o assumes that the atrium i~ operating completely normally, and ~o it is not possible to use thi~ system in many patients with ~inuq bradycardia or atrial fibrillation.
Finally, U.S. Patent No. 4,600,017, to Schroeppel, teaches the use of a pressure ~ensor in the right ventricle to sense khe closing of the trlcuspid valve, and provides a pacing pulse thereafter. once again, if the atrium i8 not operating completely normally it is nok possible to usa thi~ ~ystem.
It may there~ore be appreciated khat there exists a substantial need for a physiological response variable rate 1 3303~ 1 pacemaker which has the deslrable features of fast re~ponse, long term reliability, and high specificity. The fast response of the system insurès that the heart rate will be varied according to current demand, not the demand of some previous time averaged period. Long term reliability is of course needed in order to maXe the device suitable for human implant. Finally the system must respond at times when a response is appropriate, and not respond when a response ia not appropriate. This combination of ob~ectives must of course be achieved with no ralative 10 disadvantage.
- SUMMARY OF THE INVENTION
The disadvantages and limitations of the background art discus~ed above are overcome by the present invention. With this invention, blood pre9~ure i~ used to regulate heart rate. It has been known for some tlme that an lnverse relationship exi~ts between blood pressure and heart rate, and that tha baroreceptor reflex, which is the blood pressure control system o~ the body, causes changes in cardiac rate to provide short term aontrol o~
blood pre6sure.
The invention is an artificial pacemaker system which in the preferred embodlment ad~ust~ heart rate using a mlcroprocessor-based method that is similar to the human body's natural baroreceptor reflex. (It wlll be apparent to those skllled in the art that other circuits could be utillzed instead of a microprocessor-based cir~uit without departing from the spirit of the present invention.) In the aystam of the present invention, arterial blood pressure is measured by a transducer which is preferably extravascular and located at an easily accessible artery such as the proximal axillary artery. A first embodiment o~ the invention uses a ~eedbacX type 6y8tem having a relatively fa6t re6pon6e ~eedback loop which ad~usts heart rate, within minimum and maximum limits, quickly, with a re~ponse time of a few seconds. The preferred ambodiment of the present invention also has this first fast feedback loop to adjust heart rate quickly, and also has a second relatively 810w feedback loop which tends to maintain resting heart rat~ at a nominal value over an extended period of time.
The fast loop is deslgned based on physiological investigations of the baroreceptor reflex that has shown a linear relationship between heart interval (the reciprocal of heart rate) and arterial pressure. The feedback signal used in the fast loop of the preferred embodiment is the mean arterial pressure, due to lts ease of measurement, low noise, lower frequencies, ease of calibration, and insensitivity to systolic pulse amplification due to pressure wave reflections in the arterial network. Using mean arterial pressure also provides the system with a fast response time, and a relatively high degree of system speciflcity.
This s~cond or slow loop is an electrical analog o~ the baroreceptor reset mechanism, and it accounts for long term changes in ~ensed blood pres~ure levels of the patient, such as those caused by hyperten~lon or by drift ln the output of the pressure transducar. The 810w loop of the pre~erred embodiment has a response time of hours or days, whereas the ~ast loop has a response time of seconds.
Implementation of the preferred embodiment utilize~ an extravascular arterial blood pres~ure tran~ducer on the pro~imal axillary artery. The pressure signal may be low pass filtered, and then sampled using an A/D converter. ~he microproaessor then .: :
computes the desired heart interval or heart rate based on the sampled blood pressure and other parameters.
The microprocessor would then output thi~ heart rate to a stimulator circui~, which i8 entirely conventional. Parameters sst by the phy~ician using telemetry would include ~ensitivity (gain of the fast ~eedback loop), intercept (a constant which may ' ~.
~8- ~
be used together with the sensitivity value in the control equation to determine the nominal resting hear~ rate), minimum and maximum heart rate, calibration constant~, as well as the usual pacemaker parameters. Telemetry may be used for calibratlon and for monitoring blood pressure, heart rate, and set parameters.
It may thus be appreciated that the invention enables a system having di~tinct advantage~ over previously exi~ting variable rate pacing systems. The system has the most important characteristic of a fast response to physiological need, making it closely follow the operation of a normal healthy heart. The system features a high degree of speci~icity, unlike most of the previously known systems. In addition, the present invention is highly reliable, and will operate over an extended li~etime. The system of the present invention achieves these advantages without lncurring any relative disadvantage, making it a highly desirable and marketable device.
DESCRIPTION OF THE DRAWINGS
~0 These and other advantages of the present invention are best understood with reference to the drawings, in which:
Figure 1 is a diagramatic illu~tration of the natural stretch barorecept~rs of a human being, showing the innervation of the carotid sinu~ and the aortic arch~
Figure 2 is a diagramakic illustration of the installation o~ the sy~tem o~ the present invention in the abdominal region of a human being;
Figure 3 is a graph approximating the linear portion of heart interval as a ~unction of arterial pres~ure;
Fi~ure 4 is a graph approximating the linear portion of heart rate as a function of arterial pres~ure;
Figura 5 i~ a block diagram o~ the ba6ic operation of the system of the present invention: and --Figure 6 is a block diagram of the operation of the preferred embodiment of the present invention, in which the sys~em shown in FigurP 3 is modified to maintain resting heart ~`~
rata at a nominal value.
--DETAILED DESCRIPTIO~ OF THE PREFERRED EMBODIMENT
~efor~ discussing the preferred embodiment of the present invention, it iB helpful to briefly discuss the natural baroreceptor heart rate control system, which iB shown in Figure 1. The heart 10 pumps oxygenated blood out through the aortic arch 12, which leads to the right subclavian artery 14, the rlght common carotid 16, the left common carotid 18, the left subclavian artery 20, and ~he thoractic aorta 22. The body's system utilizes stretch receptors located in arterial walls in tha aortic arch 12 and at the bifurcation of the carotid arteries 16, 18 in the carotid sinus portion of the neck. The bifurcation of the carotid arteries 16, 18 leads to exterior carotid arteries 24, 26, respectively, and to interior carotid arteries 28, 30, respectively.
Nerve fibers extending from ~tretch receptors in the aortic arch 12 join the left and right vagus nerves 32, 34, respectively, with these fibers being referred to as cardiac depressor nerves 36, 38. A number of nerves extend ~rom the stretch receptors at the bifurcation of th~ carotid arteries 16;
18 in the carotid ~inu~, with the area immediately above the btfurcations being referred to as the aarotid bodies 40, 42.
Nerve branches 44, 4~ extending from the carotid bodies 40, 42, respectively~ ~oin the ganglions of vagus 48, 50, respectively.
Other nerve fiber~ compri~ing the sinus nerve branches 52, 54 (ganerally r~ferred to a~ ~'Hering's nerves) o~ th~
glos~opharyngeal nerve~ 56, 58, respectively, also extend from the carotid bodies 40, 42, respectively, to the medulla ~not shown).
1 3303~ 1 Although the exact mechanism by which the body controls the heart rate in responsa to blood pressure is not well understood, it is known that nerve signals are generated in response to dlstortion, which varies in direct response to varying arterial blood pressure. Nerve pulses are generated at pressures typically above 50 mmHg, a~d occur at ever-increa~ing frequency until blood pressure reache~ approximately 170 mmHg. Heart rate varies inversely with the frèquPncy of the nerve impulses. The slope of the relationship between nerve impulse freguency a~ a function of carotid sinu~ pressure is greateet at the normal level of mean arterial pressure, which means that the body's system responds most effectively when blood pressure ls within a normal range.
The system of the present invention mimics the body's natural response by controlling heart rate in response to arterial blood pressure. As shown ln Figure 2, the present invention ha~ threa components- an electronic pulse generator 60, a pacing lead 62 implanted in a vein leading to the heart, and a pressure sensor 64 connected to the pulse generator 60 by a lead 66. In Figure 2, the pulse generator is ~hown implanted in ths right upper chest cavity. As is the case with a conventional pacemaker, the pulse ~enera~or 60 could be implanted in either side of the body.
The lead 62 illustrated in Figure 2 is a bipolar ventricular lead, although the ~ystem could also utilize a unipolar lead, or even an atrial lead in some instances. Likewise, in the case of a few prospective recipients, it may be even desirable to use a dual chamber pacemaker ~y~tem. It will be appreciated by those skilled in the art that the pacemaker technology used in the present invention i8 entirely ~tandard with the exception of the component3 utilized to provide a variable rata command to the paclng circuitry.
The pressure sensor 64 is used to monitor the pre~sure in an easily accessible artery such as the proxlmal axillary artery 68.
Any artery which i~ relatively close to the heart may be used, with the proximal axillary artery 68 being the preferred artery due to its location. Since the pre~erred location to implant the pulse generator 60 is the location shown in Figure 2 (although on either side of the chest), it is desirable to use an artery which is easily accessible through the incision used to implant the lead 62 and the pulse generator 60. The proximal axillary artery meets these requirements. Only a short distance away and even closer to the heart, the subclavian artery 70 may also be used, although it is less convenient to use the subclavian artery 70.
The pressure sensor 64 used must be located external to the artery, since placing a transducer within the artery would likely lead to necrosis of the artery. The transducar may sense the stretch in the arterial wall caused by pressure changa of blood within the artery and thereby produce a variable output indicative of or proportional to arterial pressure, much the same as the body's natural method o~ response. The pre~sure sensor 64 may operate by surrounding the artery and detecting pressure change with a strain detector device. Such pressure ~ensors are described in "ImplantablQ Sensors for Closed-Loop Prosthetic Systems,~ Edited by Wen H. Ko, Futura Publishing Co., Inr., (New York, 1985), on page~ 35-88. Alternatlvely, the pre~sure ~ensor 64 may measure pulse transi~ time, w~ich i6 indicative of arterial pressure.
Figure 3 illustrate~ a linear approximation of the relationship between heart interval (the reciproaal of heart rate) and arterial pressure. Between the minimum and maximum limitg, the relationship may ba expressed as a linear regresslon:
HI = a-P + b (1) where HI is heart interval, P is arterial pressure, a i8 slope, and b is the HI axis intercept. See also "Comparison of the Reflex Heart Response to Rising and Falling Arterial Pressure in Man,~' T. G. Pickering, ~. Grlbbin, and P. Sleight, Cardiovascular Research, Vol. 6, pp. 277-283 (1972~. It will of course be realized by those skilled in the art that the parameters a and b may vary from individual to individual, and will also be dependent on the measuremen~ of arterial pressure. In the graph, HI typically has a range of values between .35 seconds and 1 second.
~ikewise, Figure 4 depicts the same approximated linear relationship (using a ~aylor serie6 approximation), but with heart rate graphed as a function of arterial pressure. Between the minimum and maximum limits, the relationship may be expre~sed as a linear regression:
HR = c-P + d (2) where HR i8 heart rate, P i~ arterial pressure, c is slope, and d is the HR axis intercept. In the graph, HR typically has a range ; ~;
of values between 60 beats per second and 170 beats per second.
The linear relationshlp describing Figure 3 and expressed in equation 1 may be used to set up a proportional control loop, a6 shown ln Figure 5. The ~ystem ~hown in Figure 5 illustrates in simplified fashion the operation of the system o~ the present invention used to control the fre~uency of the ~timulus supplied by the pulse generator 60 (Figure 2) to the heart. A pulse generator 72 paces the heart tnot 6hown) to bump blood throughout the cardiovascular system 74. An output of the cardiovasaular system 74 is arterial blood pressure, which ~is monitored by a transducer 76 which produces an electrical output proportional to arterial blood pre~sure.
An error which could ~e introduced into the system at this point i8 peak amplification o~ the arterial wave oocurring as a result of rQflections in the arterial network (su~h a~ those occurring o~ bifurcations in the arteries). Another closely related error i5 high noise content of systolic arterial pressure -- . . . .... .... . . . .
-` 1 330361 due to the peak detection method of measurement, and aleo the inherent variability in the signal from one heartbeat to the next. It i8 therefore advantagaous to u3e mean pressure, which is smoother, has less noise, is not subject to peak amplification, and is ln fact easier to measure.
Measurement of mean p~essure may be made by filtering the output of the transducer 76 through a low pass ~ilter 78. A
suitable low pass filter i8, fox example, a second or third order filter (a ~utterworth filter would work well) with a time constant of approximately 0.3 ~econds to 1.6 6econds. Such a filter would have a cutoff frequency between approximately 0.1 Hz to 0.5 Hz. The tradeoff involved in selecting a cutoff frequency is that while lower cutoff frequencies minimize harmonics due to the pulsatile nature of the arterial pressure waveform, such lower frequencies al60 introduce more phase shift into the feedback loop and make the sy~tem less stable, The mQan pressure ~gnal output from ~he low pass filter 78 i8 ~upplied to an amplifier 80, in which a gain of a i~ provided.
The output of the amplifier a is then added to the constant input b and the eummed 5ignal ~s provided to a limiting device 82. Tha limiting device 82 will output the summed signal input to it, except when the summed signal i8 below a minimum value (for example, 0.35 seconds as æhown in the graph of Flgure 3) or above a maximum value (for example, l.o second a~ ~hown in the graph of ~5 Figure 3). In such cases where the summed ~ignal exceeds these limits, the limiting device 82 will output the llmit of the value.
The output of the limiting device 82 is HI, and it i8 supplied to the pulse generator 72. The pulse generator 72 will then pace the heart at a rate which is the rec~procal of HI. of course, as is well known in the art, the pulse generator 72 may operat4 as a demand pacer, pacing the heart only when the natural rate does not meet the calculated rate. Also, the pulse generator will have other inputs, all of which are well known in the art. The variables a and b are set by the physician to provide the desired response, and the minimum and maximum values of the limiting device 82 may also be set by the phy~ician in the preferred embodiment. All such settings may be made by two way telemetry, as is known in t~e art.
It will also be appreciated by those skilled in the art that the system shown in Figure 5 may be modified to utilize equation 2 above and reflect the control shown in Figure 4, by lo substituting c and d for a and b, respectively, and by using the limits shown in Figure 4 rather than those of Figure 3. Such limits on HR are typically between 60 and 170 beats per second.
In this case, the limiting device supplies HR to the pulse generator rather than HI.
It will be realized by those skilled in the art that since a microprocessor is used in the preferred embodiment to implement the control schema, the relationship between blood pressure and HR (or HI) need not be a linear approximation, but rather could be a nonlinear transfer function. By utilizing this approach, the system may be made to ~imulate the normal healthy response even more closely.
The system discusssd to this point in con;unction with Figure 5 is a fa~t acting system which varies heart rate as a function of arterial pressure. This system has a disadvantage in that it has no means for keeping the resting heart rate at a preset level. For example, if an individuals blood pressure changes over a relatively long period of time, the ~ystem of Figure 5 would also change the individual' 8 resting heart rate.
The practical effect of this is that with elevated blood pres~ure, the heart rate would rema~n low even when the physiological demands of the body were relatively high. In any case, the system would no longer closely mimic the normal functions of the body.
The system of Figure 5 may be modifled to overcome thi~
problem, a6 shown in Figure 6. Figure 6 operate~ the same as Figurs 5 with a single exception- the ~ixed value b i5 replaced with a variable b' which functions to maintain resting heart rate at a consistent value over an extended period of time~ The desired resting heart raté RHR is 6upplied to a reciprocal function device 84, which has as its output the desired heart interval RHI. The output of the l~miting device 82, which i8 heart interval HI, i~ ~ubtracted from RHI, tha desired resting heart interval, to produce an instantaneous error signal which i6 supplied to a reset controller 86. ~Note that if the system models equation 2 and Figure 4, the output of the limiting device 82 would be HR, and the reciprocal function device 84 would not not be needed to obtain RHI.) The reset controller would function over an extended time period, on a scale of days to weeks. It functions to assure that in the long term, resting heart rate remains constant. The response i8 closed loop, and preferably includes nonllnearities to ensure safety. The reset controller 86 may be, ~or example/ a proportional or proportional-plus-integral controller with nonlinearities, or it may alternatively utilize lead-lag or pole-placement to accompllsh the re~et ~unction. This system reset~
the intercept b' of the linear ~unction, while maintaining the slope a, and has the effect of moving the linear transfer function shown in Figure 3 up or down.
It may al80 be desirabla to ad~ust the ~lope a, and this i~
within the contemplation of the present invention. It would require only an additional control line from the ~eset controller 86 to the amplifier 80 to control gain a'. It also may be dasirable to limit eithar or both of a' and b~ within a range, but 6uch limit8 constituta fin~ tuning of a degree which does not need to be 6peci~1cally addre6sed herein, but which will be readily apparent to tho~e skilled in the art.
It will be apparent to those skilled in the art that the electronics of the system described above are easily attainable uslng available technology. The electronics may be contained in the same case as the pulse generator and a power source, and therefore may use the same telemetry, power, and control systems.
It wlll thus be appreciated that the present inventlon as descrlbed above defines a system having distinct advantages over prevlously existing variable rate pacing systems. The system has a fast response to physiologlcal need, and lt closely follow the operation of a nor~al he~lthy heart, while maintaining a desired resting heart rate. The system ~eatures a high degree of specificity, unlike previously known systems. In addition, it is highly reliable while remaining relatively simple to implant, and will operate over an extended lifetime. The system of the present invention achieves all of these advantages without incurring any relative disadvantage, there~ore making it a highly desirable improvement in the state of the art.
Although an exemplary embodiment of the present invention has been shown and descrlbed, it will be apparent to those having ordinary skill in the art that a number of changes, modifications, or alterations to the invention as described herein may be made, none of which depart ~rom the splrit of the present invention. All such changes, modifications, and alterations should therefore be seen as withln the scope of the present invention.
. . .
Claims (21)
1. An artificial cardiac pacemaker for stimulating a heart to beat at a rate which is variable in response to physiological need, comprising:
a pulse generator for generating periodic electrical pulses at a rate determined by a rate input signal supplied to said pulse generator;
-17a-a pacing lead for delivering said periodic electrical pulse generated by said pulse generator to said heart:
means for sensing arterial blood pressure and generating a signal indicative of arterial blood pressure; and means for generating said rate input signal in response to said signal indicative of arterial blood pressure.
a pulse generator for generating periodic electrical pulses at a rate determined by a rate input signal supplied to said pulse generator;
-17a-a pacing lead for delivering said periodic electrical pulse generated by said pulse generator to said heart:
means for sensing arterial blood pressure and generating a signal indicative of arterial blood pressure; and means for generating said rate input signal in response to said signal indicative of arterial blood pressure.
2. A cardiac pacemaker as defined in claim 1, wherein said pulse generator generates said periodic electrical pulses only in the absence of naturally occurring heartbeats.
3. A cardiac pacemaker as defined in Claim 1, wherein said sensing means senses arterial blood pressure at the proximal axillary artery.
4. A cardiac pacemaker as defined in Claim 2, wherein said sensing means is located externally of the proximal axillary artery.
5. A cardiac pacemaker as defined in Claim 1, wherein said sensing means comprises:
a transducer located externally about an artery for producing a variable output indicative of the stretch in the arterial wall caused by pressure change of blood within said artery.
a transducer located externally about an artery for producing a variable output indicative of the stretch in the arterial wall caused by pressure change of blood within said artery.
6. A cardiac pacemaker as defined in Claim 5, wherein said transducer is a strain detector device.
7. A cardiac pacemaker as defined in Claim 5, wherein said transducer operates by measuring pulse transit time.
8. A cardiac pacemaker as defined in Claim 1, wherein said generating means has as an input the signal generated by said sensing means, said generating means using a transfer function to produce said rate input signal in response to said signal generated by said sensing means.
9. A cardiac pacemaker as defined in Claim 8, wherein said transfer function is nonlinear.
10. A cardiac pacemaker as defined in Claim 8, wherein said transfer function is linear.
11. A cardiac pacemaker as defined in Claim 10, wherein said rate input signal is commanded heart interval, and said transfer function produces a value of commanded heart interval which is equal to a first constant a times the signal indicative of arterial pressure, plus a second constant b.
12. A cardiac pacemaker as defined in Claim 10, wherein said rate input signal is commanded heart rate, and said transfer function produces a value of commanded heart rate which is equal to a third constant a times the signal indicative of arterial pressure, plus a fourth constant d.
13. A cardiac pacemaker as defined in Claim 1, wherein said generating means comprises:
means for amplifying the signal indicative of arterial blood pressure, said amplifying means having a gain a and producing an amplified output; and means for summing said amplified output with a constant b to produce said rate input signal.
means for amplifying the signal indicative of arterial blood pressure, said amplifying means having a gain a and producing an amplified output; and means for summing said amplified output with a constant b to produce said rate input signal.
14. A cardiac pacemaker as defined in Claim 13, additionally comprising:
means for maintaining the long term resting heart rate at a predetermined rate.
means for maintaining the long term resting heart rate at a predetermined rate.
15. A cardiac pacemaker as defined in Claim 14, wherein said maintaining means comprises:
means for generating an error signal between said predetermined rate and said rate input signal; and means for periodically resetting said constant b in response to said error signal.
means for generating an error signal between said predetermined rate and said rate input signal; and means for periodically resetting said constant b in response to said error signal.
16. A cardiac pacemaker as defined in Claim 13, additionally comprising:
means for filtering said signal indicative of blood pressure to minimize harmonics due to the pulsatile nature of the arterial pressure waveform.
means for filtering said signal indicative of blood pressure to minimize harmonics due to the pulsatile nature of the arterial pressure waveform.
17. A cardiac pacemaker as defined in Claim 16, wherein said filtering means has a time constant of between 0.3 and 1.6 seconds.
18. A cardiac pacemaker as defined in Claim 1, wherein said generating means produces a rate input signal corresponding to a heart rate of between 60 and 170 beats per minute.
19. A cardiac pacemaker as defined in Claim 1, additionally comprising:
means for maintaining the long term resting heart rate at a predetermined level.
means for maintaining the long term resting heart rate at a predetermined level.
20. An artificial cardiac pacemaker for stimulating a heart to beat at a rate which is variable in response to physiological need, comprising:
a pulse generator or generating periodic electrical pulses at a rate determined by a rate input signal supplied to said pulse generator;
a pacing lead for delivering said periodic electrical pulse generated by said pulse generator to said heart;
a transducer located about an artery for producing a variable output signal indicative of arterial blood pressure in response to the stretch in the arterial wall caused by pressure change of blood within said artery;
means for amplifying the signal indicative of arterial blood pressure, said amplifying means having a gain a and producing an amplified output; and means for summing said amplified output with a constant b to produce said rate input signal.
a pulse generator or generating periodic electrical pulses at a rate determined by a rate input signal supplied to said pulse generator;
a pacing lead for delivering said periodic electrical pulse generated by said pulse generator to said heart;
a transducer located about an artery for producing a variable output signal indicative of arterial blood pressure in response to the stretch in the arterial wall caused by pressure change of blood within said artery;
means for amplifying the signal indicative of arterial blood pressure, said amplifying means having a gain a and producing an amplified output; and means for summing said amplified output with a constant b to produce said rate input signal.
21. An artificial cardiac pacemaker for stimulating a heart to beat at a rate which is variable in response to physiological need, comprising:
a pulse generator for generating periodic electrical pulses at a rate determined by a rate input signal supplied to said pulse generator;
a pacing lead for delivering said periodic electrical pulse generated by said pulse generator to said heart;
a transducer located about an artery for producing a variable output signal indicative of arterial blood pressure in response to the stretch in the arterial wall caused by pressure change of blood within said artery;
means for amplifying the signal indicative of arterial blood pressure, said amplifying means having a gain a and producing an amplified output;
means for summing said amplified output with a constant b to produce said rate input signal; and means for maintaining the long term resting heart rate at a predetermined rate.
a pulse generator for generating periodic electrical pulses at a rate determined by a rate input signal supplied to said pulse generator;
a pacing lead for delivering said periodic electrical pulse generated by said pulse generator to said heart;
a transducer located about an artery for producing a variable output signal indicative of arterial blood pressure in response to the stretch in the arterial wall caused by pressure change of blood within said artery;
means for amplifying the signal indicative of arterial blood pressure, said amplifying means having a gain a and producing an amplified output;
means for summing said amplified output with a constant b to produce said rate input signal; and means for maintaining the long term resting heart rate at a predetermined rate.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US07/085,421 US4791931A (en) | 1987-08-13 | 1987-08-13 | Demand pacemaker using an artificial baroreceptor reflex |
| US085,421 | 1987-08-13 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1330361C true CA1330361C (en) | 1994-06-21 |
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Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000571785A Expired - Fee Related CA1330361C (en) | 1987-08-13 | 1988-07-12 | Demand pacemaker using an artificial baroreceptor reflex |
Country Status (5)
| Country | Link |
|---|---|
| US (1) | US4791931A (en) |
| EP (1) | EP0307093B1 (en) |
| JP (1) | JPS6468279A (en) |
| CA (1) | CA1330361C (en) |
| DE (1) | DE3854775T2 (en) |
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| US3358690A (en) * | 1964-11-18 | 1967-12-19 | Marvin M Cohen | Heart stimulator utilizing a pressuresensitive semiconductor |
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| SE346468B (en) * | 1969-02-24 | 1972-07-10 | Lkb Medical Ab | |
| US4052991A (en) * | 1970-03-24 | 1977-10-11 | Fred Zacouto | Method of stimulating the heart |
| US3857399A (en) * | 1970-03-24 | 1974-12-31 | F Zacouto | Heart pacer |
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-
1987
- 1987-08-13 US US07/085,421 patent/US4791931A/en not_active Expired - Lifetime
-
1988
- 1988-07-12 CA CA000571785A patent/CA1330361C/en not_active Expired - Fee Related
- 1988-08-12 EP EP88307509A patent/EP0307093B1/en not_active Expired - Lifetime
- 1988-08-12 JP JP63201836A patent/JPS6468279A/en active Pending
- 1988-08-12 DE DE3854775T patent/DE3854775T2/en not_active Expired - Fee Related
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|---|---|
| EP0307093B1 (en) | 1995-12-13 |
| US4791931A (en) | 1988-12-20 |
| JPS6468279A (en) | 1989-03-14 |
| DE3854775D1 (en) | 1996-01-25 |
| DE3854775T2 (en) | 1996-08-08 |
| EP0307093A1 (en) | 1989-03-15 |
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