Transcript ENTC 4350
ENTC 4350 Pacemakers A pacemaker is a prosthetic device for the heart, first conceived in 1932 by Albert S. Hymen, an American cardiologist. • In 1952 the pacemaker was used clinically by Paul M. Zoll as an external device. With the advent of solid-state circuitry in the early 1960s, it was made into a battery-operated prosthesis that was implantable into the patient. • Credit for the implantable pacemaker is given to the American physicians William Chardack and Andrew Gage and to the engineer Wilson Greatbatch. Other heart prostheses, or spare parts, include coronary bypass vessels and artificial heart valves. • An especially innovative recent heart prosthesis is the artificial heart, the best known example of which is the Jarvik-7, designed by Robert K. Jarvik and implanted into Barney Clark by William DeVries in 1982. • Another implantable artificial heart was developed in 1985 at Penn State University by a team headed by William Pierce. A pacemaker is a prosthesis specifically for the sinoatrial (SA) node of the heart. • The SA node may become ineffective for several reasons, among them: 1. the SA node tissue or atrium may become diseased; or 2. the path of the heart depolarization—specifically, the atrioventricular (AV) node from the atrium to the ventricles—may become diseased, producing a heart block. Furthermore, bradycardia, a slowing of the heart rate generally to below 50 or 60 beats per minute (bpm), may develop because of aging or other reasons. • These diseases may be treated either with a pacemaker or with medicine, depending upon the case. In the case that the SA node fails to pace the heart properly, the ventricles may beat at their own self-paced rate, normally about 40 bpm. • At this heart rate, a patient may survive, but may not be able to function normally. Because the pacemaker is batteryoperated and surgically implanted, battery lifetime is one of the most important considerations. • The lifetime is determined primarily by the stimulus requirements, as well as the current caused by the pacemaker circuitry. The use of complementary metal-oxide semiconductor (CMOS) integrated circuits has dramatically reduced the current drain, but the stimulus requirements are determined by physiology and cannot be reduced effectively. As is usually the case with physiological stimuli, there is a curve of stimulus intensity versus duration associated with the physiological response of heart depolarization. The figure shows the stimulus voltage, Vs, at the tissueelectrode interface. Vs (V) Time, TD (ms) It has a stimulus duration TD, measured in milliseconds. • Such curves depend upon the electrode-heart resistance, RH, which may range from 100 to 1400 W. The value of RH may change over time because of tissue scarring at the electrode-tissue interface. • In order to produce a stimulus pulse, it is necessary to deliver energy to the electrode with a pacemaker circuit. A pacemaker in its simplest configuration is essentially a battery-operated digital pulse generator. A digital pulse has a voltage Vs that may be made variable to allow adjustments in the energy, EP, delivered by the pacemaker to the heart during each pulse. During the pulse duration, TD , the stimulus voltage drives energy into the heart. • When the pulse is OFF, it causes an energy drain given by VsIDT, where T is the time period between successive pulses, and ID is the current drain on the battery when the pulse is OFF. Therefore, the energy delivered by the pacemaker during each pulse is given as Vs2 EP TD Vs I DT (in J / pulse) RH EXAMPLE Using the figure, compute the energy per pulse when the pacemaker pulse width is 0.5 ms, the circuit-current drain is 1 mA, the heartelectrode resistance is 200 W, and the heart rate is 70 bpm. SOLUTION From the figure, Vs = 1.8 V. Also, T = (60/70) s. Then, 1.82 3 6 60 EP 0.5 10 1.8 10 ( J / pulse) 200 70 • Thus the energy used for each pulse is EP = 9.643 mJ/pulse Pacemaker Batteries Battery-operated equipment is convenient in many applications other than pacemakers because it can be used without a power cord, and it is safer because leakage currents are not usually present. • The disadvantage is that batteries are relatively large and of limited energy-storage capability. • Even so, the energy demand of the pacemaker is such that batteries with lifetimes between five and ten years are available. Mercury cells with two-year lifetimes, used in pacemakers in the past, have been made obsolete by lithium iodide cells which can last as long as 15 years before they need to be replaced. • Nuclear pacemaker batteries have been used to extend battery lifetimes to over 20 years, even for dual-chamber pacemakers that use higher amounts of battery power. Nuclear batteries pose an environmental hazard, however, because in an accident the radioactive material could be released into the environment. • Nuclear batteries are being considered for artificial implanted hearts also, because of the potential for high energy storage, but this research is only beginning. Rechargeable batteries are not widely used for low power pacemaker application, since their shelf life is no longer than that of a lithium iodide pacemaker battery in normal use. The lifetime of a storage battery depends on both its ampere-hour (A-H) rating and its shelf life. • Shelf life is limited self-discharge of the battery due to internal leakage currents, particle migration, formation of insulating layers, and internal shorts. An illustrative example of a battery A-H rating versus its current drain is given in the figure. At high current drain, polarization of the metal electrolyte boundary increases the internal resistance of the battery and decreases the A-H rating. Implantable batteries are usually encased in metal. • If they become too hot, such as when shorted, the case may rupture. • Pacemaker design should ensure that the case is strong enough to contain such a rupture and prevent toxic materials from entering the body of the patient. Illustrative Pacemaker Characteristics The pacemaker consists of three major components: • the lead wire, • the electronic pulsing circuit, and • the battery. The lead can cause a failure due to metal fatigue, introduced by the motion and beating of the heart. • To avoid such fatigue, the lead may be constructed by winding platinum ribbon around polyester yarn. • Each lead may have three such wires for redundancy. The pacemaker electrode must make a secure contact with the heart for several years. To ensure this, two methods of implantation are used under the following classifications: • (1) endocardial lead, in which the pacemaker • lead is inserted through a major vein through a catheter guide into the right ventricle of the heart; and (2) epicardial lead, in which the pacemaker electrode is sutured to the external wall of the heart during open-heart surgery, and a wire electrode is thereby secured into the tissue. For endocardial lead implantation the electrode may be attached with tines. The tines are pushed into the Purkinje muscle fibers of the ventricle and latch themselves in place. • The porous electrode tip minimizes motion between the tip and the tissue so as to reduce the scar tissue buildup. • This tends to keep the contact resistance low. The electrode may also be held in place with a helical wire that is screwed into the tissues with a twisting motion. In this case a bipolar electrode a few centimeters behind the contact electrode serves as a return path for current to the pacemaker. In the unipolar pacemaker lead, the second electrode is eliminated, and the return conductive path to the pacemaker is made through body fluids. • A unipolar lead electrode may also be held in place by either sutures, tines, or a helical wire. The electrode-muscle contact can change after a time because of (1) polarization by ionic current flow; (2) tissue and scar growth; or (3) mechanical motion of the heart. A symptom of such change may be an increased electrode impedance. • The problem may be fixed by increasing the pulse voltage from the pacemaker or by lengthening its duration. • Loss of contact altogether may require surgical reimplantation. PROGRAMMABLE PACEMAKERS The implantable pacemaker is presented as a battery-powered, digital pulse generator, and it may be considered an asynchronous type of unit. • Other types of pacemaker include the R-wave synchronous, R-wave inhibited, and P-wave synchronous pacemakers. The asynchronous pacemaker produces a pulse at a preset rate, for example 70 bpm, and delivers pulses to the heart regardless of the heart’s natural beating tendency and independent of the QRS complex. • This pacemaker does not increase the heart rate in response to the body demand for more blood during exertion. However, a P-wave synchronous pacemaker does. • The SA node depolarization responds to body demands through the vagus nerve and hormones transported in the blood. In a P-wave synchronous pacemaker, the SA node triggers the pacer, which in turn drives the ventricle. • It is used when the AV node is blocked because of disease. As shown, this pacemaker requires two leads. • • The atrial lead feeds the atrial pulse back to a sensing amplifier. The driver, connected to the ventricle, delivers the pacing pulse. The R-wave inhibited pacemaker also allows the heart to pace at its normal rhythm when it is able to. • However, if the R-wave is missing for a preset period of time, the pacer will supply a stimulus. Therefore, if the pulse rate falls below a predetermined minimum, the pacemaker will turn on and provide the heart a stimulus. • For this reason it is called a demand pacemaker. Another type of demand pacemaker uses a piezoelectric sensor shielded inside the pacemaker casing. • When this sensor is slightly stressed or bent by the patient’s body activity, the pacemaker will automatically increase or decrease its rate. According to Medtronic, Inc., their model will react to a movement of one-millionth of an inch. • It will change heart rates to as high as 150 bpm during vigorous activity or as low as 60 bpm during rest periods. A programmable pacemaker is one that can be altered both in its block diagram and in the size and rate of the pulse it delivers. A pacemaker that can be reconfigured into four different block diagrams, after having been implanted. A magnet may be placed over the pacemaker on the skin of the patient in order to activate a reed switch, which switches the pacemaker into one of the four configurations shown. • Another kind of programming is done to alter the delivered stimulus and the pacemaker sensitivity to feedback signals. A programmable pacemaker is shown in the figure. The telemetric programmer may be placed over the pacemaker to select pulse rates ranging from 30 to 155 bpm, feedback sensitivities from 0.7 to 4.5 V, pulse amplitudes from 2.5 to 10 V, and pulse widths from 0.25 to 1 ms, among other parameters. A hard copy of the programming record is provided by the printer shown. When temporary heart pacing is needed, an external pacemaker may be used. • Since this device is not implanted, there is no need for extensive surgery. A temporary pacing lead uses a balloon tip, so that the flow of blood will carry the pacing electrode into the heart when the balloon is inflated. The ECG Pattern and Cardiac Pacing The figure shows the appearance of the normal ECG signal as measured in the atrium. Notice the large P wave, which is almost as high as the normal QRS complex. In contrast, this figure shows the effect of adding a continuously operating pacemaker signal to the normal atrium. Now the heart is responding only to the pacemaker, and the pacemaker is said to have “captured” the heart rate. • Note that the QRS wave follows the pacemaker-generated P wave at a fixed interval, and that there are no beats generated sinoatrial (SA) node. The pacemaker signal is large enough and occurs at a high enough rate to keep the SA node in the depolarized condition so that it cannot fire. • This is important, because an occasional, ectopic, SA-node beat would be entirely out of synchronization with the regularly occurring pacemaker beat. Eventually, a pacemaker-induced pulse would occur during the latter part of the QRS complex or during the T wave from the ectopic SA-node beat, and this would be trouble. • It turns out that disease-weakened hearts are more sensitive than healthy hearts to any signal that arrive during the latter part of the QRS complex or the T wave, and such a weakened heart will go into fibrillation if a pacemaker beat and either of these signals happen to coincide. To avoid this hazard, the pacemaker signal is set large enough to preclude the occurrence of any inadvertent SAnode beats. The above mode of pacemaker operation was always used when pacemakers were first invented, and it is still used if the P or QRS waves are weak, very irregular, or entirely absent. • This mode has its problems in that no adjustment can be made for the normal change in heart rate from resting to exercise. Usually a rate of 70 heats per minute is used as a compromise. • The requirement for continuous operation is reflected in reduced battery life, and the pacemaker has to be changed more often. If a patient has a more nearly normal heart, there may be no need for continuous pacing, and the unit is set in the “demand” mode. • In this mode, the pacemaker detects the peak of the QRS wave and begins “counting.” If the next QRS wave occurs within what is called the “capture interval,” the pacemaker does nothing. • If the QRS wave is late or absent, the pacemaker stimulates the heart. • Here again, the locus of can be in the atrium or in the right ventricle, as necessary. If the QRS wave stops entirely, the pacemaker will stimulate the heart at about 70 beats per minute; • One might say that it switches from the “demand” mode to the “continuous” mode. Demand operation results in longer battery life and allows the patient to benefit from the normal heart-rate control system that adjusts the beat to the demands of the body. • The pacemaker is available for action if and when the patient’s own heart-rate control system should fail.