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					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 battery-
    operated 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 tissue-
    electrode 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, T D , 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 heart-
    electrode 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,

    EP 
         1.82
         200
                      3
                              
              0.5  10  1.8 106 60
                                  70
                                     ( J / pulse)

    • 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 SA-
    node 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.

				
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posted:11/24/2011
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