Download 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.