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Pacemaker II
Lecture (6)
Lead wires and electrodes:
• Lead wires in addition to being good electrical
conductors, it must be mechanically strong.
• It should maintain good electrical insulation.
• The lead wires consists of
helical coils of spring-wire
alloy molded in a siliconrubber or polyurethane
cylinder.
• The helical coiling of the
wire minimizes stresses
applied to it, and the
multiple strands serve as
insurance against failure
of the pacemaker
• The soft compliant siliconrubber encapsulation both
maintains flexibility of the
lead-wire and provides
electrical insulation and
biological compatibility.
• Cardiac pacemaker are
either of unipolar or
bipolar type.
• Bipolar and unipolar
pacing have different
appearances on surface
ECGs.
• If the current flows
between
the
two
electrodes
on
the
pacemaker lead (the tip
and the ring), this is
referred to as bipolar
pacing.
• If the current flows between the tip of the lead and the
pacemaker generator, this is referred to as unipolar
pacing.
• In unipolar pacing, the current travels through a large
area of the body between the tip of the lead and the
pulse generator.
• Unipolar pacing, therefore, creates a large stimulus
artifact on the surface ECG.
• It may stimulate electrically excitable tissue, other than
the heart, which lies in the path of the current.
• The bipolar pacing stimulus may be very
difficult to see on the surface ECG, because in
bipolar pacing the distance between the two
poles that deliver current (i.e. the tip and the
ring of the pacemaker lead) is very small
(about a centimeter).
• It will also be noted that leads that are used
for bipolar pacing must have two insulated
wires within its outer insulation: one wire for
the negative pole (the tip), and one for the
positive pole (the ring).
Single and dual chamber:
• The word chamber in dual or single chamber
pacing refers to a chamber of the heart in which a
lead is placed.
• A single chamber pacemaker usually has a pacing
lead in either the right atrium, or the right
ventricle. These would be called, respectively, an
atrial single chamber pacemaker and a
ventricular single chamber pacemaker.
• A standard dual chamber pacemaker has a lead in
the right atrium and a lead in the right ventricle.
Synchronous pacemaker
• Many patients requires cardiac pacing only
intermittently, because they can establish a
normal cardiac rhythm between periods of
block.
• For these patients, it is not necessary to
stimulate the ventricles continuously.
Types of synchronous pacemakers:
(1) Demand pacemaker.
(2) Atrial-synchronous pacemaker.
(3) Rate-responsive pacemaker.
• A demand pacemaker consists of timing circuit, an
output circuit, and electrodes, just like those of the
asynchronous pacemaker but it has a feedback loop
as well.
• The timing circuit is set to run at a fixed rate,
usually 60 to 80 beats/min.
• After each stimulus, the timing circuit resets itself
waits the appropriate interval to provide the next
stimulus, and then generates the next pulse.
• If during this interval a natural beat occurs in the
ventricle, the feedback circuit detects the QRS
complex of the ECG signal from the electrodes
and amplifies it.
• This signal is then used to reset the timing circuit.
It waits its assigned interval before producing the
next stimulus.
• If the heart beats again before this stimulus is
produced, the timing circuit is again reset and the
process repeats itself.
• Thus we see that, when the heart conduction
system is operating normally and the heart has a
natural rate that is greater than the rate set for
the timing circuit, the pacemaker remains in a
standby mode, and the heart operates under its
own pacing control.
Atrial synchronous pacemaker:
• It is a more complicated circuit, the pacemaker is
designed to replace the blocked conduction system
of the heart.
• The heart’s physiological pacemaker located at the
SA node, initiates the cardiac cycle by stimulating the
atria to contract and then providing a stimulus to the
AV node which after appropriate delay stimulates the
ventricle.
• If the SA node is able to stimulate the atria, the
electric signal corresponding to atrial contraction can
be detected by an electrode implanted In the atrium
and used to trigger the pacemaker in the same way
that is triggers the AV node.
• The figure below shows the voltage V1 that is
detected by the atrial electrodes.
• The voltage is a pulse that corresponds to each beat.
• The atrial signal is then amplified and passed through a gate
to a monostable multivibrator giving a pulse V2 of 120-ms
duration, the approximate delay of the AV node.
• Another monostable multivibrator giving a pulse duration of
500 ms is also triggered by the atrial pulse.
• It produces V4, which causes the gate to block any signals
from the atrial electrode for a period of 500-ms following
contraction.
• This eliminates any artifact caused by the ventricular
contraction from stimulating additional ventricular
contraction.
• The pulse V2 acts as a delay, allowing the ventricular
stimulus pulse V3 to be produced 120-ms following atrial
contraction.
• Then V3 controls an output circuit that applies the
stimulus to appropriate ventricular electrodes.
Rate-responsive pacing:
• The demands of the body during stressful activities
such as exercise cannot be fully met by the previous
pacemakers.
• A new type of pacemaker system that can overcome
these limitations is evolving.
• This pacemaker includes a control system.
• A sensor is used to convert a physiological variable in the
patient to an electric signal that serves as an input to the
controller circuit.
• This type of pacemaker is programmed to control the
heart rate on the basis of the physiological variable that
is sensed.