3/27/2012
UNIVERSITY
OF SYDNEY
CONTROL SYSTEM OF A PACEMAKER
AMME3500 Assignment ONE |
AMME3500
Operation of a Pacemaker
Control systems have become an extremely important part of modern society, these systems can exist
naturally in the world as common natural processes are fulfilled or they are automatically created systems
by man. A control system is simply designed to produce a desired output with desired performances given
a specified input.
What are the Objectives and Specifications of a
Pacemaker?
Each control system has its own objectives and
specifications depending on the purpose of the
product. In the biomedical industry, there is a
constant need to create systems and devices that
perform and adjust tasks in the human body that may
have problems operating naturally. Arrhythmia for
example is the condition whereby the human heart
either beats too slow, too fast, or with an irregular
rhythm. With such a condition comes the need to
introduce a device that can resume normal heart
rhythm and allow the patients to live an active
lifestyle.
A pacemaker is a small device that’s created to deal
with conditions such as arrhythmia; its objective is to
restore natural heart rhythm. A pacemaker is placed
in the chest or abdomen and helps control abnormal
heart rhythms. This device uses electrical pulses to
prompt the heart to beat at a normal rate.
What are the actuators that are used by a
pacemaker?
In almost all systems there exists both sensors and
actuators, which brings rise to explaining the
difference between the two in order to avoid
misconception or ambiguity. A sensor is any device
that detects a change in the surrounding
environment, whereas an actuator is a device that
performs an action, or makes an adjustment in an
operational system based on the information received
from the sensors.
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How does a Pacemaker work?
A pacemaker system consists of a battery, a
computerized generator, and wires with sensors
called electrodes on one end. The battery powers the
generator, and both are surrounded by a thin metal
box. The wires connect the generator to the heart.
A pacemaker monitors and helps control your
heartbeat. The electrodes detect your heart's
electrical activity and send data through the wires to
the computer in the generator, hence the electrodes
act as sensors.
If your heart rhythm is abnormal, the computer will
direct the generator to send electrical pulses to your
heart. The pulses then travel through the wires to
reach your heart, hence the generator acts as the
actuator by implementing the electrodes to perform
an action or make an adjustment.
Draw a block diagram explaining the system. Label
the sensors, actuators and feedback paths.
A simply pacemaker is comprised of at least 3
different parts; the electrical pulse generator, a power
source (battery) and an electrode (lead) system. With
these three parts in consideration, a basic pacemaker
functional block diagram can be organised as shown in
the figure below.
However throughout the development in technology,
the system in a pacemaker has become more
intellectual and slightly complicated. With the need to
maintain battery life, sensors were introduced to
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AMME3500
Operation of a Pacemaker
measure some relevant parameters from the body
and an algorithm in the pacemaker, which is able to
adjust the pacemaker response in accordance with
the measured quantity.
A block diagram of modern pacemaker is given below.
The system is based on a pacemaker having a demand
pulse generator, which is sensitive to the measured
parameter.
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appropriate pacing therapy to be delivered by
the output driver.
If the pacing is correct, then it is fed through
to the electrodes and the electrodes stimulate
the heart, and the adjustment is made. If
however the pacing is incorrect, then it is
returned to the sense amplifier, where the
current state of the heart is determined again.
A simplified mathematical model of the plant
dynamics of a Pacemaker. The assumptions made in
preparing the mathematical model.
Before determining an appropriate mathematical
model, certain assumptions need to be taken into
consideration.
1) Primary pacemaking cannot be attributed to any
single current but arises from both the lack of a
background K+ current and a complex interaction
between Ca2+, delayed-rectifier K+, and background
leak currents.
How is the feedback in the pacemaker control
system implemented?
In the block diagram above it can be illustrated that:
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The sensors (electrodes) detect a change in
the rhythm of the heart, this change excites
the pacemaker creating a feedback system.
A signal (via electrical current) is sent to the
control section, informing it of the change.
The control section then informs the
generator (actuator) that an action needs to
be performed.
The generator has its own block diagram (loop)
within in order to determine the correct
adjustment that will restore the heart to its
normal environment, and perform the action
by sending the appropriate current to the
electrodes.
The sense amplifier plays a fundamental role
in providing information about the current
state of the heart.
The sense amplifier feeds a single event to the
timing control, which decides upon the
2) Ca2+ current displays complex behavior and is
important during repolarization.
3) Because of Ca2+ buffering by myoplasmic proteins,
the Na(+)-Ca2+ exchanger current is small and has
little influence on action potential repolarization but
may modulate the maximum diastolic potential.
4) The Na(+)-K+ pump current does not play an active
role in repolarization but is of sufficient size to
modulate the rate of diastolic depolarization.
With the effect of ion channels at hand Yasutaka
Kurata along with his team developed a mathematical
model in 2003 to describe the dynamics of the
pacemaker and show the effects of ions on the pacing
itself.
The complete model for the normal pacemaking
includes 12 membrane current components. The
differential equation however for the membrane
potential (V) is:
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AMME3500
Operation of a Pacemaker
where ICa,L and ICa,T represent the L-type and T-type
Ca2 channel currents, respectively, and the rapid and
slow components of the delayed-rectifier K current (IK)
are denoted as IKr and IKs, respectively.
References
1. R. Sutton and I. Bourgeois, “The foundations
of cardiac pacing. Part I,” Futura Publishing
Company, 1991.
2. B. V. Berkovits, “Atrial and Ventricular
Demand Pacer,” US Patent 3,595,242, Jul.
1971.
3. Sandro A. P. Haddad1, Richard Houben2 and
Wouter A. Serdijn1 “Smart Pacemakers”, The
evolution of pacemakers.
4. National heart lung and blood institute,
pacemakers
www.nhlbi.nih.gov/health//dci/Diseases/pac
pace_duringsurgery.html
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