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ELECTROCARDIOGRAPHY
(ECG)
By: Mohammed Attia
SBE-310 Medical Equipment I

2.
Outlines
■ ECG definition
■ Medical uses
■ Cardiac electrophysiology
■ ECG machine
■ ECG machine types
■ Safety and protection
■ Sources of artifacts
■ Electrocardiogram grid
■ 12-lead EKG
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3.
Outlines
■ ECG definition
■ Medical uses
■ Cardiac electrophysiology
■ ECG machine
■ ECG machine types
■ Safety and protection
■ Sources of artifacts
■ Electrocardiogram grid
■ 12-lead EKG
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4.
ECG definition
■ Electrocardiography (ECG or
EKG) is the process of recording
the electrical activity of the heart
over a period of time using
electrodes placed on the skin.
■ These electrodes detect the tiny
electrical changes on the skin that
arise from the heart muscle's
electrophysiologic pattern of
depolarizing and repolarizing
during each heartbeat.
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5.
Outlines
■ ECG definition
■ Medical uses
■ Cardiac electrophysiology
■ ECG machine
■ ECG machine types
■ Safety and protection
■ Sources of artifacts
■ Electrocardiogram grid
■ 12-lead EKG
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Medical uses
■ The goal of performing electrocardiography is to:
– obtain information about the structure and function of the
heart.
■ Numerous diagnosis and findings can be made based upon
electrocardiography:
– Suspected myocardial infarction (heart attack)
– Suspected pulmonary embolism (it is a blockage of an artery in
the lungs by a substance that has traveled from elsewhere in the
body through the bloodstream, this results on shortness of
breath)
– A third heart sound, fourth heart sound (Extra heart sound due
to abnormality in the heart)
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Medical uses (continue…)
– A cardiac murmur (are heart sounds produced when blood
flows across one of the heart valves that are loud enough to
be heard with a stethoscope.)
– Several heart arrhythmias (irregular heartbeat)
– Epileptic seizure
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8.
Outlines
■ ECG definition
■ Medical uses
■ Cardiac electrophysiology
■ ECG machine
■ ECG machine types
■ Safety and protection
■ Sources of artifacts
■ Electrocardiogram grid
■ 12-lead EKG
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Cardiac electrophysiology
■ During each heartbeat, a healthy heart has
an orderly progression of depolarization.
■ It starts with pacemaker cells in the
sinoatrial node (SA node), spreads out
through the atrium
■ It passes through the atrioventricular node
(AV node) down into the bundle of His
and into the Purkinje fibers, spreading
down and to the left throughout the
ventricles.
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Cardiac electrophysiology (continue…)
■ The p-wave represents depolarization of
the atria. Atrial depolarization spreads
from the SA node towards the AV node,
and from the right atrium to the left atrium.
■ The QRS complex represents the rapid
depolarization of the right and left
ventricles.
■ The T wave represents the repolarization
of the ventricles.
■ The U wave is hypothesized to be caused
by the repolarization of the interventricular
septum.
– It normally has a low amplitude, and
even more often is completely absent.
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Cardiac electrophysiology (continue…)
■ ECG pattern recognition is based on four factors:
– depolarization of the heart toward the positive
electrode produces a positive deflection
– depolarization of the heart away from the
positive electrode produces a negative deflection
– repolarization of the heart toward the positive
electrode produces a negative deflection
– repolarization of the heart away from the
positive electrode produces a positive deflection
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12.
Outlines
■ ECG definition
■ Medical uses
■ Cardiac electrophysiology
■ ECG machine
■ ECG machine types
■ Safety and protection
■ Sources of artifacts
■ Electrocardiogram grid
■ 12-lead EKG
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13.
ECG machine
■ An electrocardiograph is a machine that is used to perform
electrocardiography, and produces the electrocardiogram.
■ Its major components:
1. ECG main unit
2. Cables
3. ECG electrodes
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ECG machine
1. ECG main unit:
– The fundamental component to electrocardiograph is the
Instrumentation amplifier,
■ which is responsible for taking the voltage difference between leads
and amplifying the signal.
■ ECG voltages measured across the body are on the order of hundreds of
microvolts up to 1 millivolt (the small square on a standard ECG is 100
microvolts).
– Early electrocardiographs were constructed with analog electronics.
– Today, electrocardiographs use analog-to-digital converters to
convert to a digital signal that can then be manipulated with digital
electronics.
■ This permits digital recording of ECGs and use on computers.
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ECG machine
2. Electrodes
– It is a conductive pad in contact with the body that makes an
electrical circuit with the electrocardiograph.
– The most common type is self-adhesive circular pad.
– Each electrode consists of an electrically conductive electrolyte gel
and a silver/silver chloride conductor.
3. Cables
– It connects the main unit with electrodes
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16.
Outlines
■ ECG definition
■ Medical uses
■ Cardiac electrophysiology
■ ECG machine
■ ECG machine types
■ Safety and protection
■ Sources of artifacts
■ Electrocardiogram grid
■ 12-lead EKG
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ECG Machine Types
1. The 12-lead ECG: is the standard ECG machine used for medical diagnostic
testing. It uses 10 electrodes.
2. A 5-lead ECG machine: it uses 5 electrodes, 4 of which are placed on each of
the limbs and 1 on the chest.
 It is usually used for continuous monitoring, like during a major surgical
procedure or while a patient is being transported in an ambulance.
3. A 3-lead ECG machine: it uses 4 electrodes placed on each of the limbs, which
is enough to produce adequate data on heart rhythm monitoring.
4. Portable handheld ECG monitors: are the smallest ECG machines in the market
today. In spite of their portable and handheld size, they can display and record
heart rate like their larger counterparts.
5. Wireless ECGs: are similar to a standard ECG machine, but without the wires.
 The electrodes used have wireless units that transmit data to the computer
or telemetry station.
 A wireless ECG provides more functionality and comfort for patients, who
wouldn’t have to worry about wires attached to their bodies.
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18.
Outlines
■ ECG definition
■ Medical uses
■ Cardiac electrophysiology
■ ECG machine
■ ECG machine types
■ Safety and protection
■ Sources of artifacts
■ Electrocardiogram grid
■ 12-lead EKG
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Safety and protection
■ Voltage protection for the patient and operator:
– Since the machines are powered by mains power, it is conceivable that
either person could be subjected to voltage capable of causing death.
■ Defibrillation protection:
– Any ECG used in healthcare may be attached to a person who requires
defibrillation and the electrocardiograph needs to protect itself from this
source of energy.
■ Electrostatic discharge:
– is similar to defibrillation discharge and requires voltage protection up to
18,000 volts.
■ The right leg driver circuit:
– can be used to reduce common-mode interference which is Interference
that appears on both signal leads (typically the 50/60 Hz mains power).
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20.
Outlines
■ ECG definition
■ Medical uses
■ Cardiac electrophysiology
■ ECG machine
■ ECG machine types
■ Safety and protection
■ Sources of artifacts
■ Electrocardiogram grid
■ 12-lead EKG
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Sources of artifacts
■ Artifacts are distorted signals caused by a secondary internal or external
sources, such as:
– muscle movement
– interference from an electrical device
■ Improper lead placement
– for example, reversing two of the limb leads
– It has been estimated to occur in 0.4% to 4% of all EKG recordings
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22.
Outlines
■ ECG definition
■ Medical uses
■ Cardiac electrophysiology
■ ECG machine
■ ECG machine types
■ Safety and protection
■ Sources of artifacts
■ Electrocardiogram grid
■ 12-lead EKG
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Electrocardiogram grid
■ ECGs are normally printed on a grid.
■ The horizontal axis represents time and
the vertical axis represents voltage.
■ The standard values on this grid are
shown in the adjacent image:
– A small box is 1 mm x 1 mm big and
represents 0.1 mV x 0.04 seconds.
– A large box is 5 mm x 5mm big and
represents 0.5 mV x 0.2 seconds
wide.
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24.
Outlines
■ ECG definition
■ Medical uses
■ Cardiac electrophysiology
■ ECG machine
■ ECG machine types
■ Safety and protection
■ Sources of artifacts
■ Electrocardiogram grid
■ 12-lead EKG
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12-lead EKG
■ 12 lead ECG machine uses 10 electrodes !!!
■ What is the difference between Electrodes and leads ???
– Electrode is a conductive pad in contact with the body that
makes an electrical circuit with the electrocardiograph,
– a lead is a connector to an electrode.
– Since leads can share the same electrode, a standard 12-
lead EKG happens to need only 10 electrodes
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12-lead EKG (continue…)
■ Electrode placement:
1. RA
2. LA
3. RL
4. LL
5. V1
6. V2
7. V3
8. V4
9. V5
10. V6
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12-lead EKG (continue…)
■ The leads used in an ECG machine can be
divided into two types:
– Bipolar leads:
■ record voltage difference between two
electrodes.
■ EX: lead I, II, and III
– Unipolar leads:
■ record the voltage difference between a
reference electrode and the body surface
to which they are attached.
■ EX: lead aVR, aVL, aVF, V1, V2, V3,
V4, V5, and V6
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12-lead EKG (continue…)
■ Leads are broken down into three sets:
1. Limb leads
 Lead I, II, and III
2. Augmented limb leads
 Lead aVR, aVL, and aVF
3. Precordial Leads
 Lead V1, V2, V3, V4, V5, and V6
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12-lead EKG (continue…)
1. Limb leads:
– Lead I is the voltage between the (positive) left arm (LA) electrode
and right arm (RA) electrode: I = LA – RA
– Lead II is the voltage between the (positive) left leg (LL) electrode
and the right arm (RA) electrode: II = LL – RA
– Lead III is the voltage between the (positive) left leg (LL) electrode
and the left arm (LA) electrode: III = LL - LA
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12-lead EKG (continue…)
2. Augmented limb leads:
– Lead augmented vector right (aVR)' has the positive electrode on the right arm.
The negative pole is a combination of the left arm electrode and the left leg
electrode.
– Lead augmented vector left (aVL) has the positive electrode on the left arm. The
negative pole is a combination of the right arm electrode and the left leg electrode.
– Lead augmented vector foot (aVF) has the positive electrode on the left leg. The
negative pole is a combination of the right arm electrode and the left arm
electrode.
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12-lead EKG (continue…)
3. Precordial leads:
– The precordial leads lie in the transverse (horizontal) plane,
perpendicular to the other six leads.
– The six precordial electrodes act as the positive poles for the six
corresponding precordial leads: (V1, V2, V3, V4, V5 and V6).
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– Wilson's central terminal is used as the
negative pole.
■ The common lead, Wilson's central terminal VW,
is produced by averaging the measurements from
the electrodes RA, LA, and LL to give an
average potential across the body.

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