ECG Basics Module 1
Dr. Jeffrey Elliot Field, HBSc. DDS,
Fellow, American Dental Society of Anesthesia
Diploma, the National Dental Board of Anesthesia.
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Introduction to Module 1
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Objectives
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1) To learn how to properly set up your ECG leads.
2)To Learn What a lead is.
3)To learn the anatomy of a normal ECG.
4) To define Normal Sinus Rhythm.
5) To quantify the various components of a normal
ECG
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EQUIPMENT
The “Three Lead” ECG utilized
in most offices for dental
procedures. However more and
more practioners are using 5
lead ECG’s.
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What IS A LEAD ?
The term lead refers to the placement of
electrodes in relationship to the heart.
By looking at the electrical potential
differences from different placements of
positive and negative leads/electrodes
one can get a view of the electrical
activity of different areas of the heart.
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 So think of lead one, lead two, lead three etc. simply
as different views of the heart.
 By knowing which area of the heart you are looking
at you can more easily pinpoint the areas where
arrhythmias originate
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 The five lead ECG is becoming a standard feature on
all new monitors.
 The 7 leads you can monitor are:
I II III
AVR AVL AVF and one precordial lead (usually)V5
 This allows more precise diagnosis of cardiac events
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Augmented Voltage Leads: aVR,
aVL aVF; unipolar ; form a set of axes
60° apart but are rotated 30° from
the axes of the standard limb leads.
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 Chest Leads: Vl, V2, V3, V4, V5, V6, explore the
electrical activity of the heart in the horizontal
plane; i.e., as if looking down on a cross section of
the body at the level of the heart.
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This is a 12 lead ECG or simply 12 different
views of the heart.
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Lead Placement for a 3 Lead ECG
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Lead Placement for Five Lead
WHITE RIGHT, RED RIBS,
BLACK LEFTOVER, PLUS
GREEN RIGHT RIB AND
BROWN MID CHEST
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The Lead you are looking at depends on
the charge of the leads in relationship to
their position in the triangle. The
following picture shows how the ECG
machine changes the charges to show
different leads. But the physical position
of the white red and black leads does
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not change.
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G
G
G
Note the ground lead is
in the 3rd position of the
triangle ( G)
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In Emergency
 Patients can be monitored with only 2 Leads attached.
 These is done either with the Defibrillator Paddles or
with Defibrillator Patches
Note the placement in each case is upper right
and lower left chest which will sandwich the
heart in between the electrodes.
Which coincidentally is one of the correct
placements for defibrillation and will also work
for external pacing .
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Remember all that an ECG is
looking at is the electrical
activity and electical activity is
not always associated with
contraction. ( SEE EMD/PEA
LATER).
So never forget to check a manual
pulse in an emergency.
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The depolarization wave produces a wave of
atrial contraction, which is called the P wave
The ventricular depolarization is represented by
an abrupt waveform called QRS wave
Ventricular repolarization is represented by the T
wave
-P-waves are regular and upright
-Each P-wave is followed by a QRS Complex
-QRS complex are regular at a rate of 60-100
beats per minute
-T-waves are upright and follow the QRS
complexes
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Pacemaker Cells and Sites
 Each area in the conduction system has its own
inherent rate of firing in descending order from the SA
Node.
 If the area above a site fails to send an impulse ( or that
impulse is blocked) the next pacemaker site will take
over.
 Therefore by knowing the rates of each site you can get
another clue as to the area of damage
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THERE ARE 5 COMPONENTS TO A RYTHYM
STRIP
P
Q
R
S
T
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P WAVE
The P wave represents atrial
depolarization
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Q WAVE
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Q wave is the first negative deflection prior to any R
wave
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This wave represents depolarization of the
intraventricular septum
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R WAVE
 R wave is the first positive deflection
 This represents depolarization of the bulk of the
ventricular muscle.
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S WAVE
S wave is the negative deflection
following and R wave
It represents the late
depolarization of the last bit of
ventricular muscle.
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T wave
 T wave represents ventricular repolarization. The ventricle
prepares to fire again
 Normally upright in leads I, II, and V3-V6
 Variable in the other leads III, AVL, AVF, and V1-V2
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Further Defining Normal Sinus
Rhythm
Anatomy of an ECG (Normal Cardiac
Timing/Intervals)
 There are 6 intervals /timings during the cardiac
cycle. All are important except for the T-wave
interval which is usually not measured.
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-P wave ( 0.1 seconds)
-PR interval ( 0.12-0.2 seconds)
-Q wave ( 1 small box deep {0.04 sec} or less than 25%
of the R-wave)
-QRS interval ( 0.10 second)
QT interval (0.425 seconds)
-T wave ( not usually measured)
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Time Sequences on ECG Strips
The strip is read from
left to right in seconds
and up and down on
millivolts.
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Cardiac Intervals
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The Cardiac Cycle In Detail
P Wave Size and Morphology
 Normal duration is less than 0.11 seconds wide( or 3
small boxes) and less than 2.5 mv high or less than 2.5
boxes high.
 The P-wave should be upright in leads II, III, and AVF
 Over 0.12 suggests an intra-atrial conduction defect
 The normal p-wave morphology looks like this.
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Q wave
 The Q-wave is the first negative deflection after the p-
wave
 It should not exceed 0.03-0.04 millivolts in length or 1
small box.
 Pathological Q waves
are defined as those that
are 25% or more of the
height of the R wave and/or
greater than 0.04 seconds in height.
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T WAVE
Not usually measured but its
morphology is looked at in
evaluating potassium levels
in patients-see a later
module.
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Cardiac Intervals
PR INTERVAL
Normal duration is 0.12-0.20
seconds or 4-5 small boxes
This interval is measured from the
beginning of the p-wave to the
beginiing of the Q-wave
This interval is used to diagnose
heart blocks and accessory
pathways
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QRS INTERVAL
Normal is 0.10 or less than 3
small boxes.
Wide QRS complexes are
indicative of a blockage at or
above the AV node.
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QT Interval
 Normal is below 0.425 seconds or
around 10 small boxes.
 If abnormally prolonged or shortened,
there is a risk of developing Ventricular
Arrhythmias.
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Cardiac Intervals
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Thank you for viewing this
presentation.