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Understanding EKGs
A Practical Approach
For the Dental Hygienist
CHAPTER
5
The Electrocardiogram
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
The Electrocardiogram
• Objectives
– Define the term electrocardiogram
– Describe the basics of EKG monitoring
– List the types of EKG leads
– Discuss the relevance of Einthoven’s
triangle
– Identify and explain the grids and markings
on a representative strip of EKG graph
paper
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
The Electrocardiogram
• Objectives (continued)
– Describe the relationship of the following EKG
waveforms to the electrical events in the heart
 P wave
 PR interval
 QRS complex
 ST segment
 T wave
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
The Electrocardiogram
• Electrocardiogram
– Graphic representation of the electrical
activity of the heart
• Electrocardiograph
– Machine used to record the
electrocardiogram, or EKG machine
• EKG
– Graphic tracing of electrical activity of the
heart, not mechanical activity
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
The Electrical Basis of the EKG
• Electrical activity is
sensed by electrodes
placed on the skin
surface
• Recorded in the form of
an electrocardiogram
• Cardiac monitor depicts
electrical impulses on
monitor screen or
oscilloscope
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
The Electrical Basis of the EKG
• Electrical impulses present on the skin
surface are very low voltage; impulses are
amplified by an EKG machine
• The printed record of the electrical activity
of the heart is called a rhythm strip or an
EKG strip
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
EKG Leads
• Electrode
– An adhesive pad that contains conductive gel
and attaches to patient’s skin
• Leads
– How electrodes are connected to the cardiac
monitor
– 3 leads must have a positive, a negative,
and a ground
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
EKG Leads
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
EKG Leads
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
EKG Leads
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
EKG Leads
• Einthoven’s triangle
– Imaginary inverted triangle
formed around heart by
proper placement of bipolar
leads
– Top formed by lead I, right
side formed by lead III, and
left side formed by lead II
– Each lead represents a
different look at, or view,
of the heart
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
EKG Graph Paper
• Leaves the machine at
constant speed of 25
mm/s
• Time
– Measured on horizontal
line
• Amplitude or voltage
– Measured on vertical line
on graph paper
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
EKG Graph Paper
• Electrocardiographic
– Paper divided into
small squares
– 1 mm in width
– Time interval of 0.04
seconds
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
EKG Graph Paper
• Electrocardiographic
paper
– Darker lines divide paper
every fifth square
– Vertically and horizontally
– Large squares measure
5 mm in height and width
– Represents time interval
of 0.20 seconds
– Five small squares in
each large square
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
EKG Waveforms
• Wave or waveform
refers to movement
away from the
baseline or isoelectric
line (beginning and
ending of all waves)
• Positive deflection
– Above isoelectric line
• Negative deflection
– Below isoelectric line
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
EKG Waveforms
• Electrical impulse
leaves SA node,
produces waveform
on graph paper
• One complete cardiac
cycle = P, Q, R, S,
(QRS complex) and
T wave
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
P Wave
• First wave produced
by electrical impulse
from SA node
• Smooth, rounded
upward deflection
• Depolarization of left
and right atria
• 0.10 seconds in
length
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
PR Interval
• Time impulse travels
from SA node
through internodal
pathways in atria
toward ventricles
• Time interval from start
of P wave to start of
QRS
• 0.12-0.20 seconds in
length
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
QRS Complex
• Consists of Q, R,
and S waves
• Represents
conduction of impulse
from bundle of His
through ventricular
muscle
• Ventricular
depolarization
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
QRS Complex
• Q wave
– First downward deflection
• R wave
– First upward deflection
– Largest deflection seen in
lead I and II
• S wave
– Downward deflection after
R wave
• Measures less than 0.12
seconds (3 small boxes)
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
ST Segment
• Time interval during
which ventricles
depolarized and
repolarization of
ventricles begin
• Isoelectric or
consistent with
baseline
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
T Wave
• Follows ST segment
• Represents
ventricular
repolarization
• Slightly rounded,
positive deflection
• Resting phase of
cardiac cycle
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Summary of EKG Waveforms
• P wave
– Atrial depolarization
• QRS complex
– Ventricular
depolarization, atrial
repolarization
• T wave
– Ventricular
repolarization
Understanding EKGs: A Practical Approach, Third Edition
Brenda M. Beasley
Copyright ©2010 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.