Ecg workout jane huff 7th edition pdf free download

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The clinical treatments described and
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ECGWO06-010810
Library of Congress Cataloging-in-Publication
Data
Huff, Jane, RN.
ECG workout : exercises in arrhythmia
interpretation / Jane Huff.6th ed.
p. ; cm.
Includes index.
ISBN 978-1-4511-1553-6
1. ArrhythmiaDiagnosisProblems,
exercises, etc. 2. Electrocardiography
InterpretationProblems, exercises, etc.
I. Title.
[DNLM: 1. Arrhythmias, Cardiac
diagnosisProblems and Exercises.
2. ElectrocardiographyProblems and
Exercises. WG 18.2]
RC685.A65H84 2012
616.1'2807547076dc23
2011014268
ECG workout_FM.indd ii 5/17/2011 6:45:26 PM
ECG
WORKOUT
EXERCISES IN ARRHYTHMIA
INTERPRETATION
SI XTH EDI TI ON
Jane Huff, RN, CCRN
Education Coordinator, Critical Care Unit
Arrhythmia Instructor
Advanced Cardiac Life Support (ACLS) Instructor
White County Medical Center
Searcy, Arkansas
Guest Faculty, Physician Assistant Program
Harding University
Searcy, Arkansas
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iii
Contents
Preface iv
1 Anatomy and physiology of the heart 1
2 Electrophysiology 8
3 Waveforms, intervals, segments, and complexes 13
4 Cardiac monitors 25
5 Analyzing a rhythm strip 34
6 Sinus arrhythmias 44
7 Atrial arrhythmias 85
8 Junctional arrhythmias and AV blocks 138
9 Ventricular arrhythmias and bundle-branch block 197
10 Pacemakers 256
11 Posttest 284
Answer key to Chapter 3 322
Answer key to Chapters 5 through 11 325
Glossary 369
Index 376
Arrhythmia ash cards pull-out section
Electrocardiographic conversion table for heart rate
ECG workout_FM.indd iii 5/17/2011 6:45:26 PM
iv
ECG Workout: Exercises in Arrhythmia Interpretation, Sixth Edition, was written to assist
physicians, nurses, medical and nursing students, paramedics, emergency medical techni-
cians, telemetry technicians, and other allied health personnel in acquiring the knowledge
and skills essential for identifying basic arrhythmias. It may also be used as a reference for
electrocardiogram (ECG) review for those already knowledgeable in ECG interpretation.
The text is written in a simple manner and illustrated with gures, tables, boxes, and ECG
tracings. Each chapter is designed to build on the knowledge base from the previous chapters so
that the beginning student can quickly understand and grasp the basic concepts of electrocardiog-
raphy. An effort has been made not only to provide good quality ECG tracings, but also to provide
a sufcient number and variety of ECG practice strips so the learner feels condent in arrhythmia
interpretation. There are over 600 practice strips more than any book on the market.
Chapter 1 provides a discussion of basic anatomy and physiology of the heart. The electri-
cal basis of electrocardiology is discussed in Chapter 2. The components of the ECG tracing
(waveforms, intervals, segments, and complexes) are described in Chapter 3. This chapter also
includes practice tracings on waveform identication. Cardiac monitors, lead systems, lead
placement, ECG artifacts, and troubleshooting monitor problems are discussed in Chapter 4.
A step-by-step guide to rhythm strip analysis is provided in Chapter 5, in addition to practice
tracings on rhythm strip analysis. The individual rhythm chapters (Chapters 6 through 9)
include a description of each arrhythmia, arrhythmia examples, causes, and management
protocols. Current advanced cardiac life support (ACLS) guidelines are incorporated into each
arrhythmia chapter as applicable to the rhythm discussion. Each arrhythmia chapter also
includes approximately 100 strips for self-evaluation. Chapter 10 presents a general discussion
of cardiac pacemakers (types, indications, function, pacemaker terminology, malfunctions,
and pacemaker analysis), along with practice tracings. Chapter 11 is a posttest consisting of a
mix of rhythm strips that can be used as a self-evaluation tool or for testing purposes.
The text has been thoughtfully revised and expanded to include new gures, updated boxes
and tables, additional glossary terms, and even more practice rhythm strips. Skillbuilder
rhythm strips, which are new to this edition, appear immediately following the practice
rhythm strips in Chapters 7, 8, and 9. Each Skillbuilder section provides a mix of strips that
test not only your understanding of information learned in that arrhythmia chapter but also
the concepts and skills learned in the chapter(s) immediately preceding it. For example, the
Skillbuilder strips in Chapter 7 (Atrial arrhythmias) include atrial rhythm strips as well as
strips on sinus arrhythmias (covered in Chapter 6); Chapter 8 (Junctional arrhythmias and
AV blocks) includes junctional arrhythmias and AV blocks, as well as atrial and sinus arrhyth-
mias; and Chapter 9 (Ventricular arrhythmias and bundle-branch block), a mix of all of the
arrhythmias covered in Chapters 6 through 9. Such practice with mixed strips will enhance
your ability to differentiate between rhythm groups as you progress through the book a
denite advantage when you get to the Posttest. A handy pull-out section consisting of 48
individual ashcards further challenges your ability to identify different types of arrhythmias.
The ECG tracings included in this book are actual strips from patients. Above each rhythm
strip are 3-second indicators for rapid-rate calculation. For precise rate calculation, an ECG con-
version table for heart rate is printed on the inside back cover. For convenience, a removable plas-
tic version is also attached to the inside back cover. The heart rates for regular rhythms listed in the
answer keys were determined by the precise rate calculation method and will not always coincide
with the rapid-rate calculation method. Rate calculation methods are discussed in Chapter 5.
The author and publisher have made every attempt to check the content, especially drug
dosages and management protocols, for accuracy. Medicine is continually changing, and
the reader has the responsibility to keep informed of local care protocols and changes in
emergency care procedures.
Preface
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This book is dedicated to
Novell Grace, a busy little girl.
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1
Anatomy and physiology
of the heart
Description and location of the heart
The heart is a hollow, four-chambered muscular organ that
lies in the middle of the thoracic cavity between the lungs,
behind the sternum, in front of the spinal column, and just
above the diaphragm (Figure 1-1). The top of the heart (the
base) is at approximately the level of the second intercostal
space. The bottom of the heart (the apex) is formed by the
tip of the left ventricle and is positioned just above the dia-
phragm to the left of the sternum at the fth intercostal
space, midclavicular line. There, the apex can be palpated
during ventricular contraction. This physical examination
landmark is referred to as the point of maximal impulse
(PMI) and is an indicator of the hearts position within the
thorax.
The heart is tilted forward and to the left so that the
right side of the heart lies toward the front. About two-
thirds of the heart lies to the left of the bodys midline and
one-third extends to the right. The average adult heart is
approximately 5 (12 cm) long, 3 (8 to 9 cm) wide, and
2 (6 cm thick) a little larger than a normal-sized st.
The heart weighs between 7 and 15 oz (200 and 425 grams).
Heart size and weight are inuenced by age, weight, body
build, frequency of exercise, and heart disease.
Function of the heart
The heart is the hardest working organ in the body. The heart
functions primarily as a pump to circulate blood and supply
the body with oxygen and nutrients. Each day the average
heart beats over 100,000 times. During an average lifetime,
the human heart will beat more than 3 billion times.
The heart is capable of adjusting its pump performance
to meet the needs of the body. As needs increase, as with
exercise, the heart responds by accelerating the heart rate
to propel more blood to the body. As needs decrease, as
with sleep, the heart responds by decreasing the heart rate,
resulting in less blood ow to the body.
The heart consists of:
four chambers
two atria that receive incoming blood
two ventricles that pump blood out of the heart
four valves that control the ow of blood through the heart
an electrical conduction system that conducts electrical
impulses to the heart, resulting in muscle contraction.
Heart surfaces
There are four main heart surfaces to consider when dis-
cussing the heart: anterior, posterior, inferior, and lateral
(Figure 1-2). The heart surfaces are explained below:
anterior the front
posterior the back
inferior the bottom
lateral the side.
Structure of the heart wall
The heart wall is arranged in three layers (Figure 1-3):
the pericardium the outermost layer
the myocardium the middle muscular layer
the endocardium the inner layer.
Enclosing and protecting the heart is the pericardium,
which consists of an outer brous sac (the brous pericar-
dium) and an inner two-layered, uid-secreting membrane
(the serous pericardium). The outer brous pericardium
comes in direct contact with the covering of the lung (the
pleura) and is attached to the center of the diaphragm infe-
riorly, to the sternum anteriorly, and to the esophagus,
trachea, and main bronchi posteriorly. This position
1
1
1
2
3
4
5
6
7
8
9
10
11
12
Clavicle
Heart
Diaphragm
Xiphoid process
of sternum
12th thoracic
vertebra
Sternum
Rib
Figure 1-1. Location of the heart in the thorax.
ECG workout_Chap01.indd 1 4/28/2011 2:04:23 AM
2 Anatomy and physiology of the heart
anchors the heart to the chest and prevents it from shift-
ing about in the thorax. The serous pericardium is a con-
tinuous membrane that forms two layers: the parietal layer
lines the inner surface of the brous sac and the visceral
layer (also called epicardium) lines the outer surface of the
heart muscle. Between the two layers of the serous peri-
cardium is the pericardial space, or cavity, which is usually
lled with 10 to 30 mL of thin, clear uid (the pericardial
uid) secreted by the serous layers. The primary function
of the pericardial uid is to provide lubrication, preventing
friction as the heart beats. In certain conditions, large
accumulations of uid, blood, or exudates can enter the
pericardial space and may interfere with ventricular lling
and the hearts ability to contract.
The myocardium is the thick, middle, muscular layer
that makes up the bulk of the heart wall. This layer is com-
posed primarily of cardiac muscle cells and is responsible
for the hearts ability to contract. The thickness of the
myocardium varies from one heart chamber to another.
Chamber thickness is related to the amount of resist-
ance the muscle must overcome to pump blood out of the
chamber.
The endocardium is a thin layer of tissue that lines the
inner surface of the heart muscle and the heart chambers.
Extensions and folds of this tissue form the valves of the
heart.
Circulatory system
The circulatory system is required to provide a continuous
ow of blood to the body. The circulatory system is a closed
system consisting of heart chambers and blood vessels.
The circulatory system consists of two separate circuits,
the systemic circuit and the pulmonary circuit. The sys-
temic circuit is a large circuit and includes the left side of
the heart and blood vessels, which carry oxygenated blood
to the body and deoxygenated blood back to the right heart.
The pulmonary circuit is a small circuit and includes the
right side of the heart and blood vessels, which carry deox-
ygenated blood to the lungs and oxygenated blood back to
the left heart. The two circuits are designed so that blood
ow is pumped from one circuit to the other.
Figure 1-2. Heart surfaces.
Endocardium
Myocardium
Epicardium (visceral layer
of serous pericardium)
Parietal layer
of serous pericardium
Fibrous pericardium
Pericardial cavity
Figure 1-3. Heart wall.
ECG workout_Chap01.indd 2 4/28/2011 2:04:23 AM
Heart valves 3
Heart chambers
The interior of the heart consists of four hollow chambers
(Figure 1-4). The two upper chambers, the right atrium
and the left atrium, are divided by a wall called the inter-
atrial septum. The two lower chambers, the right ven-
tricle and the left ventricle, are divided by a thicker wall
called the interventricular septum. The two septa divide
the heart into two pumping systems a right heart and
a left heart.
The right heart pumps venous (deoxygenated)
blood through the pulmonary arteries to the lungs
(Figure 1-5). Oxygen and carbon dioxide exchange takes
place in the alveoli and arterial (oxygenated) blood
returns via the pulmonary veins to the left heart. The
left heart then pumps arterial blood to the systemic
circulation, where oxygen and carbon dioxide exchange
takes place in the organs, tissues, and cells; then venous
blood returns to the right heart. Blood ow within the
body is designed so that arteries carry oxygen-rich blood
away from the heart and veins carry oxygen-poor blood
back to the heart. This role is reversed in pulmonary
circulation: pulmonary arteries carry oxygen-poor blood
into the lungs, and pulmonary veins bring oxygen-rich
blood back to the left heart.
The thickness of the walls in each chamber is related
to the workload performed by that chamber. Both atria
are low-pressure chambers serving as blood-collecting
reservoirs for the ventricles. They add a small amount of
force to the moving blood. Therefore, their walls are rela-
tively thin. The right ventricular wall is thicker than the
walls of the atria, but much thinner than that of the left
ventricle. The right ventricular chamber pumps blood a
fairly short distance to the lungs against a relatively low
resistance to ow. The left ventricle has the thickest wall,
because it must eject blood through the aorta against a
much greater resistance to ow (the arterial pressure in
the systemic circulation).
Heart valves
There are four valves in the heart: the tricuspid valve,
separating the right atrium from the right ventricle; the
pulmonic valve, separating the right ventricle from the
pulmonary arteries; the mitral valve, separating the left
atrium from the left ventricle; and the aortic valve, sepa-
rating the left ventricle from the aorta (Figure 1-5). The
primary function of the valves is to allow blood ow in
one direction through the hearts chambers and prevent
a backow of blood (regurgitation). Changes in cham-
ber pressure govern the opening and closing of the heart
valves.
The tricuspid and mitral valves separate the atria from
the ventricles and are referred to as the atrioventricular
(AV) valves. These valves serve as in-ow valves for the ven-
tricles. The tricuspid valve consists of three separate cusps
or leaets and is larger in diameter and thinner than the
mitral valve. The tricuspid valve directs blood ow from
the right atrium to the right ventricle. The mitral valve (or
bicuspid valve) has only two cusps. The mitral valve directs
blood ow from the left atrium to the left ventricle. Both
valves are encircled by tough, brous rings (valve rings).
The leaets of the AV valves are attached to thin strands
of brous cords called chordae tendineae (heart strings)
(Figure 1-6). The chordae tendineae are then attached to
papillary muscles, which arise from the walls and oor of
the ventricles. During ventricular lling (diastole) when
the AV valves are open, the valve leaets, the chordae
tendineae, and the papillary muscles form a funnel, pro-
moting blood ow into the ventricles. As pressure increases
during ventricular contraction (systole), the valve cusps
close. Backow of blood into the atria is prevented by con-
traction of the papillary muscles and the tension in the
chordae tendineae. Dysfunction of the chordae tendineae
or a papillary muscle can cause incomplete closure of an AV
valve. This may result in a regurgitation of blood from the
ventricle into the atrium, leading to cardiac compromise.
The rst heart sound (S
1
) is the product of tricuspid and
mitral valve closure. S
1
is best heard at the apex of the heart
located on the left side of the chest, fth intercostal space,
midclavicular line.
The aortic and pulmonic valves have three cuplike cusps
shaped like a half-moon and are referred to as the semi-
lunar (SL) valves. These valves serve as out-ow valves
for the ventricles. The cusps of the SL valves are smaller
and thicker than the AV valves and do not have the sup-
port of the chordae tendineae or papillary muscles. Like
the AV valves, the rims of the semilunar valves are sup-
ported by valve rings. The pulmonary valve directs blood
ow from the right ventricle to the pulmonary artery.
The aortic valve directs blood ow from the left ventri-
cle to the aorta. As pressure decreases during ventricular
Left atrium
Left
heart
Left
ventricle
Right
ventricle
Right atrium
Right
heart
Interatrial septum
Interventricular septum
Figure 1-4. Chambers of the heart.
ECG workout_Chap01.indd 3 4/28/2011 2:04:25 AM
4 Anatomy and physiology of the heart
relaxation (diastole), the valve cusps close. Backow of
blood into the ventricles is prevented because of the cusps
brous strength, their close approximation, and their
shape. The second heart sound (S
2
) is produced by closure
of the aortic and pulmonic SL valves. It is best heard over
the second intercostal space on the left or right side of the
sternum.
Blood ow through the
heart and lungs
Blood ow through the heart and lungs is traditionally
described by tracing the ow as blood returns from the sys-
temic veins to the right side of the heart, to the lungs, back
to the left side of the heart, and out to the arterial vessels
Figure 1-6. Papillary muscles and chordae tendineae.
Aortic arch
Pulmonic valve
Branches of left pulmonary artery
Left atrium
Left pulmonary veins
Mitral valve
Myocardium
Aortic valve
Left ventricle
Superior vena cava
Branches of right pulmonary artery
Right atrium
Right pulmonary veins
Tricuspid valve
Chordae tendineae
Interventricular septum
Right ventricle
Papillary muscle
Inferior vena cava
Descending aorta

Figure 1-5. Chambers, valves, blood ow.
RA, right atrium; RV, right ventricle;
LA, left atrium; LV, left ventricle.
ECG workout_Chap01.indd 4 4/28/2011 2:04:25 AM
Coronary circulation 5
of the systemic circuit (Figure 1-5). The right atrium
receives venous blood from the body via two of the bodys
largest veins (the superior vena cava and the inferior vena
cava) and from the coronary sinus. The superior vena cava
returns venous blood from the upper body. The inferior
vena cava returns venous blood from the lower body. The
coronary sinus returns venous blood from the heart itself.
As the right atrium lls with blood, the pressure in the
chamber increases. When pressure in the right atrium
exceeds that of the right ventricle, the tricuspid valve
opens, allowing blood to ow into the right ventricle. As
the right ventricle lls with blood, the pressure in that
chamber increases, forcing the tricuspid valve shut and the
pulmonic valve open, ejecting blood into the pulmonary
arteries and on to the lungs. In the lungs, the blood picks
up oxygen and excretes carbon dioxide.
The left atrium receives arterial blood from the pulmo-
nary circulation via the pulmonary veins. As the left atrium
lls with blood, the pressure in the chamber increases.
When pressure in the left atrium exceeds that of the left
ventricle, the mitral valve opens, allowing blood to ow into
the left ventricle. As the left ventricle lls with blood, the
pressure in that chamber increases, forcing the mitral valve
shut and the aortic valve open, ejecting blood into the aorta
and systemic circuit, where the blood releases oxygen to the
organs, tissues, and cells and picks up carbon dioxide.
Although blood ow can be traced from the right side of
the heart to the left side of the heart, it is important to realize
that the heart works as two pumps (the right heart and the left
heart) working simultaneously. As the right atrium receives
venous blood from the systemic circulation, the left atrium
receives arterial blood from the pulmonary circulation. As
the atria ll with blood, pressure in the atria exceeds that of
the ventricles, forcing the AV valves open and allowing blood
to ow into the ventricles. Toward the end of ventricular ll-
ing, the two atria contract, pumping the remaining blood
into the ventricles. Contraction of the atria during the nal
phase of diastole to complete ventricular lling is called the
atrial kick. The ventricles are 70% lled before the atria con-
tract. The atrial kick adds another 30% to ventricular capac-
ity. In normal heart rhythms, the atria contract before the
ventricles. In abnormal heart rhythms, the loss of the atrial
kick results in incomplete lling of the ventricles, causing a
reduction in cardiac output (the amount of blood pumped
out of the heart). Once the ventricles are lled with blood,
pressure in the ventricles increases, forcing the AV valves
shut and the SL valves open. The ventricles contract simul-
taneously, ejecting blood through the pulmonary artery into
the lungs and through the aortic valve into the aorta.
Coronary circulation
The blood supply to the heart is supplied by the right cor-
onary artery, the left coronary artery, and their branches
(Figure 1-7). There is some individual variation in the
pattern of coronary artery branching, but in general, the
right coronary artery supplies the right side of the heart and
the left coronary artery supplies the left side of the heart.
The right coronary artery arises from the right side
of the aorta and consists of one long artery that travels
downward and then posteriorly. The major branches of the
right coronary artery are:
conus artery
sinoatrial (SA) node artery (in 55% of population)
anterior right ventricular arteries
acute marginal artery
AV node artery (in 90% of population)
posterior descending artery with septal branches
(in 90% of population)
posterior left ventricular arteries (in 90% of population).
Dominance is a term commonly used to describe coro-
nary vasculature and refers to the distribution of the terminal
portion of the arteries. The artery that gives rise to both the
posterior descending artery with its septal branches and the
posterior left ventricular arteries is considered to be a domi-
nant system. In approximately 90% of the population, the
right coronary artery (RCA) is dominant. The term can be
confusing because in most people the left coronary artery is of
wider caliber and perfuses the largest percentage of the myo-
cardium. Thus, the dominant artery usually does not perfuse
the largest proportion of the myocardium. The left coronary
artery arises from the left side of the aorta and consists of the
left main coronary artery, a short stem, which divides into
the left anterior descending artery and the circumex artery.
The left anterior descending (LAD) travels downward over
the anterior surface of the left ventricle, circles the apex, and
ends behind it. The major branches of the LAD are:
diagonal arteries
right ventricular arteries
septal perforator arteries.
The circumex artery travels along the lateral aspect of
the left ventricle and ends posteriorly. The major branches
of the circumex are:
SA node artery (in 45% of population)
anterolateral marginal artery
posterolateral marginal artery
distal left circumex artery.
In 10% of the population, the circumex artery gives
rise to the posterior descending artery with its septal
branches, terminating as the posterior left ventricular
arteries. A left coronary artery with a circumex that gives
rise to both the posterior descending artery and the pos-
terior left ventricular arteries is considered a dominant
left system. When the left coronary artery is dominant, the
entire interventricular septum is supplied by this artery.
Table 1-1 summarizes the coronary artery distribution to
the myocardium and the conduction system.
The right and left coronary artery branches are intercon-
nected by an extensive network of small arteries that provide
the potential for cross ow from one artery to the other.
These small arteries are commonly called collateral vessels
or collateral circulation. Collateral circulation exists at birth
ECG workout_Chap01.indd 5 4/28/2011 2:04:26 AM
6 Anatomy and physiology of the heart
Table 1-1.
Coronary arteries
Coronary artery and its branches Portion of myocardium supplied Portion of conduction system supplied
Right coronary artery
Right atrium
Right ventricle
Inferior wall of left ventricle (90%)*
Posterior one-third of interventricular septum (90%)*
Sinotrial (SA) node (55%)*
Atrioventricular (AV) node and bundle of His (90%)*
Left coronary artery
Left anterior descending (LAD) Anterior wall of left ventricle
Anterolateral wall of left ventricle
Anterior two-thirds of interventricular septum
Right and left bundle branches
Circumex Left atrium
Anterolateral wall of left ventricle
Posterolateral wall of left ventricle
Posterior wall of left ventricle
Inferior wall of left ventricle (10%)*
Posterior one-third of interventricular septum (10%)*
SA node (45%)*
AV node and bundle of His (10%)*
* = of population
Right coronary artery Left coronary artery
Sinoatrial node artery
Left main coronary artery
Left circumflex coronary artery
Left anterior descending artery
Anterolateral marginal artery
Posterolateral marginal artery
Distal left circumflex artery
Diagonal arteries
Right ventricular artery
Septal perforator artery
Conus artery
Acute marginal artery
Anterior right ventricular
artery
AV node artery
Posterior left
ventricular arteries
Posterior descending
coronary artery
Septal branch
Figure 1-7. Coronary circulation.
ECG workout_Chap01.indd 6 4/28/2011 2:04:26 AM
Cardiac innervation 7
but the vessels do not become functionally signicant until
the myocardium experiences an ischemic insult. If a block-
age occurs in a major coronary artery, the collateral vessels
enlarge and provide additional blood ow to those areas of
reduced blood supply. However, blood ow through the col-
lateral vessels isnt sufcient to meet the total needs of the
myocardium in most cases. In other vascular beds of the body,
arterial blood ow reaches a peak during ventricular contrac-
tion (systole). However, myocardial blood ow is greatest dur-
ing ventricular diastole (when the ventricular muscle mass
is relaxed) than it is during systole (when the hearts blood
vessels are compressed). The blood that has passed through
the capillaries of the myocardium is drained by branches of
the cardiac veins whose path runs parallel to those of the
coronary arteries. Some of these veins empty directly into the
right atrium and right ventricle, but the majority feed into
the coronary sinus, which empties into the right atrium.
Cardiac innervation
The heart is under the control of the autonomic nerv-
ous system located in the medulla oblongata, a part of
the brain stem. The autonomic nervous system regu-
lates functions of the body that are involuntary, or not
under conscious control, such as blood pressure and
heart rate. It includes the sympathetic nervous system
and the parasympathetic nervous system, each produc-
ing opposite effects when stimulated. Stimulation of
the sympathetic nervous system results in the release
of norepinephrine, a neurotransmitter, which acceler-
ates the heart rate, speeds conduction through the AV
node, and increases the force of ventricular contrac-
tion. This system prepares the body to function under
stress (ght-or-ight response). Stimulation of the
parasympathetic nervous system results in the release
of acetylcholine, a neurotransmitter, which slows the
heart rate, decreases conduction through the AV node,
and causes a small decrease in the force of ventricular
contraction. This system regulates the calmer functions
of the body (rest-and-digest response). Normally a bal-
ance is maintained between the accelerator effects of
the sympathetic system and the inhibitory effects of the
parasympathetic system.
ECG workout_Chap01.indd 7 4/28/2011 2:04:26 AM
8
Electrophysiology
Cardiac cells
The heart is composed of thousands of cardiac cells. The
cardiac cells are long and narrow, and divide at their ends
into branches. These branches connect with branches of
adjacent cells, forming a branching and anastomosing
network of cells. At the junctions where the branches join
together is a specialized cellular membrane of low electri-
cal resistance, which permits rapid conduction of electrical
impulses from one cell to another throughout the cell net-
work. Stimulation of one cardiac cell initiates stimulation
of adjacent cells and ultimately leads to cardiac muscle
contraction.
There are two basic kinds of cardiac cells in the heart:
the myocardial cells (or working cells) and the pace-
maker cells. The myocardial cells are contained in the
muscular layer of the walls of the atria and ventricles. The
myocardial working cells are permeated by contractile
laments which, when electrically stimulated, produce
myocardial muscle contraction. The primary function of
the myocardial cells is cardiac muscle contraction, fol-
lowed by relaxation. The pacemaker cells are found in the
electrical conduction system of the heart and are primar-
ily responsible for the spontaneous generation of electrical
impulses.
Cardiac cells have four primary cell characteristics:
automaticity the ability of the pacemaker cells to
generate their own electrical impulses spontaneously; this
characteristic is specic to the pacemaker cells.
excitability the ability of the cardiac cells to respond
to an electrical impulse; this characteristic is shared by all
cardiac cells.
conductivity the ability of cardiac cells to conduct
an electrical impulse; this characteristic is shared by all
cardiac cells.
contractility the ability of cardiac cells to cause car-
diac muscle contraction; this characteristic is specic to
myocardial cells.
Depolarization and repolarization
Cardiac cells are surrounded and lled with an electrolyte
solution. An electrolyte is a substance whose molecules
dissociate into charged particles (ions) when placed in
water, producing positively and negatively charged ions.
An ion with a positive charge is called a cation. An ion with
a negative charge is called an anion. Potassium (K
+
) is the
primary ion inside the cell and sodium (Na
+
) is the primary
ion outside the cell.
A membrane separates the inside of the cardiac cell
(intracellular) from the outside (extracellular). There is a
constant movement of ions across the cardiac cell mem-
brane. Differences in concentrations of these ions deter-
mine the cells electric charge. The distribution of ions
on either side of the membrane is determined by several
factors:
Membrane channels (pores) The cell membrane has
openings through which ions pass back and forth between
the extracellular and intracellular spaces. Some channels
are always open; others can be opened or closed; still others
can be selective, allowing one kind of ion to pass through
and excluding all others. Membrane channels open and
close in response to a stimulus.
Concentration gradient Particles in solution move,
or diffuse, from areas of higher concentration to areas of
lower concentration. In the case of uncharged particles,
movement proceeds until the particles are uniformly dis-
tributed within the solution.
Electrical gradient Charged particles also diffuse, but
the diffusion of charged particles is inuenced not only by
the concentration gradient, but also by an electrical gradi-
ent. Like charges repel; opposite charges attract. Therefore,
positively charged particles tend to ow toward negatively
charged particles and negatively charged particles toward
positively charged particles.
Sodium-potassium pump The sodium-potassium
pump is a mechanism that actively transports ions across
the cell membrane against its electrochemical gradient.
This pump helps to reestablish the resting concentrations
of sodium and potassium after cardiac depolarization.
Electrical impulses are the result of the ow of ions (pri-
marily sodium and potassium) back and forth across the
cardiac cell membrane (Figure 2-1). Normally there is an
ionic difference between the two sides. In the resting car-
diac cell, there are more negative ions inside the cell than
outside the cell. When the ions are so aligned, the rest-
ing cell is called polarized. During this time, no electrical
2
2
ECG workout_Chap02.indd 8 4/28/2011 1:09:30 AM
Electrical conduction system of the heart 9
activity is occurring and a straight line (isoelectric line) is
recorded on the ECG (Figure 2-5).
Once a cell is stimulated, the membrane permeability
changes. Potassium begins to leave the cell, increasing
cell permeability to sodium. Sodium rushes into the cell,
causing the inside of the cell to become more positive
than negative (cell is depolarized). Muscle contraction
follows depolarization. Depolarization and muscle con-
traction are not the same. Depolarization is an electrical
event that results in muscle contraction, a mechanical
event.
After depolarization, the cardiac cell begins to recover.
The sodium-potassium pump is activated to actively trans-
port sodium out of the cell and move potassium back into
the cell. The inside of the cell becomes more negative than
positive (cell is repolarized) and returns to its resting state.
Depolarization of one cardiac cell acts as a stimulus on
adjacent cells and causes them to depolarize. Propagation
of the electrical impulses from cell to cell produces an
electric current that can be detected by skin electrodes and
recorded as waves or deections onto graph paper, called
the ECG.
Electrical conduction system
of the heart
The heart is supplied with an electrical conduction system
that generates and conducts electrical impulses along
specialized pathways to the atria and ventricles, causing
them to contract (Figure 2-2). The system consists of the
sinoatrial node (SA node), the interatrial tract (Bach-
manns bundle), the internodal tracts, the atrioventricular
node (AV node), the bundle of His, the right bundle branch,
the left bundle branch, and the Purkinje bers.
The SA node is located in the wall of the upper right
atrium near the inlet of the superior vena cava. Special-
ized electrical cells, called pacemaker cells, in the SA node
discharge impulses at a rate of 60 to 100 times per minute.
Pacemaker cells are located at other sites along the con-
duction system, but the SA node is normally in control and
is called the pacemaker of the heart because it possesses
the highest level of automaticity (its inherent ring rate
is greater than that of the other pacemaker sites). If the
SA node fails to generate electrical impulses at its normal
rate or stops functioning entirely, or if the conduction
of these impulses is blocked, pacemaker cells in second-
ary pacemaker sites can assume control as pacemaker of
the heart, but at a much slower rate. Such a pacemaker is
called an escape pacemaker because it usually only appears
(escapes) when the faster ring pacemaker (usually the
SA node) fails to function. Pacemaker cells in the AV junc-
tion generate electrical impulses at 40 to 60 times per
minute. Pacemaker cells in the ventricles generate elec-
trical impulses at a much slower rate (30 to 40 times per
minute or less). In general, the farther away the impulse
originates from the SA node, the slower the rate. A beat or
series of beats arising from an escape pacemaker is called
an escape beat or escape rhythm and is identied according
to its site of origin (for example, junctional, ventricular).
As the electrical impulse leaves the SA node, it is con-
ducted through the left atria by way of Bachmanns bundle
and through the right atria via the internodal tracts, caus-
ing electrical stimulation (depolarization) and contraction
of the atria. The impulse is then conducted to the AV node
located in the lower right atrium near the interatrial sep-
tum. The AV node relays the electrical impulses from the
atria to the ventricles. It provides the only normal conduc-
tion pathway between the atria and the ventricles. The AV
node has three main functions:
To slow conduction of the electrical impulse through the
AV node to allow time for the atria to contract and empty
its contents into the ventricles (atrial kick) before the ven-
tricles contract. This delay in the AV node is represented on
the ECG tracing as the at line of the PR interval.
To serve as a backup pacemaker, if the SA node fails, at a
rate of 40 to 60 beats per minute
To block some of the impulses from being conducted to
the ventricles when the atrial rate is rapid, thus protecting
the ventricles from dangerously fast rates.
Figure 2-1. Depolarization and repolarization of a cardiac cell.
ECG workout_Chap02.indd 9 4/28/2011 1:09:30 AM
10 Electrophysiology
Figure 2-2. Electrical conduction system of the heart.
SA node
Interatrial tract (Bachmann's bundle)
Internodal
tracts
AV node
Bundle of His
Interatrial septum
Anterior fascicle of left bundle branch
Interventricular septum
Purkinje fibers
Right bundle branch
Posterior fascicle of left bundle branch
After the delay in the AV node, the impulse moves
through the bundle of His. The bundle of His divides into
two important conducting pathways called the right bundle
branch and the left bundle branch. The right bundle branch
conducts the electrical impulse to the right ventricle. The
left bundle branch divides into two divisions: the anterior
fascicle, which carries the electrical impulse to the anterior
wall of the left ventricle, and the posterior fascicle, which
carries the electrical impulse to the posterior wall of the
left ventricle. Both bundle branches terminate in a network
of conduction bers called Purkinje bers. These bers
make up an elaborate web that carry the electrical impulses
directly to the ventricular muscle cells. The ventricles are
capable of serving as a backup pacemaker at a rate of 30 to
40 beats per minute (sometimes less). Transmission of the
electrical impulses through the conduction system is slow-
est in the AV node and fastest in the His-Purkinje system
(bundle of His, bundle branches, and Purkinje bers).
The hearts electrical activity is represented on the
monitor or ECG tracing by three basic waveforms: the
P wave, the QRS complex, and the T wave (Figure 2-3).
A U wave is sometimes present. Between the waveforms
are the following segments and intervals: the PR interval,
the PR segment, the ST segment, and the QT interval.
Although the letters themselves have no special signi-
cance, each component represents a particular event in the
depolarizationrepolarization cycle. The P wave depicts
atrial depolarization, or the spread of the impulse from
the SA node throughout the atria. A waveform represent-
ing atrial repolarization is usually not seen on the ECG
because atrial repolarization occurs during ventricular
depolarization and is hidden in the QRS complex. The PR
interval represents the time from the onset of atrial depo-
larization to the onset of ventricular depolarization. The PR
segment, a part of the PR interval, is the short isoelectric
line between the end of the P wave to the beginning of the
QRS complex. It is used as a baseline to evaluate elevation
or depression of the ST segment. The QRS complex depicts
ventricular depolarization, or the spread of the impulse
throughout the ventricles. The ST segment represents
early ventricular repolarization. The T wave represents
P
R
Q
S
T
U
ST segment
QT interval
PR segment
PR interval
Figure 2-3. Relationship of the electrical conduction system to
the ECG.
ECG workout_Chap02.indd 10 4/28/2011 1:09:31 AM
Refractory and supernormal periods of the cardiac cycle 11
can be applied to the P wave, the QRS complex, and the
T wave deections.
Waveforms and current ow
A monitor lead, or ECG lead, provides a view of the hearts
electrical activity between two points or poles (a positive
pole and a negative pole). The direction in which the elec-
tric current ows determines how the waveforms appear
on the ECG tracing (Figure 2-6). An electric current ow-
ing toward the positive pole will produce a positive deec-
tion; an electric current traveling toward the negative pole
produces a negative deection. Current owing away from
the poles will produce a biphasic deection (both positive
and negative). Biphasic deections may be equally positive
and negative, more negative than positive, or more positive
than negative (depending on the angle of current ow to
the positive or negative pole).
The size of the wave deection depends on the magni-
tude of the electrical current owing toward the individual
pole. The magnitude of the electrical current is determined
by how much voltage is generated by depolarization of a par-
ticular portion of the heart. The QRS complex is normally
larger than the P wave because depolarization of the larger
muscle mass of the ventricles generates more voltage than
does depolarization of the smaller muscle mass of the atria.
Refractory and supernormal
periods of the cardiac cycle
There is a period of time in the cardiac cycle during which
the cardiac cells may be refractory, or unable to respond,
to a stimulus. Refractoriness is divided into three phases
(Figure 2-7):
ventricular repolarization. The U wave, which isnt always
present, represents late ventricular repolarization. The QT
interval represents total ventricular activity (the time from
the onset of ventricular depolarization to the end of ven-
tricular repolarization).
The cardiac cycle
A cardiac cycle consists of one heartbeat or one PQRST
sequence. It represents a sequence of atrial contraction
and relaxation followed by ventricular contraction and
relaxation. The basic cycle repeats itself again and again
(Figure 2-4). Regularity of the cardiac rhythm can be
assessed by measuring from one heartbeat to the next
(from one R wave to the next R wave, also called the R-R
interval). Between cardiac cycles, the monitor or ECG
recorder returns to the isoelectric line (baseline), the at
line in the ECG during which electrical activity is absent
(Figure 2-5). Any waveform above the isoelectric line is
considered a positive (upright) deection and any wave-
form below this line a negative (downward) deection.
A deection having both a positive and negative compo-
nent is called a biphasic deection. This basic concept
R
T
QS
P
Figure 2-4. The cardiac cycle.
Figure 2-6. Relationship between current ow and waveform
deections.
Biphasic
deflections
Electric
current
Negative
deflection

Positive
deflection
+
R R
T
P
Q
Q
S
T
P
S
Figure 2-5. Relationship between waveforms and the isoelectric
line.
ECG workout_Chap02.indd 11 4/28/2011 1:09:31 AM
12 Electrophysiology
Supernormal period During this period the cardiac
cells will respond to a weaker than normal stimulus. This
period occurs during a short portion near the end of the
T wave, just before the cells have completely repolarized.
ECG graph paper
The PQRST sequence is recorded on special graph paper
made up of horizontal and vertical lines (Figure 2-8). The
horizontal lines measure the duration of the waveforms in
seconds of time. Each small square measured horizontally
represents 0.04 second in time. The width of the QRS com-
plex in Figure 2-9 extends across for 2 small squares and
represents 0.08 second (0.04 second 2 squares). The ver-
tical lines measure the voltage or amplitude of the wave-
form in millimeters (mm). Each small square measured
vertically represents 1 mm in height. The height of the
QRS complex in Figure 2-9 extends upward from baseline
16 small squares and represents 16 mm voltage (1 mm
16 squares).
Absolute refractory period During this period the
cells absolutely cannot respond to a stimulus. This period
extends from the onset of the QRS complex to the peak of
the T wave. During this time the cardiac cells have depolar-
ized and are in the process of repolarizing. Because the car-
diac cells have not repolarized to their threshold potential
(the level at which a cell must be repolarized before it can
be depolarized again) they cannot be stimulated to depolar-
ize. In other words, the myocardial cells cannot contract,
and the cells of the electrical conduction system cannot
conduct an electrical impulse during the absolute refrac-
tory period.
Relative refractory period During this period the
cardiac cells have repolarized sufciently to respond to
a strong stimulus. This period begins at the peak of the
T wave and ends with the end of the T wave. The relative
refractory period is also called the vulnerable period of
repolarization. A strong stimulus occurring during the
vulnerable period may usurp the primary pacemaker of
the heart (usually the SA node) and take over pacemaker
control. An example might be a premature ventricular con-
traction (PVC) that falls during the vulnerable period and
takes over control of the heart in the form of ventricular
tachycardia.
Figure 2-7. Refractory and supernormal periods.
QRS complex
T wave
absolute
refractory
period
relative
refractory
period
supernormal
period
P wave
Figure 2-8. Electrocardiographic paper. Figure 2-9. QRS width: 0.08 second; QRS height: 16 mm.
ECG workout_Chap02.indd 12 4/28/2011 1:09:32 AM
13
Waveforms, intervals,
segments, and
complexes
Much of the information that the ECG tracing provides is
obtained from the examination of the three principal wave-
forms (the P wave, the QRS complex, and the T wave) and
their associated segments and intervals. Assessment of this
data provides the facts necessary for an accurate cardiac
rhythm interpretation.
P wave
The rst deection of the cardiac cycle, the P wave,
is caused by depolarization of the right and left atria
(Figure 3-1). The rst part of the P wave represents depo-
larization of the right atrium; the second part represents
depolarization of the left atrium. The waveform begins as
the deection leaves baseline and ends when the deection
returns to baseline. A normal sinus P wave originates in
the sinus node and travels through normal atria, resulting
in normal depolarization. Normal P waves are smooth and
round, positive in lead II (a positive lead), 0.5 to 2.5 mm
in height, 0.10 second or less in width, with one P wave
to each QRS complex. More than one P wave before a
QRS complex indicates a conduction disturbance, such as
that which occurs in second and third-degree heart block
(discussed in Chapter 8).
There are two types of abnormal P waves:
Abnormal sinus P wave An abnormal sinus P wave
originates in the sinus node and travels through enlarged
atria, resulting in abnormal depolarization of the atria.
Abnormal atria depolarization results in abnormal-looking
P waves.
Impulses traveling through an enlarged right atrium
(right atrial hypertrophy) result in P waves that are tall
and peaked. The abnormal P wave in right atrial enlarge-
ment is sometimes referred to as p pulmonale because the
atrial enlargement that it signies is common with severe
pulmonary disease (for example, pulmonary stenosis and
insufciency, chronic obstructive pulmonary disease,
acute pulmonary embolism, and pulmonary edema).
Impulses traveling through an enlarged left atrium (left
atrial hypertrophy) result in P waves that are wide and
notched. The term p mitrale is used to describe the abnormal
P waves seen in left atrial enlargement because they were rst
seen in patients with mitral valve stenosis and insufciency.
Left atrial enlargement can also be seen in left heart failure.
Ectopic P wave The term ectopic means away from its
normal location. Therefore, an ectopic P wave arises from a
site other than the SA node. Abnormal sites include the atria
and the AV junction. P waves from the atria may be positive
or negative; some are small, pointed, at, wavy, or sawtooth
in appearance. P waves from the AV junction are always neg-
ative (inverted) and may precede or follow the QRS complex
or be hidden within the QRS complex and not visible.
Examples of P waves are shown in Figure 3-2.
PR interval
The PR interval (sometimes abbreviated PRI) represents
the time from the onset of atrial depolarization to the onset
of ventricular depolarization. The PR interval (Figure 3-3)
includes a P wave and the short isoelectric line (PR seg-
ment) that follows it. The PR interval is measured from the
beginning of the P wave as it leaves baseline to the begin-
ning of the QRS complex. The duration of the normal PR
interval is 0.12 to 0.20 seconds.
Abnormal PR intervals may be short or prolonged:
Short PR interval A short PR interval is less than
0.12 seconds and may be seen if the electrical impulse
originates in an ectopic site in the AV junction. A short-
ened PR interval may also occur if the electrical impulse
progresses from the atria to the ventricles through one
of several abnormal conduction pathways (called acces-
sory pathways) that bypass a part or all of the AV node.
Wolff-Parkinson-White syndrome (WPW) is an example of
such an accessory pathway.
Prolonged PR interval A prolonged PR interval is
greater than 0.20 seconds and indicates that the impulse
3
3
Figure 3-1. The P wave.
ECG workout_Chap03.indd 13 4/28/2011 5:23:15 AM
14 Waveforms, intervals, segments, and complexes
A B
C D
Wide, notched P wave Tall, peaked P wave E F
G Flat P wave H
Small, pointed
P wave I Sawtooth P waves J Wavy P waves
Figure 3-2. P wave examples.
ECG workout_Chap03.indd 14 4/28/2011 5:23:15 AM
QRS complex 15
Finding the beginning of the QRS complex usually isnt
difficult. Finding the end of the QRS complex, however,
is at times a challenge because of elevation or depres-
sion of the ST segment. Remember, the QRS complex
ends as soon as the straight line of the ST segment