Chapter 20
The
Electrocardiogram
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Review Tip
Refer to Chapter 6, “Anatomy and Physiology,” and review the
cardiovascular system, including the functions, components,
cardiac cycle, and cardiac conduction. Exam questions may involve
identifying a cardiac rhythm or PQRST waves from a picture of an
electrocardiogram strip or identifying cardiac leads.
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Overview
The electrocardiogram (ECG or EKG) is the graphic representation of
the electrical activity that passes through the heart. It is monitored at
the skin surface with sensors called electrodes that produce specific
leads or views of the heart, which is a three-dimensional organ. The
ECG is a painless and noninvasive tool used to collect baseline
information (e.g., during a routine physical exam) and to diagnose and
monitor various heart diseases such as myocardial infarctions and
other ischemia, heart blocks and other conduction defects, benign and
life-threatening arrhythmias, and the effects of cardiac drugs.
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Overview, cont’d.
The electrical activity of the heart begins at the cellular level. It then
follows the conductive pathway bringing about the cardiac cycle, which
results in the heart pumping blood throughout the body.
■
Cardiac polarity—electrical status of cardiac muscle cells; an
attempt to maintain electronegativity (ability to attract electrons)
inside these cells to ensure an appropriate distribution of ions (e.g.,
potassium, sodium, chloride, calcium)
• Polarization—resting cardiac muscle cells
• Depolarization—charged and contracting cardiac muscle cells
• Repolarization—recovering cardiac muscle cells; returning to
equilibrium
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Overview, cont’d.
■
Cardiac cycle—the pumping of the heart in a rhythmic cycle of
contraction and relaxation (the sound through a stethoscope is often
described as “lub dub”)
• Normally 60 to 100 cycles or beats per minute (normal adult heart
rate)
• Phases of atrial and ventricle contractions (systole) and
relaxation (diastole)
- Atrial systole—contraction of atria, forcing blood into
ventricles through tricuspid and mitral valves
- Ventricle diastole—relaxation of ventricles, allowing them to
fill with blood from atria
- Ventricle systole—contraction of the ventricles, forcing blood
through the aortic and pulmonic valves to the aorta and
pulmonary artery
- Atrial diastole—relaxation of atria, allowing them to fill with
blood from the vena cava and pulmonary veins
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Overview, cont’d.
■
ECG complex—a full cardiac electrical cycle (one heartbeat)
represented by PQRST waves (and sometimes U wave) working
together as a complex
• P wave—an upward curve representing atrial contraction; used
to measure the atrial rate
• Q wave—a downward deflection after the P wave
• R wave—a large upward spike after the Q wave
• S wave—a downward deflection after the R wave
• T wave—an upward curve after the S wave, representing the
repolarization and resting of the ventricles
• U wave—a small upward curve sometimes following the T
wave, representing slow repolarization or return to resting
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Overview, cont’d.
• QRS complex—the QRS waves representing contraction of
the ventricles
• PR interval—the P wave and the line connecting it to the
QRS complex, representing the time the electrical impulse
travels from the sinoatrial (SA) node to the atrioventricular
(AV) node
• QT interval—the QRST waves representing a full cardiac
electrical cycle
• ST segment—a slight upward line connecting the QRS
waves to the T wave and representing the time between
contraction of the ventricles and relaxation or recovery
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Overview, cont’d.
Figure 20-1. Normal cardiac electrical cycle.
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Overview, cont’d.
■
Types of ECGs
• Single-lead ECG—information recorded from one view of
the heart; a lead is a specific view of the heart, which is a
three-dimensional organ; usually, lead II is selected
• 12-lead (multichannel) ECG—information recorded from 10
electrodes, representing 12 views of the heart from 12
different angles
• Telemetry—single-lead or 12-lead ECGs transmitted via
radio, electronic, or telephone waves to another site for
monitoring or interpretation
• Interpretive ECG—a computerized ECG machine that is
programmed to analyze data and produce a printed
interpretation with the graph
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Electrodes, Placement, and
Leads
The terms “electrodes” and “leads” are sometimes used synonymously.
This is not correct. An electrode is the sensor attached to the ECG
machine that adheres to the skin. A lead is the view of the heart
produced by a standard combination of electrode placements. The 12lead ECG is produced using 10 electrodes. The correct placement of
these electrodes influences the quality and accuracy of the rhythm
strip. The right leg (RL) is the grounding electrode and not used as part
of any lead. Electrodes should never be placed over a bony
prominence or clothing.
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Electrodes, Placement, and
Leads
Figure 20-2. Twelve-lead electrocardiogram electrode placement.
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Electrodes, Placement, and
Leads, cont’d.
Placement of Electrodes (10 Sensors)
■ Chest
electrodes (6)
• V1—fourth intercostal space at right margin of sternum
• V2—fourth intercostal space at left margin of sternum
• V3—midway between V2 and V4
• V4—fifth intercostal space at left midclavicular line
• V5—placed midway between V4 and V6
• V6—fifth intercostal space at the left midaxillary line
■ Limb electrodes (4)
• RA—right arm
• LA—left arm
• RL—right leg (ground)
• LL—left leg
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Electrodes, Placement, and
Leads, cont’d.
ECG Leads (12 Views of the Heart)
■ Limb
leads (6)
• Bipolar limb leads—record cardiac electrical activity between
two electrodes
- Lead I—heart view between LA and RA electrodes
- Lead II—heart view between LL and RA electrodes
- Lead III—heart view between LL and LA electrodes
- Lead I, lead II, and lead III with RL (ground) form Einthoven
triangle
• Unipolar (augmented voltage) limb leads
- aVR (RA electrode)
- aVL (LA electrode)
- aVF (LL electrode)
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Electrodes, Placement, and
Leads, cont’d.
■ Chest
(precordial) leads (6)—all chest leads are unipolar and equate
to the six chest electrodes
• V1
• V2
• V3
• V4
• V5
• V6
The national certification examinations frequently ask questions about
identification of ECG leads.
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Electrodes, Placement, and
Leads
Figure 20-3. Einthoven triangle representing views of the heart in
leads I, II, and III.
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Electrodes, Placement, and
Leads, cont’d.
ECG Paper
ECG paper is standardized paper designed for ECG machines. It has a
combination of small and large blocks to measure the cardiac electrical
activity demonstrated on the graph.
■ Horizontal
line—time
■ Vertical line—voltage or amplitude
■ Small block—1 mm 1 mm, representing 0.1 millivolt (mV) on the
vertical axis and representing 0.04 second on the horizontal axis
■ Large block—5 mm 5 mm, representing 0.5 mV on the vertical axis
and representing 0.20 second on the horizontal axis; five large
horizontal blocks represent1 second
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Electrodes, Placement, and
Leads, cont’d.
■ Vertical slashes above the graph—mark 3-second intervals (15 large
blocks); used to calculate heart rate
■ Paper speed—25 mm/sec is the usual speed for adults; 50 mm/sec is usual
speed for children
■ Calibration—10-mm (two large blocks) vertical mark is the normal standard;
it is sometimes referred to as the standardization mark; the calibration is
changed to 5 mm (one-half standard) in situations where the R wave is too
large and the ECG machine amplitude must be decreased to allow the
tracing to fit on the paper
■ Marking codes—symbols of dots and dashes representing the leads on an
ECG tracing; most model machines identify the leads by placing I, II, III,
aVR, aVL, aVF, V1, V2, V3, V4, V5, or V6 in the portion of the graph
representing that lead
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Electrodes, Placement, and
Leads, cont’d.
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Stylus
A stylus is a heated pen like instrument of the ECG machine that
receives impulses via electrodes and moves on ECG paper,
recording the electrical activity of the heart.
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Normal Sinus Rhythm
Normal sinus rhythm (NSR) is a standard cardiac cycle that begins in
the SA node. The role of the medical assistant is not to interpret an
ECG. The expectation is that the medical assistant can differentiate
normal sinus rhythm from abnormal cardiac rhythms and notify the
physician of irregularities. The physician may order a rhythm strip, a
long tracing of lead II, as opposed to a complete ECG. The purpose
is to evaluate a longer interval of complexes that may be compared
to a previous ECG. Criteria for NSR are as follows:
Regular rhythm—same number of spaces between all R waves
■ Heart rate—normal adult heart rate is 60 to 100 beats per minute
■
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Normal Sinus Rhythm,
cont’d.
• Calculation method 1—count number of large blocks between
two R waves and divide into 300 (e.g., five large blocks
between two R waves = 300/5 = 60 beats per minute)
• Calculation method 2—count number of R waves between 6second marks (30 large blocks) and multiply by 10 (e.g., 6 R
waves in 6 seconds = 6 × 10 = 60 beats per minute)
■ P waves—P waves present before each QRS complex
■ Normal PR interval—0.12 to 0.20 second (three to five small
blocks)
■ Normal-shaped QRS complex—0.06- to 0.10-second duration
(1.5 to 2.5 small blocks)
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Normal Sinus Rhythm,
cont’d.
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Cardiac Arrhythmias
Cardiac arrhythmias or dysrhythmias are irregular heart activities resulting
in loss of a regular rhythm. If all the criteria for normal sinus rhythm, as
stated previously, are not met, the beat is considered abnormal.
Bradycardia—a heart rate slower than 60 beats per minute
■ Tachycardia—a heart rate faster than 100 beats per minute
■ Asystole—absence of a heart rate, no complexes; flat line; cardiac
arrest
■ Ectopic beat—a beat originating outside the SA node, the
pacemaker of the heart
■ Bigeminy—every other beat is ectopic and/or premature
■
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Cardiac Arrhythmias, cont’d.
■
Ventricular arrhythmias—irregularities in the ventricular activity
• Premature ventricular contraction (PVC)—a contraction of the
ventricles occurring early; may be life threatening, depending
on the ratio of PVCs to normal ventricular contractions; report to
physician
• Ventricular tachycardia—ventricular rate of more than 100 to
150 beats per minute, a wide QRS complex; considered a lifethreatening arrhythmia and should be reported to the physician
immediately
• Ventricular flutter—ventricular rate of 150 to 300 beats per
minute; considered a life-threatening arrhythmia and should be
reported to the physician immediately
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Cardiac Arrhythmias, cont’d.
■ Atrial
arrhythmias—irregularities in the atrial activity
• Premature atrial contraction (PAC)—a contraction of the atria
occurring early
• Atrial tachycardia—also called AT; atrial rate of 150 to 250 beats
per minute; P waves are often unidentifiable or hidden in
previous T wave
• Paroxysmal atrial tachycardia (PAT)—atrial tachycardia that
starts, and often stops, suddenly
• Atrial flutter—atrial rate of 250 to 350 beats per minute; “saw
tooth” pattern on ECG; the ventricular rate is dependent on the
number of nonconducted beats
• Atrial fibrillation—atrial rate of 350 to 500 beats per minute; P
waves not distinct because of rapid rate; R to R waves are
usually irregular; often with a rapid ventricular rate
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Cardiac Arrhythmias, cont’d.
Figure 20-4. Select electrocardiogram strips.
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Artifacts
Artifacts are interruptions or disturbances in the ECG strip resulting from
activity outside the heart.
Somatic tremors or movement—involuntary or voluntary muscle or
other movement by the patient
■ Alternating current (AC) interference—caused by other sources of
electricity in the room, such as other equipment including some cell
phones, crossed wires, improper grounding
■ Wandering baseline—movement of the stylus from the center of the
ECG paper in a “roaming” or “wandering” manner; causes may be
electrodes that are too loose or too tight, corroded or dirty
electrodes, oil or lotion on the patient’s skin
■
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Artifacts, cont’d.
■ Interrupted baseline—a break between complexes, usually resulting
from a wire becoming disconnected from an electrode or a broken
wire
■ Heat or pressure—imprint on the ECG paper caused by a hot or
sluggish stylus
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Artifacts, cont’d.
Figure 20-5. Electrocardiogram artifacts.
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Closure Materials, cont’d.
■ Staples—metal clips used to approximate skin edges during healing
or occlude internal structures; materials vary per use (e.g.,
stainless steel used on skin, silver used for neurosurgery); external
staples must be removed with a staple remover
■ Steri-Strips—adhesive strips of material used in minor lacerations
or as a follow-up to sutures to hold wound edges together during
healing
■ Glue—bonding material used externally to approximate skin edges
or internally to affix structures; often used in neurologic or
orthopaedic surgeries
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Other Common Cardiac Tests
Holter monitor—a portable ECG device worn by a patient for 24
hours; monitors heart activity during normal activities of daily living
and requires a patient to keep a diary of activities to help physician
with diagnosis; electrodes are placed in the following locations:
• Fourth intercostal space, right sternal margin
• Right clavicle, lateral to sternal notch
• Left clavicle, lateral to sternal notch
• Fifth intercostal space, left axillary line
• Lower right chest wall
■ Stress test—ECG recordings taken while the patient exercises using
a treadmill, stationary bicycle, or stair climber; monitors the
response of the heart to increased demand
■
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Other Common Cardiac Tests,
cont’d.
■ Echocardiogram—sound waves transmitted through the heart
producing a picture on a screen; used to test the heart for structural
or functional abnormalities
■ Angiogram—an x-ray visualization with contrast material injected
into a blood vessel to determine the presence of structural or
functional abnormalities
■ Cardiac catheterization—insertion of a catheter into a major blood
vessel to visualize the heart’s activity, to measure pressures, and to
identify abnormalities, especially blockages
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