Download Fundamentals of Electrocardiography Part I

Fundamentals of
Electrocardiography
Part I
Mary Patricia English
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The Beginning
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Basic Electrophysiology
Objectives
• Describe the characteristics of cardiac cells
• Describe the normal sequences of electrical
conduction through the heart
• Explain why the SA node is the dominate
pacemaker in the normal heart
• State the intrinsic rate of the SA node AV
junction and the Purkinje fibers
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Types of Cardiac Cells
•Myocardial cells
– Working or mechanical cells
– Contain contractile filaments
•Pacemaker cells
– Specialized cells of the electrical conduction system
– Responsible for the spontaneous generation and conduction of
electrical impulses
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Properties of Cardiac Cells
•Conductivity
– Ability of a cardiac cell to receive an electrical stimulus and conduct that
impulse to an adjacent cardiac cell
•Contractility
– Ability of cardiac cells to shorten, causing cardiac muscle contraction in
response to an electrical stimulus
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The Conduction System
•Conduction system
– Specialized electrical (pacemaker) cells in the heart arranged in a
system of pathways
•Normally, the pacemaker site with the
fastest firing rate controls the heart
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Sinoatrial (SA) Node
• Located at the junction
of the superior vena
cava and the right
atrium
• Initiates electrical
impulses at a rate of 60
to 100 beats/min
• Normally the primary
pacemaker of the heart
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Atria
• Fibers of SA node
connect directly with
fibers of atria
• Impulse leaves SA
node and is spread
from cell to cell across
the atrial muscle
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Internodal Pathways
• Conduction through
the AV node begins
before atrial
depolarization is
completed
• Impulse is spread to
AV node via internodal
pathways
– Pathways merge gradually with
cells of AV node
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AV Junction
• Area of
specialized
conduction
tissue
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AV Node
•Located in the posterior septal wall of the
right atrium
– Supplied by right coronary artery in most individuals
•As the impulse from the atria enters the AV
node, there is a delay in conduction of the
impulse to the ventricles
– Allows time for atria to empty contents into ventricles
– Provides electrical links between atrium and ventricle
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AV Node
•Divided into three
functional regions
according to their action
potentials and responses
to electrical and chemical
stimulation
– Atrionodal (AN) or upper junctional
region Nodal (N) region Nodal-His (NH)
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AV Node
• The primary delay
in the passage of
the electrical
impulse from the
atria to the
ventricles occurs in
the AN and N
areas of the AV
node
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Bundle of His
•Also called the “common bundle” or the
“AV bundle”
•Normally the only electrical connection
between the atria and the ventricles
– Connects AV node with bundle branches
– Has pacemaker cells capable of discharging at an intrinsic rate of 40 to
60 beats/min
– Conducts impulse to right and left bundle branches
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Right & Left Bundle Branches
• Right bundle branch
– Innervates the right ventricle
• Left bundle branch
– Spreads the electrical impulse to the interventricular septum and left
ventricle
– Divides into three divisions (fascicles)
• Anterior fascicle
• Posterior fascicle
• Septal fascicle
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Purkinje Fibers
•Elaborate web of fibers that penetrate
about 1/3 of the way into the ventricular
muscle mass
– Become continuous with cardiac muscle fibers
•Receive impulse from bundle branches and
relay it to ventricular myocardium
•Intrinsic pacemaker ability of 20 to 40
beats/min
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Conduction System Activation
•Sequence of
Activation
through the
Conduction
System
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Conduction Disturbances
•May occur because of:
– Trauma
– Drug toxicity
– Electrolyte disturbances
– Myocardial ischemia or infarction
•Conduction may be too rapid or too slow
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The ECG
•The ECG is a voltmeter
– Records electrical voltages (potentials) generated by depolarization of
heart muscle
•Electrical activity within the heart can be
observed by means of electrodes
connected by cables to an ECG machine
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The ECG
•Can provide information about:
– The orientation of the heart in the chest
– Conduction disturbances
– The electrical effects of medications and electrolytes
– The mass of cardiac muscle
– The presence of ischemic damage
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The ECG
•Does not provide information about the
mechanical (contractile) condition of the
myocardium
– Evaluated by assessment of pulse and blood pressure
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Electrodes
• Disposable disk
electrodes contain
conductive media
– Conductive media conducts the
skin surface voltage change
through color-coded wires to a
cardiac monitor
• Applied at specific
locations on the patient's
chest wall and
extremities
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Leads
•A lead is a record of electrical activity
between two electrodes
– Allow viewing of the heart’s electrical activity in two different planes:
frontal (coronal) or horizontal (transverse)
•Each lead records the average current flow
at a specific time in a portion of the heart
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ECG Paper
•ECG paper is graph paper made
up of small and larger, heavylined squares
– Smallest squares are 1 mm wide and 1 mm high
– 5 small squares between the heavier black lines
– 25 small squares within each large square
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Horizontal Axis = Time
• Width of each small box = 0.04 second
• Width of each large box (5 small boxes) = 0.20
second
–
5 large boxes (each consisting of 5 small boxes) = 1 second
• 15 large boxes = 3 seconds
• 30 large boxes = 6 seconds
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Vertical Axis =
Voltage/Amplitude
•Size or amplitude of a waveform is
measured in millivolts (voltage) or
millimeters (amplitude)
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Waveforms
•A waveform or deflection is
movement away from the baseline
in either a positive (upward) or
negative (downward) direction
– A waveform that is partly positive and partly negative is
“biphasic”
– A waveform or deflection that rests on the baseline is
“isoelectric”
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P Wave
• The first wave in the cardiac cycle
• Represents atrial depolarization and spread
of the electrical impulse throughout the right
and left atria
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The Normal P Wave
• Smooth and rounded
• Usually no more than 2.5 mm in height and 0.11
second in duration
• Positive in leads I, II, aVF, and V2 through V6
• May be positive, negative, or biphasic in leads
III, aVL, and V1
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Abnormal P waves
• May be notched, tall and pointed (peaked), or
inverted (negative)
• May be seen in conditions such as chronic
obstructive pulmonary disease (COPD), congestive
heart failure (CHF), or valvular disease
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Terminology
•Waveform
– Movement away from the baseline in either a positive or negative
direction
•Segment
– A line between waveforms
– Named by the waveform that precedes or follows it
•Interval
– A waveform and a segment
•Complex
– Several waveforms
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PR Segment
•Part of the PR interval
– Horizontal line between the end of the P wave and the beginning of the
QRS complex
•Normally isoelectric (flat)
– Used as a baseline from which to evaluate ST segment elevation or
depression
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PR Interval
• The P wave plus the
PR segment equals the
PR interval
• Begins with the onset
of the P wave and
ends with the onset of
the QRS complex
• Normally measures
0.12 to 0.20 second
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PR Interval
• Reflects:
– Depolarization of the right
and left atria (P wave)
– Spread of the impulse
through the AV node,
bundle of His, right and left
bundle branches, and
Purkinje fibers (PR
segment)
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Abnormal PR Interval
•Long PR interval (greater than 0.20 sec)
– Indicates the impulse was delayed as it passed through the atria or AV
junction
•Short PR interval (less than 0.12 sec)
– May be seen when the impulse originates in the atria close to the AV
node or in the AV junction
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QRS Complex
• A QRS complex normally
follows each P wave
• Consists of Q wave, R
wave, and S wave
• Represents the spread of
electrical impulse through
the ventricles (ventricular
depolarization)
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Q Wave
•The first negative, or downward, deflection
following the P wave
•Always a negative waveform
•Represents depolarization of the
interventricular septum
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Q Wave
• Physiological Q waves
– A normal Q wave is less than 25% of
the amplitude of the R wave
– Normal Q wave duration does not
exceed 0.04 second
• Pathological Q waves
– More than 0.04 second in duration
– More than 25% of the amplitude of the
following R wave in that lead
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R Wave
•The first
positive,
or
upward,
deflection
following
the P
wave
• Always positive
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S Wave
• A negative
waveform
following the R
wave
• Always negative
• R and S waves
represent
simultaneous
depolarization
of the right and
left ventricles
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QRS Measurement
•The width of a QRS complex is most
accurately determined when it is viewed
and measured in more than one lead
– Measure the QRS complex with the longest duration and clearest onset
and end
•Normal QRS duration in an adult varies
between 0.06 and 0.10 second
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ST Segment
• The portion of
the ECG
tracing
between the
QRS complex
and the T
wave
Represents the
early part of
repolarization of
the right and
left ventricles
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ST Segment
•The point at which the QRS complex
and the ST segment meet = “J point”
or junction
•Normally isoelectric (flat) in the limb
leads
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Normal ST Segment
• Begins with the end of the QRS complex and
ends with the onset of the T wave
• Limb leads
– Normal ST segment is isoelectric (flat)
– May normally be slightly elevated or depressed (usually less than 1 mm)
• Precordial leads
– In some precordial leads, ST segment may be normally elevated by as much as
2 to 3 mm
– In the left precordial leads, ST segment elevation is not normally greater than 1
mm
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ST Segment
•The PR segment used as the baseline
from which to evaluate the degree of
displacement of the ST segment from
the isoelectric line (elevation or
depression)
– Measure at a point 0.04 second (one small box) after the end of
the QRS complex (J point)
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ST Segment
•The ST segment is considered:
– “Elevated” if the segment deviates above the baseline of the
segment
– “Depressed” if the segment deviates below it
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PR
Abnormal ST Segment
•ST segment depression of more than 1 mm
is suggestive of myocardial ischemia
•ST segment elevation of more than 1 mm
is suggestive of
myocardial injury
– Pericarditis causes ST-segment elevation in virtually all leads
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Abnormal ST Segment
•A horizontal ST segment (forms a
sharp angle with the T wave) is
suggestive of ischemia
•Digitalis causes a depression (scoop)
of the ST segment
– “Dig dip”
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T Wave
•Represents
ventricular
repolarization
– The beginning of the T wave
is identified as the point
where the slope of the ST
segment appears to
become abruptly or
gradually steeper
– The T wave ends when it
returns to the baseline
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Abnormal T Waves
•The T wave following an abnormal QRS
complex is usually opposite in the direction
of the QRS
•Negative T waves suggest myocardial
ischemia
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Abnormal T Waves
•Tall, pointed (peaked) T waves are
commonly seen in hyperkalemia
•Significant cerebral disease (e.g.,
subarachnoid hemorrhage) may be
associated with deeply inverted T waves
– “Cerebral T waves”
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QT Interval
•Measured from the
beginning of the QRS
complex to end of the T
wave
– In the absence of a Q wave, measure
the QT interval from the beginning of the
R wave to the end of the T wave
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U Wave
•Significance is not definitely known
– Thought to represent repolarization of Purkinje fibers
•Not easily identified due to its low
amplitude
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U Wave - Normal Characteristics
• Usually less than 2 mm
in amplitude
• In general, a U wave of
more than 1.5 mm in
height in any lead is
considered abnormal
• Rounded and
symmetrical
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Review
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Causes of Artifact
• Loose electrodes
• Broken ECG cables or broken wires
• Muscle tremor
• Patient movement
• External chest compressions
• 60-cycle interference
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Artifact – Loose Electrodes
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Artifact – Muscle Tremor
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Artifact – 60-cycle Interference
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Rate Measurement
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Six-Second Method
• Ventricular rate
– Count the number of complete
QRS complexes within a period of
6 seconds
– Multiply that number by 10 to
determine the number of QRS
complexes in 1 minute
• May be used for
regular and irregular
rhythms
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Large Box Method
•Count the number of large boxes between
the consecutive R waves and divide into
300
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Sequence Method
•Select an R wave that
falls on a dark vertical
line
– Number the next 6 consecutive
dark vertical lines as follows:
• 300, 150, 100, 75, 60, and 50
– Note where the next R wave falls
in relation to the 6 dark vertical
lines already marked—this is the
heart rate
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Analyzing a Rhythm Strip
•Assess the Rate
•Assess Rhythm/Regularity
•Identify & Examine P Waves
•PR Interval (PRI)
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PR Interval (PRI)
• Measured from the point where the P wave leaves the
baseline to the beginning of the QRS complex
• Normal PR interval is 0.12 to 0.20 second
• If the PR intervals are the same, they are said to be
constant
• If the PR intervals are different, is there a pattern?
– Lengthening
– Variable (no pattern)
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Assess the Rate
•What is the rate?
– Determine ventricular rate (R-R intervals)
– Determine atrial rate (P-P intervals)
•A “tachycardia” exists if the rate is greater
than 100 bpm
•A “bradycardia” exists if the rate is less
than 60 bpm
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Assess Rhythm/Regularity
•Ventricular rhythm
– Measure the distance between two consecutive R-R intervals
– Compare with other R-R intervals
•Atrial rhythm
– Measure the distance between two consecutive P-P intervals
– Compare with other P-P intervals
• When analyzing a rhythm strip, determine:
–
Atrial (P-P intervals) rhythm
–
Ventricular (R-R intervals) rhythm
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Identify & Examine P Waves
•Look to the left of each QRS complex
•Normally:
– One P wave precedes each QRS complex
– P waves occur regularly and appear similar in size, shape,
position
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and
PR Interval (PRI)
• Measured from the point where the P wave leaves the
baseline to the beginning of the QRS complex
• Normal PR interval is 0.12 to 0.20 second
• If the PR intervals are the same, they are said to be
constant
• If the PR intervals are different, is there a pattern?
– Lengthening
– Variable (no pattern)
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QRS Complex
•Identify the QRS complexes and measure
their duration
– Narrow (normal) if it measures 0.10 second or less
– Wide if it measures more than 0.10 second
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QT Interval
•Measure in the leads that show the largest
amplitude T waves
– Measured from the beginning of the QRS complex to end of the T wave
– If the measured QT interval is less than half the R-R interval, it is
probably normal
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ST Segment
•Usually isoelectric in the limb leads
•To determine ST segment elevation or
depression, measure at a point 0.04
second (one small box) after the end of
the QRS complex
– Use the PR segment as the baseline
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T Waves
• Are the T waves upright and of normal height
• The T wave following an abnormal QRS complex is
usually opposite in direction of the QRS
• Negative T waves suggest myocardial ischemia
• Tall, pointed (peaked) T waves are commonly seen in
hyperkalemia
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Questions?
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