Download After reading this chapter you will be able to

Chapter 11
Interpretation of
Electrocardiogram Tracings
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Learning Objectives
After reading this chapter you will be able to:
 Describe the clinical value of the
electrocardiogram (ECG)
 Describe the clinical findings that indicate
the need for an ECG recording
 Identify key components of the electrical
conduction system of the heart and the
role of each component
 Define depolarization and repolarization
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Learning Objectives (cont’d)
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Describe the specific electrical activity of
the heart associated with each wave and
interval of the normal ECG
Identify the normal values for the PR
interval and the QRS complex
Identify the ventricular rate and position of
the mean QRS vector from an ECG
recording
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Learning Objectives (cont’d)
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
List the steps for ECG interpretation
Describe ECG criteria for:
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Sinus bradycardia
Sinus tachycardia
Sinus dysrhythmia
Premature atrial contraction
Atrial flutter
Atrial fibrillation
Premature ventricular contractions
Ventricular tachycardia
Ventricular fibrillation
Asystole
First, second, and third-degree atrioventricular (AV)
block
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Learning Objectives (cont’d)



Describe the ECG abnormalities
associated with chronic lung disease
Identify the signs of ischemia, injury, and
infarction using the 12-lead ECG tracing
Explain how to assess and help treat
patients with chest pain
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Introduction

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RTs observing the onset of an ischemic
cardiac event may be the first link in the
chain of survival
Early recognition of a serious cardiac
problem may minimize cardiac damage or
prevent death
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Introduction (cont’d)

ECG reflects electrical activity of the heart
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12-lead ECGs provide 12 different views of
that activity
Diagnostic tool to detect abnormalities such as:
• Myocardial infarctions
• Ventricular hypertrophy
• Dysrhythmias
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When Should an ECG Be
Obtained?
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Obtain an ECG when there are
signs/symptoms of acute or chronic
cardiac disorders
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CHF
Angina
Acute myocardial infarction
Prior to surgery as a screening tool
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Cardiac Anatomy and Physiology
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The heart is composed of 4 chambers
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2 upper chambers: atria
2 lower chambers: ventricles
Heart divided down the middle to form the
right and left side
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Right atria and ventricle receive venous blood
and circulate it to the lungs for gas exchange
Left atria and ventricle receive oxygenated
blood from lungs and circulate it to entire body
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Cardiac Anatomy and Physiology
(cont’d)

Electrophysiology
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Electrical activity starts in the sinoatrial (SA)
node because it has the greatest level of
automaticity
 Signal travels throughout atria and via
internodal pathways to atrioventricular (AV)
node
 Signal delayed in AV node to allow atrial
contraction prior to ventricular contraction
 Then travels through right and left bundle
branches and to Purkinje fibers of the
ventricles
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Basic ECG Waves
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Depolarization: sudden loss of negative
charge in polarized cells when stimulated
Repolarization: return of the negative
charge within the cell
ECG tracings record the waves of
depolarization and repolarization traveling
across the myocardium

Each wave can be identified and analyzed
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Basic ECG Waves (cont’d)
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P wave: atrial depolarization
PR interval: time for impulse to travel to
ventricles
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Normally 0.12 to 0.20 second
Allows atrial contraction and “priming of the
pump” to precede ventricular contraction
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Basic ECG Waves (cont’d)
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QRS complex: ventricular depolarization
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Normally occurs in <0.12 second
ST segment: between QRS and T wave
T wave: ventricular repolarization
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Basic ECG Waves (cont’d)
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Basic ECG Waves (cont’d)
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Evaluating heart rate (HR)
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If HR regular, count number of large boxes (0.2
sec) between QRS complexes and divide this
number into 300
• 300: number of large boxes occurring in 1 minute
• 1 box between QRS would be rate of 300
• 2 boxes between would be rate of 150 etc.
If HR irregular, average obtained by counting
QRS complexes in a 6-sec strip times 10
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ECG Leads
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12-lead ECG provides different views of
same event
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6 limb leads:
• I, II, III, aVR, aVL, aVF
• View heart on frontal plane used to determine axis
6 chest leads:
• V1, V2, V3, V4, V5, V6
• View the heart on the horizontal plane
• Overlie the RV (V1, V2), ventricular septum (V3, V4)
and LV (V5, V6 )
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Evaluating the Mean QRS Axis
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The QRS axis reflects the general
direction of electrical flow during
depolarization
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Normally the axis points to the left and
downward
Between 0 and +90 degrees
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Evaluating the Mean QRS Axis
(cont’d)
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An upward deflection is seen on the ECG
when the axis is flowing toward a positive
electrode
A downward deflection is made when the
axis travels toward a negative electrode
The axis points toward the lead with the
tallest QRS
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Evaluating the Mean QRS Axis
(cont’d)
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If the tallest QRS complexes are equal in
two leads the axis falls midway between
the two leads.
Figure 11-12 shows the degree at which
each lead lies and thus it is easy to
determine the axis

If lead II had the tallest QRS, then the axis is at
+60˚
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Steps of ECG Interpretation
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First evaluate patient’s overall condition
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Clinical signs and symptoms may aid in
identifying the dysrhythmia
Interpretation can be done on three levels
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Ventricular response
Origin of the impulse
Electrophysiology
• Ectopic beats or rhythms, escape beats or rhythms
• AV blocks or bundle branch blocks
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Steps of ECG Interpretation
(cont’d)
Use a systematic method:
Identify the heart rate
Evaluate the rhythm
Note the presence of P waves
Measure the PR interval
5. Measure the width of the QRS complex
6. Inspect the ST segment in all leads
7. Identify the mean QRS axis
8. Assess the waveform morphology
9. Evaluate the Q wave
10. Look for signs of chamber enlargement
1.
2.
3.
4.
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Sinus Bradycardia
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Meets all the criteria for NSR but is too
slow
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Rate: less than 60 beats/min
Rhythm: regular
P waves: normal and followed by a QRS
complex
PR interval: 0.12 to 0.2 second
QRS: less than 0.12 second in width
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Sinus Tachycardia
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Meets all criteria for NSR but is too fast
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Rate: 100 to 150 beats/min
Rhythm: regular
P waves: normal and followed by a QRS
complex
PR interval: 0.12 to 0.2 second
QRS: less than 0.12 second in width
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Sinus Dysrhythmia
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Meets all criteria for NSR but is irregular
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Rate: 60 to 100 beats/min; may also be
bradycardia
Rhythm: irregular
P waves: normal and followed by a QRS
complex
PR interval: 0.12 to 0.2 second in length
QRS: less than 0.12 second in width
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Atrial Flutter
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Distinct rapid sawtooth pattern between
normal QRS
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Rate: atrial rates 180 to 400; ventricular rate is
slower
Rhythm: regular
P waves: sawtooth and uniform
PRI: not measurable
QRS: less than 0.12 second in width
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Atrial Fibrillation
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Characterized by chaotic baseline
between QRSs
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Rate: variable (count QRSs in 6-second strip)
Rhythm: irregularly irregular
P waves: fibrillatory waves that all vary
PRI: not measurable
QRS: less than 0.12 second in width
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Premature Ventricular
Contractions
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Underlying rhythm is interrupted by wide
QRS (>0.12 s) not preceded by a P wave,
with an inverted T
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Rate: that of the underlying rhythm
Rhythm: regular rhythm is interrupted by PVC
P waves: not associated with the PVC
PR interval: not measurable
QRS: >0.12 second, premature, abnormal
configuration, followed by compensatory pause
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Ventricular Tachycardia
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Wide QRSs occurring rapidly without P
waves
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Rate: 140 to 300 beats/min
Rhythm: regular
P waves: not associated with the QRS
complexes
PR interval: not measurable
QRS: abnormal and >0.12 second in width
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Ventricular Fibrillation
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Chaotic, characterized by wavy irregular
pattern
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Rate: none
Rhythm: irregular, chaotic waves
P waves: none
PRI: none
QRS: none or sporadic low-amplitude waves
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Asystole
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Characterized by a straight or almost flat
line
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Rate: none
Rhythm: none
P waves: none
PRI: none
QRS: none
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AV Heart Block
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General term: problems conducting
impulses from the atrial to the ventricles
Blocks can occur at the AV node, bundle
of His, or the bundle branches
Complete heart block may be associated
with hypotension
Milder forms of heart block often cause no
symptoms
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First-Degree AV Block
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Looks like NSR but a prolonged PR
interval (>0.2 sec)
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Rate: underlying rhythm rate
Rhythm: regular
P waves: normal each preceding a QRS
complex
PRI: >0.2 second
QRS: < 0.12 second in width
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Second-Degree AV Block Type I
(Wenckebach)
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Recurrent lengthening PRI followed by a
dropped QRS
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Rate: varies, ventricular rate less than atrial
rate
Rhythm: irregular
P waves: normal not always followed by a
QRS
PR interval: varies, lengthens, then none
conducted
QRS: < 0.12 second in width
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Second-Degree AV Block Type I
(Wenckebach)
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Second-Degree AV Block Type II
(Wenckebach) (cont’d)
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Characterized by nonconducted P waves
followed by a P wave that is conducted
thus has an associated QRS
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Rate: varies, ventricular rate less than atrial
rate
Rhythm: atrial rate is regular, ventricular rate
may be regular or irregular
P waves: normal not always followed by a
QRS
PR interval: normal or prolonged but constant
QRS: < 0.12 second in width
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Second-Degree AV Block Type II
(Wenckebach) (cont’d)
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Third-Degree AV Block (Complete
Heart Block)
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No association between P waves and QRS
complexes
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Rate: slow, ventricular rate less than atrial rate
Rhythm: atrial and ventricular rates are regular
P waves: normal not always followed by a
QRS
PRI: varies, no relationship to QRS complexes
QRS: generally but not always >0.12 second
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Evidence of Cardiac Ischemia,
Injury, or Infarction
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Cardiac ischemia
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Indicated by depressed ST segment (≥1 mm
below baseline) or inversion of the T waves
Injury is potentially reversible at this point
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Evidence of Cardiac Ischemia,
Injury, or Infarction (cont’d)
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Acute myocardial injury
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Noted by elevated ST segment changes over
the affected myocardium
• Generally indicates acute myocardial infarction
ST segment returns to baseline with restored
perfusion
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Evidence of Cardiac Ischemia,
Injury, or Infarction (cont’d)

Myocardial infarction
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Noted early by elevated ST segments and T
wave changes
Once fully evolved pathologic Q waves appear
• Appear hours to days following AMI
• Generally remain for the duration of patient’s life
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Assessing Chest Pain
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Assess S/S by asking the patient:
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O: onset of pain
P: provoked by …
Q: quality of pain
R: radiation of pain to …
S: severity of pain between 0 and 10
T: time frame of symptoms (acute or chronic)
U: what do You perceive as wrong?
AHA says suspect AMI if nonresponsive to
nitroglycerin
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ECG Patterns with Chronic Lung
Disease
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Multiple ECG changes with severe COPD
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Right axis deviation is common
P waves larger due to right atrial enlargement
• Leads II, III, and aVF
Prominent and negative P wave in lead I
May have changes associated with cor
pulmonale
• Increase R-wave size on leads V1, V2, and V3
• Reduced size of QRS in leads I, II, III, V5, and V6
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Summary
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An ECG provides a picture of heart’s
electrical activity
ECG can identify the condition of the
heart’s electrical conduction system
The ECG can also identify abnormal
rhythms that may be of little consequence
or very serious and life threatening
ECG does not measure pumping ability of
the heart
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