Chapter 11
Interpretation of
Electrocardiogram
Tracings
Introduction
• 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
Introduction
• ECG reflects electrical activity of the heart
• 12-lead ECGs provide 12 different views
of that activity
• Diagnostic tool to detect abnormalities
such as:
• Myocardial infarctions
• Ventricular hypertrophy
• Dysrhythmias
When Should an ECG Be Obtained?
• Obtain an ECG when there are
signs/symptoms of acute or chronic
cardiac disorders
• CHF
• Angina
• Acute myocardial infarction
• Prior to surgery as a screening tool
Cardiac Anatomy and Physiology
• The heart is composed of 4 chambers
• 2 upper chambers: atria
• 2 lower chambers: ventricles
• Heart divided down the middle to form
the right and left side
Cardiac Anatomy and
Physiology
• 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
Conduction Pathway in the Heart
• 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
Conduction Pathway
• 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
Basic ECG Waves
• Depolarization: sudden loss of
negative charge in polarized cells
when stimulated
• Repolarization: return of the
negative charge within the cell
Basic ECG waves
• ECG tracings record the waves of
depolarization and repolarization
traveling across the myocardium
• Each wave can be identified and
analyzed
ECG Waves
• P wave: atrial depolarization
• PR interval: time for impulse to travel to
ventricles:
• Normally 0.12 to 0.20 second
• Allows atrial contraction and “priming of
the pump” to precede ventricular
contraction
Basic ECG Waves
• QRS complex: ventricular depolarization
• Normally occurs in <0.12 second
• ST segment: between QRS and T wave
• T wave: ventricular repolarization
Basic ECG Waves
Evaluating heart rate (HR)
• 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.
Evaluating HR
• If HR irregular, average obtained by
counting QRS complexes in a 6-sec strip
times 10
ECG Leads
• 12-lead ECG provides different views of
same event
• 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 )
Steps of ECG Interpretation
• First evaluate patient’s overall condition
• Clinical signs and symptoms may aid in
identifying the dysrhythmia
• Interpretation can be done on three levels
• Ventricular response
• Origin of the impulse
• Electrophysiology
• Ectopic beats or rhythms, escape beats or
rhythms
• AV blocks or bundle branch blocks
Steps of ECG Interpretation
Use a systematic method:
1. Identify the heart rate
2. Evaluate the rhythm
3. Note the presence of P waves
4. 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
Sinus Bradycardia
• Meets all the criteria for NSR but is too
slow
• 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
Sinus Tachycardia
• Meets all criteria for NSR but is too fast
• 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
Atrial Flutter
• Distinct rapid sawtooth pattern between
normal QRS
• 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
Atrial Fibrillation
• Characterized by chaotic baseline between
QRSs
• 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
Premature Ventricular Contractions
• Underlying rhythm is interrupted by wide
QRS (>0.12 s) not preceded by a P wave,
with an inverted T
• 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
PVC
Ventricular Tachycardia
• Wide QRSs occurring rapidly without
P waves
• 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
V-Tach
Ventricular Fibrillation
• Chaotic, characterized by wavy irregular
pattern
• Rate: none
• Rhythm: irregular, chaotic waves
• P waves: none
• PRI: none
• QRS: none or sporadic low-amplitude
waves
V-Fib
Asystole
• Characterized by a straight or almost flat
line
• Rate: none
• Rhythm: none
• P waves: none
• PRI: none
• QRS: none
AV Heart Block
• 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
First-Degree AV Block
• Rate: underlying rhythm rate
• Rhythm: regular
• P waves: normal each preceding a QRS
complex
• PRI: >0.2 second* (Key Feature)
• QRS: < 0.12 second in width
Second-Degree AV Block Type I
(Wenckebach)
• Recurrent lengthening PRI followed by a
dropped QRS
• 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
Second-Degree AV Block Type I
(Wenckebach)
Second-Degree AV Block Type II
• Characterized by nonconducted P waves
followed by a P wave that is conducted thus
has an associated QRS
• 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
Second-Degree AV Block Type II
Third-Degree AV Block (Complete Heart
Block)
• No association between P waves and QRS
complexes
• 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
Evidence of Cardiac Ischemia, Injury, or
Infarction
• Cardiac ischemia
• Indicated by depressed ST segment (≥1
mm below baseline) or inversion of the T
waves
• Injury is potentially reversible at this
point
Evidence of Cardiac Ischemia, Injury, or
Infarction
• Acute myocardial injury
• Noted by elevated ST segment changes
over the affected myocardium
• Generally indicates acute myocardial
infarction
• ST segment returns to baseline with
restored perfusion
Evidence of Cardiac Ischemia, Injury, or
Infarction
• Myocardial infarction
• 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
Assessing Chest Pain
• Assess S/S by asking the patient:
• 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
ECG Patterns with Chronic Lung Disease
• Multiple ECG changes with severe COPD
• 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
Summary
• 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