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Myocardial Ischemia, Injury, and
Infarction
Fast & Easy ECGs, 2nd E – A SelfPaced Learning Program
Fast & Easy ECGs, 2E
1
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Myocardial Oxygen Supply
• Because the heart’s oxygen and nutrient
demand is extremely high it requires its own
continuous blood supply
Fast & Easy ECGs, 2E
2
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Myocardial Oxygen Supply
• Coronary arteries deliver blood to myocardial
cells
• Coronary veins return deoxygenated blood to
RA via coronary sinus
• Coronary blood flow can be increased through
vasodilation to meet increased myocardial
oxygen demands
Fast & Easy ECGs, 2E
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© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
PR Segment
• Flat line that
extends from P
wave to Q wave
(or R wave in
absence of a Q
wave)
Fast & Easy ECGs, 2E
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Q Wave
• First part of QRS
complex
• First downward
deflection from
baseline
I
Fast & Easy ECGs, 2E
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ST Segment
• Flat line that follows
the QRS complex
and connects it to T
wave
I
Fast & Easy ECGs, 2E
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T Wave
• Slightly asymmetrical
and oriented in same
direction as
preceding QRS
complex
Fast & Easy ECGs, 2E
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Ischemia, Injury, and Infarction
• Occurs with
interruption of
coronary artery
blood flow
• Often a progressive
process
I
Fast & Easy ECGs, 2E
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© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Myocardial Ischemia
• Results from
decreased oxygen
and nutrient delivery
to myocardium
• Can be reversed if
supply of oxygen and
nutrients is restored
I
Fast & Easy ECGs, 2E
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© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Myocardial Ischemia - Causes
•
•
•
•
Atherosclerosis
Vasospasm
Thrombosis and embolism
Decreased ventricular filling time
– Tachycardia
• Decreased filling pressure in coronary arteries
– Severe hypotension or aortic valve disease
Fast & Easy ECGs, 2E
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© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Myocardial Injury
• Results if ischemia
progresses
unresolved or
untreated
Fast & Easy ECGs, 2E
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Myocardial Infarction
• Death of myocardial
cells
I
Fast & Easy ECGs, 2E
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ECG Indicators
I
Fast & Easy ECGs, 2E
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© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Myocardial Ischemia
• Characteristic signs:
Fast & Easy ECGs, 2E
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T Wave Inversion
• Occurs because
ischemic tissue
does not
repolarize
normally
I
Fast & Easy ECGs, 2E
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T Wave Inversion
I
Fast & Easy ECGs, 2E
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Peaked T Waves
• May be seen in early
stages of acute
myocardial infarction
• T waves invert within
a short time (two
hours)
Fast & Easy ECGs, 2E
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ST Segment Depression
• May or
may not
include T
wave
inversion
I
Fast & Easy ECGs, 2E
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Flat ST Segment Depression
• Results from
subendocardial
infarction
Fast & Easy ECGs, 2E
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ST Segment Elevation
• Earliest
reliable
sign that
myocardial
infarction
has
occurred
I
Fast & Easy ECGs, 2E
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ST Segment Elevation
• May also be seen in:
– Ventricular hypertrophy
– Conduction abnormalities
– Pulmonary embolism
– Spontaneous pneumothorax
– Intracranial hemorrhage
– Hyperkalemia
– Pericarditis
Fast & Easy ECGs, 2E
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ST Segment Elevation - Pericarditis
I
Fast & Easy ECGs, 2E
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Pathologic Q Waves
• Indicate
presence of
irreversible
myocardial
damage or
myocardial
infarction
I
Fast & Easy ECGs, 2E
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© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Pathologic Q Waves
• Develop
because
infarcted areas
of heart become
electrically
silent (fail to
depolarize) as
they are
functionally
dead
Fast & Easy ECGs, 2E
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© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Progression of Myocardial Infarction
• During MI the ECG
often evolves
through three
stages:
– Ischemia
– Injury
– Infarction
Fast & Easy ECGs, 2E
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Criteria for Diagnosing MI
• Based on the presence of at least two of the
following three criteria:
1. Clinical history of ischemic-type chest
discomfort/pain
2. Rise and fall in serum cardiac markers
3. Changes on serially obtained ECG tracings
Fast & Easy ECGs, 2E
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© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
ECG Changes in MI
• 12-lead ECG should be immediately
performed on anyone even remotely
suspected of experiencing MI
• Because early ECGs do not always reveal MI, it
is important to obtain serial 12-lead ECGs
throughout patient assessment and treatment
– Particularly true if first ECG is obtained during a
pain-free episode
Fast & Easy ECGs, 2E
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Identification of MI
• ECG changes need to be present in two or
more contiguous leads
Fast & Easy ECGs, 2E
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Identification of MI
• Reciprocal
changes seen
on 12-lead
ECG may assist
with
distinguishing
between MI
and conditions
that mimic it
Fast & Easy ECGs, 2E
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© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Identification of MI
• Closely scrutinizing the contour of the ST
segment may also be helpful
– With MI the ST segment tends to be straight or
upwardly convex (nonconcave)
Fast & Easy ECGs, 2E
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Additional Indicators of MI
• A new (or presumably) new bundle branch
block can be another indicator of MI
– However, the patient’s old ECGs must be used to
confirm this
• Left bundle branch block (as well as pacing)
can interfere with identifying acute MI by
making it difficult to accurately interpret the
ST segment
Fast & Easy ECGs, 2E
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Identifying Myocardial Infarction
Location
• 12-lead ECG can help identify which coronary
artery or branch is occluded as well as the
area of the heart which is ischemic, injured,
and/or infarcted
• Leads II, III, and aVF provide a view of the
tissue supplied by the right coronary artery,
whereas leads I, aVL,V1,V2,V3,V4,V5, and V6 view
the tissue supplied by the left coronary artery
Fast & Easy ECGs, 2E
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Septal Ischemia, Injury, Infarction
• Identified though
ECG changes in seen
in leads V1 and V2
Fast & Easy ECGs, 2E
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Anterior Ischemia, Injury, Infarction
• Involves anterior
surface of left
ventricle
• Identified though
ECG changes in seen
in leads V3 and V4
I
Fast & Easy ECGs, 2E
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Lateral Ischemia, Injury, Infarction
• Involves left lateral
ventricular wall
• Identified though
ECG changes in seen
in leads I, aVL, V5,V6
Fast & Easy ECGs, 2E
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Lateral Ischemia, Injury, Infarction
• The positive electrode for leads I and aVL
should be located distally on the left arm and
because of which, leads I and aVL are
sometimes referred to as the high lateral leads
• Because the positive electrodes for leads V5
and V6 are on the patient's chest, they are
sometimes referred to as the low lateral leads
Fast & Easy ECGs, 2E
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© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Inferior Ischemia, Injury, Infarction
• Involves inferior
surface of the heart
(diaphragmatic
surface of heart)
• Identified though
ECG changes in seen
in leads II, III, aVF
Fast & Easy ECGs, 2E
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© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Posterior Ischemia, Injury, Infarction
• Involve posterior
surface of the heart
• Look for reciprocal
changes in leads V1
and V2
Fast & Easy ECGs, 2E
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Posterior Ischemia, Injury, Infarction
• Can be identified through leads V7, V8 and V9
Fast & Easy ECGs, 2E
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Right Ventricular Ischemia, Injury,
Infarction
• Can be identified using leads V3R, V4R, V5R,
V6R
Fast & Easy ECGs, 2E
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© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Practice Makes Perfect
• Determine the likely location of ischemia,
injury or infarction
I
Fast & Easy ECGs, 2E
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© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Practice Makes Perfect
• Determine the likely location of the ischemia,
injury or infarction
I
Fast & Easy ECGs, 2E
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© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Practice Makes Perfect
• Determine the likely location of the ischemia,
injury or infarction
I
Fast & Easy ECGs, 2E
43
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Practice Makes Perfect
• Determine the likely location of the ischemia,
injury or infarction
I
Fast & Easy ECGs, 2E
44
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Practice Makes Perfect
• Determine the likely location of the ischemia,
injury or infarction
I
Fast & Easy ECGs, 2E
45
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Summary
• Coronary arteries deliver blood to the
myocardial cells while the coronary veins
return deoxygenated blood to the right atrium
via the coronary sinus
• By increasing coronary blood flow, mostly
through vasodilation, the coronary arteries
satisfy increased myocardial oxygen demands
Fast & Easy ECGs, 2E
46
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Summary
• The ST segment can be compared to the PR
segment to evaluate ST segment depression or
elevation
• The Q wave is the first downward deflection from
the baseline
– It is not always present
• The ST segment is the flat line that follows the
QRS complex and connects it to the T wave
• The T wave is slightly asymmetrical and oriented
in the same direction as the preceding QRS
complex
Fast & Easy ECGs, 2E
47
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Summary
• Myocardial ischemia, injury and death can
occur with Interruption of coronary artery
blood flow
• Myocardial ischemia may cause the
appearance of T waves and ST segments to
change
• A flat depression of the ST segment results
from subendocardial infarction
Fast & Easy ECGs, 2E
48
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Summary
• ST segment elevation occurs with myocardial
injury
– It is the earliest reliable sign that myocardial infarction
has occurred and tells us the myocardial infarction is
acute
• Pathologic Q waves indicate the presence of
irreversible myocardial damage or myocardial
infarction
• Leads V3, and V4 provide the best view for
identifying anterior myocardial infarction
Fast & Easy ECGs, 2E
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© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Summary
• Lateral infarction is identified by ECG changes
such as ST segment elevation, T wave
inversion, and the development of pathologic
Q waves in leads I, aVL, V5 and V6
• Inferior infarction is determined by ECG
changes such as ST segment elevation, T wave
inversion, and the development of pathologic
Q waves in Leads II, III, and aVF
Fast & Easy ECGs, 2E
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© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Summary
• Posterior infarction can be diagnosed by
looking for reciprocal changes in leads V1 and
V2 or by using the posterior leads V7, V8 and V9
• Right ventricular infarction can be identified
using leads V3R, V4R, V5R, V6R
Fast & Easy ECGs, 2E
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© 2013 The McGraw-Hill Companies, Inc. All rights reserved.