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Fast & Easy ECGs, 2EFast & Easy ECGs, 2E© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Myocardial Ischemia, Injury, and Infarction
Fast & Easy ECGs, 2nd E – A Self-Paced Learning Program
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Fast & Easy ECGs, 2EFast & Easy ECGs, 2E© 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
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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
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PR Segment
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• Flat line that extends from P wave to Q wave (or R wave in absence of a Q wave)
Fast & Easy ECGs, 2EFast & Easy ECGs, 2E© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Q Wave
• First part of QRS complex
• First downward deflection from baseline
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ST Segment
• Flat line that follows the QRS complex and connects it to T wave
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T Wave
• Slightly asymmetrical and oriented in same direction as preceding QRS complex
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Ischemia, Injury, and Infarction
• Occurs with interruption of coronary artery blood flow
• Often a progressive process
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Myocardial Ischemia
• Results from decreased oxygen and nutrient delivery to myocardium
• Can be reversed if supply of oxygen and nutrients is restored
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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
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Myocardial Injury
• Results if ischemia progresses unresolved or untreated
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Myocardial Infarction
• Death of myocardial cells
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ECG Indicators
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Myocardial Ischemia
• Characteristic signs:
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T Wave Inversion
• Occurs because ischemic tissue does not repolarize normally
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T Wave Inversion
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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)
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ST Segment Depression
• May or may not include T wave inversion
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Flat ST Segment Depression
• Results from subendocardial infarction
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ST Segment Elevation
• Earliest reliable sign that myocardial infarction has occurred
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ST Segment Elevation
• May also be seen in:– Ventricular hypertrophy – Conduction abnormalities – Pulmonary embolism – Spontaneous pneumothorax – Intracranial hemorrhage – Hyperkalemia– Pericarditis
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ST Segment Elevation - Pericarditis
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Pathologic Q Waves
• Indicate presence of irreversible myocardial damage or myocardial infarction
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Pathologic Q Waves
• Develop because infarcted areas of heart become electrically silent (fail to depolarize) as they are functionally dead
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Progression of Myocardial Infarction
• During MI the ECG often evolves through three stages:– Ischemia– Injury– Infarction
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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/pain2. Rise and fall in serum cardiac markers3. Changes on serially obtained ECG tracings
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Fast & Easy ECGs, 2EFast & Easy ECGs, 2E© 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
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Identification of MI
• ECG changes need to be present in two or more contiguous leads
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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
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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)
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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
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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
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Septal Ischemia, Injury, Infarction
• Identified though ECG changes in seen in leads V1 and V2
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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
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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
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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
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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
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Posterior Ischemia, Injury, Infarction
• Involve posterior surface of the heart
• Look for reciprocal changes in leads V1 and V2
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Posterior Ischemia, Injury, Infarction
• Can be identified through leads V7, V8 and V9
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Right Ventricular Ischemia, Injury, Infarction
• Can be identified using leads V3R, V4R, V5R, V6R
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Practice Makes Perfect
• Determine the likely location of ischemia, injury or infarction
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Practice Makes Perfect
• Determine the likely location of the ischemia, injury or infarction
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Practice Makes Perfect
• Determine the likely location of the ischemia, injury or infarction
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Practice Makes Perfect
• Determine the likely location of the ischemia, injury or infarction
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Practice Makes Perfect
• Determine the likely location of the ischemia, injury or infarction
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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
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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
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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
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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
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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
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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
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