Myocardial Ischemia, Injury, and Infarction

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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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)


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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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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• Reciprocal changes seen on 12-lead ECG may assist with distinguishing between MI and conditions that mimic it

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• 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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• 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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Myocardial Ischemia, Injury, and Infarction