Myocardial Ischemia, Injury, and Infarction 20 Myocardial Ischemia, Injury, and Infarction Fast & Easy ECGs, 2nd E – A Self-Paced Learning Program

Myocardial Oxygen Supply Because the heart’s oxygen and nutrient demand is extremely high it requires its own continuous blood supply

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

PR Segment 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 Instructional point: The Q wave is not always present. I

ST Segment Flat line that follows the QRS complex and connects it to T wave Instructional point: The ST segment can be compared to the PR segment to evaluate ST segment depression or elevation. I

T Wave Slightly asymmetrical and oriented in same direction as preceding QRS complex

Ischemia, Injury, and Infarction Occurs with interruption of coronary artery blood flow Often a progressive process Instructional point: Progressive process which includes: (1) Arteries narrow from atherosclerosis and form plaque, (2) narrowing causes a decrease in perfusion and ischemia to cells, (3) causes angina or ischemia to the muscle cells. I

Myocardial Ischemia Results from decreased oxygen and nutrient delivery to myocardium Can be reversed if supply of oxygen and nutrients is restored Instructional points: Myocardial ischemia typically occurs when the heart has a greater need for oxygen than the narrowed coronary arteries can deliver or there is a loss of blood supply to the myocardium. Ischemia can be transient such as in angina or it can be related to myocardial infarction. I

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

Myocardial Injury Results if ischemia progresses unresolved or untreated

Myocardial Infarction Death of myocardial cells Instructional point: The area of the heart which is normally supplied by the blocked artery goes through a characteristic sequence of events described as “zones” of ischemia, injury and infarction. Each zone is associated with characteristic ECG changes. I

ECG Indicators Instructional point: Although the ECG usually progresses through these three stages during an acute myocardial infarction, any one of these changes may be present without any of the others. As an example, it is not unusual to see ST segment depression without T wave inversion. I

Myocardial Ischemia Characteristic signs:

T Wave Inversion Occurs because ischemic tissue does not repolarize normally Instructional point: T wave inversion also be present during a myocardial infarction as the tissue around the infarct becomes ischemic. I

T Wave Inversion Instructional point: Because the chest leads are closest to the ventricles they are the best for checking for T wave inversion as it will be most pronounced in these leads. Used in this context the term symmetrical means that both right and left sides are mirror images. I

Peaked T Waves May be seen in early stages of acute myocardial infarction T waves invert within a short time (two hours)

ST Segment Depression May or may not include T wave inversion Instructional point: Remind the student that ST segment depression or elevation can be evaluated by comparing the ST segment to the PR segment. I

Flat ST Segment Depression Results from subendocardial infarction

ST Segment Elevation Earliest reliable sign that myocardial infarction has occurred Instructional point: Myocardial injury produces ST segment elevation in the leads facing the affected area as depolarization of the injured myocardium is incomplete with the tissue remaining electrically more positive than the uninjured areas surrounding it. I

ST Segment Elevation May also be seen in: Ventricular hypertrophy Conduction abnormalities Pulmonary embolism Spontaneous pneumothorax Intracranial hemorrhage Hyperkalemia Pericarditis

ST Segment Elevation - Pericarditis Instructional points: With pericarditis, an inflammation of the heart, the ST segment elevation is usually flat or concave (the middle sags downward) and seen in all leads except aVR. This resolves with time. It also seems to elevate the entire T wave off the baseline, with baseline gradually slanting back downward all the way to the next QRS complex. The P wave is often a part of this downward slanting of the baseline. A ventricular aneurysm, which is a weakening and bulging of the ventricular wall should be suspected if ST segment elevation following a myocardial infarction persists. I

Pathologic Q Waves Indicate presence of irreversible myocardial damage or myocardial infarction Instructional point: Myocardial infarction can occur without the development of Q waves. I

Pathologic Q Waves Develop because infarcted areas of heart become electrically silent (fail to depolarize) as they are functionally dead

Progression of Myocardial Infarction During MI the ECG often evolves through three stages: Ischemia Injury Infarction

Criteria for Diagnosing MI Based on the presence of at least two of the following three criteria: Clinical history of ischemic-type chest discomfort/pain Rise and fall in serum cardiac markers Changes on serially obtained ECG tracings

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

Identification of MI ECG changes need to be present in two or more contiguous leads

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

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)

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

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

Septal Ischemia, Injury, Infarction Identified though ECG changes in seen in leads V1 and V2

Anterior Ischemia, Injury, Infarction Involves anterior surface of left ventricle Identified though ECG changes in seen in leads V3 and V4 Instructor note: Leads V5 and V6, while located more laterally, may also help identify anterior infarction. I

Lateral Ischemia, Injury, Infarction Involves left lateral ventricular wall Identified though ECG changes in seen in leads I, aVL, V5,V6

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

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

Posterior Ischemia, Injury, Infarction Involve posterior surface of the heart Look for reciprocal changes in leads V1 and V2

Posterior Ischemia, Injury, Infarction Can be identified through leads V7, V8 and V9

Right Ventricular Ischemia, Injury, Infarction Can be identified using leads V3R, V4R, V5R, V6R

Practice Makes Perfect Determine the likely location of ischemia, injury or infarction Answer: Anteroseptal infarct – age undetermined (pathologic Q waves in V1, V2 and V3, ST & T wave abnormality in V1, V2 and V3, ST segment depression in V4) and marked ST segment abnormality, possible inferior subendocardial injury (depressed ST segment in lead II, III and aVF). I

Practice Makes Perfect Determine the likely location of the ischemia, injury or infarction Answer: Inferior wall MI (pathologic Q waves in II, III and aVF, ST segment elevation in II, III and aVF). I

Practice Makes Perfect Determine the likely location of the ischemia, injury or infarction Answer: Anterior wall injury (ST segment elevation in V2 and V3). I

Practice Makes Perfect Determine the likely location of the ischemia, injury or infarction Answer: Inferolateral wall ischemia (ST segment depression and T wave abnormality in leads II, III aVF, V5 and V6) I

Practice Makes Perfect Determine the likely location of the ischemia, injury or infarction Answer: Consider lateral wall ischemia (ST segment depression and T wave abnormality in leads V5 and V6) I

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

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

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

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

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

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