1. Myocardial Ischemia, Injury, and Infarction20
Myocardial Ischemia, Injury, and Infarction
Fast & Easy ECGs, 2nd E – A Self-Paced Learning Program

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

3. 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

4. PR Segment
Flat line that extends from P wave to Q wave (or R wave in absence of a Q wave)

5. Q Wave First part of QRS complex
First downward deflection from baseline
Instructional point:
The Q wave is not always present. I

6. 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

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

8. 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

9. 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

10. 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

11. Myocardial Injury
Results if ischemia progresses unresolved or untreated

12. 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

13. 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

14. Myocardial Ischemia
Characteristic signs:

15. 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

16. 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

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

18. 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

19. Flat ST Segment Depression
Results from subendocardial infarction

20. 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

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

22. 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

23. 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

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

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

26. 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

27. 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

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

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

30. 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)

31. 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

32. 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

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

34. 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

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

36. 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

37. 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

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

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

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

41. 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

42. 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

43. 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

44. 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

45. 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

46. 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

47. 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

48. 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

49. 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

50. 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

51. 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