Myocardial Ischemia, Injury, and Infarction

1. 20 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 I

6. ST Segment • Flat line that follows the QRS complex and connects it to T wave 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 I

9. Myocardial Ischemia • Results from decreased oxygen and nutrient delivery to myocardium • Can be reversed if supply of oxygen and nutrients is restored 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 I

13. ECG Indicators I

14. Myocardial Ischemia • Characteristic signs:

15. T Wave Inversion • Occurs because ischemic tissue does not repolarize normally I

16. T Wave Inversion 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 I

19. Flat ST Segment Depression • Results from subendocardial infarction

20. ST Segment Elevation • Earliest reliable sign that myocardial infarction has occurred 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 I

23. Pathologic Q Waves • Indicate presence of irreversible myocardial damage or myocardial infarction 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) newbundle 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 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 I

42. Practice Makes Perfect • Determine the likely location of the ischemia, injury or infarction I

43. Practice Makes Perfect • Determine the likely location of the ischemia, injury or infarction I

44. Practice Makes Perfect • Determine the likely location of the ischemia, injury or infarction I

45. Practice Makes Perfect • Determine the likely location of the ischemia, injury or infarction 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