Myocardial Ischemia and Infarction PDF

Myocardial Ischemia and Infarction Prepared by Dr. Salwa Maghrabi Updated by Dr. Hayam Asfour Clinical Nursing Practices Department  Intended Learning Outcomes By the end of this session the trainee should be able to: 1.Def...

Myocardial Ischemia and Infarction Prepared by Dr. Salwa Maghrabi Updated by Dr. Hayam Asfour Clinical Nursing Practices Department  Intended Learning Outcomes By the end of this session the trainee should be able to: 1.Define the concept of Acute coronary syndrome 2.Recognize ST elevation myocardial infarction 3.Recognize Non-ST elevation Myocardial infarction 4.Distinguish the Ischemic ECG changes 1.T wave changes 2.ST depression 3.Other ischemic changes 5.Identify ECG definition of LV aneurysm 6.Differentiate other ST elevation causes, e.g. Early repolarization pattern  Acute Coronary Syndrome The term Acute Coronary Syndrome is used to describe urgent situations when the blood supply to the heart is acutely compromised. It is most often caused by acute rupture or erosion of an atherosclerotic plaque which in turn prompts the formation of a thrombus in the coronary artery, further limiting or completely blocking blood flow. The result can be either what is called unstable angina or a myocardial infarction (aka heart attack). (Thaler, 2019,p.241)  How to diagnose MI (1) History and physical examination Typical symptoms: Sudden onset of prolonged, crushing substernal chest pain radiating to the jaw, shoulders, or left arm, associated with nausea, diaphoresis, and shortness of breath Atypical symptoms: burning, a knot in the throat, or a sensation of fullness in the neck (2) Cardiac Enzyme Determinations Elevated blood levels of Creatine Kinase (CK) → rise after 6 hrs of infarction and return normal within 48 hrs. Elevated levels of Cardiac Troponin Enzymes → rise earlier within 2 to 3 hrs and stay elevated for several days. (3) The EKG (Thaler, 2019,p.244)  MI: STEMI vs NSTEMI Myocardial infarctions occur in two basic varieties. 1- ST-segment elevation myocardial infarction or STEMI→ If blood flow through a coronary artery is totally occluded. The most characteristic feature is elevation of the ST segments on the EKG. A STEMI is a true emergency, because the heart muscle is starved of blood supply. 2- Unstable angina or a non–ST-segment myocardial infarction (non- STEMI or NSTEMI) → If blood flow is reduced but not totally blocked In non-STEMIs and unstable angina, the ST segments do not elevate, may remain normal, but most often are depressed (in the morphologic, not emotional, sense). (Thaler, 2019,p.242) Recognize ST Elevation Myocardial Infarction During an acute STEMI, the EKG evolves through three stages: 1. T-wave peaking followed by T-wave inversion (A and B, below) 2. ST-segment elevation (C) 3. The appearance of new Q waves(D) (Thaler, 2019,p.246) Distinguish the Ischemic ECG changes: T wave The T waves is peaking (i.e., hyperacute T wave) → When T wave become tall, nearly equaling or even exceeding the height of the QRS complexes in the same lead (Positive peaked) (Fig A) A few hours later→ the T waves invert (T waves will become negative) (Fig B) These T-wave changes reflect myocardial ischemia, the lack of adequate blood flow to the myocardium. T wave revert to normal → if blood flow restored or O2 demands of heart are eased T wave inversion persist for months or year → actual myocardial cell death occur T wave inversion can be seen in both bundle branch block, ventricular hypertrophy, and hyperventilation (Thaler, 2019,p.247-249) Distinguish the Ischemic ECG changes: T wave One helpful diagnostic feature is that the T waves of myocardial ischemia are inverted symmetrically, with a gentle downslope and rapid upslope. Distinguish the Ischemic ECG changes: ST Segment ST-segment elevation often signifies myocardial injury ST-segment elevation is a reliable sign that true infarction has occurred and that the complete electrocardiographic picture of infarction will evolve unless there is immediate and aggressive therapeutic intervention Even in the setting of a true infarction, the ST segments usually return to baseline within a few hours (Thaler, 2019,p.247-248)  When you unsure about the ST-segment elevation on patient’s ECG 1.If you have access to a previous EKG, just compare the old one to the new one—if the ST elevation is new, you are most likely dealing with an acute coronary syndrome. 2.If the patient is stable and in a monitored environment where emergency care is available, obtain serial EKGs. Any increase in the ST-segment elevation over the ensuing 15 to 60 minutes is indicative of cardiac ischemia. J point elevation will not change. (Thaler, 2019,p.254)  ST- Segment Depression (i.e. Non-STEMI) Non-STEMIs are more common than STEMIs Caused by either: 1. Nonocclusive thrombosis of a major coronary artery; small, incomplete infarctions 2. Complete occlusion of a small offshoot of one of the major coronary arteries. The only EKG changes seen with non-STEMIs are: 1. T-wave inversion 2. ST-segment depression (not elevation). They have a lower initial mortality rate; but a higher risk for further infarction and mortality than STEMIs. They are initially treated medically including coronary angiography and proceeding with revascularization right away. (Thaler, 2019,p.272-273) ST- segment depression (i.e. Non-STEMI) (Thaler, 2019,p.273) Other Ischemic Changes : [reciprocal] Q wave New [reciprocal] Q waves indicates that irreversible myocardial cell death has occurred The presence of [reciprocal] Q waves is diagnostic of myocardial infarction. It is usually appeared within several hours of the onset of a STEMI, but in some patients, they may take several days to evolve [reciprocal] Q waves usually persist for the lifetime of the patient. Pathophysiology of Q wave in MI (Thaler, 2019,p.274-275)  Reciprocal Changes Located some distance from the site of infarction, will see an apparent increase in the electrical forces moving toward them. They will record tall positive R waves. A lead distant from an infarct may record ST-segment depression. (Thaler, 2019,p.275-276)  Pathologic Q waves Pathologic Q waves signifying infarction are wider and deeper. They are often referred to as significant Q waves. The criteria for significance are the following: 1. The Q wave must be greater than 0.04 seconds in duration. 2. The depth of the Q wave must beat least 25% the height of the R wave in the same QRS complex. (Thaler, 2019,p.277)  Identify ECG Definition of LV Aneurysm Left Ventricular Aneurysm is well- known structural complication of acute MI. It is a weakening and blusing out of the ventricular wall Persistent ST-segment elevation often indicates the formation of a ventricular aneurysm Also, you could see small R waves in the leads of the distribution of the left ventricular wall. (Thaler, 2019,p.247-248)  Small R wave Small R wave Differentiate other ST elevation causes, e.g. Early repolarization pattern Early repolarization (ER), also recognized as “J- waves” or “J-point elevation”’ It is an electrocardiographic abnormality consistent with elevation of the junction between the end of the QRS complex and the beginning of the ST segment in 2 contiguous leads J point elevation is a very common type of ST- segment elevation that can be seen in normal hearts. (Thaler, 2019,p.251) Differentiate other ST elevation causes, e.g. Early repolarization pattern The J point, or junction point, is the place where the ST segment takes off from the QRS complex. J point elevation is very, very common, so pay close attention to what follows! Have no pathological significance and carries no risk to the patient It is often seen in young, healthy individuals, particularly in leads V1, V2, and V3 (Thaler, 2019,p.251) J point ST-segment elevation Vs MI ST- segment elevation With myocardial injury, the elevated ST segment has a distinctive configuration. It is bowed upward (convex downward) and tends to merge imperceptibly with the T wave (Fig 1). In J point elevation, the T wave maintains its independent waveform. Fig 1 Fig 2 (Thaler, 2019,p.252-253) Criteria to distinguish the ST elevation of true cardiac ischemia from J point elevation  References Thaler, M. S. (2019). The only EKG book you’ll ever need (Ninth edition). Wolters Kluwer.