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Myocardial Infarction and Ischemia MEHDI BAKHSHI MSN PhD ICNS ST Elevation and non-ST Elevation MIs When myocardial blood supply is abruptly reduced or cut off to a region of the heart, a sequence of injurious events occur beginning with ischemia (inadequate tissue perfusion), followed by necrosis (infarction), and eventual fibrosis (scarring) if the blood supply isn't restored in an appropriate period of time. The ECG changes over time with each of these events… ECG changes in a MI ECG Changes Ways the ECG can change include: ST elevation & depression T-waves peaked Appearance of pathologic Q-waves flattened inverted Cross sectional analysis of an area of infarcted myocardium reveals the three electrically differentiated zones. E INFARCTION INJURY ISCHAEMIA E INFARCTION INJURY ISCHAEMIA INFARCTED MYOCARDIUM (STEMI) myocardium The electrically dead electrode lying over the area of infarction has the effect of looking through the infarcted area as a window. This therefore will detect and record potentials from the myocardium directly opposite. INJURED MYOCARDIUM myocardium The is never completely polarized electrode lying over the area of injury will record ST Segment elevation on the ECG because of the myocardium retaining its polarity. ISCHAEMIC MYOCARDIUM myocardium exhibits impaired repolarisation The electrode lying over the area of ischaemia will record T wave changes on the ECG STAGE 1 ACUTE STAGE - HOURS OLD stage of injury – The myocardium is not yet dead and unless rapid intervention is possible then death of the affected area of muscle will certainly follow. In the case of rapid intervention then the area of death may be reduced although even with treatment some necrosis will take place Acute The typical shape of the ECG leads which are positioned directly over the injured area of myocardium will show significant ST segment elevation of greater than 2 mm, there may also be a reduction in the size of the R wave. There will be ST segment depression in the areas of myocardium opposite the injured area these are known as RECIPROCAL CHANGES STAGE 2 LATER PATTERN - DAYS OLD In stage 2 the injured myocardium is now starting to necrose and this results in Q waves beginning to appear on the ECG which are representations of depolarization on the opposite wall of the heart, this is due to the window effect over the area of dead myocardium The electrode is looking through the electrical window where no electrical activity occurs The ST segment elevation will lessen as the area of injury either becomes Ischaemic or dies T waves now begin to appear representing the area of ischaemia which is surrounding the infarcted STAGE 3 LATE PATTERN - WEEKS OLD In stage three, the zone of injury has now evolved into infarcted myocardium There is a pathological Q wave seen on the ECG due to the electrical window being present The ST segment has now returned to normal/Iso-electric line because the injured area has now necrosed or become ischaemic There is now a symmetrically inverted T wave present on the ECG which represents persistent ischaemia surrounding the area of infarct STAGE 4 OLD INFARCT -MONTHS TO YEARS In stage 4 the zone of ischaemia has recovered and the ECG returns to almost normal However there are changes which allow us to identify a previous infarct on the ECG The pathological Q wave is considered the finger print for life of a previous myocardial infarction The R wave height is reduced in the leads positioned directly over the area of infarct fingerprint For for life of MI - Q wave the Q wave to be significant Q wave must be 25 % in depth of the following R wave height ST Elevation Infarction Here’s a diagram depicting an evolving infarction: A. Normal ECG prior to MI B. Ischemia from coronary artery occlusion results in ST depression (not shown) and peaked T-waves C. Infarction from ongoing ischemia results in marked ST elevation D/E. Ongoing infarction with appearance of pathologic Q-waves and T-wave inversion F. Fibrosis (months later) with persistent Qwaves, but normal ST segment and Twaves Right Coronary Artery (RCA) Supplies blood to • Right atrium • Right ventricle • Posterior and inferior walls of left ventricle (if dominant) VI EDUCATION Not to be copied or re-distributed Source: data on file at Guidant Left Anterior Descending (LAD) Supplies blood to the left ventricle •Anterior wall •Lateral wall •Apical wall •Ventricular septum VI EDUCATION Not to be copied or re-distributed Source: data on file at Guidant LEFT CIRCUMFLEX (LCX) Supplies blood to •Left atrium •Left ventricle •Lateral wall •Posterior wall •Inferior wall (if dominant) VI EDUCATION Not to be copied or re-distributed Source: data on file at Guidant Location Of infarction ??????????????????? Arrangement of Leads on the ECG Anatomic Groups (Septum) Anatomic Groups (Anterior Wall) Anatomic Groups (Lateral Wall) Anatomic Groups (Inferior Wall) Anatomic Groups (Summary) ST Elevation Infarction Here’s an ECG of an inferior MI: Look at the inferior leads (II, III, aVF). Question: What ECG changes do you see? ST elevation and Q-waves Extra credit: What is the rhythm? Atrial fibrillation (irregularly irregular with narrow QRS)! Non-ST Elevation Infarction Here’s an ECG of an inferior MI later in time: Now what do you see in the inferior leads? ST elevation, Q-waves and T-wave inversion Non-ST Elevation Infarction Here’s an ECG of an evolving non-ST elevation MI: Note the ST depression and T-wave inversion in leads V2-V6. Question: What area of the heart is infarcting? Anterolateral