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9. Myocardial Infarction

Topics for study:

  1. Introduction (Read this first)
  2. Inferior MI Family
  3. Anterior Q-Wave MI Family
  4. MI + Bundle Branch Block
  5. Non Q-Wave MI
  6. The Pseudoinfarctions
  7. Miscellaneous QRS Abnormalities


Introduction to ECG Recognition of Myocardial Infarction

When myocardial blood supply is abruptly reduced or cut off to a region of the heart, a sequence of injurious events occur beginning with subendocardial or transmural ischemia, followed by necrosis, and eventual fibrosis (scarring) if the blood supply isn't restored in an appropriate period of time. Rupture of an atherosclerotic plaque followed by acute coronary thrombosis is the usual mechanism of acute MI. The ECG changes reflecting this sequence usually follow a well-known pattern depending on the location and size of the MI. MI's resulting from total coronary occlusion result in more homogeneous tissue damage and are usually reflected by a Q-wave MI pattern on the ECG. MI's resulting from subtotal occlusion result in more heterogeneous damage, which may be evidenced by a non Q-wave MI pattern on the ECG. Two-thirds of MI's presenting to emergency rooms evolve to non-Q wave MI's, most having ST segment depression or T wave inversion.

Most MI's are located in the left ventricle. In the setting of a proximal right coronary artery occlusion, however, up to 50% may also have a component of right ventricular infarction as well. Right-sided chest leads are necessary to recognize RV MI.

In general, the more leads of the 12-lead ECG with MI changes (Q waves and ST elevation), the larger the infarct size and the worse the prognosis. Additional leads on the back, V7-9 (horizontal to V6), may be used to improve the recognition of true posterior MI.

The left anterior descending coronary artery (LAD) and it's branches usually supply the anterior and anterolateral walls of the left ventricle and the anterior two-thirds of the septum. The left circumflex coronary artery (LCX) and its branches usually supply the posterolateral wall of the left ventricle. The right coronary artery (RCA) supplies the right ventricle, the inferior (diaphragmatic) and true posterior walls of the left ventricle, and the posterior third of the septum. The RCA also gives off the AV nodal coronary artery in 85-90% of individuals; in the remaining 10-15%, this artery is a branch of the LCX.

Usual ECG evolution of a Q-wave MI; not all of the following patterns may be seen; the time from onset of MI to the final pattern is quite variable and related to the size of MI, the rapidity of reperfusion (if any), and the location of the MI.

  1. Normal ECG prior to MI
  2. Hyperacute T wave changes - increased T wave amplitude and width; may also see ST elevation
  3. Marked ST elevation with hyperacute T wave changes (transmural injury)
  4. Pathologic Q waves, less ST elevation, terminal T wave inversion (necrosis)
    • (Pathologic Q waves are usually defined as duration ≥ 0.04 s or ≥ 25% of R-wave amplitude)
  5. Pathologic Q waves, T wave inversion (necrosis and fibrosis)
  6. Pathologic Q waves, upright T waves (fibrosis)

image 01-10

Inferior MI Family

(includes inferior, true posterior, and right ventricular MI's)

Inferior MI

Example #1: Acute inferior wall ST segment elevation MI (STEMI); note ST segment elevation in leads II, III, aVF; ST segment depression in V1-3 represents true posterior injury.

image 10-32

Example #2: Old inferior Q-wave MI; note largest Q in lead III, next largest in aVF, and smallest in lead II (indicative of right coronary artery occlusion).

image 10-33

True posterior MI

ECG changes are seen in anterior precordial leads V1-3, but are the mirror image of an anteroseptal MI:

True posterior MI is often seen with inferior MI (i.e., "inferoposterior MI")

Example #1: 15-lead ECG with acute posterior MI due to left circumflex coronary artery occlusion.  Note ST depression in leads V1-6, ST segment elevation in V8-9 (true posterior leads), and slight ST segment elevation in leads I and aVL.  ST segment depression in Lead V4R (right chest lead) also indicates left circumflex occlusion.

image 10-35

Example #2: Old inferoposterior MI; note tall R waves in V1-3 (mirror image of posterior Q-waves), and deep Q waves in leads II, III, aVF.  Residual ST-T wave abnormalities are also evident.

image 10-34

Example #3: Old posterolateral MI (precordial leads): note tall R waves and upright T's in V1-3, and loss of R in V6

image 10-11

Right Ventricular MI (only seen with proximal right coronary occlusion; i.e., with inferior family MI's)

Anterior Family of Q-wave MI's

Anteroseptal MI

Example: Fully evolved anteroseptal MI (note QS waves in V1-2, qrS complex in V3, plus ST-T wave changes)

image 10-01

Anterior MI (similar changes, but usually V1 is spared; if V4-6 involved call it "anterolateral")

Example: Acute anterior or anterolateral MI (note Q's V2-6 plus hyperacute ST-T changes)

image 10-03

High Lateral MI (typical MI features seen in leads I and/or aVL)

Example: note Q-wave, slight ST elevation, and T inversion in lead aVL

image 10-27

MI with Bundle Branch Block

MI + Right Bundle Branch Block

MI + Left Bundle Branch Block

Often a difficult ECG diagnosis because in LBBB the right ventricle is activated first and left ventricular infarct Q waves may not appear at the beginning of the QRS complex (unless the septum is involved).

Non-Q Wave MI

Example: Anterolateral ST-T wave changes

image 10-12

ECG Evidence of Acute Left Main Coronary Artery Occlusion

The electrocardiographic changes suggestive of acute left main coronary occlusion are not to be missed!  These include ST segment elevation in lead aVR that is greater than any ST segment elevation in lead V1 plus ST segment depression in 7 or more other leads.  These are illustrated in the image below.  Patients with these findings need urgent attention in the cardiac catheterization lab.

image 10-37

The Pseudoinfarcts

These are ECG conditions that mimic myocardial infarction either by simulating pathologic Q or QS waves or mimicking the typical ST-T changes of acute MI.

Miscellaneous Abnormalities of the QRS Complex:

The differential diagnosis of these QRS abnormalities depend on other ECG findings as well as clinical patient information

Poor R Wave Progression - defined as loss of, or no R waves in leads V1-3 (R £2mm):

Prominent Anterior Forces - defined as R/S ration >1 in V1 or V2

Test your knowledge on lesson 9!