Accurate ECG interpretation in a patient with chest pain is critical. Basically, there can be three types of problems - ischemia is a relative lack of blood supply (not yet an infarct), injury is acute damage occurring right now, and finally, infarct is an area of dead myocardium. It is important to realize that certain leads represent certain areas of the left ventricle; by noting which leads are involved, you can localize the process. The prognosis often varies depending on which area of the left ventricle is involved (i.e. anterior wall myocardial infarct generally has a worse prognosis than an inferior wall infarct).
|II, III, aVF||inferior wall|
|I, aVL||lateral wall|
|V1-V2||posterior wall (reciprocal)|
|1. Ischemia||Represented by symmetrical T wave inversion (upside down). The definitive leads for ischemia are: I, II, V2 - V6.|
|2. Injury||Acute damage - look for elevated ST segments. (Pericarditis and cardiac aneurysm can also cause ST elevation; remember to correlate it with the patient.|
|3. Infarct||Look for significant "patholgic" Q waves. To be significant, a Q wave must be at least one small box wide or one-third the entire QRS height. Remember, to be a Q wave, the initial deflection must be down; even a tiny initial upward deflection makes the apparent Q wave an R wave.|
Figure 34: Ischemia: Note symmetric T wave inversions in leads I, V2-V5.
Figure 35: Injury: Note ST segment elevation in leads V2-V3 (anteroseptal/anterior wall).
Figure 36: Infarct: Note Q waves in leads II, III, and aVF (inferior wall).
For the posterior wall, remember that vectors representing depolarization of the anterior and posterior portion of the left ventricle are in opposite directions. So, a posterior process shows up as opposite of an anterior process in V1. Instead of a Q wave and ST elevation, you get an R wave and ST depression in V1.
Figure 37: Posterior wall infarct. Notice tall R wave in V1. Posterior wall infarcts are often associated with inferior wall infarcts (Q waves in II, III and aVF).
Two other caveats: One is that normally the R wave gets larger as you go to V1 to V6. If there is no R wave "progression" from V1 to V6 this can also mean infarct. The second caveat is that, with a left bundle branch block, you cannot evaluate "infarct" on that ECG. In a patient with chest pain and left bundle branch block, you must rely on cardiac enzymes (blood tests) and the history.
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