Infarcts and Ischemia ECG 5
- Normal sinus rhythm
- Inferior ST segment elevation myocardial infarction
- Posterior myocardial infarction
- Left atrial enlargement
A posterior wall MI frequently occurs along with an inferior wall MI due to the shared blood supply from the right coronary artery. The posterior wall demonstrates ECG changes the opposite of other myocardial segments due to the location; thus, instead of ST segment elevation, a posterior myocardial infarction is characterized by ST segment depression in lead V1 and frequently V2. The R wave is large in lead V1. Turn the ECG upside down, and you will see ST segment elevation in V1 and a large Q wave — normal findings of a myocardial infarction.
Note that the causes for the R wave being larger than the S wave in lead V1 include a posterior MI, right bundle branch block, Wolff-Parkinson-White Syndrome Type A, right ventricular hypertrophy, ventricular tachycardia with a right bundle branch block pattern and isolated posterior wall hypertrophy (can occur with Duchenne’s muscular dystrophy).