Infarcts and Ischemia ECG Quiz #1 Answer

1. Normal sinus rhythm
2. Second-degree type I AV block (Wenkebach)
3. Inferior myocardial infarction

This is a classic example of atrioventricular nodal blocks that can occur with inferior wall myocardial infarctions. There are two mechanisms behind this phenomenon. An inferior wall MI results in vagal stimulation, which enhances vagal tone. This activates the parasympathetic nervous system which causes slowing of the AV node and AV nodal blocks. This is part of the Bezold-Jarisch reflex.

Also recall that the right coronary artery, in addition to supplying the right ventricle, inferior and posterior walls, also supplies the sinoatrial node and the AV node. A decrease in blood flow to the AV nodal artery (a branch of the RCA) can result in AV nodal ischemia and AV conduction abnormalities such as the second-degree type II AV block seen in the ECG. In order to cause SA node dysfunction, the occlusion in the RCA must be very proximal, as the SA nodal artery is one of the first branches of the RCA. The AV nodal artery, however, is more distal and quite commonly affected in acute coronary syndromes involving the RCA.

Remember, a majority of people (90%) are right coronary dominant meaning the AV nodal artery originates from the RCA. This patient is likely right coronary dominant because the RCA is involved (ST segment elevation in inferior leads), and there are AV conduction abnormalities, indicating likely AV nodal artery ischemia. If the patient was left coronary dominant (10%), an infarct due to RCA occlusion would not be as likely to cause AV nodal ischemia.