ECG Case #1 - Question #3 Answer

Answer: Enhanced parasympathetic tone and ischemia to the AV nodal artery.

This ECG represents an AV nodal block that can occur with inferior wall myocardial infarctions, specifically second degree type I AV block (Wenckebach). Enhanced parasympathetic tone can occur with inferior MIs. The proposed mechanisms include stimulation of the vagus nerve increasing the parasympathetic nervous system which causes slowing of the AV node and AV nodal blocks and the Bezold-Jarisch reflex. AV nodal ischemia may also contribute. Remember that AV blocking medications such as beta-blockers can cause 1st degree AV block or second degree type I AV block as seen in this scenario.

Also recall that the right coronary artery (RCA), in addition to supplying the right ventricle and inferior walls, also supplies the sinoatrial node (SA node) and the atrioventricular node (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 2nd degree type I AV block seen in the ECG. In order to cause SA node dysfunction, the occlusion in the RCA must be very proximal since the SA nodal artery is one of the first branches of the RCA. The AV nodal artery, however, is more distal and this is quite commonly affected in acute coronary syndromes involving the RCA.

A majority of people (90%) are "right coronary dominant" meaning the AV nodal artery originates from the RCA. In this case, the patient must be right coronary dominant since the RCA is involved (ST elevation in inferior leads) AND there is AV conduction abnormalities indicating AV nodal artery ischemia. If the patient was left coronary dominant (10%), an infarct due to RCA occlusion would not cause AV nodal ischemia. While the inferior wall can be involved in a left dominant circumflex occlusion, the lateral wall would likely also be involved.