USMLE Step 3 - Question 28

A 56 year old male presents to the emergency department with chest pains, nausea and vomiting. His temperature is 37.2, blood pressure 110/70, heart rate 50, respirations 20, and oxygen saturation normal on room air. Physical examination reveals normal breath sounds, an S4 heart sound, and a regularly irregular rhythm. Laboratory studies reveal an elevated troponin I level and appropriate therapy for acute coronary syndrome is begun. His ECG is below. What is the treatment for his heart rhythm?

2ndDegreeAVB-TypeI

A. Atropine

B. Adenosine

C. Emergency pacemaker placement

D. Observation

This patient is having an inferior wall ST segment elevation myocardial infarction (inferior STEMI, note the ST elevation in leads II, III, and aVF and reciprocal depression in I and aVL) and his rhythm is second degree type I AV nodal block (Wenkebach). This is a benign rhythm even in the setting of an MI. During an inferior STEMI, the vagus nerve is stimulated due to the proximity of the inferior wall and the diaphragm resulting in nausea, vomiting, and enhanced vagal tone to the heart. Vagal hypertonicity results in bradycardia and AV nodal blocks, frequently second degree Type I. Treating the inferior STEMI should relieve the vagal hypertonicity and restore AV nodal function, thus no emergency pacemaker would be needed. While atropine can be used to temporarily relieve the vagal hypertonicity, our patient is doing fine with no symptoms of bradycardia (which would be related to hypotension such as dizziness or syncope).

Remember than an inferior STEMI is usually due to a right coronary artery (RCA) occlusion and 80% of people are "right dominant" in their coronary anatomy. Dominance is determined by which coronary system (left or right) supplies the AV node. Thus, an RCA occlusion (causing an inferior STEMI) usually results in AV nodal ischemia as well as the vagal hypertonicity, thus acting synergistically to cause AV conduction problems, which on occasion (mostly in revascularization is not quickly achieved) it may necessitate permanent pacemaker implantation.