Review Question - QID 217291

This elderly man with a history of hypertension, hyperlipidemia, and smoking presenting with sudden chest pain, hypotension, bradycardia, and jugular venous distention with ST elevations in leads II, III, and aVF most likely has an inferior wall ST elevation myocardial infarction (STEMI) due to a blockage in the right coronary artery (RCA).

Inferior wall STEMIs occur due to a blockage of the RCA which feeds the posterior descending artery (PDA), which then supplies the inferior myocardium. Significant involvement of the right ventricle in addition to the inferior wall can also occur and may precipitate cardiogenic shock. Diagnosis of an inferior wall STEMI can be made primarily with an electrocardiogram (ECG) demonstrating ST elevations in leads II, III, and aVF. Cardiac biomarkers such as troponin can also be used to support the diagnosis. Importantly, the AV node also receives its blood supply from the RCA and so inferior wall STEMIs can cause significant heart block. As with any STEMI, emergent cardiac catheterization provides the highest mortality benefit if instituted early. Medical management includes aspirin, heparin, and a 2nd anti-platelet agent such as clopidogrel.

Warner et al. outline the pathophysiology, evaluation, and management of inferior wall STEMIs. Of note, medical management of inferior wall STEMIs deviates from typical STEMI protocols due to the preload dependent state of patients who have significant right ventricle damage. In these patients, the nitrate-induced reduction in preload actually worsens cardiac function rather than improving it. In light of this effect, patients with inferior wall STEMIs should be managed with volume resuscitation in order to preserve preload.

Figure/Illustration A is an ECG demonstrating ST-elevations in leads II, III, and aVF (red arrows) as well as bradycardia due to an inferior wall MI with heart block. Note: standard 12-lead ECG convention is 1 large box = 0.2 seconds. Using this knowledge, heart rate can be calculated by dividing the number of large boxes between 2 consecutive QRS complexes into 300. In this patient, approximately 5.5 large boxes separate adjacent QRS complexes, and so 300/5.5 = 55/min.

Incorrect Answers:
Answer 1: Intimal layer of the ascending aorta can be damaged, leading to Stanford type A aortic dissections which present with "tearing" chest or back pain and hypertension, rather than hypotension. Although Stanford type A dissections can extend into the coronary ostium and result in a STEMI with elevated cardiac biomarkers, this is an extremely rare occurrence (1-2% of Stanford type A dissections) and would be unlikely in a patient presenting with crushing, rather than tearing, chest pain. Preferred treatment for Stanford type A aortic dissections is emergency surgical correction.

Answer 2: Left anterior descending (LAD) artery occlusions result in ST-elevations in leads V1, V2, V3, and V4 and would not be expected to present with heart block as the AV node is supplied by the RCA. Lesions in the LAD can lead to ischemia and subsequent dysfunction of the left ventricle (LV), resulting in signs of acute left-sided heart failure such as pulmonary edema, hypotension, and cold extremities.

Answer 3: Left circumflex artery (LCA) occlusions most commonly result in ST elevations in leads V5 and V6 without heart block. Of note, in approximately 20% of patients, the posterior descending artery (PDA) is a branch of the LCA rather than the RCA and so a similar pattern of injury to the inferior myocardium is possible via an LCA occlusion.

Answer 4: Main pulmonary artery bifurcation occlusions occur due to a massive saddle embolus and may present with chest pain and hemodynamic instability, but would cause tachycardia rather than bradycardia as seen in this patient. Treatment options depend on clot burden and hemodynamic stability of the patient and includes therapeutic anticoagulation, targeted thrombolysis, and in some cases suction thrombectomy.

Bullet Summary:
Patients with inferior wall STEMIs often present with heart block in addition to chest pain due to involvement of the AV node and can be diagnosed with ST elevations in leads II, III, and aVF on ECG.