A 58-year-old man with a history of hypertension and hyperlipidemia comes to the emergency department due to substernal chest pain that began 6 hours ago and has gradually worsened. Blood pressure is 104/72 mm Hg and pulse is 88/min. ECG shows normal sinus rhythm with ST-segment depression and T-wave inversion, and troponin levels are elevated. The patient is diagnosed with non-ST-segment elevation myocardial infarction and admitted to the hospital for medical management, with coronary angiography scheduled the following morning. That evening, telemetry monitoring shows a new Mobitz type 1 second-degree atrioventricular block. Which of the following arteries was most likely obstructed during this patient's myocardial infarction?

Question

A 58-year-old man with a history of hypertension and hyperlipidemia comes to the emergency department due to substernal chest pain that began 6 hours ago and has gradually worsened.  Blood pressure is 104/72 mm Hg and pulse is 88/min.  ECG shows normal sinus rhythm with ST-segment depression and T-wave inversion, and troponin levels are elevated.  The patient is diagnosed with non-ST-segment elevation myocardial infarction and admitted to the hospital for medical management, with coronary angiography scheduled the following morning.  That evening, telemetry monitoring shows a new Mobitz type 1 second-degree atrioventricular block.  Which of the following arteries was most likely obstructed during this patient's myocardial infarction?

Quizplus · Accepted Answer

11ec160b_355c_fb5b_9249_f3a051c86970_MD0001_00 Low-degree atrioventricular (AV) block, including first-degree and Mobitz type I second-degree, usually involves conduction disruption within the AV node. This patient's development of Mobitz type I second-degree AV block in the setting of acute myocardial infarction (MI) suggests obstruction of the right coronary artery (RCA). In approximately 90% of patients, the RCA supplies blood to the AV node while coursing around the right side of the heart to perfuse the right ventricle and inferior portion of the left ventricle. Because the RCA also usually supplies blood to the sinoatrial node, sinus bradycardia is also commonly seen with MI involving the RCA. In contrast, the left anterior descending (LAD) artery (Choice A), which courses down the anterior left ventricle, typically supplies blood to the left and right bundle branches and sometimes the bundle of His. MI involving the LAD can lead to high-degree AV block (ie, Mobitz type II second-degree, infranodal third-degree) involving conduction disruption below the AV node. Because the LAD does not supply blood to the AV node, low-degree AV block is not typically seen. (Choices B and C) The left main coronary artery bifurcates into the LAD artery and the left circumflex artery, which courses around the left side of the heart to supply the lateral left ventricle. The left main coronary artery does not directly supply blood to the conduction system. The left circumflex artery frequently supplies blood to the sinoatrial node after coursing around to the back of the heart, but only supplies the AV node in about 10% of patients (ie, those with left-dominant coronary circulation). (Choice D) Present in approximately 20% of the population, a ramus intermedius is a third branch off of the left main coronary artery. It originates at the bifurcation of the LAD and left circumflex arteries and courses between them to supply the anterolateral left ventricular wall. It does not supply blood to the conduction system. Educational objective: Low-grade atrioventricular block (ie, first-degree, Mobitz type I second-degree) involves conduction disruption within the AV node. The occurrence of low-grade AV block in the setting of myocardial infarction suggests obstruction of the right coronary artery because it supplies blood to the AV node in approximately 90% of patients.

A 58-Year-Old Man with a History of Hypertension and Hyperlipidemia