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USMLE® Step 2 CK Question of the Day: Second Degree Mobitz type II AV block

Osmosis Team
Published on May 4, 2022. Updated on May 3, 2022.

We're back with a USMLE® Step 2 CK Question of the Day! Today's case involves a 61-year-old man with two weeks of fevers and chills. Medical history is significant for chronic kidney disease (CKD) stage II, due to hypertension. Physical exam shows a regularly irregular heartbeat with a high-pitched, early diastolic decrescendo murmur. Which of the following is the most appropriate management of this patient?

A 61-year-old man comes to the emergency department with two weeks of fevers and chills. He also reports a poor appetite and has noticed a weight loss of 2.3 kg (5 lb) over the past month. He also complains of malaise with intermittent headaches, muscle aches, and night sweats over this same time period. Medical history is significant for chronic kidney disease (CKD) stage II, due to hypertension. Medications include lisinopril and hydrochlorothiazide. He was born in India, and he does not drink alcohol or use recreational drugs. His temperature is 39.4°C (102.9°F), pulse is 45/min, respirations are 23/min, blood pressure is 100/50 mmHg, and oxygen saturation is 95% on room air. He appears ill. Physical exam shows a regularly irregular heartbeat with a high-pitched, early diastolic decrescendo murmur. Lung auscultation reveals crackles at the bilateral lung bases. Laboratory studies show renal function at baseline and normal cardiac enzymes. An ECG is obtained and shows the following:

ECG image from Wikipedia.

Reproduced from: Wikimedia Commons

Which of the following is the most appropriate management of this patient?

A. Left heart catheterization

B. Intubation

C. Defibrillation

D. Transcutaneous pacing

E. No treatment is required

Scroll down for the correct answer!

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The correct answer to today's USMLE® Step 2 CK Question is...

D. Transcutaneous pacing

Before we get to the Main Explanation, let's see why the answer wasn't A, C, or E. Skip to the bottom if you want to see the correct answer right away!

Incorrect answer explanations

Today's incorrect answers are...

A. Left heart catheterization

Incorrect: Left heart catheterization lets interventional cardiologists visualize the anatomy and patency of the coronary arteries and deploy stents to obstructed vessels. Patients with acutely obstructed coronary arteries who need left heart catheterization would instead present with ST-segment changes and elevated cardiac enzymes.

B. Intubation

Incorrect: Patients with acute or worsening hypoxia, severely altered mental status, or airway obstruction should be intubated to provide mechanical ventilation while the underlying problem is fixed. This patient has a peripheral oxygen saturation greater than 90% on room air and does not require mechanical ventilation.

C. Defibrillation

Incorrect: Defibrillation refers to the application of electricity across the chest to shock the heart back into sinus rhythm, and it only works on ventricular tachycardia (VT) and ventricular fibrillation (VF). Defibrillating a patient who does not have VT or VF can cause a deadly arrhythmia.

E. No treatment is required

Incorrect: Second-degree type II AV block is a dangerous arrhythmia that can develop into a complete heart block and cause circulatory collapse at any time. It must be treated urgently and will require pacemaker placement.

Main Explanation

This patient’s ECG shows a second-degree Mobitz type II atrioventricular (AV) block. This heart block is characterized by “all-or-nothing” conduction of electrical impulses through the AV node, leading to dropped beats that appear as a P wave without a subsequent QRS complex. Compared to the lengthening PR intervals in second-degree Mobitz type I (Wenckebach) AV block, Mobitz type II AV block demonstrates a stable PR interval.

Osmosis illustration of 2nd degree av block.

Causes of Mobitz type II AV block overlap with the causes of other AV blocks, including myocardial ischemia, cardiomyopathy (e.g., amyloidosis, sarcoidosis), myocarditis (e.g. Lyme disease), hyperkalemia, and medication effects. This patient is presenting with signs and symptoms of infective endocarditis, including high fevers, night sweats, anorexia, weight loss, and myalgias. Infective endocarditis of the aortic valve may lead to a periannular abscess at the base of the valve, which places pressure on the adjacent AV node, leading to heart block. An abscess in this location may also lead to aortic insufficiency, which is consistent with the murmur present on this patient’s exam.



Initial management of all patients with Mobitz type II AV block involves the placement of transcutaneous pacer pads, as this block may cause profound bradycardia and progress to third-degree heart block at any time. If no reversible cause is found, patients will require permanent pacemaker implantation.

Major Takeaway

Second-degree Mobitz type II AV block is characterized by a stable PR interval with dropped beats, and it most often occurs in patients with underlying heart disease. Treatment typically requires permanent pacemaker implantation.

References

Puech P, Wainwright RJ. Clinical electrophysiology of atrioventricular block. Cardiol Clin. 1983;1(2):209-224.

Guideline on the Evaluation and Management of Patients With Bradycardia and Cardiac Conduction Delay 

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