Review Question - QID 216573

This patient with bradycardia, hypotension, cannon A waves (marked pulsations in the jugular vein), bibasilar crackles, altered mental status, and intermittent non-conducted P waves without PR prolongation has Mobitz type II second degree atrioventricular (AV) block. Unstable patients with high-grade AV block should have transcutaneous pacing performed.

In Mobitz type II second degree AV block, atrial impulses intermittently fail to reach the ventricles usually due to conduction disease below the level of the AV node. Signs and symptoms may include fatigue, dyspnea, chest pain, presyncope, syncope, bradycardia, and an irregular pulse. ECG findings show intermittently dropped P waves without PR prolongation. Unstable patients should receive atropine. Treatment for a Mobitz type II involves initiating pacing as soon as this rhythm is identified. Type II blocks imply structural damage to the AV conduction system. This rhythm often deteriorates into complete heart block. These patients require transvenous pacing until a permanent pacemaker is placed. Unlike Mobitz type I (Wenckebach), patients that are bradycardic and hypotensive with a Mobitz type II rhythm often do not respond to atropine. If patients continue to be unstable, norepinephrine for cardiogenic shock, dobutamine for heart failure symptoms, or epinephrine infusion can be started. Stable patients should be evaluated for reversible causes, such as myocardial ischemia, use of drugs like calcium channel blockers and beta-blockers, hypoxia, or hyperkalemia. Definitive management, once patients are stable, is with a permanent pacemaker.

Mangi et al. note that Mobitz type II is rarely seen in patients without structural heart disease. It is often associated with myocardial ischemia and fibrosis or sclerosis of the myocardium. This rhythm often progresses to third-degree atrioventricular block

Figure/Illustration A is the rhythm strip of an ECG showing a Mobitz II second-degree AV block. The blue box shows two successive PR intervals without increasing PR prolongation. The orange box shows a non-conducted P wave.

Incorrect Answers:
Answer 1: Atropine is generally avoided in patients with Mobitz type II second-degree AV block, as the block is generally infranodal. Rarely, atropine or other agents can worsen infranodal block by increasing sinus rate without improving conduction.

Answer 2: Carotid sinus massage stretches baroreceptors in the carotid wall and mimics high blood pressure, which in turn increases parasympathetic outflow and decreases blood pressure and heart rate. This technique may help distinguish Mobitz type I from Mobitz type II second-degree AV block in patients with a 2:1 AV block by unmasking higher degree AV block. In this patient with a history of carotid artery disease and unstable vitals, a carotid artery massage would not be indicated.

Answer 3: Synchronized electrical cardioversion delivers energy synchronized to the QRS complex that depolarizes all excitable myocardial tissue. Synchronized cardioversion can be used to treat atrial fibrillation, atrial flutter, and stable ventricular tachycardia. This unstable patient’s ECG is consistent with a Mobitz type II second-degree AV block which should be initially managed with atropine and temporary cardiac pacing.

Answer 5: Vasopressin may slightly improve the patient's blood pressure, however, it would not treat the underlying cause which is the heart block. Patients with Mobitz type II second-degree AV block who are hemodynamically unstable could be treated with a beta-adrenergic agonist (eg, isoproterenol, dopamine, dobutamine, or epinephrine) if myocardial ischemia is unlikely and if there is a delay in pacing.

Bullet Summary:
Unstable patients with second-degree Mobitz Type II AV block should be placed on temporary cardiac pacing.