A 67-year-old man presents to the emergency department with shortness of breath. His dyspnea began 1 hour ago while he was sitting in a chair watching television. He also endorses lightheadedness and fatigue but denies chest pain. Two weeks ago, he had a mitral valve repair for mitral insufficiency secondary to myxomatous degeneration. His medical history is otherwise significant for hypertension and diabetes mellitus, for which he takes metformin and lisinopril. His temperature is 98.8°F (37.1°C), blood pressure is 85/54 mmHg, pulse is 48/min, and respirations are 22/min. On physical exam, he appears to be in moderate distress. He has normal cardiac sounds with an irregular rhythm. His lungs are clear to auscultation bilaterally, and his electrocardiogram can be seen in Figure A. The patient is given an initial dose of atropine but this has no effect. Which of the following is the most appropriate next step in management?
Cardioversion
Atropine
Procainamide
Transvenous pacing
Transvenous pacing
This patient who presents with lightheadedness has an ECG consistent with a Mobitz type 2 2nd-degree heart block. Because the patient is symptomatic from his dysrhythmia and it did not respond to atropine, the most appropriate next step is transvenous pacing.

Mobitz type 2 2nd-degree heart block can be distinguished from Mobitz type 1 (Wenckebach) by the length of the PR interval. Whereas the PR interval in Mobitz type 1 becomes progressively longer with each beat, the PR intervals in Mobitz type 2 are consistent in length and the QRS complexes drop suddenly. While Mobitz type 1 heart block is usually caused by atrioventricular (AV) node dysfunction, Mobitz type 2 is caused by dysfunction below the AV node in the His-Purkinje system. Patients with either type of heart block can present with lightheadedness or syncope. Since this pattern has a risk of progressing into 3rd-degree heart block, hemodynamic compromise, and asystole, Mobitz type 2 heart block is an indication for placement of a permanent pacemaker. Unstable patients who do not respond to atropine and/or other medical management should undergo transcutaneous or transvenous pacing.

Clark and Prystowsky present the pathophysiology of heart block. They discuss how dysfunction at the infranodal level leads to type 2 2nd-degree heart block. They recommend pacing in symptomatic patients.

Figure/Illustration A is an ECG that is characterized by consistent PR intervals (red lines) and sudden drops in the QRS complex. This pattern is classically seen with a Mobitz type 2 2nd-degree heart block.

Incorrect Answers:
Answer A: Adenosine can be used to terminate supraventricular tachycardia. These patients will present with tachycardia rather than bradycardia and will experience palpitations in addition to lightheadedness. The rhythm is rapid and regular with a narrow QRS complex.

Answer C: Cardioversion can be used to convert atrial flutter, atrial fibrillation, stable monomorphic ventricular tachycardia, or recalcitrant supraventricular tachycardia (non-responsive to adenosine or vagal maneuvers) to sinus rhythm, but it is not used for 2nd-degree heart block.

Answer B: Atropine can improve Mobitz type 1 2nd-degree heart block but tends not to be effective in Mobitz type 2 heart block in a stable patient. Atropine might be an appropriate treatment in symptomatic 3rd-degree heart block or Mobitz type 2 heart block; however, if it fails to improve a patient's symptoms, pacing is the most appropriate next step.

Answer D: Procainamide is a class 1A antiarrhythmic medication used to treat Wolff-Parkinson-White syndrome. This would present with an upgoing delta wave that occurs prior to the QRS complex. This medication does not play a role in the management of 2nd-degree heart block.

Bullet Summary:
Hemodynamically unstable patients with Mobitz type 2 2nd-degree heart block should be managed with transvenous pacing followed by a permanent pacemaker.