Answer

Atrial fibrillation in a paced rhythm: The rhythm depicted in the most recent ECG (A) is irregular and has an average rate of 84 beats per minute. Atrial and ventricular spike waves are present, indicating that the dual-chamber pacer is firing. However, no P waves follow the atrial spikes. The ventricular spike is followed by a wide QRS complex; its morphology is consistent with the typical left bundle branch block pattern of a ventricular paced rhythm. In contrast, an ECG obtained 3 months ago (B) reveals a regular rhythm with distinct P waves following the atrial spike.

This patient has recurrent atrial fibrillation with a dual-chamber pacer. The ECG in B shows distinct atrial activity (P waves) that is transmitted to the ventricle and results in a coordinated, paced, regular rhythm. This type of rhythm allows the patient to achieve the atrial stimulation necessary to maintain a cardiac output, which allows her to lead a functional life. The patient now presents to the hospital with atrial fibrillation and no distinct P waves that can be conducted to the ventricle. Hence, her rhythm has become irregular. Because of a reduction in cardiac output due to a loss of atrial stimulation, the patient developed fatigue and exertional dyspnea. She recovered completely after direct-current (DC) cardioversion was performed to restore a sinus rhythm. Reference: Tintinalli JE. Emergency Medicine: A Comprehensive Study Guide. 5th ed. New York, NY: McGraw-Hill Book Co; 2000:174-193.

BACKGROUND

An 80-year-old woman with a history of paroxysmal atrial fibrillation presents with fatigue that has lasted 5 days. She has a dual-chamber pacemaker that was placed 3 months ago. She denies chest pain but complains of shortness of breath on exertion. The patient is afebrile, her heart rate is 84 beats per minute, her blood pressure is 100/60 mm Hg, and her respiratory rate is 16 breaths per minute.

Hint

Review both ECGs for their rhythms and look for P waves. ECG A is current. ECG B was obtained 3 months ago.

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