A 55-year-old woman presents for follow-up of recent episodes of atrial fibrillation. Her medical history is significant for tetralogy of Fallot, surgically corrected when the patient was age 24. During the last six months, she has noticed an increase in fatigue with normal activities. Three times in the last two months, she has presented to the emergency department in atrial fibrillation, and each time she was successfully cardioverted to sinus rhythm.

An echocardiogram performed two days ago shows mild left atrial enlargement, moderate right ventricular enlargement, mild aortic insufficiency, moderate tricuspid regurgitation, moderate to severe pulmonic regurgitation, and moderate pulmonic stenosis. The surgical patch repair of her ventricular defect is intact. Her estimated pulmonary artery pressure is 36 mm Hg, and her left ventricular ejection fraction is 64%.

Current medications include benazepril (20 mg in the morning and 40 mg in the evening), buspirone (5 mg bid), furosemide (40 mg/d), metoprolol (50 mg tid), and warfarin (2.5 and 5 mg, on alternating days). She has no drug allergies and does not use alcohol or tobacco.

An ECG is performed and reveals the following: a ventricular rate of 137 beats/min; PR interval, unmeasurable; QRS duration, 168 ms; QT/QTc interval, 342/516 ms; R axis, 190°; and T axis, 11°. What is your interpretation of this ECG?

ANSWER

The ECG reveals a supraventricular tachycardia at a rate of 137 beats/min. A right bundle branch block (RBBB) is evident with a QRS duration of 0.12 sec or more, a terminal broad S wave in lead I, and an RSR’ in lead V₁. Careful inspection of the T wave reveals retrograde P waves.

The correct interpretation of this ECG is atrioventricular (AV) nodal reentry, or AVNRT. AVNRT is a reentrant tachycardia occurring within the AV node. The driving impulse exits the AV node antegrade through the normal bundle (His-Purkinje system) and retrograde back into the atria. Because conduction occurs faster through the His-Purkinje system than through the atria, retrograde P waves are either buried within the QRS complex or occur shortly afterward.

AVNRT is a common supraventricular tachycardia with a female predominance. It has an abrupt onset and termination, and the ventricular rate is typically between 150 and 200 beats/min in adults. This patient’s rate (137 beats/min) is blunted due to beta-blocker therapy. Finally, the RBBB is present as a result of prior surgical repair of a ventricular septal defect from tetralogy of Fallot.