Unusual source of tachycardia in an adolescent

Electrophysiologic study then confirmed the diagnosis of Mahaim fiber tachycardia. Treatment was successful with mapping of the accessory pathways followed by radiofrequency ablation.Patients who present with wide complex tachycardia are always challenging both diagnostically and therapeutically. There can be disagreement among physicians over the ECG interpretation and the best treatment option for the patient. Mahaim fiber is an uncommon cause of tachycardia in which cardiac pre-excitation occurs via slow-conducting, long accessory pathways that terminate in the right ventricular free wall or into the adjacent right bundle. It was first reported by Mahaim and Bennett who found accessory conducting tissues that originated from the Bundle of His and terminated in the right ventricle. Subsequently, other investigators have elucidated the electrophysiologic properties of this pathway leading to the currently accepted concept of slow and decremental anterograde fiber conduction.A 17-year-old previously healthy female presented to the emergency department in the early morning hours with a feeling that her heart was racing. Her symptoms had been ongoing for several hours with accompanying shortness of breath, lightheadedness, nausea, and vomiting. She had experienced recurrent palpitations that usually spontaneously resolved within 20 min over the past year. On the evening prior to presentation, she had drunk multiple cans of caffeinated soda and was up most of the night. She denied drug and alcohol use, fever, recent illness, or any other significant past medical history.At presentation, her heart rate was 220 beats per minute. The electrocardiogram (ECG) revealed a wide complex tachycardia with left bundle branch morphology, a superior axis, an rS in lead III, an R wave in V1, and late QRS transition (after V5) (Figure 1). She was treated with 6 mg rapid-infusion adenosine intravenously, which reduced her heart rate to 78 beats per minute. The repeat ECG showed a sinus