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A 30-year-old man presented to the emergency department with palpitations and tachycardia. He had been experiencing sore throat, fevers, and myalgias for the past day. He became alarmed when he awoke from sleep with strong palpitations and a heart rate greater than 200/min documented on his smartwatch. He had similar symptoms 1 year ago and was diagnosed with and treated for supraventricular tachycardia (SVT). A subsequent outpatient echocardiogram revealed a structurally normal heart; results of a follow-up electrocardiogram (ECG) were also normal (Figure 1, top).
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Idiopathic fascicular left ventricular tachycardia (IFLVT) arising from the left anterior fascicle
D. Stop diltiazem drip and administer intravenous verapamil
The differential diagnosis for this patient’s narrow-complex tachycardia includes SVT, SVT with aberrancy, and IFLVT. The key to the correct diagnosis is to differentiate IFLVT from SVT with or without aberrancy by recognizing 3 factors—change in axis from sinus rhythm; QRS morphology typical of IFLVT arising from the left anterior fascicle; and atrioventricular (AV) dissociation.
Figure 1 (top) shows the patient’s baseline ECG, which has a normal QRS axis and duration of 82 ms. This changes in the next ECG, which shows a wider QRS with an incomplete right bundle-branch block–like morphology and a right inferior axis. This QRS morphology and axis is typical of an IFLVT arising from the left anterior fascicle. Unlike most ventricular tachycardias, the QRS duration in IFLVTs is often only slightly prolonged (around 120 ms), and those arising from the left anterior and upper septal fascicles can even be narrow-complex with QRS durations less than 110 ms, which adds to the diagnostic difficulty.
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Corresponding Author: Michael Wu, MD, Rhode Island Hospital, 593 Eddy St, APC 814, Providence, RI 02903 (firstname.lastname@example.org).
Published Online: October 25, 2019. doi:10.1001/jama.2019.16560
Conflict of Interest Disclosures: None reported.
Additional Contributions: We thank the patient for providing permission to share his information.
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