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.