Fast broad irregular tachycardia (FBIT): Not always atrial tachyarrhythmias with aberrancy or pre-excitation

BACKGROUND: Fast broad irregular tachycardia (FBIT) is traditionally attributed to atrial arrhythmias with aberrancy or pre-excitation, and ventricular tachycardia (VT) is not typically considered in differential diagnosis. PURPOSE: We present a series of FBIT cases that presented with no evidence of pre-excitation or typical atrial arrhythmia in the baseline ECG and were ultimately diagnosed as VT. METHODS AND RESULTS: This is a retrospective case series where the data was extracted from myocarditis registry (May 2020 - November 2023) consisting of 615 patients. Out of which 177 patients had ventricular tachycardia and seven patients had FBIT as initial clinical presentation. The cohort comprised of seven patients (mean age 39.85 ± 9.8 years; 3 males) with no known comorbidities There was no evidence of pre-excitation on baseline ECG or VT morphology mimicking baseline ECG. Two-dimensional echocardiography revealed structurally normal heart in all patients. Four out of seven patients underwent catheter ablation for multidrug resistant VT storm and non-clinical VTs were inducible in three patients. Spontaneous onset and termination along with changing morphology of arrhythmias were observed. No abnormal substrate was identified to perform modification, as per bipolar and unipolar voltage mapping during sinus rhythm. There was no evidence of inducible atrial arrhythmias in this subset. All these patients responded to disease specific management. CONCLUSION: Hence, in cases presenting with FBIT without evidence of atrial arrhythmias on ECG, a high index of suspicion for VT as a differential diagnosis should be maintained.