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Broad Complex Tachycardia: A Broad Differential

A 76-year-old male presented with troponin positive (troponin T 0.51 ng/ml) pulmonary edema and an intermittent broad complex tachycardia (Fig. 1). In addition to hypertension and type II diabetes, he had previously suffered three myocardial infarctions and had undergone both surgical and percutaneous coronary revascularization. A dual-chamber pacemaker had recently been fitted for sinus bradycardia with syncope. The current presentation with a broad complex tachycardia and a left ventricular ejection fraction of 29% prompted initial treatment with amiodarone and referral for an implantable cardiac defibrillator (ICD). The patient was randomized to preemptive ventricular tachycardia (VT) ablation prior to implantation of an ICD. During electrophysiological study, VT could not be initiated by programmed stimulation. A transseptal puncture was performed to permit left ventricular arrhythmia substrate mapping and immediately following which, the clinical arrhythmia was initiated by catheter manipulation within the left atrium. Why does the phenomenon demonstrated in Figure 2 refute the original diagnosis and confirm another?