Young Patient with Palpitations on Exertion

A 20-year-old runner was seen in the general cardiology clinic for episodic palpitations and dyspnea on exertion of two-month duration. The episodes of palpitations would last for "a couple of minutes" and were associated with a recent decline in functional capacity for the last month. She denied any exertional chest pain, dizziness, or syncope. Physical examination was grossly normal. Resting 12-lead electrocardiogram is shown in Figure 1. Subsequent to the initial evaluation, an event monitor and echocardiogram were performed. The event monitor (Figure 2) demonstrated an episode of wide complex tachycardia at 185 beat per minute, which correlated with symptoms. Complete 2-D echocardiogram revealed global left ventricular hypokinesis with an ejection fraction of 35-40% and subsequent cardiac magnetic resonance confirmed the presence of mild to moderate left ventricular dilatation with mild global hypokinesis and estimated ejection fraction of 45%. There was no evidence of fibrosis or inflammation. During the initial clinical evaluation, the patient reported a family history of cardiomyopathy in two generations; however, follow-up genetic testing in the patient was unrevealing. No further functional exercise stress testing or stress echocardiogram was performed prior to the referral to electrophysiology for further evaluation of the recorded wide complex QRS tachycardia. After our initial evaluation, the patient proceeded with an electrophysiology study. Decremental atrial pacing resulted in transition from narrow QRS to wide QRS with left bundle branch morphology (Figures 3-5). Programmed ventricular stimulation induced tachycardia with typical left bundle branch block (LBBB) morphology.

Figure 1: Baseline electrocardiogram

Figure 1

Figure 2: Event monitor recording

Figure 2

Figure 3: Baseline AH and HV intervals

Figure 3

Figure 4: Rapid atrial pacing demonstrating prolongation of AH interval, and antegrade His activation

Figure 4

Figure 5

Figure 5
Rapid atrial pacing demonstrating change in QRS complex to typical left bundle branch block and retrograde activation of right bundle branch and prolonged AH interval. With progressively shorter atrial pacing cycle length, His bundle potential activated retrogradely (inscribed in QRS), as wavefront travels down the bypass tract and retrograde up the right bundle to His bundle.

Which of the following is the correct diagnosis?

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