Alternating and gradually changing narrow QRS complex tachycardia in a patient with heart failure: What is the mechanism?

A 23-year-old woman with palpitations for 9 years was referred for catheter ablation. ECG showed an irregular narrow complex tachycardia with alternating and gradually changing QRS morphologies after alternating and changing RR intervals, with a clear pattern of 2 alternating QRS complexes. An electrophysiology study was performed and confirmed that the mechanism of tachycardia was an automatic left-side His-Purkinje system (HPS) ventricular tachycardia. The gradually changing type-2 QRS complexes was the conduction delayed in the left anterior fascicle due to the short RR interval or the short left-side HH interval. Nine months after the index electrophysiology study, the patient encounter a progressive of heart failure with increased heart rate to 130-150 bpm during rest. Radiofrequency ablation was performed at the upper-septum for eliminating the tachycardia and resulted in complete atrioventricular block. A permanent pacemaker with left bundle branch pacing was implanted. Twelve months after the ablation, the enlarged heart shrink to normal with normal left ventricular ejection fraction. In conclusion, careful interpretation of the ECG can identify the sinus P waves followed by irregular narrow complexes, thus avoiding misdiagnosis and unnecessary treatment. Unifocal HPS tachycardia could present with alternating and gradually changing narrow QRS complexes tachycardia and lead to tachycardia cardiomyopathy. Electrophysiology study and catheter ablation were useful for the diagnosis and treatment of HPS tachycardia but with high risk of atrioventricular block. However, successfully elimination the tachycardia would resolve and reverse the enlarged heart and deteriorative heart function.