Supraventricular tachycardia with alternating QRS morphology and cycle length

The presence of a single atrioventricular (AV) node and accessory pathway associated with tachycardia is common. However, tachycardia with alternating QRS morphology and cycle length is uncommon, and may indicate the presence of a double loop. We reported the case of a 45-year-old man with tachycardia and continually alternating QRS morphology and cycle length. The man with intermittent palpitations and tachycardia for 2 years was referred for an electrophysiologic study. During tachycardia, a continually alternating QRS morphology and cycle length were observed on a twelve-lead electrocardiogram (ECG) (Fig. 1). He denied a history of hypertension or coronary heart disease and conventional drug therapy was not effective. The ECG and echocardiogram were otherwise normal. The atrio-His (AH) interval, His-ventricular (HV) interval, and sinus rhythm cycle length were 822 ms, 226 ms, and 48 ms, respectively. Programmed ventricular stimulation showed 1:1 ventriculoatrial (VA) conduction with the preceding coronary sinus 1,2 (CS1,2), indicating VA conduction through concealed left accessory pathways.