The following tracings were obtained in the EP lab during a SVT study. Fig. 1 shows a 12 lead recording of ECG of a patient showing termination of supraventricular tachycardia (SVT). The first 8 beats are narrow QRS short RP tachycardia - with the differential diagnosis of AVNRT or AVRT or less commonly atrial tachycardia. This train is followed by a single ventricular premature beat (VPB) delivered from a catheter in right ventricle, which terminates the tachycardia. On the 12 lead ECG, the single paced beat is seen as a fusion beat as it has some resemblance to a paced ventricular beat and some to the SVT beat. The fusion can result only if both the paced and SVT beats have excited the His-Purkinje system and confirms that the paced beat is His refractory. Fig. 2 shows the corresponding intracardiac recordings of the same event. The first 3 complexes are the SVT beats, where the atrial activation of distal coronary sinus appears to precede that of the His and proximal coronary sinus suggesting possible involvement of a left sided accessory pathway.
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