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. 2013 Jan 10;36(4):508–532. doi: 10.1111/pace.12064

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Resetting with fusion. Left panel: a left ventricular extrastimulus (St) is delivered during a regular orthodromic tachycardia mediated by a left-sided accessory pathway. Right panel: left ventricular stimulation at identical site and rate is started during sinus rhythm. Note that the extrastimulus resets the tachycardia, but the QRS is totally different from stimulation during sinus rhythm at the same site (right panel); the QRS of the extrastimulus is a fused QRS complex, actually resembling more the tachycardia QRS morphology (there are only minor changes—see III, aVL, and aVF) and duration (is narrow). Measurements were obtained at high screen speed for accuracy. Courtesy of Dr. M. Pachon and Dr. M.A. Arias.