REVIEW Diagnostic Pacing Maneuvers for Supraventricular Tachycardias: Part 2 GEORGE D. VEENHUYZEN, M.D., F. RUSSELL QUINN, M.R.C.P., PH.D., STEPHEN B. WILTON, M.D., ROBIN CLEGG, M.D., and L. BRENT MITCHELL, M.D. From the Libin Cardiovascular Institute of Alberta, University of Calgary and Calgary Health Region, Alberta, Canada The approach to supraventricular tachycardia (SVT) diagnosis can be complex because it involves synthesizing baseline electrophysiologic features, features of the SVT, and the response(s) to pacing maneuvers. In this two-part review, we will mainly explore the latter while recognizing that neither of the former can be ignored, for they provide the context in which diagnostic pacing maneuvers must be correctly chosen and interpreted. Part 1 involved a detailed consideration of ventricular overdrive pacing, since this pacing maneuver provides the diagnosis in the majority of cases. In Part 2, other diagnostic pacing maneuvers that might be helpful when ventricular overdrive pacing is not diagnostic or appropriate, including attempts to reset SVT with single atrial or ventricular beats, para-Hisian pacing, apex versus base pacing, and atrial overdrive pacing, are discussed, as are some specific diagnostic SVT challenges encountered in the electrophysiology lab. There is considerable literature on this topic, and this review is by no means meant to be all-encompassing. Rather, we hope to clearly explain and illustrate the physiology, strengths, and weaknesses of what we consider to be the most important and commonly employed diagnostic pacing maneuvers, that is, those that trainees in cardiac electrophysiology should be well familiar with at a minimum. (PACE 2012; 35:757–769) ablation, electrophysiology - clinical, SVT , pacing In part 1 of this review on diagnostic pac- ing maneuvers for supraventricular tachycardia (SVT), we explored ventricular overdrive pacing (VOP) in detail, since it provides a firm SVT diagnosis in the majority of cases. 1 We will now consider pacing maneuvers that can be performed when VOP is not diagnostic, including ones that can be performed when sustained, regular SVT cannot be induced. These will include single- paced ventricular beats during ongoing SVT, para- Hisian pacing, and apex versus base pacing. We will also explore some challenging specific situations in SVT diagnosis including differen- tiating atrioventricular node reentry tachycardia (AVNRT) from atrial tachycardia (AT) and junc- tional tachycardia (JT), SVT with atrioventricular (AV) dissociation, and differentiating AVNRT with a leftward atrionodal exit from orthodromic atrioventricular reciprocating tachycardia (AVRT) employing a left-sided accessory pathway (AP). Address for reprints: George D. Veenhuyzen, M.D., F.R.C.P.C., Libin Cardiovascular Institute of Alberta, University of Calgary and Calgary Health Region, Foothills Medical Centre, Rm C836, 1403-29 St. N.W., Calgary, Alberta, T2N 2T9, Canada. Fax: 403- 944-1592; e-mail: george.veenhuyzen@calgaryhealthregion.ca Received September 19, 2011; revised December 22, 2011; accepted January 5, 2012. doi: 10.1111/j.1540-8159.2012.03352.x Scanning diastole with ventricular premature beats (VPBs) Single VPB introduced decrementally during diastole in SVT offer an opportunity to determine the relationship between altered timing of ven- tricular depolarization and the timing of atrial depolarization. For example, if a VPB is able to terminate tachycardia without atrial depolariza- tion, then AT can be excluded, provided this is not a coincidence. Furthermore, VPBs that occur during SVT at a time when the stimulated wavefront would be expected to collide with the SVT wavefront in the His-Purkinje network or in ventricular myocardium cannot possibly affect atrial timing during either AVNRT or AT (unless a bystander AP is present). Accordingly, such His- refractory VPBs (HRVPBs) should only be capable of affecting AVRT circuits (again, in the absence of a bystander AP). VPBs that occur before His bundle refractoriness are potentially capable of affecting atrial timing (including terminating SVT) in any of AT, AVNRT, or AVRT. How does one determine if a paced VPB is His-refractory? If the QRS complex morphology of the VPB shows evidence of fusion (i.e., the QRS complex morphology of the VPB shows some features of the QRS complex morphology of a paced VPB and some features of the QRS complex morphology of the SVT), then the paced VPB must be His-refractory, since the SVT wavefront that the C 2012, The Authors. Journal compilation C 2012 Wiley Periodicals, Inc. PACE, Vol. 35 June 2012 757