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Radiological and Electrocardiographic Characterization of Right Ventricular 1 2 3 4 Outflowtract Pacing. J.C. Balt , N.M. van Hemel , H.J.J. Wellens and W.G. de Voogt 1 Department of Cardiology, St. Antonius Hospital, Nieuwegein, The Netherlands 3 Cardiovascular Research Institute Maastricht, Maastricht, The Netherlands 2 Utrecht University, Heart Lung Center Utrecht, Utrecht, The Netehrlands 4 Department of Cardiology, St Lucas Andreas Hospital, Amsterdam, The Netherlands Background A. Septal B. Anterior The RVOT is used as an alternative pacing site, but its superiority to the RV-apex remains to be established. The aim of the present study is to characterize the RVOT in terms of fluoroscopic and electrocardiographic parameters. C. Free Wall fd Methods 143 patients underwent pacemaker implantation in the RVOT. Lead position was determined by fluoroscopy. The RVOT was divided into four areas: Anterior, Posterior, Septal and Free Wall (figures 1 and 2). During ventricular pacing, QRS-duration, configuration and amplitude was determined. Figure 1. Fluoroscopy images after lead implantation. LAO 45° projection. Septal placement is assumed when the RV-lead points rightward (A). If the leads points upward, the lead is assumed to be in the Anterior position (B). Free wall placement is assumed when the RV-lead points leftward (towards the sternum) (B). Figure 2. Cross-section of the chest, adapted with permission from Mond et al. The different areas of the RVOT are schematically outlined. Results fg Figures 4 and 5. QRS vector and morphology in lead I. A QRScomplex with an amplitude of 0.05 mV is defined as positive. Regarding QRS vector, differences between pacing sites are statistically significant (2-test, p<0.05). Regarding QRSmorphology, differences between pacing sites are not statistically significant (2-test, p=0.07). Figure 3. The Transitional Zone Index (TZI) is used to quantify precordial R-transition. #: p < 0.05 compared with septal. *: p < 0.001 compared with septal The TZI is defined as the abscissa of the point at which the R-wave amplitude and S-wave amplitude are equal. Adated from Shima et al. Conclusions We conclude that (1) in patients in whom lead implantation in the RVOT was performed, in 1/3 of patients, a septal position was achieved and (2) the paced QRS complexes resulting from different stimulation sites within the RVOT differ significantly. However, the overlap of QRS patterns is considerable. Therefore, we could not define clear cut-off point or devise flow-charts to match ECG and pacing site.