The relationship between the P wave and local atrial electrogram in predicting conduction block during catheter ablation of cavo-tricuspid isthmus-dependent atrial flutter Miki Yokokawa 1 & Mohamad C. Sinno 1 & Mohammed Saeed 1 & Rakesh Latchamsetty 1 & Hamid Ghanbari 1 & Thomas Crawford 1 & Krit Jongnarangsin 1 & Ryan Cunnane 1 & Frank Pelosi Jr 1 & Frank Bogun 1 & Aman Chugh 1 & Fred Morady 1 & Hakan Oral 1,2 Received: 9 January 2018 /Accepted: 18 April 2018 # Springer Science+Business Media, LLC, part of Springer Nature 2018 Abstract Purpose The endpoint for radiofrequency catheter ablation (RFA) of cavo-tricuspid isthmus (CTI)-dependent atrial flutter (AFL) is complete conduction block along the CTI. The purpose of this study is to evaluate the utility of the temporal relationship between the P wave and the local atrial electrograms in determining complete CTI block. Methods RFA of CTI was performed in 125 patients (age 63 ± 11 years). During pacing from the coronary sinus (CS), the intervals from the peak of the P wave (P peak ) in lead V 1 to the second component of the local atrial electrogram (A 2 ) along the ablation line (P peak -A 2 ) and from the end of the P wave (P end ) to A 2 (P end -A 2 ) were investigated before and after complete block in the first 100 patients (training set). In the next 25 patients (validation set), P peak -A 2 and P end -A 2 intervals were prospectively assessed to determine CTI block. Results The mean P peak -A 2 and P end -A 2 immediately before complete block were - 15±24 and - 39±23 ms compared to 49 ± 17 and 21 ± 16 ms after CTI block (P < 0.0001). P peak -A 2 ≥ 20 ms and P end -A 2 ≥ 0 ms predicted CTI block with 98% sensitivity and 95% specificity and 96% sensitivity and 100% specificity, respectively. In the validation set, the positive and negative predictive values of P peak -A 2 ≥ 20 ms or P end -A 2 ≥ 0 ms were 100 and 96%, respectively. The diagnostic accuracy was 98%. Conclusions During pacing from the CS, the temporal relationship between the P wave in lead V 1 and A 2 is a simple and reliable indicator of complete block during RFA of CTI-AFL. Keywords Atrial flutter . Catheter ablation . Cavo-tricuspid isthmus 1 Introduction Radiofrequency catheter ablation (RFA) of the cavo-tricuspid isthmus (CTI) is often the preferred treatment modality to eliminate CTI-dependent atrial flutter (AFL) [1–9]. A number of maneuvers have been proposed to predict conduction block along the CTI, including the reversal of activation sequence during pacing from the contralateral side of the ablation line, changes in local electrogram polarity, high-density activation mapping, and differential pacing [6, 8, 10–14]. The presence of a parallel corridor of wide-split potentials along the ablation line has also been proposed to predict conduction block. However, identifying double potentials can be challenging after extensive ablation along the CTI. The essence of all of these techniques is to assess whether trans-isthmus conduction along the CTI is slower than non-trans-isthmus conduction in the atrium. When there is complete conduction block along the CTI, activation recorded along the CTI ablation line during pacing from the coronary sinus (CS) should be the latest in the atria and recorded after the paced P wave. The purpose of this study was to investigate whether the relationship between the P wave and activation of CTI immediately lateral to the ablation line during CS pacing is useful for identifying complete con- duction block along the CTI. * Hakan Oral oralh@umich.edu 1 Cardiac Arrhythmia Service, Division of Cardiovascular Medicine, University of Michigan, Ann Arbor, MI, USA 2 Cardiovascular Center, SPC 5853, 1500 E. Medical Center Dr., Ann Arbor, MI 48109-5853, USA Journal of Interventional Cardiac Electrophysiology https://doi.org/10.1007/s10840-018-0378-3