Usefulness of unipolar electrograms to detect isthmus block after radiofrequency ablation of typical atrial flutter

Background-RS morphology of the unipolar electrogram is associated with pro pagation of the wave front through the exploring electrode, whereas positiv e uniphasic (R) unipolar electrograms are characteristic of the end of acti vation. Methods and Results-Unipolar electrograms were recorded in 45 consecutive p atients with atrial flutter who were undergoing radiofrequency ablation (RF A). Bidirectional cavotricuspid isthmus (CTI) block was achieved in 44 pati ents, The unipolar electrogram obtained before RFA at the low anterolateral right atrium during coronary sinus pacing changed from RS, rS, or QS to R or Rs in all patients after clockwise CTI block was obtained,The morphology of unipolar electrograms recorded close to the coronary sinus during pacin g from the low anterolateral right atrium changed from RS or rS to R or Rs in all but 4 patients after counterclockwise CTI block. In the patient in w hom CTI block was not achieved, the RS morphology of the unipolar electrogr am remained unchanged. In 18 patients, the results of the RFA were assessed with only the unipolar electrogram, The unipolar electrogram correctly pre dicted 100% and 89% of the cases of clockwise and counterclockwise CTI bloc k, respectively. Conclusions-The creation of CTI block is associated with an easily detectab le loss of negative components and development of an R or Rs pattern of the unipolar electrogram recorded close to the ablation line while pacing at t he opposite side of the CTI.