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Notes: Slower Conduction in the TV-IVC Isthmus. Introduction: In human type I atrial flutter, the electrophysiologic substrate is unclear. In order to determine if slow conduction is mechanistically important, we evaluated conduction velocity in the tricuspid valve-inferior vena cava (TV-IVC) isthmus, right atriai free wall, and interatrial septum in patients with and without a history of atrial flutter undergoing electrophysiologic study. Methods and Results: Nine patients with (group 1) and nine without a history of type I atrial flutter (group 2) were studied. Conduction time (msec) in the right atrial free wall. TV-IVC isthmus (bidirectional), and interatrial septum was measured during pacing in sinus rhythm at cycle lengths of 600, 500, 400, and 300 msec from the low lateral right atrium and coronary sinus ostium. Conduction velocity (cm/sec) was calculated by dividing the distance between pacing electrodes and sensing electrodes (cm) by the conduction time (sec). Conduction velocity was slower in the TV-IVC isthmus in group 1 (range 37 ± 8 to 42 ± 8 cm/sec) versus group 2 (range 50 ± 8 to 55 ± 9 msec) at all pacing cycle lengths (P 〈 0.05). However, conduction velocity was not different in the right atrial free wall or interatrial septum between groups 1 and 2. Conduction velocity was also slower in the TV-IVC isthmus than in the right atrial free wall and interatrial septum in group 1 patients, at all pacing cycle lengths (P 〈 0.05). Atrial flutter cycle length correlated with total atrial conduction time (r ≥ 0.832, P 〈 0.05). Conclusion: Slow conduction in the TV-IVC isthmus may be mechanistically important for the development of human type I atrial flutter.

Notes: Ablation of Atrial Fibrillation. Introduction: Atrial fibrillation (AF) is often refractory to antiarrhythmic drugs, and patients who are intolerant of AF may require the maze operation for cure. As a less Invasive alternative, a catheter-based, right atrial compartmentalization procedure was evaluated. Methods and Results: Twelve patients with AF refractory to Class I and III antiarrhythmic drugs were studied. Four linear right atrial radiofrequency ablations were performed, from superior to inferior vena cava in the posterior wall and interatrial septum, anteriorly from the superior vena cava to the tricuspid annulus through the appendage, and across the tricuspid valve-inferior vena cava isthmus. The radiofrequency catheter was dragged along each line three to four times, until the atrial electrogram amplitude decreased by 75% and there was bidirectional conduction block in the tricuspid valve-inferior vena cava isthmus. One complication occurred: sinus node dysfunction requiring a pacemaker. Eight patients were discharged from the hospital on no antiarrhythmic drugs, and four were discharged on previously ineffective antiarrhythmic drugs. Total duration of follow-up was 21.3 ± 11.2 months. Four patients discharged on previously ineffective antiarrhythmic drugs had no recurrence of AF. One patient discharged off antiarrhythmic drugs had no recurrence of AF. Seven patients discharged off antiarrhythmic drugs had recurrent AF by 12.6 ± 13.0 months (median 6, range 1 to 39); 3 of these 7 responded to previously ineffective antiarrhythmic drugs without further AF and 4 did not. Thus, 8 of 12 patients (67%) had suppression of AF after ablation on previously ineffective medication or no medication. Conclusion: Right atrial compartmentalization may alter the substrate for AF, thus improving the efficacy of previously ineffective antiarrhythmic drugs. Because it is relatively safe, it may be a reasonable adjunctive intervention to maintain sinus rhythm in patients with drugrefractory AF.

Notes: Abstract Background: The upper limit of vulnerability (ULV) is the stimulus strength above which ventricular fibrillation cannot be induced, even when the stimulus occurs during the vulnerable period of the cardiac cycle. Determination of ULV using T-wave shocks during ventricular pacing has been shown to closely correlate with the defibrillation threshold (DFT) at ICD implantation. However, there are no data correlating ULV determined in sinus rhythm at ICD implantation, with DFT determined at implantation or during long-term follow-up. This is of clinical importance since ULV may be used to estimate DFT during ICD implantation, both during ventricular pacing or sinus rhythm. Methods and Results: Twenty-one patients receiving a transvenous ICD system were studied prospectively. There were 16 males and 5 females, mean age 68 ± 15 years, with mean ejection fraction 37.4 ± 17.4%. All had structural heart disease. The ULV was defined as the lowest energy that did not induce ventricular fibrillation with shocks at 0, 20 and 40ms before the peak of the T-wave, using a step-down protocol. The initial energy tested was 15J and the lowest energy 2J. DFT was determined following a similar step-down protocol. The DFT was defined as the lowest energy that successfully defibrillated the ventricles. The linear correlation coefficient between ULV and DFT was r = 0.73 (p 〈 0.001). At implant, mean ULV was 9.2 ± 5J, not statistically different from mean DFT 9.4 ± 4J. ULV plus 5J successfully defibrillated 19 of 21 patients. During long-term follow-up of 10.1 ± 1.8 months in eight patients, DFT was 8.8 ± 5.8J, not significantly different than the DFT of 7.5 ± 4.1J or ULV of 8.0 ± 5.3 at implant. Conclusion: 1) When determined during normal sinus rhythm the ULV significantly correlates with DFT. 2) ULV testing might be used in lieu of standard DFT testing to confirm adequate lead placement thus minimizing or eliminating VF inductions, particularly in hemodynamically unstable patients. 3) Since ULV + 5J has a high probability of successful defibrillation in most patients, programming ICD first shock energy for VF at ULV + 5J may result in lower first shock energies compared to the standard methods of programming first shock energy at twice DFT. Condensed Abstract. The purpose of this study was to determine if the upper limit of vulnerability (ULV) determined during normal sinus rhythm correlates with the defibrillation threshold (DFT), as has been previously shown when determined during ventricular pacing. The linear correlation coefficient between the ULV and DFT was r = 0.73 (p 〈 0.001). Mean ULV at implant was 9.2 ± 5J, not statistically different from mean DFT of 0.4 ± 4J. During long-term follow-up of 10.1 ± 1.8 months in 8 patients, DFT was 8.75 ± 8J, not significantly different than the DFT of 7.5 ± 4.1J or ULV of 8.0 ± 5.3 at implant. Shocks energies of ULV + 5J successfully defibrillated 19 of 21 patients at implant and 8 of 8 at follow-up. This study indicates that the ULV determined in normal sinus rhythm closely correlates with the DFT, and that ULV + 5J defibrillated most patients. ULV testing could be used to predict DFT and reduce or eliminate the need for DFT testing and VF induction. Programming ICD first shock energy for VF to ULV + 5J will result in lower energy than that used with standard DFT testing.