Halo电极远端电位方向逆转作为典型心房扑动消融终点标准的观察

目的探讨一种通过Halo电极远端电位方向发生逆转来判断典型心房扑动(简称房扑)三尖瓣环峡部线性消融成功的方法。方法 20例典型房扑患者接受射频消融治疗,将Halo电极经右股静脉置于三尖瓣环上,从远端电极至近段电极呈顺时针方向。于窦性心律下,行三尖瓣环至下腔静脉之间线性消融,与此同时,于冠状窦口行S1S1500ms起搏。消融过程中,Halo电极标测显示远端电极及近端电极呈双向传导。以Halo电极远端电位方向逆转作为消融成功终点。结果 20例均消融成功,无消融并发症发生。消融后,反复电刺激未诱发房扑。术后随访6个月,未见房扑复发。结论冠状窦口起搏刺激下,Halo电极远端电位方向出现逆转可作为典型房扑三尖瓣环峡部线性消融成功的可靠指标。     

低右房电图极性改变评价典型心房扑动射频消融结果

目的　在典型心房扑动 (房扑 )射频消融中观察临近峡部的低位右房心内电图极性改变 ,来迅速判断是否形成峡部双向阻滞。方法　对 10例典型房扑患者 ,沿三尖瓣环放置Halo电极 ,Halo远端紧邻峡部消融线 ,房扑发作中消融在房扑终止后行冠状窦起搏 ,窦性心律下消融则同时行冠状窦电极起搏 ,观察紧邻消融线低右房侧的心房电极电图起始部分的主波方向 (H电位 )的极性表现 ,并与最终双向阻滞评价结果比较。结果　 8例在窦性心律下行冠状窦电极起搏时消融 ,2例在房扑发作中消融。 10例患者最终消融结果均为双向阻滞。 2例房扑发作时H电位极性均为负向 ,8例窦性心律下H电位极性均为正向 ,峡部缓慢传导时该极性仍为正向 ,形成双向阻滞后H电位极性变为负向。结论　典型心房扑动行峡部线性消融时 ,行冠状窦起搏观察到紧邻峡部消融线低右房侧H电位的极性改变可能是峡部消融成功的新指标。该指标简单、快速、可靠性高。     

慢传导带传导阻滞和为心房扑动消融终点的意义

应用Ｈａｌｏ导管标测技术，结合冠状动脉窦与希氏束电图识别心房扑动折返环的慢传导带，在下腔静脉口到三尖瓣环峡部作射频线性消融，并以慢传导带出现双向阻滞作为心房扑动消融成功的标志，治疗了１例Ｉ型心房扑动患者。随访１个月心动过速未发。     

导管射频消融Ⅰ型心房扑动的方法学评价

本文比较在心房扑动 (房扑 )发作时、冠状静脉窦口 (冠状窦口 )起搏和窦性心律 3种情况下对 型房扑进行导管消融治疗的优缺点 ,消融终点是达到后峡部 (三尖瓣环至下腔静脉口 )双向阻滞。　　资料和方法　将 17例阵发性房扑患者随机分为房扑组、起搏组和窦性心律组。房扑组在房扑发作中按下列步骤消融 :1标测三尖瓣隔瓣和下腔静脉口内侧壁的小心房波(A波 )确定消融线路 ;2对上述线路的三尖瓣侧 1/3、中 1/3和下腔静脉侧 1/3分别以 40 W、2 0 W、10 W做连续的线形消融 ;3重复上述过程直到房扑终止 ;4冠状窦口起搏后峡部传导无变化者按起搏…     

慢传导带传导阻滞作为心房扑动消融终点的意义（附Halo导管应用一例报告）

应用Ｈａｌｏ导管标测技术，结合冠状静脉窦与希氏束电图识别心房扑动折返环的慢传导带，在下腔静脉口到三尖瓣环峡部作射频线性消融，并以慢传导带出现双向阻滞作为心房扑动消融成功的标志，治疗了１例Ｉ型心房扑动患者。随访１个月心动过速未发。由于Ｈａｌｏ导管能在右房内全面记录右房激动顺序，便于了解峡部的传导情况，在心房扑动消融中有助于明确其诱发与终止的机制，并为慢传导带传导阻滞作为成功消融终点提供了可靠的手段。     

第46课射频消融后诊断下腔静脉—三尖瓣环峡部阻滞的简易起搏方法

前言下腔静脉-三尖瓣环峡部(CTI)消融已成为典型心房扑动(AFL)的治疗方法.最初,治疗的目标是中止心房扑动且不能诱发,而现在接受的终点是下腔静脉-三尖瓣环峡部双向阻滞,以防止复发.鉴别下腔静脉-三尖瓣环峡部阻滞与缓慢传导是困难的,一直提倡"鉴别起搏",该方法关注的是逐渐远离下腔静脉-三尖瓣环峡部的起搏对消融线或附近记录到的电图(EGM)时间的影响.为分析前-后(逆钟向, CCW)传导,选用多极导管于右心房前壁多个层面起搏,用同一导管远端电极对或消融电极导管于下腔静脉-三尖瓣环峡部处作记录.后-前(顺钟向, CW)传导常由冠状窦近端起搏实施,另需一根电极导管起搏右心房间隔中部作鉴别起搏.在后-前传导检测中,下腔静脉-三尖瓣环峡部电图因电压低或多个成分(指多个波,译者加注),难以解释.此外,在下腔静脉-三尖瓣环峡部阻滞的情况下,起搏冠状窦时跨过界嵴的传导酷似下腔静脉-三尖瓣环峡部传导.我们将报道一种简单的方法,无需冠状窦电极导管,根据经下腔静脉-三尖瓣环峡部消融线间隔缘起搏时右心房前壁的激动顺序,评估下腔静脉-三尖瓣环峡部后-前阻滞.这种方法使得整个手术的完成只需两根电极导管:多极参考导管和消融电极导管.     

射频消融右房后位峡部和间隔峡部治疗典型心房扑动方法的对比研究

报道对23例典型心房扑动(房扑)患者的后位峡部或间隔峡部作线性消融,并比较两种消融方法的疗效。23例患者中,男性19例,女性4例。年龄16～72(55±14)岁。房扑病史6个月～11年。按消融方法随机分组:后位峡部组12例,间隔峡部组11例。两组患者的年龄、房扑病程、心胸比值差异均无显著性。经静脉放置Ｈａｌｏ、希氏束和冠状静脉窦电极导管,进行电生理检查。房扑心律或持续冠状静脉窦口起搏下(窦性心律者),4或8ｍｍ消融电极导管线性消融后位峡部或间隔峡部。后位峡部消融方法:消融线径为三尖瓣环至下腔静脉。间隔峡部消融方法:消融线径为三尖瓣环…     

左心室起搏下标测并消融左侧隐匿性旁路一例

本文报道１例左侧隐匿性旁路患者,右心室起搏的冲动只经房室结逆传。为此,在消融电极导管的第３、４极起搏左心室的同时,采用消融电极导管的１、２极标测靶点,并消融成功。现报道如下：１　　临床资料　　　　患者男性,５４岁。因反复阵发性室上性心动过速而于２００１年３月５日入院行射频导管消融术。局部麻醉下经静脉放置冠状静脉窦、高位右心房、希氏束、右心室电极。室上速发作时冠状静脉窦远端Ａ波明显提前,诊断为左侧游离壁隐匿性旁路参与逆传的房室折返性心动过速。用不同频率行右心室起搏,均为希氏束处Ａ波领先,提示冲动不…     

欧氏瓣对老年人Ⅰ型心房扑动射频消融的影响

目的探讨欧氏瓣对Ⅰ型心房扑动(简称房扑)导管射频消融即刻效果的影响。方法28例老年Ⅰ型房扑患者(呈逆钟向折返18例,顺钟向折返10例)在透视解剖标志和Halo电极三尖瓣环标测电图指引下,在房扑发作或冠状窦口起搏时以温控方式消融位于三尖瓣口和下腔静脉口之间的后峡部,消融方向从三尖瓣叶右室侧到下腔静脉开口。预设温度70℃,每点消融30s,每次移动消融电极3～5mm。观察下列指标:①房扑终止和后峡部阻滞时消融电极在消融线上所处的位置;②房扑终止后峡部残存传导间隙在消融线上所处的位置;③房扑终止后继续消融致后峡部完全阻滞的最终消融部位。结果根据右前斜位30°透视影像测得后峡部平均弧长(即消融线)为38.6±9.7mm。28例全部达到后峡部完全阻滞的消融终点,无并发症。与欧氏瓣有关的房扑终止率为100%(17/17),与欧氏瓣有关的后峡部完全阻滞发生率为92.9%(26/28)。结论欧氏瓣是Ⅰ型房扑后峡部消融线终点的重要标志,线性消融时欧氏瓣心室侧易残存传导间隙,消融该部位的残存传导间隙是Ⅰ型房扑后峡部消融的重要环节。     

典型心房扑动对心房颤动导管消融复发的影响

目的 评价典型心房扑动(房扑)对心房颤动(房颤)导管消融复发的影响.方法 120例药物治疗无效的阵发性房颤患者在三维电解剖标测系统和肺静脉环状标测电极导管联合指导下行环肺静脉电隔离.其中17例(14.2%)合并典型房扑(房扑组,其余作为对照组),行三尖瓣环峡部消融,三尖瓣环峡部消融终点为三尖瓣环峡部双向阻滞.房颤复发定义为导管消融3个月后发生房性快速心律失常.结果 房扑组房颤病程(9.8±10.7)年,长于对照组(5.9±6.3)年,差异有统计学意义(P=0.036).房扑组与对照组相比,年龄、性别、合并器质性心脏病、左心房直径、左心室射血分数差异无统计学意义.随访91～401(237±79)d,房扑组房颤复发率为47.1%,对照组房颤复发率为12.6%,两组间差异有统计学意义(P=0.001).经校正年龄、房颤病程、合并器质性心脏病、左心房直径等因素,Cox多因素分析发现消融术前合并房扑是房颤复发的独立危险因素(危险比3.52,95%可信区间1.32～9.34,P=0.012).结论 典型房扑可能增加房颤导管消融术后房颤的复发,房颤导管消融前应对患者是否合并典型房扑进行认真评价.     

Cavotricuspid isthmus mapping to assess bidirectional block during common atrial flutter radiofrequency ablation

BACKGROUND: We sought to compare published methods to an alternative approach ascertaining cavotricuspid isthmus (CTI) block during atrial flutter ablation. METHODS AND RESULTS: In 39 consecutive patients who underwent an atrial flutter ablation procedure, a 24-pole mapping catheter was positioned so that 2 adjacent dipoles were bracketing the targeted CTI line of block (LOB), with proximal dipoles lateral to the LOB and distal dipoles in the coronary sinus. Two pacing sites were lateral (positions A and B) and 2 were septal (positions C and D) to the LOB, with locations A and D closest to the LOB. A resulting CTI block was accepted when 3 criteria were fulfilled: (1) complete reversal of the right atrial depolarization on the 24-pole catheter when pacing in the coronary sinus, (2) conduction delays from A to D greater than from B to D, and (3) conduction delays from D to A greater than from C to A. A successful CTI block was obtained in all patients. Before CTI block was obtained, a progressive CTI conduction delay was observed in 11 patients (28.2%). During the procedure, the 3 criteria defined above were either all present or all absent. CONCLUSIONS: This study establishes that reversal of the atrial depolarization sequence up to the LOB is a definitive and mandatory criteria of successful atrial flutter ablation.     

Correlation between electrogram morphology and standard criteria to validate bidirectional cavotricuspid block in common atrial flutter ablation.

AIM: Assessment of a bidirectional conduction block within the cavotricuspid isthmus (CTI) is critical during radiofrequency (RF) atrial flutter (AF) ablation. We investigated the use of bipolar atrial electrogram (BAE) morphology as an additional criterion identifying CTI block and tested it against two recognized criteria: differential pacing and reversal of the right atrial depolarization sequence during coronary sinus (CS) pacing. METHODS AND RESULTS: An RF ablation procedure was performed during 600 ms CS pacing in 100 consecutive patients with a common AF. BAE recorded along the CTI were continuously monitored. CTI conduction block was achieved by RF ablation in all patients and a clear change in BAE polarity in the Electrogram recorded by the dipoles located on the CTI and immediately lateral to the intended line of block (RS to QR pattern) associated with a confirmed CTI conduction block was observed in all cases. BAE morphology changes predicted bidirectional CTI conduction blocks with a 100% positive and a 100% negative predictive value. At a mean follow-up of 33 +/- 11 months, there was a 5% AF recurrence rate. CONCLUSIONS: Our study suggests that morphological changes in BAE recorded at sites lateral and adjacent to the target line of block may be used as a unique and robust criterion to validate CTI conduction block during AF ablation procedure.     

Clinical implication of the double potentials at the cavotricuspid isthmus for radiofrequency catheter ablation of typical atrial flutter

An 87-year-old man was referred to our institution to undergo radiofrequency catheter ablation for typical atrial flutter. A decapolar electrode catheter with an interelectrode spacing of 1mm positioned at the cavotricuspid isthmus (CTI) exhibited 2 discrete potentials. The first electrogram was recorded at a site close to the tricuspid annulus and the second at a site close to the inferior vena cava at the CTI. Entrainment study showed that the first component was crucial to maintain the artrial flutten; however, the second one was found to function as a bystander portion of the reentrant circuit. Radiofrequency energy application at the site where the first potential was inscribed could terminate the atrial flutter creating a complete bidirectional conduction block at the CTI.     

Randomized comparison of anatomical versus voltage guided ablation of the cavotricuspid isthmus for atrial flutter.

OBJECTIVES: The purpose of this prospective study was to compare radiofrequency catheter ablation of the cavotricuspid isthmus using a strictly anatomic approach to an approach guided by a bipolar voltage map to avoid high voltage zones in the cavotricuspid isthmus. BACKGROUND: It is not clear whether local atrial electrogram amplitude influences the achievement of complete cavotricuspid isthmus block during radiofrequency catheter ablation for atrial flutter. METHODS: Thirty-two patients with atrial flutter were randomized to cavotricuspid isthmus ablation using an anatomical approach (group I, 16 patients) or guided by a bipolar voltage map (group II, 16 patients). A 3-dimensional electroanatomic mapping system and an 8-mm-tip ablation catheter were used in all patients. With the anatomical approach, an ablation line was created in the cavotricuspid isthmus at a 6 o'clock position in the 45 degree left anterior oblique projection. During voltage-guided ablation, a high-density bipolar voltage map of the cavotricuspid isthmus was created, and then contiguous applications of radiofrequency energy were delivered to create an ablation line through the cavotricuspid isthmus sites with the lowest bipolar voltage. RESULTS: Complete cavotricuspid isthmus conduction block was achieved in 100% of patients in each group. The mean maximum voltages along the line were 3.6 +/- 1.5 mV in group I, and 1.2 +/- 0.9 mV in group II (P < .01). Creating a high-density voltage map was associated with approximately 15-minute increase in the total procedure time (P = .2). During a mean follow-up of 177 +/- 40 days, there were no recurrences of atrial flutter in either group. There were no complications in either group. CONCLUSIONS: When cavotricuspid isthmus ablation for atrial flutter is performed with an 8-mm-tip catheter, complete block can be achieved in all patients regardless of local voltage. Ablation of high voltage zones is not associated with a higher recurrence rate. Therefore, anatomic ablation without voltage mapping is the preferred initial approach for cavotricuspid isthmus ablation.     

Characteristics of the Cavotricuspid Isthmus in Predicting Recurrent Conduction in the Long-Term Follow-Up

Background: The characteristics of cavotricuspid isthmus (CTI) in patients with atrial fibrillation (AF) and flutter that may predict recurrence of flutter is not known. We aimed to investigate the CTI characteristics in patients who underwent a second ablation procedure for recurrent AF after previous combined pulmonary vein (PV) and CTI ablation.
Methods: Among 196 consecutive patients with drug-refractory symptomatic AF who underwent PV isolation and CTI ablation with bidirectional isthmus block, 49 patients (age 50 ± 12 years, 43 males) had recurrent AF and received a second procedure 291 ± 241 days after the first procedure. Right atrial angiography for the evaluation of the CTI morphology, and the biatrial contact bipolar electrograms were obtained before both procedures.
Results: In the second procedure, 11 (group 1) of the 49 patients demonstrated recovered CTI conduction. Compared with the patients without CTI conduction (group 2, n = 38), group 1 patients had a higher frequency of a pouch-type anatomy (82% vs 13%, P < 0.001), longer CTI (34.0 ± 8.6 vs 25.5 ± 7.5 mm, P = 0.01), longer ablation time, and larger number of radiofrequency applications; furthermore, the preablation bipolar voltage decreased along both the CTI and ablation line in group 2, whereas it remained similar in group 1 in the second procedure.
Conclusions: A high (22%) percentage of CTIs exhibited recurrent conduction in the long-term follow-up. The CTIs with recurrent conduction had a higher incidence of a pouch and longer length compared with those without recurrent conduction.     

Unidirectional conduction block at cavotricuspid isthmus created by radiofrequency catheter ablation in patients with typical atrial flutter

INTRODUCTION: Although unidirectional conduction block at the cavotricuspid isthmus can be created by radiofrequency ablation for atrial flutter, its underlying mechanism has not been elucidated. METHODS AND RESULTS: Twenty-seven patients (22 men and 5 women; mean age 59 +/- 11 years) who met the following criteria were studied: (1) bidirectional isthmus conduction demonstrable at baseline; (2) at least one linear lesion attempted on the cavotricuspid isthmus with radiofrequency catheter ablation; and (3) conduction times at anterolateral and posteromedial portions of the isthmus measured for both clockwise and counterclockwise directions before the ablation procedure. Unidirectional conduction block was observed before achieving bidirectional block in 9 patients (group I); the remaining 18 patients did not exhibit unidirectional conduction block (group II). All unidirectional conduction blocks were demonstrated in the counterclockwise direction. Anterolateral isthmus conduction time in group I was significantly longer than that in group II in both directions. However, there were no significant differences in posteromedial isthmus conduction time between groups I and II in either direction. Anterolateral isthmus conduction time was significantly longer than posteromedial conduction time in group I but not in group II. CONCLUSION: There were significant differences in conduction properties at the cavotricuspid isthmus between patients who developed unidirectional conduction block and those who did not. Our results support the notion that anisotropy contributes to the genesis of unidirectional conduction block at the cavotricuspid isthmus during the radiofrequency ablation procedure.     

Concealed Automaticity From an Island of Atrial Myocardium Post Cavotricuspid Ablation

Island of Atrial Myocardium Post Cavotricuspid Ablation. We report the case of a patient with paroxysmal atrial fibrillation in whom 2 previous cavotricuspid isthmus (CTI) ablations were performed for recurrent type I counterclockwise atrial flutter. One year after the last CTI ablation, the patient underwent pulmonary vein isolation for AF and reassessment of conduction block in the CTI was performed during the procedure. While mapping the CTI, activations were documented within the CTI that were dissociated from both right atrial and ventricular activity during sinus rhythm and pacing maneuvers. This dissociated activity was confined to a region delimited by the 2 previous ablation lines, the tricuspid annulus and the inferior vena cava. These findings suggest that an island of atrial myocardium with automatic activity was created within the CTI by previous ablation lines. (J Cardiovasc Electrophysiol, Vol. 21, pp. 1408‐1409, December 2010)     

A New and Simple Method for Distinguishing Complete from Incomplete Block Through the Cavotricuspid Isthmus

Background: A complete line of block (CLOB) in the cavotricuspid isthmus (CTI) is the endpoint of typical atrial flutter ablation. Before CTI block is obtained, a progressive CTI conduction delay due to an incomplete line of block (InLOB) can be difficult to distinguish from CLOB. The purpose of this study was to assess a new simple approach based on the changes in atrio-ventricular (AV) conduction delays during septal and lateral right atrial pacing, to distinguish a CLOB from an InLOB during typical atrial flutter (AFL) ablation. Methods and Results: Forty patients who presented an InLOB before a CLOB, and a stable (AV) conduction delay at 600 ms cycle length pacing (when in sinus rhythm), during AFL ablation were included in this study. A 24-pole mapping catheter was positioned so that 2 adjacent dipoles bracketed the targeted CTI line of block (LOB), with proximal dipoles lateral to the LOB and distal dipoles in the coronary sinus. Two pacing sites were lateral (position L1 and L2) and one was septal (position S) to the LOB, with locations L1 and S closest to the LOB. During L1, L2 and S site pacing, the delay between the pacing artefact and the peak of the R wave in a surface ECG (lead II) was measured. We measured the following conduction delays (mean ± SD in ms), during InLOB versus CLOB: (L1 to R) 320.5 ± 68.0 versus 367.0 ± 62.0, p = 0.001; (L2 to R) 333.0 ± 59.0 versus 338.0 ± 62.0, p = 0.663, (S to R) 259.4 ± 51.5 versus 247.1 ± 55.5, p = 0.987. We calculated the following data during an InLOB versus a CLOB: (L1R–L2R) − 12.3 ± 7 versus 20.2 ± 12.7, p = 0.001; (L1R–SR) 51.1 ± 21.5 versus 120.1 ± 16.6, p < 0.05. The sensitivity, specificity, positive and negative predictive values for CLOB with (L1R-SR > 94 ms) and with (L1R-L2R > 0 ms) were respectively; 100%, 98%, 98% and 100%. Conclusions: This study establishes that lateral versus septal right atrial pacing sites combined with the measure of AV conduction delay on a surface ECG can be useful to distinguish a CLOB from an InLOB during AFL ablation.     

Preliminary results with percutaneous transcatheter microwave ablation of typical atrial flutter

Background: Linear microwave ablation has been shown to be effective for treatment of atrial fibrillation during open-heart surgery by producing transmural lesions in the atrium to isolate the pulmonary veins. However, the safety and efficacy of percutaneous, transcatheter, linear microwave ablation for atrial arrhythmias, while demonstrated in animal models, is unknown in humans. Therefore, we studied the safety and efficacy of linear microwave ablation of the cavotricuspid isthmus (CTI) in humans with typical atrial flutter, utilizing a 2-cm long microwave antenna mounted on a steerable 9-French catheter.
Methods and Results: In seven consecutive patients, multielectrode catheters were positioned at the His bundle (quadripolar) and around the TV annulus (duo-decapolar) for pacing and recording atrial activation sequence before and after ablation. The microwave antenna was withdrawn gradually from tricuspid annulus towards inferior vena cava to ablate the CTI. Intracardiac ultrasound was used to ensure adequate endocardial contact of the microwave ablation catheter with the CTI. Microwave energy was applied at a power of 18 to 21 W at each ablation point for 120 seconds. Ablation was repeated until bidirectional CTI block was confirmed by demonstrating a descending activation wavefront in the contralateral atrial wall during pacing from the coronary sinus ostium or low lateral right atrium, respectively. Bidirectional isthmus block was achieved in all patients, after a mean number of 27.4 ± 14.7 energy applications per patients. There were no acute procedural complications.
Conclusions: Percutaneous, transcatheter microwave ablation of CTI dependent atrial flutter was demonstrated to be safe and effective in this preliminary feasibility study.     

Right ventricular pacing to assess transisthmus conduction in patients undergoing isthmus-dependent atrial flutter ablation: a new useful technique?

BACKGROUND: Successful radiofrequency (RF) ablation of typical, isthmus-dependent atrial flutter requires establishment and confirmation of bidirectional conduction block across the cavotricuspid isthmus. Low atrial pacing usually is performed from the bipoles of the 20-pole Halo catheter, septal and lateral to the cavotricuspid isthmus ablation line. However, occasionally this is difficult because of high pacing thresholds and/or saturation of the atrial electrograms recorded near the pacing catheter. OBJECTIVES: The purpose of this study was to assess if right ventricular (RV) pacing and resulting retrograde atrial activation can be used to assess conduction block from the septum to the lateral wall in a clockwise direction. METHODS: Thirty-five consecutive male patients (mean age 64 +/- 10 years; mean ejection fraction 42 +/- 13%; mean left atrial dimension 44 +/- 6 mm) with typical isthmus-dependent atrial flutter were studied. The following electrophysiology catheters were used: 20-pole catheter along the tricuspid annulus, quadripolar catheters at the His and/or RV apex, and 8-mm ablation catheter. Following RF ablation of the cavotricuspid isthmus, bidirectional conduction block was confirmed in all 35 patients by pacing at a cycle length of 600 ms from bipoles septal and lateral to the cavotricuspid isthmus ablation line. Conduction times from pacing artifact to adjacent bipolar atrial electrograms and reversal of atrial activation pattern were analyzed. RV pacing was performed and retrograde atrial activation pattern assessed. If retrograde AV nodal conduction was absent, isoproterenol was infused intravenously at 2 microg/min, and RV pacing was repeated. The conduction time between the double potentials across the cavotricuspid isthmus ablation line was measured. RESULTS: Mean conduction times across the isthmus during septal (S), lateral (L), and RV pacing were 145 +/- 21 ms, 144 +/- 24 ms, and 129 +/- 20 ms, respectively. Retrograde AV nodal conduction was present in 34 of 35 patients (isoproterenol 8 patients). Evidence of conduction block by a clear change in activation pattern across the isthmus was seen during RV pacing in 33 of 35 patients with bidirectional conduction block. CONCLUSION: RV pacing is a simple and easy maneuver that can be performed to assess isthmus conduction in most patients.