特殊类型“慢——快”型室上性心律失常的临床特点及射频消融治疗后的随访

目的探讨特殊类型"慢--快"型室上性心律失常的临床特点与治疗方法.方法以射频导管消融为主的方法控制快速性心律失常治疗4例特殊类型"慢--快"型室上性心律失常患者.结果4例特殊类型"慢--快"型室上性心律失常,2例为房室折返性心动过速(AvRT),1例房室结折返性心动过速(AVNRT),于长时间心动过速发作终止时均有心脏停搏伴有晕厥或黑朦,经射频导管消融根除室上性心动过速后,由其引起的症状不复存在,随访窦房结功能正常;另1例左侧显性旁道、AVRT,心房扑动伴晕厥,在临时心脏起搏保护下,同步直流电复律终止心房扑动,口服乙胺碘酮有效控制心动过速发作,未再发生缓慢性心律失常和晕厥.结论以射频导管消融为主的方法控制快速性折返性心律失常对特殊类型"慢--快"型室上性心律失常患者具有十分重要的临床意义.     

快速性心律失常终止后长时间窦性停搏的电生理特征

目的 分析快速性心律失常终止后出现长时间窦性停搏的电生理特征.方法 观察有快速性心律失常终止后出现长时间窦性停搏(＞3 s)的25例患者,25例患者根据年龄分为2组,≤60岁14例(中青年组),＞60岁11例(老年组).对这些患者行导管消融治疗,在消融前后行心内电生理检查.在消融术后的1、3、6、12和24个月行动态心电图检查.结果 25例患者中,12例为房室折返性心动过速(AVRT),2例为房室结折返性心动过速(AVNRT),3例为房性心动过速,3例为Ⅰ型心房扑动(房扑),5例为阵发性心房颤动(房颤).在快速性心律失常终止时,记录到窦性停搏时间为3.2 ～9.4(5.6±2.7)s.窦房结恢复时间(SNRT),矫正SNRT及窦房传导时间(SACT)等在两组间差异无统计学意义(P＞0.05).两组患者消融前后的心内电生理检查比较发现,中青组患者消融前后,SNRT、矫正的SNRT及SACT差异均无统计学意义(P＞0.05).老年组患者消融前后,SNRT差异有统计学意义(P＜0.05),而矫正的SNRT及SACT差异无统计学意义(P＞0.05).23例患者射频消融术成功后未再发心动过速,也没有出现＞3 s的窦性停搏.1例持续性房扑患者在射频消融术后半个月,房扑反复发作并在终止时出现长时间的窦性停搏并伴晕厥,植入永久性心脏起搏器.1例阵发性房颤患者在术后仍有房颤间断发作,房颤终止时仍有窦性停搏现象,但停搏时间＜3 s,并且没有晕厥或近似晕厥的症状.结论 快速心律失常后出现窦性停搏患者的窦房结功能大多是正常的,消融治疗快速性心律失常后患者心律失常相关的窦性停搏随之消失了,如果患者存在窦房结病变,则可考虑植入永久性心脏起搏器.     

阵发性室上性心动过速患者经导管射频消融术治疗后的临床效果

目的分析阵发性室上性心动过速患者经导管射频消融术治疗后的临床效果。方法本研究选取2014年3月~2016年2月于周至县人民医院所收治104例阵发性室上性心动过速患者,全部患者均给予导管射频消融术治疗,观察分析经导管射频消融术后的即时成功率和并发症(血管穿刺形成的血肿、心包填塞、房室传导阻滞)发生情况;随访3个月,观察阵发性室上性心动过速是否复发。结果在104例患者中,45例为房室折返性心动过速,59例为房室结折返性心动过速;经导管射频消融术治疗后,手术平均时间为(97.1±22.7)min;102例手术即时成功,成功率98.1%;2例发生手术并发症(1例股动脉穿刺部位血肿,对症处理后恢复。1例发生不可逆性Ⅲ度房室传导阻滞,术后植入永久型心脏起搏器);随访3个月,3例出现术后复发。结论导管射频消融术治疗阵发性室上性心动过速具有较显著的临床效果,安全性较高,并发症发生率有效降低。     

国产与进口射频消融导管治疗阵发性室上性心动过速的疗效对比

正阵发性室上性心动过速属于临床较为常见的一种心律失常类型,其中房室折返性心动过速及房室结折返性心动过速占到阵发性室上性心动过速90%以上。射频消融术(RFCA)是治疗本病较为有效的一种方法,能够从根本上根治阵性室上性心动过速,从而避免长期服用抗心律失常药物所带来的不良反应。我国临床上使用的诊断标测导管及射频消融导管主要依赖于进口,其价格相     

儿童心律失常的射频消融治疗

目的总结儿童心律失常患儿射频消融的疗效。方法回顾性研究2016年1月1日至2018年9月30日期间,因心律失常接受心内电生理检查+射频消融术治疗患儿的心律失常类型及疗效。结果 95例患儿进行了电生理检查,其中4例在术中未诱发心律失常,未行射频消融治疗。91例患儿中,阵发性室上性心动过速67例,其中房室折返性心动过速48例、房室结折返性心动过速19例,房性心动过速3例,室性心律失常21例。91例患儿射频消融术后随访(15±8)个月。84例消融成功,成功率92.31%;复发7例(7.69%),其中左侧隐匿性旁路复发3例(3/16),B型预激复发1例(1/23),左后分支室性心动过速3例(3/7)。只有1例患儿出现股浅动脉闭塞的并发症。结论射频消融治疗患儿心律失常安全有效。     

导管射频消融治疗室上性心律失常对阵发性心房颤动自然发作的影响

目的　观察导管射频消融治疗室上性心律失常对阵发性心房颤动 (房颤 )自然发作的影响 ,探讨房颤发生的可能电生理机制。方法　 31例室上性心律失常合并阵发性房颤患者 ,其中男性2 0例 ,女性 11例 ,年龄 5 4± 12 (2 4～ 6 9)岁 ,病程 9± 5 (1～ 19)年。 31例患者经电生理检查证实 :5例为典型心房扑动 (AFL) ;17例为房室折返性心动过速 (AVRT) ,包括显性预激综合征 7例 ;9例为房室结折返性心动过速 (AVNRT)。术前 1年内房颤平均发作 6± 5 (2～ 18)次。对于典型AFL者 ,线性消融下腔静脉 三尖瓣环峡部致双向传导阻滞 ;AVRT者行旁道消融术 ;AVNRT者行慢径改良术。平均随访39± 19(12～ 72 )月。结果　 31例患者中 ,2 3例消融术后房颤不再发生 ,占研究组的 74 19%。 5例AFL者术后 3例不再有AFL及房颤发生 ,2例术后房颤发生的频度与术前无变化 (其中 1例动态心电图检查提示有频繁房性早搏并有房早触发房颤现象 )。 2 6例室上性心动过速患者术后 2 0例无房颤发作 ;6例术后仍有阵发性房颤 ,但 2例发作频度较术前明显减少 ,3例无变化 (其中 1例为高血压合并左心房轻度扩大者 ) ,1例动态检查示有频繁房性早搏及短阵房性心动过速。结论　AFL可能与房颤具有共同的发生基质 ,也可以是房颤的触发因素 ,成功消融后可     

导管射频消融阵发性心房颤动合并快慢综合征的疗效观察

目的:探讨射频消融治疗阵发性心房颤动(房颤)合并快慢综合征患者的临床疗效。方法:2012年1月至2016年6月共入选150例阵发性房颤伴发作终止时长间歇≥3.0s的患者,拟行导管射频消融术后1、3、6、9、12个月随访常规心电图和动态心电图检查。结果:127例患者接受了心内电生理检查及射频消融术,平均RR长间歇为(4.3±1.0)s,9例患者在射频消融术后14d至31.5个月接受了永久起搏器置入术;与未接受永久起搏器置入术的患者相比,行永久起搏器置入的患者的最长RR间歇明显延长[(6.7±1.1)s对(4.2±0.7)s,P0.01)];在调整了性别、年龄、病程、左室射血分数、左房内径等因素后,射频消融失败(OR=49.21,95%CI:7.28~332.51,P0.001)、最长RR间歇≥6.5s(OR=7.32,95%CI:1.13~47.39,P=0.037)增加术后置入起搏器风险。结论:房颤发作抑制窦房结功能导致RR长间歇,射频消融治疗使窦房结功能有一定程度的恢复,但较长的RR间歇提示患者需接受永久起搏器置入治疗。     

射频消融治疗阵发性心房颤动伴长间歇患者的长期随访研究

目的 随访射频消融治疗阵发性心房颤动（房颤）伴长间歇的临床转归.方法 2006年5月至2008年9月共入选18例（男12例,女6例）阵发性房颤伴发作终止时窦性停搏≥3 s的患者,年龄37～72（56.8±11.7）岁,病程2～276（69.6±71.3）个月.所有患者均有房颤终止时窦性停搏≥3 s的临床资料,平均长RR间期3.1～8.0（4.5±1.6）s,部分患者伴有黑矇、晕厥先兆或者晕厥症状.首次消融患者,在三维标测系统指导下行肺静脉前庭隔离术达到肺静脉-左心房电学隔离;术后常规程序刺激诱发,若合并室上性心律失常或者非肺静脉触发灶,同时消融.术前,术后3d,1、3、6个月分别常规检查动态心电图,检测心率变异性（HRV）指标.结果 18例患者共进行了27次消融,8例1次手术成功,5例行2次手术,2例行3次手术,3例复发未再行手术治疗.首次消融术中,3例诱发典型心房扑动,同时行后位峡部消融,达到双向阻滞;3例术中出现显性去迷走[血压＜ 90/60 mm Hg（1 mm Hg=0.133 kPa）,心率＜60次/min].10例复发患者中,1例合并左心房后壁的局灶房颤,第3次消融时成功.平均随访（34.1±7.5）个月,术后2例发生窦性心动过速;3例房颤复发,其中1例为起搏器植入术后,1例房颤复发患者终止后有10.4 s的长间歇,植入单腔起搏器,另外1例房颤复发但无症状,未作特殊处理,临床密切随访.共12例完成6个月动态心电图随访的患者符合分析要求.术前反应迷走神经功能的HRV指标相邻RR间期差的均方根（rMSSD）,RR间期平均值的标准差（SDNN）,高频（HF）,低频（LF）/HF分别为（42.0±11.1）ms,（136.0±24.9）ms,12.5±3.9,1.32±0.26.术后随访发现其指标均下降并持续至少3个月,但在6个月时恢复.结论 房颤发作时能够抑制窦房结功能,导致长间歇;射频消融治疗祛除房颤这一原始因素后,窦房结功能常有一定程度的恢复.     

“快－慢”型室上性心律失常的射频导管消融治疗及随访

目的：探讨射频导管消融在治疗“快—慢”型室上性心律失常中的作用。方法：用射频导管消融的方法治疗了４例“快—慢”型室上性心律失常病人。结果：１９９１年１月至１９９６年１月间，对２４１例次各种快速性心律失常进行了射频导管消融治疗。其中４例为“快—慢”型室上性心律失常，２例为反复发作性房室结折返性心动过速（ＡＶＮＲＴ），终止时有长时间的心脏停搏并引起晕厥，原准备安装永久性心脏起搏器，ＡＶＮＲＴ根治后，由其引起的症状不复存在，电生理检查窦房结功能正常，故未安装起搏器；另２例均已植入永久性心脏起搏器，１例频繁发作快速心室率心房颤动并经常引起急性心功能不全，１例反复发作ＡＶＮＲＴ、心房扑动和心房颤动且有明显的症状，射频导管消融治疗后症状均消失，射频导管消融术对起搏器的功能无影响。３例平均随访３１±２个月，１例随访２个月未见并发症和临床症状复发。结论：射频导管消融法治疗“快—慢”型室上性心律失常具有重要的临床价值。     

心房颤动患者左右下肺静脉共干合并左侧房室旁道一例

1例52岁男性,阵发性心房颤动(简称房颤)患者,因抗心律失常药物疗效不佳入院行房颤射频消融术,术前检查肺静脉CTA显示下肺静脉共干,术中电生理检查发现左侧房室旁道,成功消融旁道,然后隔离各肺静脉后,随访24个月内无房颤发作。     

Radiofrequency ablation for treating paroxysmal supraventricular tachycardia complicated by atrial fibrillation:A single-center retrospective analysis

Background The effect of selective radiofrequency ablation for treating paroxysmal supraventricular tachycardia(PSVT) and its associated paroxysmal atrial fibrillation(PAF) was assessed. Methods Data were collected retrospectively from patients diagnosed of PSVT and subsequently treated with radiofrequency ablation. Regular monthly follow-up by dynamic electrocardiography(ECG) was performed. Incident rates of atrial fibrillation before and after ablation were compared. Results 382 PSVT patients with 58 having atrial fibrillation were enrolled. The order of complicated PAF from high to low in these patients was displayed as: atrial tachycardia(AT),atrioventricular reentrant tachycardia(AVRT) and atrioventricular nodal reentrant tachycardia(AVNRT). Among AVRT patients, PAF was more frequent in patients having accessory pathways. AVNRT patients had significantly lower PAF rate comparing to other patients. PAF incident rate was significantly reduced by radiofrequency ablation therapy. Conclusion We advise regular dynamic ECG for PSVT patients, especially those with atrial flutter, AT or pre-excitation syndrome. Selective radiofrequency ablation is a feasible approach for treating AF complicated PSVT patients.     

儿童和青少年快速性心律失常的临床特点

研究儿童和青少年快速性心律失常的临床特点。选择 1995～ 2 0 0 2年在我院行射频消融 (RFCA)治疗的儿童和青少年快速性心律失常患者 ,共 32 1例 ,男 2 10例、女 111例 ,年龄中位数 13.4± 3.6 (1.5～ 18)岁 ;其中 ,房室折返性心动过速 (AVRT) 2 0 4例、房室结折返性心动过速 (AVNRT) 74例、特发性室性心动过速 (IVT) 35例、房性心动过速 (AT) 5例、心房扑动 (AFL) 2例、不适当窦性心动过速 (IST) 1例。记录所有病例术前未发作心动过速及心动过速发作时的体表 12导联心电图 ,结合电生理检查 ,分析其临床特点。结果 :AVRT、AVNRT和IVT分别占6 3.6 %、2 3.1%和 10 .9%。年岁较小的儿童和青少年 ,右侧旁道较多 ,随着年龄的增加 ,左侧旁道相对越来越多。B型预激合并多旁道较常见。 35例IVT ,其中 2 3例为左室IVT ;6例为右室IVT。 12例合并先天性心脏病 ;13例并发心动过速性心肌病 ,心功能及心脏大小在RFCA术后 3～ 6个月恢复正常。结论 :①AVRT、AVNRT和IVT是儿童和青少年快速性心律失常中最常见的 3种类型。②心动过速性心肌病经早期适当的治疗是可逆的。     

Effects of diltiazem in supraventricular paroxysmal tachyarrhythmias

Diltiazem (0.3 mg/kg body weight intravenous in 2 minutes) was administered to 40 patients (24 males, 16 females, mean age 51.55 years) with paroxysmal supraventricular tachyarrhythmias: 7 patients with atrial fibrillation, 6 patients with atrial flutter, 25 patients with paroxysmal supraventricular tachycardia, 2 patients with uncommon atrioventricular reciprocating tachycardia. In patients with atrial fibrillation intravenous diltiazem produced a significant decrease of ventricular response (from 160 +/- 11 to 113.57 +/- 10.34--p less than 0.01). In patients with atrial flutter intravenous diltiazem produced variable effects: an increase in atrio-ventricular block (from 2:1 to 3:1 atrio-ventricular conduction (2 patients); conversion to sinus rhythm (1 patient); change to atrial fibrillation (1 patient); no appreciable change of the basic rhythm (2 patients). In paroxysmal supraventricular tachycardia patients conversion to sinus rhythm occurred in 20/22 patients (91%) treated with intravenous diltiazem (mean conversion time 4.69 minutes). In the 2 patients with uncommon atrioventricular nodal reciprocating tachycardia diltiazem increased P'-R and R-P' intervals without appreciable change of the basic rhythm. No serious side effects from drug administration were noted. Intravenous diltiazem appears to be as a highly effective medication in conversion or control of paroxysmal supraventricular tachyarrhythmias.     

射频导管消融360例儿童心律失常的疗效和安全性

目的 评价射频导管消融360例儿童心律失常的疗效和安全性。方法 回顾性分析2000年1月2013年12月360例因心律失常在我院接受射频导管消融的儿童患者360(男213例、女147)例,年龄15个月14(10±3)岁,平均体质量34 kg。结果 急性期消融成功率98.9%(356/360例),4例消融失败。360例患者中,阵发性室上性心动过速308例,占85.6%,307例(99.7%)患者消融成功,其中隐匿性房室旁路伴房室折返性心动过速121例、显性旁路(预激综合征)82例、房室结折返性心动过速105例;室性心律失常42例,其中室性早搏15例(均消融成功),27例室性心动过速(其中4例并发结构性心脏病,25例消融成功);房性心律失常10例(9例消融成功,其中典型心房扑动3例,不典型心房扑动1例,局灶性房性心动过速6例)。所有患儿围术期均未出现明重要并发症。术后随访至少12个月,随访期复发率为2.2%(8/356例),其中6例经再次消融成功。结论 射频导管消融术治疗有适应证的心律失常儿童安全有效。     

心内膜导管射频消融致心律失常作用的研究

通过分别比较１０条犬右房和左室导管射频消融前后的电生理检查结果和８０例房室旁道病人射频消融前后的电生理检查、心电监测和晚电位检查结果，探讨心内膜导管射频消融是否具有近期致心律失常作用。１０条犬消融前和消融后７日右房有效不应期分别为１４３±２５和１４１±２８ｍｓ（Ｐ＞０．０５），左室有效不应期分别为２３１±５６和２３７±７４ｍｓ（Ｐ＞０．０５），均未诱发出房性心动过速、心房扑动、心房颤动、室性心动过速、心室颤动等心律失常。８０例病人消融后即刻电生理检查没有诱发出上述快速心律失常；消融后随访３个月，共行２４小时心电监测３次均未发现新的心律失常，晚电位检查均为阴性。表明采用导管射频消融术治疗室上性快速心律失常没有近期的致心律失常作用，是一种相对安全的介入性治疗方法。     

The contribution of artificial pacemaking to understanding the pathogenesis of arrhythmias

Right atrial or ventricular pacing was performed on 36 occasions in 26 patients in an attempt to terminate a variety of tachyarrhythmias. Of 16 episodes of atrial flutter, 13 were terminated successfully; in 9 of the 13, sinus rhythm or the patient's pre-flutter rhythm was restored immediately, whereas in 4 patients, intervening atrial fibrillation or unstable atrial flutter occurred. Pacing terminated paroxysmal atrioventricular junctional or paroxysmal atrial tachycardia on 3 occasions; in a fourth patient, this tachyarrhythmia terminated during catheter manipulation. Six episodes of pacemaker-induced ventricular tachycardia were abolished by ventricular pacing. In 2 patients, atrial tachycardia was only transiently suppressed, and in 1 of these patients, d-c cardioversion produced a similar effect. Atrial fibrillation, spontaneously converting to atrial flutter, resulted during pacing for atrial tachycardia with block; the latter arrhythmia returned when the atrial flutter was terminated. Atrial fibrillation in 7 patients remained unaffected by atrial pacing. Based on the different electrophysiologic mechanisms responsible for reentrant excitation and automatic pacemaker discharge, an attempt has been made to determine the pathogenesis of the tachyarrhythmia by its response to pacing.     

Provocation of supraventricular tachycardias by an intravenous class I antiarrhythmic drug.

Antiarrhythmic drugs may aggravate or induce ventricular arrhythmia. The induction of a supraventricular tachycardia or its facilitation has rarely been reported. The purpose of the study was to know whether the potential for supraventricular proarrhythmic effect of a class Ia intravenous antiarrhythmic drug can be exposed during electrophysiologic study. Ajmaline was chosen because of its short duration of action. The protocol of the study consisted of an electrophysiological study and programmed atrial stimulation using 1 and 2 extrastimuli on driven rhythm and atrial pacing up to second-degree atrioventricular block. Then 1 mg/kg of ajmaline was injected and atrial pacing was performed 3 minutes after its injection. Supraventricular proarrhythmic effect of ajmaline was defined as the spontaneous occurrence of a supraventricular tachycardia or the facilitation of its induction. Seventy patients among 1955 presented a proarrhythmic effect: 63 developed a supraventricular tachyarrhythmia (atrial flutter, fibrillation, tachycardia) and 7 an atrioventricular reentrant tachycardia, either spontaneously (n = 23) or during atrial pacing (n = 47). Risk factors were identified in most patients: old age, underlying heart disease, history of spontaneous supraventricular tachycardia and/or induction of a supraventricular tachycardia by 2 extrastimuli on driven rhythm in the control state (34 patients), sinus node dysfunction (22 patients). Compared with patients without proarrhythmic supraventricular effect only the history of spontaneous supraventricular tachycardia and the existence of a sinus node dysfunction were significantly more frequent (P less than 0.05) in patients with proarrhythmic effect of ajmaline. In conclusion, the supraventricular proarrhythmic effect of intravenous ajmaline exists and is related both to the electrophysiologic characteristics of the drug and to the arrhythmia substrate. The results indicate that a supraventricular tachyarrhythmia may be induced by a class I antiarrhythmic drug.     

An approach to therapy of supraventricular tachyarrhythmias: An algorithm versus individualized therapy

Approaches to the treatment of supraventricular arrhythmias, including atrial fibrillation, atrial flutter, atrial tachycardia, atrioventricular (AV) reentrant tachycardia, and AV nodal reentrant tachycardia, continue to evolve. Within the past two decades, many new and effective treatments have become available. These include several new antiarrhythmic agents, ablative therapies, pacing and surgical modalities, and cardioversion/defibrillation techniques. This paper provides an algorithm for the treatment of these supraventricular arrhythmias which includes therapy for the acute episode as well as the prevention of subsequent episodes of the tachyarrhythmia.     

Cardiac arrhythmias in patients with surgical repair of Ebstein's anomaly

Preoperative, perioperative and postoperative arrhythmias in 52 consecutive patients who underwent operation for Ebstein's anomaly were reviewed. There were 25 male and 27 female patients (mean age 18 years, range 11 months to 64 years). Thirty-four patients had one or more documented arrhythmias preoperatively (18 had paroxysmal supraventricular tachycardia, 10 had paroxysmal atrial fibrillation or flutter, 13 had ventricular arrhythmia and 3 had high grade atrioventricular block). Seven patients without documented arrhythmias had a history typical of tachyarrhythmias. During the perioperative and early postoperative periods, 14 patients had atrial tachyarrhythmias and 8 had ventricular tachycardia or ventricular fibrillation. There were seven deaths between day 1 and 27 months after operation. Five of these deaths were sudden (all in male patients, aged 12 to 34 years), and four of the patients had had perioperative ventricular tachycardia or ventricular fibrillation. One patient was taking one antiarrhythmic agent and another patient was taking two at the time of sudden death. Of the 18 patients with paroxysmal supraventricular tachycardia and 9 patients with paroxysmal atrial fibrillation or flutter preoperatively who were followed up for a mean of 40 and 36 months, respectively, 22 and 33% continued to have symptomatic tachycardia. Of the 11 patients (mean age 9 years) without preoperative documentation or symptoms of arrhythmia, follow-up data were obtained (range 1 to 144 months, mean 31) in 9 patients. None died suddenly or developed symptomatic arrhythmia.     

Total or partial electrical interruption of the atrioventricular pathway. Evaluation of clinical results and indications

After checking on the efficacy and innocuity of catheter ablation of the His bundle in an experimental study in the sheep, 22 patients with supraventricular tachycardia resistant to an average of 5.5 +/- 1.7 antiarrhythmic drugs per patient underwent this procedure. The most common arrhythmias were atrial fibrillation or flutter (poorly tolerated in 14 cases), and reciprocating tachycardia (8 cases, including 3 with accessory atrioventricular pathways). In 3 of the 5 patients with intranodal tachycardia, an initial attempt was made to modify one of the two AV nodal conduction pathways. Although complete atrioventricular block was obtained in all patients, atrioventricular conduction returned in 15 patients between the 30th minute and 4th day after the procedure. A second attempt at His bundle ablation was carried out in 9 patients. During follow-up ranging from 1 to 38 months, complete or an advanced degree of atrioventricular block persisted in 5 patients, the other patients being in sinus rhythm (12 cases) or slow atrial fibrillation (2 cases). The remaining patient who was in complete atrioventricular block died of septicaemia 2 months after the procedure. The clinical results were evaluated by electrophysiological investigation, exercise testing and Holter monitoring. There were 15 satisfactory clinical results (83.3 p. 100) and 3 poor results. This series shows that interruption or modification of the normal atrioventricular conduction pathway may be performed at low risk and with good results in a high percentage of cases. It is a valuable therapeutic option in patients with supraventricular tachycardia resistant to antiarrhythmic therapy.