依赖异丙肾上腺素逆传的房室折返性心动过速

目的：探讨4例依赖异丙肾上腺素逆传的房室折返性心动过速的发作情况及电生理特点。方法：对4例射频导管消融前常规电生理检查未能诱发出阵发性定性心动过速及A－H间期跳跃，亦未见旁道逆传者，静脉滴注异丙肾上腺素、右心室刺激时发现左侧旁道逆传，并均诱发出正向型房室折返性心运过速；停止注射后，左侧旁道逆传功能消失，亦不能诱发出房室折返性心动过速。结果：静脉滴注异丙肾上腺素，右心室起搏下用逆行法于二尖瓣环心室侧消融，4例均获成功；术后静脉滴注异丙肾上腺素下再行右心室起搏，未见旁道逆传现象。结论：部分隐匿性旁道构成的房室折返性心动过速发作依赖异丙肾上腺素，射频导管消融在静脉滴注异丙肾上腺素及右心室起搏下进行。     

射频消融治疗具有快频率依赖性室房逆传特性的旁道

目的报道具有快频率依赖性室房逆传特性的房室旁道电生理检查及射频消融结果。方法4例患者,均有阵发性心悸史,且发作时心电图均显示为窄QRS波心动过速,按常规方法接受心脏电生理检查及射频消融治疗。结果4例均证实存在旁道的快频率依赖性室房逆传,且均诱发了房室折返性心动过速,室房逆传最早激动部位均为左房。于快频率心室刺激下标测消融靶点,消融均获成功。结论旁道的快频率依赖性传导为一种少见电生理现象,可伴发房室折返性心动过速。     

隐匿性预激综合征房室结-希浦系的电生理特点

(?)匿性预激综(?)征在电生理检查中因旁道逆传(掩盖正常房室传导径路的逆传功能)和频繁诱发顺向型房室折返性心动过速(O-AVRT),以致难以可靠地评价该类病人房室结-希浦系(AVN-HPS)的电生理特点.木文报道20例隐匿性预激综合征经射频消融完全阻断旁道消除O-AVRT后AVN-HPS的电生理特点.资料与方法 患者20例.男12例.女8例;平均年龄40.2(21～68)岁.因反复发作室上性心动过速而接受射频消融治疗.入院体检、X线胸片和超声心动图检查其心脏结构和功能均正常.心腔内电生理检查均能诱发O-AVRT,心内膜标测证实为单旁道.分别位于左后(1)、左侧(13)和左前(3)游离壁消融电极经股动脉逆行送入左室,在冠状窦标测电     

慢旁道与快旁道并存的复杂心电图分析

本文报道1例慢传导旁道（简称慢旁道）与快传导旁道（简称快旁道）并存的病例，并分析讨论了其复杂的心内电生理现象。病例资料与电生理检查结果患者女性，30岁，于1996年12月行心内电生理和射频消融术。患者心动过速发作史5年，近1年发作频繁，发作为室上性心动过速，频率170次／分左右。术中各电生理检查导管到位后，即因导管机械刺激诱发心动过速，心内电图见最早道传A波出现于冠状窦近端（参见图2），故首先考虑为左后壁隐匿性旁道参与的房室折返性心动过速。终止心动过速后，以550－250Ins多个周长起搏右心室，发现550－450Ins周长起…     

快频率依赖性室房逆传左侧隐匿性房室旁道参与的心动过速及射频导管消融

目的探讨快频率依赖性室房逆传特性左侧隐匿性房室旁道的电生理特点及射频消融。方法对8例心电图显示窄QRS波群心动过速的患者行电生理检查,分析房室、室房传导情况、心动过速特点、旁道定位,并行射频消融。结果8例患者均证实存在快频率依赖性室房逆传特性左侧隐匿性旁道,在较慢频率起搏右心室时旁道逆传发生阻滞,而以中等频率起搏时表现为间断旁道逆传,较快频率起搏时才表现为旁道1：1传导且均诱发了房室折返性心动过速,于快频率心室刺激下标测消融靶点,消融均获成功。结论左侧隐匿性房室旁道有时可发生快频率依赖性室房逆传现象,并伴发房室折返性心动过速,在射频消融中需注意分辨,以免漏诊。     

房室多旁道的电生理特征及其射频消融治疗

目的 探讨房室多旁道的电生理特点及射频消融方法。方法 23例患者经电生理检查确定房室多旁道，应用心房和心室刺激诱发室上速，确定每条旁道的电生理特征及与心动过速的关系，按照标测部位对相关旁道逐步消融，以射频消融成功确定旁道位置。结果 23例中检出旁道49条，其中三条旁道3例；左侧多旁道12例，右侧多旁道2例，双侧多旁道9例；左侧多旁道以隐匿性为主；右侧多旁道多为显性；未见心动过速时右侧旁道前传而同侧旁道逆传现象。结论 多旁道患者应首先确定和消融与心动过速相关旁道；左侧多旁道应以诱发心动过速或快速心室起搏方法标测；右侧多旁道应同步描记12导联体表心电图，旁道消融成功可能仅见于QRS波的变化，双侧多旁道应首先消融左侧旁道。     

室上性心动过速中旁道折返的少见现象

报告阵发性室上性心动过速（PSVT）电生理检查中发现的几种少见的旁道电生理特性。例1为一左侧隐匿性房室旁道参与的房室折返性心动过速,其心室扫描示旁道逆向有效不应期260ms,但520－390ms时无逆向传导功能。例2为一慢－慢型房室交接区折返性心动过速,同时存在一条右侧隐匿性房室旁道作为旁观者。例3为一宽QRS波群心动过速,其体表心电图呈典型马海姆纤维型预激综合征,电生理揭示存在左侧房室隐匿性旁道和束室旁道两条附加旁道,前者为心动过速的逆传支,后者与房室结－希室束同为顺传支。对上述几种少见的电生理现象临床意义进行讨论。     

频率依赖性隐匿性房室旁道伴发的心动过速及射频消融

目的　研究频率依赖性隐匿性房室旁道伴发的心动过速特点及射频消融治疗。方法　6例患者 ,男性 2例 ,女性 4例 ,年龄 14～ 6 8岁。电生理检查包括采用右室心尖部和左室S1S1及S1S2起搏分析室房传导情况、心动过速特点、旁道位置确定及射频消融治疗。结果　左侧游离壁 5例 ,右侧三尖瓣环 11点处 1例。具有旁道 1∶1室房传导功能 5例 ,传导窗口 80～ 10 0ms,有偶发旁道逆传现象 4例 ,诱发心动过速 5例。在右室起搏下标测靶点 ,所有病例均消融成功。结论　隐匿性房室旁道发生 3位相或 4位相阻滞时表现为频率依赖性室房传导。电生理检查过程中应注意分辨偶发室房逆传现象 ,以免漏诊。     

逆向型房室折返性心动过速的临床特点

探讨逆向型房室折返性心动过速 (ADRT)的临床特点。 397例预激综合征患者进行常规电生理检查和导管射频消融术 ,2 2 (5 .5 % )例 (包括Mahaim纤维旁道 12例 )诱发出ADRT ,心动过速的周长为 30 2± 5 6 (2 30～ 4 10 )ms,2例心动过速时出现低血压伴有头晕 ,4例在心动过速时演化为心房颤动。通过与患者既往临床心电图比较 ,证实 17例有ADRT临床发作 ,常见于多旁道和年轻的患者 (15 / 2 2例 ) ,12例同时伴有顺向型房室折返性心动过速。 19例多旁道患者中 15例逆传经旁道 ,4例逆传经旁道和 /或房室结。 3例单旁道患者在静脉点滴异丙肾上腺素后诱发ADRT ,逆传经房室结。参与构成ADRT的 4 1条旁道 19条位于右侧游离壁 ,9条位于右后间隔 ,3条位于左后间隔 ,7条位于左侧游离壁。 12例前传经Mahaim纤维的ADRT ,其逆传旁道均位于后间隔。 7例普通旁道参与的心动过速其前传支和逆传分别位于右侧、左侧游离壁。 3例单旁道均位于右侧游离壁。结论 :ADRT最常见于多旁道患者并有多种形成机制。     

变异性房室结-希氏-浦肯野系统裂隙现象一例

患者女性，２３岁。因反复发作心动过速２年，复发１天就诊。食管电生理揭示存在房室旁路合并房室结双径路，诱发出顺向型房室折返性心动过速。入院后行射频消融术成功阻断左后隔旁房室旁路，由于快径路逆传功能较差，在食管心房起搏和心内电生理检查中反复刺激均未能诱发...     

Adenosine-5'-triphosphate test for the noninvasive diagnosis of concealed accessory pathway

OBJECTIVES: This study assessed the use of adenosine triphosphate (ATP) in the noninvasive diagnosis of concealed accessory pathway (AP) and dual atrioventricular (AV) node physiology in patients with inducible AV reentrant tachycardia (AVRT). BACKGROUND: Administration of ATP during sinus rhythm identifies dual AV node physiology in 76% of patients with inducible sustained slow/fast AV nodal reentry tachycardia (AVNRT). METHODS: Incremental doses of ATP were intravenously administered during sinus rhythm to 34 patients with inducible sustained AVRT involving a concealed AP and to 27 control patients without AP or dual AV node physiology. One study group patient could not complete the study and was excluded from analysis. RESULTS: The AV reentrant echo beats (AVRE), or AVRT, suggestive of the presence of concealed AP, were observed after ATP administration in 24 (73%) study patients and in none of the control group. Electrocardiographic signs suggestive of dual AV node physiology were observed after ATP administration in 7 (21%) study patients and in none of the control group. Most instances of AVRE/AVRT were preceded by a slight increase (<50 ms) in PR interval. In 8 of 9 patients tested, neither AVRE nor AVRT was no longer observed following ATP administration after successful radiofrequency ablation of the AP. In the remaining patient, a different AVRE due to the presence of an additional AP was observed. CONCLUSIONS: Administration of ATP during sinus rhythm may be a useful bedside test for identifying patients with concealed AP who are prone to AVRT and those with associated dual AV node pathways.     

Evolution of clinical and electrophysiologic data in patients with a preexcitation syndrome

The purpose of the study is to report the natural changes of preexcitation syndrome (PS).MethodsElectrophysiologic study was performed for syncope (n = 8), atrioventricular reentrant tachycardia (AVRT) (n = 42), atrial fibrillation (n = 3), adverse presentation (n = 4), or for asymptomatic PS (n = 22) and was repeated 1 to 21 years later.ResultsClinically, 12 patients initially asymptomatic became symptomatic (54.5%), and 12 symptomatic patients became asymptomatic (21%). At electrophysiologic study 2, maximal rate conducted over accessory pathway (AP) was slower. Anterograde conduction disappeared in 22 patients, but 10 of them had inducible AVRT. Among 27 patients with initially rapid conduction over AP, 7 had a benign form; 20 had always a rapid conduction over AP, and 3 of them initially asymptomatic developed rapid atrial fibrillation.ConclusionsAsymptomatic patients with a PS frequently became symptomatic (54.5%), whereas symptomatic patients rarely became asymptomatic (21%). Maximal rate conducted over AP decreased during life, but AVRT remained inducible.     

Autonomic dependence of ventriculoatrial conduction

To evaluate the effects of isoproterenol and atropine on patients with poor ventriculoatrial (VA) conduction, 17 patients were studied who did not have 1-to-1 VA conduction during ventricular pacing at a rate slightly faster than sinus rate (group I) and 11 patients were studied who had 1-to-1 VA conduction, but only at constant ventricular pacing cycle lengths longer than 600 ms (group II). Isoproterenol infusion at a rate causing a 20 to 30% increase in sinus rate or up to 4 micrograms/min shortened the ventricular pacing cycle lengths that induced VA block in all group II patients. Atropine administration at a dose causing a 20 to 30% increase in sinus rate or up to a total dose of 2 mg also shortened the ventricular pacing cycle lengths that induced VA block in all group II patients. At similar pacing cycle lengths, isoproterenol and atropine induced shorter VA intervals than control. Nine of 17 group I patients had demonstrable 1-to-1 VA conduction either during isoproterenol infusion or after atropine administration. Of these 9 patients, 1-to-1 VA conduction could be found only during isoproterenol infusion in 3 patients and only after atropine administration in 4 patients. The improvement of VA conduction by these drugs was related to their effects on the atrioventricular node. The change in VA conduction mediated by autonomic changes induced by these drugs may explain why some patients without demonstrable VA conduction during rest may have, under certain circumstances, "endless-loop" tachycardia or paroxysmal supraventricular tachycardia using atrioventricular nodal conduction as the retrograde limb.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of atropine and isoproterenol on tachycardia induction in asymptomatic patients with Wolff-Parkinson-White electrocardiographic pattern.

OBJECTIVE: To determine if pharmacological interventions aimed at altering autonomic tone would allow induction of orthodromic atrioventricular reentrant tachycardia in asymptomatic patients with the Wolff-Parkinson-White electrocardiographic pattern. DESIGN: Prospective interventional protocol in consecutive eligible patients. SETTINGS: University hospital. PATIENTS: Eighteen asymptomatic patients (13 male and five female) with the Wolff-Parkinson-White electrocardiographic pattern without inducible tachycardia in the drug-free state. INTERVENTION: Electrophysiological assessment was performed at baseline, after intravenous administration of atropine (0.03 mg/kg) and during isoproterenol infusion (0.5 to 2 micrograms/min). RESULTS: Orthodromic reciprocating tachycardia was not inducible at baseline because of absent retrograde accessory pathway conduction in seven patients. In five patients, orthodromic atrial echo beats could be induced (which blocked retrogradely in the accessory pathway in three patients and anterogradely in the atrioventricular node in two). In the remaining six patients, neither orthodromic echo beats nor reciprocating tachycardia could be induced despite intact retrograde accessory pathway conduction. Following atropine administration (mean dose 1.9 +/- 0.3 mg), anterograde and retrograde accessory pathway effective refractory periods decreased from 360 +/- 172 to 284 +/- 62 ms and from 340 +/- 38 to 296 +/- 32 ms, respectively (both P < 0.05 versus control). Orthodromic reciprocating tachycardia was induced in two patients (nonsustained in one). During isoproterenol infusion (mean dose 1.0 +/- 0.3 micrograms/min), anterograde and retrograde accessory pathway effective refractory periods decreased further to 243 +/- 23 and 248 +/- 22 ms, respectively (both P < 0.05 versus after atropine); two further patients had inducible orthodromic reciprocating tachycardia (nonsustained in one). No patient with absent retrograde accessory pathway conduction developed retrograde accessory pathway conduction or reciprocating tachycardia with isoproterenol and/or atropine. CONCLUSIONS: Isoproterenol and/or atropine allowed tachycardia induction in four of 18 asymptomatic patients with the Wolff-Parkinson-White electrocardiographic pattern. In the majority of these patients, tachycardia is not inducible because of deficient retrograde accessory pathway conduction which does not improve with autonomic facilitation.     

A clinical study of induction rate of atrioventricular reciprocating tachycardia in WPW syndrome]

The differences in induction rate of atrioventricular reciprocating tachycardia (AVRT) were analyzed in 76 consecutive patients of WPW syndrome with tachycardia attack, confirmed by either ECG or history, and who underwent electrophysiological study. AVRT was induced by programed electrical stimulation in 72% of patients with manifest WPW syndrome, in 77% of patients with intermittent WPW syndrome, and in 96% of patients with concealed WPW syndrome, respectively. There was a significant difference in induction rate between manifest WPW syndrome and concealed WPW syndrome (p less than 0.05). Induction rate of AVRT in patients with accessory pathway (AP) located in the ventricular septum was significantly lower (50%) than in patients with AP located in the left ventricle (88%) and in the right ventricle (72%) (p less than 0.05). Ventricular atrial conduction was found in only 56% of patients with AP located in the ventricular septum, while it was found in 94% of patients with AP located in the left ventricle, and in 100% of patients with AP located in the right ventricle (p less than 0.05). There were no significant differences in antegrade effective refractory periods of atrioventricular node and AP between patients with and without inducible AVRT. There was also no significant difference in the retrograde effective refractory periods of AP between patients with or without inducible AVRT. We concluded that the induction rate of AVRT would be affected by the location of AP and the mode of delta wave appearance in the surface electrocardiogram.     

Usefulness of invasive electrophysiologic testing to stratify the risk of arrhythmic events in asymptomatic patients with Wolff-Parkinson-White pattern: results from a large prospective long-term follow-up study

OBJECTIVES: The aim of this study was to assess in a large cohort of asymptomatic subjects with Wolff-Parkinson-White (WPW) pattern the usefulness of invasive electrophysiologic testing (EPT) in predicting the occurrence of arrhythmic events over a five-year follow-up. BACKGROUND: Sudden death may be the first clinical manifestation of the WPW syndrome in previously asymptomatic patients. Serial EPTs have been proposed to identify patients at risk. METHODS: A total of 212 consecutive asymptomatic WPW patients were enrolled after a baseline EPT; patients were followed for five years, and 162 patients (115 noninducible and 47 inducible) patients underwent a second EPT. RESULTS: After a mean follow-up of 37.7 months, 33 patients became symptomatic. Of the 115 noninducible patients, 18.2% lost anterograde accessory pathway (AP) conduction, 30% retrograde AP conduction, and only 4 (3.4%) developed symptomatic supraventricular tachycardia (SVT). Of the 47 inducible patients, 25 with sustained atrioventricular reciprocating tachycardia (AVRT) and atrial fibrillation (AF), and 4 with nonsustained AVRT and AF became symptomatic for SVT (n = 21) and AF (n = 8). They were younger, had shorter AP anterograde refractory periods, and multiple APs compared to patients who remained asymptomatic (for all comparisons, p < 0.0001). Of the eight patients with symptomatic episodes of AF and inducible sustained AF, two had a resuscitated cardiac arrest and one died suddenly; all three patients were inducible for AVRT and AF and had multiple APs. CONCLUSIONS: In asymptomatic WPW subjects, EPT may be a valuable tool to stratify the risk of symptomatic and fatal arrhythmic events.     

Electrophysiological evaluation of asymptomatic ventricular pre-excitation in children and adolescents

BACKGROUND: Diagnostic assessment and treatment have been described in detail in symptomatic WPW syndrome, but little information exists about significance and prognosis of an incidentally found ventricular pre-excitation (VPE) in asymptomatic children. The aim of the study was to evaluate, retrospectively, the role of electrophysiological study (EPS) in the assessment of the arrhythmic risk in asymptomatic patients with VPE. Material and METHODS: Sixty-two asymptomatic children and adolescents (38 M/24 F, aged 9.8+/-5.1 years) referred to our Division between 1996 and 2002 for an incidentally found VPE underwent an EPS for arrhythmic risk stratification. The following parameters were examined: anterograde effective refractory period of the accessory pathway (AP), the 1-to-1 conduction over the AP, the inducibility of atrio-ventricular re-entrant tachycardia (AVRT) and the inducibility of atrial fibrillation (AF) with measurement of minimal RR between two consecutive preexcitated QRS complexes, the average RR interval of all cycles, and the percentage of preexcitated QRS complexes. RESULT: During the EPS, 36 patients (58.1%) experienced sustained SVT. The tachycardia was initiated in the basal state in 22 patients and after isoproterenol in the other 14. Orthodromic AVRT (cycle length 305.9+/-48.5 ms) was recorded in 29 patients. In three patients, both orthodromic and antidromic AVRT were recorded, with different cycle length (CL). Antidromic AVRT alone (CL 239.5+/-13.7 ms) was recorded in four patients. AF was recorded in nine patients: in six patients, it was recorded after the induction of orthodromic or antidromic AVRT, in the other three cases AF was the first and only arrhythmic event. The minimal RR between two consecutive pre-excitated QRS ranged between 250-230 ms (mean 237.5+/-9.6 ms). In the 26 patients who presented no induced sustained tachycardia in the EPS, the 1:1 conduction over the AP ranged between 210 and 600 ms (mean 279.6+/-75.2 ms). CONCLUSIONS: Electrophysiological evaluation remains the gold standard for assessing risk of life-threatening arrhythmias in patients with VPE. However, a high proportion of healthy children and adolescents with VPE can experience sustained AVRT and/or AF during EPS. These results raise questions about the necessity of an aggressive treatment approach to prevent those "rare" cases of sudden death.     

Detection of concealed left sided accessory atrioventricular pathway by P wave signal averaged electrocardiogram

Objectives. The purpose of this study was to examine whether P wave signal-averaged electrocardiogram (P-SAECG), which detects subtle changes in P wave, detects the concealed accessory atrioventricular pathway (AP).Background. It is difficult to differentiate atrioventricular reciprocating tachycardia (AVRT) due to the AP from atrioventricular nodal reentrant tachycardia (AVNRT) when the ventricular preexcitation is absent on 12-lead electrocardiograms. By electrophysiological studies, the anterograde conduction in the concealed AP is shown to be blocked near the AP-ventricular interface during sinus rhythm.Methods. P-SAECG during sinus rhythm was performed in 20 normal volunteers (control), 21 patients with AVRT due to the concealed AP, 19 with AVNRT, 22 with paroxysmal atrial fibrillation (PAF), and 7 with automatic atrial tachycardia (AT). The filtered P wave duration (FPD) and AR20 (power spectrum area ratio of 0–20 to 20–100 Hz) were measured and repeated in AVRT, AVNRT and AT groups at one week after catheter ablation.Results. The anterograde conduction in the concealed left-sided AP was confirmed in all cases by an electrophysiological study. The FPD in AVRT group was more prolonged than that in controls or AVNRT group. Although the FPD was similar between AVRT and PAF groups, AR20 differentiated between the two groups. Ablation of the concealed AP shortened FPD in AVRT group but that of the slow pathway or the atrial focus did not shorten in the AVNRT or AT groups, respectively. The changes in FPD after ablation were correlated with those in the duration of atrial activity by an electrophysiological study (r = 0.67).Conclusions. Our findings suggest that P-SAECG detects the concealed left-sided AP, providing a clinical tool in noninvasively assessing atrial activation patterns.     

Differential Effects of Atropine and Isoproterenol on Inducibility of Atrioventricular Nodal Reentrant Tachycardia

Background: Radiofrequency ablation of the slow pathway in atrioventricular nodal reentrant tachycardia (AVNRT) relies on tachycardia non-inducibility after ablation as success criterion. However, AVNRT is frequently non-inducible at baseline. Thus, autonomic enhancement using either atropine or isoproterenol is frequently used for arrhythmia induction before ablation. Methods: 80 patients (57 women, 23 men, age 50±14 years) undergoing slow pathway ablation for recurrent AVNRT were randomized to receive either 0.01mg/kg atropine or 0.5-1.0g/kg/min isoproterenol before ablation after baseline assessment of AV conduction. The effects of either drug on ante- and retrograde conduction was assessed by measuring sinus cycle length, PR and AH interval, antegrade and retrograde Wenckebach cycle length (WBCL), antegrade effective refractory period (ERP) of slow and fast pathway and maximal stimulus-to-H interval during slow and fast pathway conduction. Results: Inducibility of AVNRT at baseline was not different between patients randomized to atropine (73%) and isoproterenol (58%) but was reduced after atropine (45%) compared to isoproterenol (93%, P<0.001). Of the 28 patients non-inducible at baseline isoproterenol rendered AVNRT inducible in 21, atropine in 4 patients. Dual AV nodal pathway physiology was present in 88% before and 50% after atropine compared to 83% before and 73% after isoproterenol. Whereas both drugs exerted similar effects on ante- and retrograde fast pathway conduction maximal SH interval during slow pathway conduction was significantly shorter after isoproterenol (300±48ms vs. 374±113ms, P=0.012). Conclusion: Isoproterenol yields higher AVNRT inducibility than atropine in patients non-inducible at baseline. This may be caused by a more pronounced effect on antegrade slow pathway conduction.     

Electrophysiologic effects of isoproterenol in patients with atrioventricular reentrant tachycardia treated with flecainide

The electrophysiologic effects of isoproterenol in patients treated with flecainide for atrioventricular (AV) reentrant tachycardia were studied to evaluate the mechanism of tachycardia inducibility after isoproterenol and the value of isoproterenol challenge as a predictor of spontaneous arrhythmia recurrence. Seventeen patients underwent electrophysiologic study before and after oral flecainide administration and after the addition of isoproterenol to flecainide. No patient had inducible sustained supraventricular tachycardia after flecainide alone. Two patients had inducible sustained and six had inducible nonsustained tachycardia after isoproterenol was added to flecainide. The patients were then followed up on the same flecainide dose they received at the time of the electrophysiologic study. Findings: 1) Flecainide treatment prolonged HV and VA intervals, and the addition of isoproterenol did not affect these variables. 2) Isoproterenol shortened anterograde and retrograde block cycle length and the refractory period of the accessory pathway and the AV node. It also decreased the tachycardia cycle length, an effect that was due solely to shortening of AV node conduction time. 3) Flecainide treatment prevented tachycardia induction by affecting retrograde conduction over the accessory pathway. Isoproterenol allowed for tachycardia induction and for more sustained episodes of tachycardia by reversing the effect of flecainide on retrograde accessory pathway conduction. 4) Tachycardia recurred during follow-up in all three patients in whom tachycardia of greater than or equal to 10 s duration was induced after isoproterenol but in no patient who had no or shorter episodes of induced tachycardia (and who did not have a change in medical regimen). Conclusions: 1) Isoproterenol reverses flecainide-induced prolongation of block cycle length and refractory periods of the accessory pathway and AV node. 2) Isoproterenol reverses flecainide-induced prevention of tachycardia induction through reversal of the effects of flecainide on the retrograde accessory pathway. 3) The addition of isoproterenol during flecainide restudy is valuable in predicting long-term drug efficacy.