aVR导联四步流程在宽QRS波心动过速鉴别诊断中的应用

目的 验证分析aVR导联四步流程对宽QRS波心动过速(WCT)进行鉴别诊断的应用价值.方法 回顾性分析113例患者发生WCT时12导联体表心电图的形态学特征.按aVR导联四步流程分步骤地对113例WCT患者的心电图进行室性心动过速(室速)和室上性心动过速(室上速)的鉴别,将分析的结果与电生理检查结果进行比较,计算每一步...     

Electrocardiogram algorithms used to differentiate wide complex tachycardias demonstrate diagnostic limitations when applied by non-cardiologists

Aims

Non-cardiologists (NCs) are often responsible for the preliminary diagnosis and early management of patients presenting with ventricular tachycardia (VT) or supraventricular wide complex tachycardia (SWCT). At present, the Vereckei aVR and Brugada algorithms are the most widely recognized and frequently relied upon wide complex tachycardia (WCT) differentiation criteria by NCs. This study aimed to determine the diagnostic efficacy of the Vereckei aVR and Brugada algorithms when applied by NCs.

Application of a new algorithm in the differential diagnosis of wide QRS complex tachycardia.

AIMS: The Brugada criteria proposed to distinguish between regular, monomorphic wide QRS complex tachycardias (WCT) caused by supraventricular (SVT) and ventricular tachycardia (VT) have been reported to have a better sensitivity and specificity than the traditional criteria. By incorporating two new criteria, a new, simplified algorithm was devised and compared with the Brugada criteria. METHODS AND RESULTS: A total of 453 WCTs (331 VTs, 105 SVTs, 17 pre-excited tachycardias) from 287 consecutive patients with a proven electrophysiological (EP) diagnosis were prospectively analysed by two of the authors blinded to the EP diagnosis. The following criteria were analysed: (i) presence of AV dissociation; (ii) presence of an initial R wave in lead aVR; (iii) whether the morphology of the WCT correspond to bundle branch or fascicular block; (iv) estimation of initial (v(i)) and terminal (v(t)) ventricular activation velocity ratio (v(i)/v(t)) by measuring the voltage change on the ECG tracing during the initial 40 ms (v(i)) and the terminal 40 ms (v(t)) of the same bi- or multiphasic QRS complex. A v(i)/v(t) >1 was suggestive of SVT and a v(i)/v(t) 相似文献   

宽QRS波心动过速的诊治进展

宽QRS波是临床常见的心律失常,常分为室性心动过速和室上性心动过速,由于两者治疗原则及方法不相同,所以鉴别宽QRS波心动过速非常重要,现介绍Brugada法、Vereckei法、aVR法、Ⅱ导联R波峰值标准法以及各种方法的特异性及敏感性。对于不同类型的宽QRS波心动过速有着不同的治疗方法,如药物、电复律及射频消融术。     

Validação de um Algoritmo Simples para Detecção de Taquicardia Ventricular no Eletrocardiograma

BackgroundThe differential diagnosis of wide QRS complex tachycardia (WCT) between ventricular tachycardia (VT) or supraventricular tachycardia with aberrant conduction (SVT-A) is sometimes difficult in the emergency room.ObjectiveThe aim of this study was to evaluate the accuracy of a new simple electrocardiographic algorithm to recognize VT in patients with wide complex tachycardia.MethodsThe 12-lead electrocardiograms (ECG) for WCT were prospectively obtained from 120 patients during electrophysiological study. Six physicians with different expertise analyzed the electrocardiographic recordings, and made the diagnosis based on the D12V16 algorithm, that involves the analysis of the predominant polarity of QRS in leads I, II, V1 and V6. The diagnosis was compared with that made using the traditional Brugada algorithm and the “gold-standard” electrophysiological study. Statistical analyses were performed with a significance level of 5% (p<0.05).ResultsAccording to the EPS study, 82 ECG recordings were VT and 38 SVT-A. Structural heart diseases were present in 71 (86.6%) patients with VT and in 8 (21.1%) with SVT-A. The Brugada algorithm had higher global sensitivity (87.2%), and the D12V16 algorithm had higher global specificity (85.1%) for VT. Both D12V16 and Brugada’s algorithms presented a high positive predictive value (90.9% vs 85.8%, respectively) and similar accuracy (73.8% vs 81.4%, respectively) for the diagnosis of VT. Experienced evaluators were more accurate using Brugada algorithm than the D12V16 algorithm, but the accuracy of both algorithms was similar according to less experienced examiners.ConclusionThe simplified algorithm may be a useful method to recognize VT in the ECG, especially for less experienced doctors. (Arq Bras Cardiol. 2021; [online].ahead print, PP.0-0)     

Are wide complex tachycardia algorithms applicable in children and patients with congenital heart disease?

Introduction

Several algorithms have been developed to help determine the etiology of wide complex tachycardias (WCTs) in adults. Sensitivity and specificity for differentiating supraventricular tachycardia (SVT) with aberration from ventricular tachycardia (VT) in adults have been demonstrated to be as high as 98% and 97%. These algorithms have not been tested in the pediatric population. We hypothesize that these algorithms have lower diagnostic accuracy in children and patients with congenital heart disease.

Methods

A retrospective review of the pediatric electrophysiology database at Stanford from 2001 to 2008 was performed. All children with WCT, a 12-lead electrocardiogram (ECG) available for review, and an electrophysiology study confirming the etiology of the rhythm were included. Patients with a paced rhythm were excluded. The ECGs were analyzed by 2 electrophysiologists blinded to the diagnosis according to the algorithms described in Brugada et al,² and Vereckei et al.⁵ Additional ECG findings were recorded by each electrophysiologist.

Results

A total of 65 WCT ECGs in 58 patients were identified. Supraventricular tachycardia was noted in 62% (40/65) and VT in 38% (25/65) of the ECGs. The mean age was 13.5 years (SD ± 5.1), the mean weight was 51.8 kg (SD ± 22.4), and 48% (31/65) were male. The mean tachycardia cycle length was 340 milliseconds (SD ± 95). Congenital heart disease (CHD) was present in 37% (24/65) of patients (7 tetralogy of Fallot, 6 Ebstein's, 4 double-outlet right ventricle, 3 complex CHD, 2 d-transposition of great arteries, 1 status-post orthotopic heart transplantation, 1 ventricular septal defect). The Brugada algorithm correctly predicted the diagnosis 69% (45/65) of the time, the Vereckei algorithm correctly predicted the diagnosis 66% (43/65) of the time, and the blinded reviewer correctly predicted the diagnosis 78% (51/65) of the time. There was no difference in the efficacy of the algorithms in patients with CHD vs those with structurally normal hearts. The findings of left superior axis deviation (P < .01) and a notch in the QRS downstroke of V₁ or V₂ (P < .01) were more common in VT than SVT, whereas a positive QRS deflection in V₁ (P = .03) was more commonly present in SVT than VT.

Conclusion

The Brugada and Vereckei algorithms have lower diagnostic accuracy in the pediatric population and in patients with congenital heart disease than in the adult population. Left superior axis deviation and a notch in the QRS downstroke were more commonly associated with VT, whereas a positive QRS deflection in V₁ was more commonly associated with SVT in this population.     

Approach to the diagnosis and initial management of the stable adult patient with a wide complex tachycardia

The initial electrocardiographic evaluation of every tachyarrhythmia should begin by addressing the question of whether the QRS complex is wide or narrow. The most important cause of wide complex tachycardia (WCT) is ventricular tachycardia. However, supraventricular tachycardia can also manifest with a wide QRS complex. The ability to differentiate between supraventricular tachycardia with a wide QRS due to aberrancy or preexcitation and ventricular tachycardia often presents a diagnostic challenge. The identification of whether WCT has a ventricular or supraventricular origin is critical because the treatment for each is different, and improper therapy may have potentially lethal consequences. In conclusion, although the diagnosis and treatment of sustained WCT often arise in emergency situations, this report focuses on a stepwise approach to the management of WCT in relatively stable adult patients, particularly the diagnosis and differentiation of ventricular tachycardia from supraventricular tachycardia with a wide QRS complex on standard 12-lead electrocardiography.     

单导联对宽QRS波鉴别诊断价值的应用

目的 验证应用Ⅱ导联QRS波第1峰时限(RWPT)、aVR导联形态及Vi/Vt比值对宽QRS波的鉴别诊断价值。方法 回顾性分析已明确诊断的261例室性早搏及127例房性早搏伴心室内差异传导患者12导联体表心电图的Ⅱ导联RWPT、aVR导联形态及Vi/Vt比值,比较3种方法鉴别诊断宽QRS波形早搏的敏感度、特异度、阳性预测值、阴性预测值、准确度,并比较aVR导联形态法前3步的鉴别价值。结果 ①Ⅱ导联RWPT、aVR导联形态法鉴别宽QRS波差异无统计学意义(P>0.05),均有较高的准确度、敏感度、特异度、阳性预测值;Vi/Vt比值法与其余两种方法相比准确度、敏感度较低(P<0.05),特异度、阳性预测值较高(P<005)。②应用aVR导联形态法第1步、第3步有较高的特异度,第3步有较高的阳性预测值。结论 鉴别诊断宽QRS波时,Ⅱ导联RWPT、aVR导联形态法较Vi/Vt比值法诊断价值高,且具有简便、快捷、准确的优点。     

New algorithm using only lead aVR for differential diagnosis of wide QRS complex tachycardia.

BACKGROUND: We recently reported an ECG algorithm for differential diagnosis of regular wide QRS complex tachycardias that was superior to the Brugada algorithm. OBJECTIVE: The purpose of this study was to further simplify the algorithm by omitting the complicated morphologic criteria and restricting the analysis to lead aVR. METHODS: In this study, 483 wide QRS complex tachycardias [351 ventricular tachycardias (VTs), 112 supraventricular tachycardias (SVTs), 20 preexcited tachycardias] from 313 patients with proven diagnoses were prospectively analyzed by two of the authors blinded to the diagnosis. Lead aVR was analyzed for (1) presence of an initial R wave, (2) width of an initial r or q wave >40 ms, (3) notching on the initial downstroke of a predominantly negative QRS complex, and (4) ventricular activation-velocity ratio (v(i)/v(t)), the vertical excursion (in millivolts) recorded during the initial (v(i)) and terminal (v(t)) 40 ms of the QRS complex. When any of criteria 1 to 3 was present, VT was diagnosed; when absent, the next criterion was analyzed. In step 4, v(i)/v(t) >1 suggested SVT, and v(i)/v(t) < or =1 suggested VT. RESULTS: The accuracy of the new aVR algorithm and our previous algorithm was superior to that of the Brugada algorithm (P = .002 and P = .007, respectively). The aVR algorithm and our previous algorithm had greater sensitivity (P <.001 and P = .001, respectively) and negative predictive value for diagnosing VT and greater specificity (P <.001 and P = .001, respectively) and positive predictive value for diagnosing SVT compared with the Brugada criteria. CONCLUSION: The simplified aVR algorithm classified wide QRS complex tachycardias with the same accuracy as standard criteria and our previous algorithm and was superior to the Brugada algorithm.     

The differential diagnosis on the electrocardiogram between ventricular tachycardia and preexcited tachycardia

The 12-lead surface electrocardiogram is a simple and useful tool for the differential diagnosis of regular wide QRS complex tachycardia. However, criteria do not as yet exist to discriminate between ventricular tachycardia and supraventricular tachycardia with anterograde conduction over an accessory pathway (preexcited tachycardia). Therefore, we designed a new stepwise approach with three criteria for the electrocardiographic differential diagnosis between ventricular tachycardia and preexcited tachycardia and prospectively studied 267 regular tachycardias with electrophysiologically proven mechanism and a wide QRS complex (≥ 0.12 s): 149 consecutive ventricular tachycardias and 118 consecutive preexcited regular tachycardias. Underlying heart disease was old myocardial infarction in 133 of 149 (89%) ventricular tachycardias. The patients presenting with preexcited tachycardia had no additional structural heart disease. Atrial fibrillation with preexcited QRS complex was not included. The criteria favoring ventricular tachycardia were: (1) presence of predominantly negative QRS complexes in the precordial leads V₄ to V₆, (2) presence of a QR complex in one or more of the precordial leads V₂ to V₆, and (3) AV relation different from 1:1 (more QRS complexes than P waves). The final sensitivity and specificity of these three consecutive steps to diagnose ventricular tachycardia were 0.75 and 1.00, respectively. This new stepwise approach is sensitive and highly specific for the differential diagnosis between ventricular tachycardia in coronary artery disease and preexcited regular tachycardia.     

Automatic wide complex tachycardia differentiation using mathematically synthesized vectorcardiogram signals

BackgroundAutomated wide complex tachycardia (WCT) differentiation into ventricular tachycardia (VT) and supraventricular wide complex tachycardia (SWCT) may be accomplished using novel calculations that quantify the extent of mean electrical vector changes between the WCT and baseline electrocardiogram (ECG). At present, it is unknown whether quantifying mean electrical vector changes within three orthogonal vectorcardiogram (VCG) leads (X, Y, and Z leads) can improve automated VT and SWCT classification.MethodsA derivation cohort of paired WCT and baseline ECGs was used to derive five logistic regression models: (i) one novel WCT differentiation model (i.e., VCG Model), (ii) three previously developed WCT differentiation models (i.e., WCT Formula, VT Prediction Model, and WCT Formula II), and (iii) one “all‐inclusive” model (i.e., Hybrid Model). A separate validation cohort of paired WCT and baseline ECGs was used to trial and compare each model''s performance.ResultsThe VCG Model, composed of WCT QRS duration, baseline QRS duration, absolute change in QRS duration, X‐lead QRS amplitude change, Y‐lead QRS amplitude change, and Z‐lead QRS amplitude change, demonstrated effective WCT differentiation (area under the curve [AUC] 0.94) for the derivation cohort. For the validation cohort, the diagnostic performance of the VCG Model (AUC 0.94) was similar to that achieved by the WCT Formula (AUC 0.95), VT Prediction Model (AUC 0.91), WCT Formula II (AUC 0.94), and Hybrid Model (AUC 0.95).ConclusionCustom calculations derived from mathematically synthesized VCG signals may be used to formulate an effective means to differentiate WCTs automatically.     

改良Brugada四步法诊断室性心动过速价值初探

为评价改良的Brugada四步法诊断室性心动过速（VT）的价值，对心电图表现为宽QRS波（QRS时限＞110ms），心动过速行射频消融术的连续病例24例（其中经心脏电生理检查证实VT19例，室上性心动过速5例）进行回顾性分析，结果显示改良的Brugada四步法对VT的敏感性为94．7％，特异性为80％，阳性预告值为94．7％。     

Characteristics of ventricular tachycardia arising from the inflow region of the right ventricle

IntroductionVentricular tachycardia (VT) arising from the right ventricular inflow (RVI) region is uncommon. There is minimal literature on the clinical and electrocardiographic characteristics of RVI VT.MethodsA retrospective analysis of patients with RVI VT who underwent electrophysiology study between 2006 and 2011 was performed. Patients with structural heart disease (including arrhythmogenic right ventricular dysplasia) were excluded.ResultsSeventy patients underwent an electrophysiology study for VT arising from the right ventricle during the study period. Nine patients (13%) met the inclusion criteria for RVI VT and were the subject of this analysis. The median age was 46 years (range, 14-71), and VT cycle length was 295 milliseconds (range, 279-400 milliseconds). All VTs had an left bundle-branch block morphology. An inferiorly directed QRS axis was noted in 7 (78%) of 9 patients and a left superior axis in 2 (22%) of 9 patients. A QS or rS pattern was noted in all patients in aVR and V₁. A transition from S to R wave occurred in V₃ to V₅ in all patients, with 78% of the patients transitioning in V₄ or V₅. Ablation was attempted in 8 (89%) of 9 patients and was successful in 6 (67%) of 9 patients. Ablation was limited in all unsuccessful patients due to the proximity to the His and risk of complete heart block.ConclusionsElectrocardiographic findings of a left bundle-branch block with a normal QRS axis, QS or rS patterns in aVR and V₁, and late S to R transition (V₄/V₅) are commonly found in RVI VT. Because of the proximity to the His, ablation of RVI VT may be more challenging than that of right ventricular outflow tract VT.     

A new criterion to differentiate atrioventricular nodal reentrant tachycardia from atrioventricular reciprocating tachycardia: Combined AVR criterion

Aim

A combined aVR criterion is described as the presence of a pseudo r′ wave in aVR during tachycardia in patients without r′ wave in aVR in sinus rhythm and/or a ≥50% increase in r′ wave amplitude compared to sinus rhythm in patients with r′ wave in the basal aVR lead. We aimed to investigate the use of combined aVR criterion in differential diagnosis of atrioventricular nodal reentrant tachycardia (AVNRT) and atrioventricular reciprocating tachycardia (AVRT).

Regelm??ige Tachykardien mit breitem Kammerkomplex

Differential diagnosis of regular tachycardia with broad QRS complex can be challenging in daily practice. There are four different arrhythmias that have to be taken into account when being confronted with a broad QRS complex tachycardia: (1) ventricular tachycardia (VT); (2) supraventricular tachycardia (SVT) with bundle branch block (BBB); (3) SVT with AV conduction over an accessory AV pathway; (4) paced ventricular rhythm. Due to potentially fatal consequences, the correct diagnosis is important in view of both the acute treatment and the long-term therapy. Since SVT with accessory conduction is rare and a paced ventricular rhythm can be identified easily by stimulation artifacts, in most cases, a VT has to be differentiated from an SVT with BBB. Several ECG criteria can be helpful: (1) QRS complex duration > 140 ms in right BBB tachycardia or > 160 ms in left BBB tachycardia; (2) ventricular fusion beats; (3)“Northwest” QRS axis; (4) ventriculoatrial dissociation; (5) absence of an RS complex or RS interval > 100 ms in leads V₁-V₆; (6) a positive or negative concordant R wave progression pattern in leads V₁-V₆; (7) absence of an initial R wave or an S wave in lead V₁ in right BBB tachycardia; (8) absence of an R wave or an R/S ratio < 1 in lead V₆ in right BBB tachycardia; (9) absence or delay of the initial negative forces in lead V₁ in left BBB pattern (R wave duration > 30 ms in V₁; interval between onset of R wave and Nadir of S wave > 60 ms in V₁); (10) presence of Q wave. Any of these variables favor VT. However, none of the criteria has both a sufficient sensitivity and specificity when utilized on its own. Therefore, various diagnostic algorithms have been proposed using a number of the above criteria consecutively. By doing so, the specificity and sensitivity of correctly identifying a VT or an SVT with BBB can be raised to > 95%.     

Brugada法联合Steurer法在宽QRS波心动过速鉴别诊断中的价值

为评价Brugada法联合Steurer法在宽QRS波心动过速 (WRT)鉴别诊断中的应用价值及存在的缺陷。对 1 0 1例WRT[室性心动过速 (VT) 5 8例 ,室上性心动过速 (SVT) 43例 ]进行分析。结果 :Brugada法诊断VT灵敏度、特异度、准确性分别为 85 .7%、89.5 %、87.1 % ;联合Steurer法后灵敏度、特异度、准确性分别升至 91 .5 %、90 .5 %、91 .1 %。进一步分析显示 :Brugada法对器质性原因所致VT、右束支阻滞型 (RBBB)特发性VT(IVT)、SVT伴室内差异性传导 (AC)或原有单侧束支阻滞 (BBB)者诊断符合率高 ( 95 .8%～ 1 0 0 .0 % ) ;对左束支阻滞型 (LBBB)特发性VT、SVT伴原有双支阻滞、心肌坏死或心肌梗死伴宽QRS波SVT及预激综合征伴旁道前传型SVT(WPW SVT)诊断的符合率低 ( 0～ 5 0 .0 % )。联合Steurer法可使WPW伴旁道前传型SVT得以明确诊断 ,但对前三者无鉴别意义 ,故不适合在前三者中应用。结论 :Brugada法联合Steurer法能提高WPW伴旁道前传型SVT的鉴别能力 ,是目前鉴别WRT的重要方法。     

The exception to Marriot's sign

Differentiation of a wide complex arrhythmia can pose as a clinical challenge in the acute care setting. Two broad differentials exist including ventricular tachycardia versus supraventricular tachycardia with aberrancy, underlying bundle branch block or intrinsic conduction defect. To aid in distinguishing between supraventricular tachycardia and ventricular tachycardia, Brugada criteria is commonly used, albeit new algorithms have become more common. Marriott's sign, a taller peak in the first R wave when compared to R' is considered a specific criterion for distinguishing between these two entities and strongly favors the diagnosis of ventricular tachycardia. In this case we present a wide complex tachycardia, which is an exception to Marriott's sign.     

Mayo Clinic VT calculator: A practical tool for accurate wide complex tachycardia differentiation

The discrimination of ventricular tachycardia (VT) versus supraventricular wide complex tachycardia (SWCT) via 12-lead electrocardiogram (ECG) is crucial for achieving appropriate, high-quality, and cost-effective care in patients presenting with wide QRS complex tachycardia (WCT). Decades of rigorous research have brought forth an expanding arsenal of applicable manual algorithm methods for differentiating WCTs. However, these algorithms are limited by their heavy reliance on the ECG interpreter for their proper execution. Herein, we introduce the Mayo Clinic ventricular tachycardia calculator (MC-VTcalc) as a novel generalizable, accurate, and easy-to-use means to estimate VT probability independent of ECG interpreter competency. The MC-VTcalc, through the use of web-based and mobile device platforms, only requires the entry of computerized measurements (i.e., QRS duration, QRS axis, and T-wave axis) that are routinely displayed on standard 12-lead ECG recordings.     

Electrophysiologic Characteristics of Wide QRS Complexes during Pharmacologic Termination of Sustained Supraventricular Tachycardias with Verapamil and Adenosine: Observations from Electrophysiologic Study

Background: In this study we evaluate wide QRS complexes observed during pharmacologic termination of supraventricular tachycardias. Methods: Patients with supraventricular tachycardia, undergoing electrophysiologic study were enrolled. 12 mg of adenosine or 10 mg of verapamil were administered during tachycardia, under continuous monitoring of intaracardiac and surface electrocardiograms. Electrocardiographic features of ventricular ectopy were noted. Results: Seventy‐four patients were enrolled. 48 patients were randomized to adenosine and 26 to verapamil. Five different appearance patterns of ventricular ectopy were observed during termination of tachycardias. All wide QRS complexes were of ventricular origin and all of them were observed during the termination of tachycardia. Adenosine more frequently resulted in appearance of ventricular beats (15.4% vs 41.7%, P = 0.003), and this was more frequently observed in patients with atrioventricular nodal reentrant tachycardia. Patients with ventricular beats were younger than those without, in both, verapamil (47.5 ± 15.6 vs 65.0 ± 8.8 years, P = 0.04) and adenosine (40.9 ± 13.8 vs 49.7 ± 16.8, P = 0.03) groups. Left bundle branch block (LBBB)/superior axis morphology was most frequent morphology in adenosine group (55%). Two of 4 patients in verapamil group displayed LBBB/inferior axis QRS morphology and another 2 patients displayed LBBB/superior axis morphology. Conclusions: Noncatheter induced, five different appearance patterns and four distinct morphologies of ventricular origin were observed. Most of them do not directly terminate tachycardia, but are associated with its termination and are not observed in ongoing tachycardia.     

Electrocardiogram during tachycardia in patients with anterograde conduction over a Mahaim fiber: old criteria revisited.

OBJECTIVES: The aim of this study was to prospectively evaluate the sensitivity, specificity, and positive and negative predictive values of previously described ECG criteria to identify preexcited tachycardia due to decrementally conducting accessory pathways (QRS axis between 0 and -75 degrees , QRS width < or = 0.15 seconds, an R wave in lead I, an rS pattern in lead V(1), RS > 1 QRS transition > V(4), and cycle length between 220 and 450 ms). BACKGROUND: Preexcited tachycardia associated with decrementally conducting right-sided accessory pathways usually shows a rather "narrow" QRS complex and can be difficult to differentiate from supraventricular tachycardia (SVT) with left bundle branch block (LBBB) aberrant conduction. METHODS: We analyzed three groups of patients: 32 patients with an atriofascicular pathway (group I); 8 patients with long (n = 3) or short (n = 5) decrementally conducting right-sided AV pathway (group II); and a control group that consisted of 35 patients with SVT and LBBB (group III). RESULTS: Presence of all six criteria had 87.5% sensitivity in group I and a 0% sensitivity in group II. There were four false negatives in group I. The negative predictive value was 82.5%, with six false positives in group III (five patients with an aberrant LBBB-shaped tachycardia with ventriculoatrial conduction over an accessory AV pathway). The criterion cycle length was not helpful. CONCLUSIONS: Criteria for identifying a tachycardia with anterograde conduction over a Mahaim fiber are helpful only in atriofascicular pathways, with a sensitivity of 87.5% and a negative predictive value of 82.5%. The major cause of false positives was a tachycardia with aberrant LBBB conduction and ventriculoatrial conduction over an accessory AV pathway.