Identification of concealed posteroseptal Kent pathways by comparison of ventriculoatrial intervals from apical and posterobasal right ventricular sites

Local VA index for the differential diagnosis of supraventricular tachycardia

BACKGROUND

Differentiating between atypical atrioventricular nodal reentrant tachycardia (AVNRT) and orthodromic reciprocating tachycardia utilizing a septal accessory pathway is a complex challenge.

OBJECTIVE

The purpose of this study was to describe the "local VA index," a straightforward method based on signals from the coronary sinus catheter, to distinguish between these arrhythmias during tachycardia and entrainment. The ventriculoatrial (VA) interval on the coronary sinus catheter is measured during tachycardia and entrainment, at the site of earliest atrial activity. The difference between these 2 situations defines the "local VA index." We also propose a mechanism to clarify the limitations of historical pacing maneuvers, such as postpacing interval minus tachycardia cycle length (PPI-TCL) and stimulus-atrial interval minus ventriculoatrial interval (SA-VA), by examining nodal decrement and intraventricular conduction delay.

METHODS

In a retrospective study of 75 patients referred for supraventricular tachycardia evaluation, 37 were diagnosed with atrioventricular reentrant tachycardia (AVRT) with orthodromic reciprocating tachycardia, and 38 with AVNRT (27 typical, 11 atypical).

RESULTS

In comparison to AVRT patients, AVNRT patients exhibited longer PPI-TCL (176 ± 47 ms vs 113 ± 42 ms; P <.01) and SA-VA (138 ± 47 ms vs 64 ± 28 ms; P <.01). The AVRT group had mean local VA index of -1 ± 13 ms, whereas the AVNRT group had a significantly longer index of 91 ± 46 ms (P <.01). An optimal threshold for differentiation was a local VA index of 40 ms. Importantly, there was no significant correlation between pacing cycle length and nodal decrement as well as intraventricular delay related to pathway location. This interindividual variability might explain misleading interpretations of PPI-TCL and SA-VA.

CONCLUSION

This novel approach is advantageous because of its simplicity and effectiveness, requiring only 2 diagnostic catheters. A local VA interval difference <40 ms provides a clear distinction for AVRT.

The Value of Programmed Ventricular Extrastimuli From the Right Ventricular Basal Septum During Supraventricular Tachycardia

BACKGROUND

The difference between the right ventricular (RV) apical stimulus-atrial electrogram (SA) interval during resetting of supraventricular tachycardia (SVT) versus the ventriculoatrial (VA) interval during SVT (ΔSA-VA_apex) is an established technique for discerning SVT mechanisms but is limited by a significant diagnostic overlap.

OBJECTIVES

This study hypothesized that the difference between the RV SA interval during resetting of SVTs versus the VA interval during SVTs (ΔSA-VA) would yield a more robust differentiation of atrioventricular nodal re-entrant tachycardia (AVNRT) from atrioventricular reciprocating tachycardia (AVRT) when using the RV basal septal stimulation (ΔSA-VA_base) as compared to the RV apical stimulation (ΔSA-VA_apex). Moreover, it was predicted that the ΔSA-VA_base might distinguish septal from free wall accessory pathways (APs) effectively.

METHODS

In this prospective study, 105 patients with AVNRTs (age 48 ± 20 years, 44% male) and 130 with AVRTs (age 26 ± 18 years, 54% male) underwent programmed ventricular extrastimuli delivered from both the RV basal septum and RV apex. The ΔSA-VA values were compared between the 2 sites.

RESULTS

The ΔSA-VA_base was shorter than the ΔSA-VA_apex during AVRT (44 ± 30 ms vs 58 ± 29 ms; P < 0.001), and the opposite occurred during AVNRT (133 ± 31 ms vs 125 ± 25 ms; P = 0.03). A ΔSA-VA_base of ≧85 milliseconds had a sensitivity of 97% and specificity of 96% for identifying AVNRT. Furthermore, a ΔSA-VA_base of 45-85 milliseconds identified AVRT with left free wall APs (sensitivity 86%, specificity 95%), 20-45 milliseconds for posterior septal APs (sensitivity 72%, specificity 96%), and <20 milliseconds for right free wall or anterior/mid septal APs (sensitivity 86%, specificity 98%).

CONCLUSIONS

The ΔSA-VA_base during programmed ventricular extrastimuli produced a robust differentiation between AVNRT and AVRT regardless of the AP location with ≧85 milliseconds as an excellent cutoff point. This straightforward technique further allowed localizing 4 general AP sites.

Variability of the VA interval at tachycardia induction: a simple method to differentiate orthodromic reciprocating tachycardia from atypical atrioventricular nodal reentrant tachycardia

BACKGROUND

The differential diagnosis between orthodromic atrioventricular reentry tachycardia (AVRT) and atypical AV nodal reentrant tachycardia (aAVNRT) is sometimes challenging. We hypothesize that aAVNRTs have more variability in the retrograde conduction time at tachycardia onset than AVRTs.

METHODS

We aimed to assess the variability in retrograde conduction time at tachycardia onset in AVRT and aAVNRT and to propose a new diagnostic tool to differentiate these two arrhythmia mechanisms. We measured the VA interval of the first beats after tachycardia induction until it stabilized. The difference between the maximum and minimum VA intervals (∆VA) and the number of beats needed for the VA interval to stabilize was analyzed. Atrial tachycardias were excluded.

RESULTS

A total of 107 patients with aAVNRT (n = 37) or AVRT (n = 64) were included. Six additional patients with decremental accessory pathway-mediated tachycardia (DAPT) were analyzed separately. All aAVNRTs had VA interval variability. The median ∆VA was 0 (0 - 5) ms in AVRTs vs 40 (21 - 55) ms in aAVNRTs (p < 0.001). The VA interval stabilized significantly earlier in AVRTs (median 1.5 [1 - 3] beats) than in aAVNRTs (5 [4 - 7] beats; p < 0.001). A ∆VA < 10 ms accurately differentiated AVRT from aAVNRT with 100% of sensitivity, specificity, and positive and negative predictive values. The stabilization of the VA interval at < 3 beats of the tachycardia onset identified AVRT with sensitivity, specificity, and positive and negative predictive values of 64.1%, 94.6%, 95.3%, and 60.3%, respectively. A ∆VA < 20 ms yielded good diagnostic accuracy for DAPT.

CONCLUSIONS

A ∆VA < 10 ms is a simple and useful criterion that accurately distinguished AVRT from atypical AVNRT. Central panel: Scatter plot showing individual values of ∆VA in atypical AVNRT and AVRT. Left panel: induction of atypical AVNRT. The VA interval stabilizes at the 5th beat and the ∆VA is 62 ms (maximum VA interval: 172 ms - minimum VA interval: 110 ms). Right panel: induction of AVRT. The tachycardia has a fixed VA interval from the first beat. ∆VA is 0 ms.

Ripple Mapping: A precise tool for atrioventricular nodal reentrant tachycardia ablation

INTRODUCTION

Ablation for atrioventricular nodal reentrant tachycardia (AVNRT) classically utilizes evaluation of signal morphology within the anatomic region of the slow pathway (SP), which involves subjectivity. Ripple Mapping (RM) (CARTO-3© Biosense Webster Inc, Irvine, CA) displays each electrogram at its 3-dimensional coordinate as a bar changing in length according to its voltage-time relationship. This allows prolonged, low-amplitude signals to be displayed in their entirety, helping identify propagation in low-voltage areas. We set out to evaluate the ability of RM to locate the anatomic site of the slow pathway and assess its use in guiding ablation for AVNRT.

METHODS

Patients ≤18 yrs with AVNRT in the EP laboratory between 2017 and 2021 were evaluated. RM was performed to define region of SP conduction in patients from 2019-2021, whereas standard electro-anatomical mapping was used from 2017-2019. All ablations were performed using cryo-therapy. Demographics, outcomes and analysis of variance in number of test lesions until success were compared between groups.

RESULTS

A total 115 patients underwent AVRNT ablation during the study; 46 patients were in the RM group and 69 were in the control group. There were no demographic differences between groups. All procedures, in both groups, were acutely successful. In RM group, 89% of first successful lesions were within 4mm of the predicted site. There was significantly reduced variability in number of test lesions until success in the RM group (p=0.01).

CONCLUSIONS

RM is a novel technique that can help identify slow pathway location, allowing for successful ablation of AVNRT with decreased variability. This article is protected by copyright. All rights reserved.

Pathophysiology, Diagnosis, and Ablation of Atrioventricular Node-dependent Long-R-P Tachycardias

Atrioventricular (AV) node–dependent long-R–P tachycardias are a unique group of supraventricular tachycardias that include atypical AV nodal reentrant tachycardia (AVNRT), atypical AVNRT with a concealed bystander nodofascicular (NF)/nodoventricular (NV) accessory pathway inserting into the slow pathway of the AV node, the permanent form of junctional reciprocating tachycardia, and orthodromic NF/NV reciprocating tachycardia. Here, we discuss the complex pathophysiology, diagnosis, and ablation of these intriguing arrhythmias.

When and how does a single ventricular premature beat initiate and terminate supraventricular tachycardia?

BACKGROUND

The differential diagnosis of a supraventricular tachycardia (SVT) is accomplished using a number of pacing maneuvers. The incidence and mechanism of a single ventricular premature beat (VPB) on initiation and termination of tachycardia were evaluated during programmed electrical stimulation (PES) of the heart in patients with the two most common regular SVTs: atrioventricular re-entrant tachycardia (AVNRT) and orthodromic atrioventricular tachycardia (AVRT).

METHODS

Three hundred and thirty-seven consecutive patients aged above 18 years with an inducible sustained AVNRT or AVRT were prospectively enrolled. Patients with more than one tachyarrhythmia mechanism were excluded. Two hundred and seventeen patients (64.4%) had typical slow/fast AVNRT and 120 (35.6%) had an orthodromic AVRT using a rapidly conducting accessory pathway for V-A conduction. In this cross-sectional study, we specifically report the analysis of tachycardia induction and termination by a single VPB.

RESULTS

Tachycardia induction with a single VPB during sinus rhythm was seen in 7 of 120 AVRT and in only one of the 217 patients with AVNRT, (5.8% vs. 0.3%, p < 0.05). When a single VPB was delivered during basic ventricular pacing these values were 28% versus 4%, respectively, (p < 0.001). Termination of tachycardia by a single VPB was observed in nine (4.1%) patients with AVNRT and in 57 (47.5%) with AVRT (p < 0.001).

CONCLUSION

Initiation of SVT by a single VPB during sinus rhythm was uncommon and favored AVRT. Termination of SVT by a single VPB was commonly seen in AVRT but rarely in AVNRT. These findings can be of help when interpreting a noninvasive arrhythmia event recording.

Delta of the local ventriculo-atrial intervals at the septal location to differentiate tachycardia using septal accessory pathways from atypical atrioventricular nodal re-entry

Aims

Tachycardia mediated by septal accessory pathways (AP) and atypical atrioventricular nodal re-entry (AVNRT) require careful electrophysiologic evaluation for differential diagnosis. We aim to describe the differential behaviour of local ventriculo-atrial (VA) intervals which predicts the tachycardia mechanism.

Methods and results

The local VA intervals at the para-Hisian septum were measured under three different situations: (i) tachycardia; (ii) sustained entrainment from the right ventricular apex (RVA); and (iii) continuous pacing from the RVA during sinus rhythm. Differences were computed as follows: Δ-VAentr = VA during entrainment - VA during tachycardia; and Δ-VApac = VA while pacing during sinus rhythm - VA during tachycardia. In contrast to AVNRT, we hypothesized that an invariable retrograde conduction through the septal AP will keep the result of the subtractions close to 0 ms in cases of ortodromic atrioventricular re-entrant tachycardia (AVRT). We analysed 55 atypical AVNRT (45% posterior type) and 82 AVRT (10 anteroseptal, 18 para-Hisian, 12 mid-septal, and 42 posteroseptal). Δ-VAentr was longer for AVNRT (98.5 ± 40.3 ms) compared with septal AP (-5.7 ± 19.3 ms; P < 0.001). A value of 50 ms showed 98.7% sensitivity and 92% specificity (AUC 0.99; 95% CI 0.98-1). According to physiological criteria, a negative Δ-VAentr remains unobserved in the case of AVNRT (positive predictive value 100% for septal AP). Δ-VApac was also longer for AVNRT (66.5 ± 14.6 ms) compared with septal AP (-9.7 ± 3.3 ms; P < 0.001). A value of 50 ms showed 100% sensitivity and 74% specificity (AUC 0.86; 95% CI 0.76-0.93).

Conclusions

Delta of the local VA intervals enables distinction between atypical AVNRT and AVRT mediated by septal AP.

A Simple Method to Differentiate Atrioventricular Node Reentrant Tachycardia from Orthodromic Reciprocating Tachycardia

Discrimination between atrioventricular node reentry tachycardia (AVNRT) and orthodromic reciprocating tachycardia (ORT) during an electrophysiological study is sometimes challenging. This study aimed to investigate if the difference in the local VA (ventricle-atrium) interval during ventricular entrainment pacing and during tachycardia (DVA, defined as the shortest local VA interval of coronary sinus [CS] during entrainment minus the shortest local VA interval of CS during tachycardia) was different in patients with AVNRT and patients with ORT.Diagnoses of AVNRT or ORT through a concealed accessory pathway (AP) were made according to conventional electrophysiological criteria and ablation results. Entrainment by right ventricular (RV) pacing was performed in each patient before ablation and patients with successful entrainment were included in the study. The DVA was compared between patients with AVNRT and patients with ORT. The DVA in patients with AVNRT was significantly longer than that in patients with ORT (120 ± 20 versus 5.7 ± 9; P < 0.001). In each patient with AVNRT of slow-fast type, fast-slow type, and slow-slow type, the DVA was more than 48 ms. In each patient with ORT using a left free wall accessory pathway (AP), right free wall AP, and septal AP, the DVA was less than 20 ms.DVA was found to be a rapid, useful test in distinguishing patients with AVNRT from those with ORT.

Classification, Electrophysiological Features and Therapy of Atrioventricular Nodal Reentrant Tachycardia

Atrioventricular nodal reentrant tachycardia (AVNRT) should be classified as typical or atypical. The term 'fast-slow AVNRT' is rather misleading. Retrograde atrial activation during tachycardia should not be relied upon as a diagnostic criterion. Both typical and atypical atrioventricular nodal reentrant tachycardia are compatible with varying retrograde atrial activation patterns. Attempts at establishing the presence of a 'lower common pathway' are probably of no practical significance. When the diagnosis of AVNRT is established, ablation should be only directed towards the anatomic position of the slow pathway. If right septal attempts are unsuccessful, the left septal side should be tried. Ablation targeting earliest atrial activation sites during typical atrioventricular nodal reentrant tachycardia or the fast pathway in general for any kind of typical or atypical atrioventricular nodal reentrant tachycardia, are not justified. In this review we discuss current concepts about the tachycardia circuit, electrophysiologic diagnosis, and ablation of this arrhythmia.

Fusion during entrainment of orthodromic reciprocating tachycardia is enhanced for basal pacing sites but diminished when pacing near Purkinje system end points

BACKGROUND

In the electrophysiological laboratory, orthodromic atrioventricular reciprocating tachycardia (ORT) can be distinguished from atrial tachycardia and atrioventricular node reentry tachycardia by identifying orthodromic and antidromic wavefront fusion during ventricular overdrive pacing (VOP). Previous work has shown that basal VOP near the accessory pathway (AP) increases the likelihood of observing fusion; however, in a third of cases, fusion is not appreciable regardless of VOP location.

OBJECTIVE

To explore the hypothesis that pacing near His-Purkinje system (PS) end points reduces fusion quality, which may explain patients with nonresponsive ORT.

METHODS

In a novel computer model of ORT, simulations were performed with a variety of AP locations and pacing sites; results were analyzed to assess factors influencing fusion quality in pseudo-electrocardiogram signals.

RESULTS

Entrainment by basal VOP near the AP was more likely to produce fusion visible on simulated electrocardiograms compared to entrainment by apical VOP, but this advantage was dramatically diminished when the pacing site was also near PS end points. Prediction of fusion quality based on AP proximity alone was dramatically improved when corrected to penalize for PS proximity.

CONCLUSIONS

These results suggest that basal VOP near the AP and far from the PS is optimal; this could be tested in patients. A denser basal ramification of PS fibers is known to exist in a minority of human hearts; our findings indicate that this unusual PS configuration is a plausible explanation for ORT cases where fusion is never observed in spite of entrainment by basal VOP near the AP.

Introduction to Supraventricular Tachycardia

Paroxysmal supraventricular tachycardia (PSVT) is a clinical syndrome characterized by a rapid tachycardia with an abrupt onset and termination cardiomyopathy. The three most common causes of PSVT are atrioventricular nodal reentrant tachycardia (50%-60%), atrioventricular reentrant tachycardia in patients with Wolff-Parkinson-White syndrome (25%-30%), and atrial tachycardia (10%). Rare causes of PSVT include focal junctional tachycardia, atriofascicular tachycardia, permanent reciprocating junctional tachycardia, and nodoventricular/nodofascicular tachycardia. This article, based on challenging PSVT cases, is a guide for clinicians dealing with diagnostic or therapeutic dilemmas in the electrophysiology laboratory.

First postpacing interval after tachycardia entrainment with correction for atrioventricular node delay: A simple maneuver for differential diagnosis of atrioventricular nodal reentrant tachycardias versus orthodromic reciprocating tachycardias

BACKGROUND

The difference between the first postpacing interval (PPI) after tachycardia entrainment from the right ventricular apex and the tachycardia cycle length (TCL) can be used as an index of proximity to the circuit.

OBJECTIVES

The purpose of this study was to determine whether the response to entrainment of tachycardia during ventricular stimulation with correction for AV node delay is a useful, simple maneuver for differentiating AV nodal reentrant tachycardia (AVNRT) from orthodromic reciprocating tachycardia (ORT) using a concealed accessory pathway.

METHODS

The study consisted of 193 consecutive patients who underwent electrophysiologic study and ablation of regular paroxysmal supraventricular tachycardia without preexcitation during sinus rhythm. Tachycardia entrainment was attempted through trains of 5 to 15 right ventricular apex pacing pulses. The increment in AV nodal conduction time in the first PPI was subtracted from the PPI-TCL difference (corrected PPI-TCL).

RESULTS

Electrophysiologic study demonstrated ORT in 84 patients and AVNRT in 109 patients. Transient entrainment was achieved in all but 12 patients. The mean corrected PPI-TCL difference was significantly shorter in 77 patients with ORT (66 +/- 27 ms) than in 104 AVNRT patients (151 +/- 28 ms; P <.0001). Patients with septal accessory pathways had shorter corrected PPI-TCL differences than patients with free-wall accessory pathways. The presence of a corrected PPI-TCL difference <110 ms identified all but one patient with ORT, and no patients with AVNRT had such a difference.

CONCLUSION

The return cycle after tachycardia entrainment by right ventricular apex stimulation with correction for AV node delay is a rapid, useful maneuver for differential diagnosis of AVNRT vs ORT in patients without preexcitation. The presence of a corrected PPI-TCL <110 ms accurately identified with high reliability those patients with ORT.

Classification and differential diagnosis of atrioventricular nodal re-entrant tachycardia

Recent evidence on atrioventricular nodal re-entrant tachycardia has identified several types of this common arrhythmia, with potential therapeutic implications. This article reviews the relevant new information, discusses the differential diagnosis of atrioventricular nodal re-entrant tachycardia, and summarizes the electrophysiological criteria for classification of the various forms of the arrhythmia.

Differentiation of atypical atrioventricular node re-entrant tachycardia from orthodromic reciprocating tachycardia using a septal accessory pathway by the response to ventricular pacing

OBJECTIVES

The purpose of this study was to determine whether the response to ventricular pacing during tachycardia is useful for differentiating atypical atrioventricular node re-entrant tachycardia (AVNRT) from orthodromic reciprocating tachycardia (ORT) using a septal accessory pathway.

BACKGROUND

Although it is usually possible to differentiate atypical AVNRT from ORT using a septal accessory pathway, a definitive diagnosis is occasionally elusive.

METHODS

In 30 patients with atypical AVNRT and 44 patients with ORT using a septal accessory pathway, the right ventricle was paced at a cycle length 10 to 40 ms shorter than the tachycardia cycle length (TCL). The ventriculo-atrial (VA) interval and TCL were measured just before pacing. The interval between the last pacing stimulus and the last entrained atrial depolarization (stimulus-atrial [S-A] interval) and the post-pacing interval (PPI) at the right ventricular apex were measured on cessation of ventricular pacing.

RESULTS

All 30 patients with atypical AVNRT and none of the 44 patients with ORT using a septal accessory pathway had an S-A-VA interval >85 ms and PPI-TCL >115 ms.

CONCLUSIONS

The S-A-VA interval and PPI-TCL are useful in distinguishing atypical AVNRT from ORT using a septal accessory pathway.

Radiofrequency ablation of right-sided accessory pathways in pediatric patients

Right free-wall and septal accessory pathways encompass the full spectrum of accessory pathway electrophysiology and are situated in complex anatomical arrangements. Understanding this diversity of physiology is necessary for the successful and safe elimination of these connections with transcatheter radiofrequency ablation. When radiofrequency catheter ablation of these pathways is attempted in children, anatomical relationships often become more complex, and spatial constraints require more adaptive techniques than in adults. It is clear that considerable progress has been made with radiofrequency catheter ablation, such that it is now first-line therapy for most children who have been diagnosed with one of the broad spectrum of clinical manifestations that result from the presence of these accessory connections. This review will discuss how accessory pathway electrophysiology and anatomy impact the clinical syndromes observed in children, and how these factors, as well as others particular to children, determine the approach, results and potential long-term consequences of radiofrequency catheter ablation of right-sided accessory pathways in the pediatric population.

Characteristics and radiofrequency catheter ablation of septal accessory atrioventricular pathways

Septal accessory AV pathways are located in the complex AV septal space that also contains the specialized conduction system. They have unique electrocardiographical and electrophysiological characteristics to be differentiated from free-wall accessory pathways. Some of the septal pathways have AV nodelike conduction properties and produce a similar activation sequence in the retrograde conduction. Several methods have been developed to distinguish them from AV nodal pathways. Radiofrequency catheter ablation using the titration method and endocardial approach without entrance into the coronary sinus is effective in eliminating most of the septal accessory pathways without impairment of AV conduction. However, some posteroseptal accessory pathways may require energy application inside the coronary sinus, thus information of the coronary sinus anatomy is important for preventing complication.

Use of only a regular diagnostic His-bundle catheter for both fast and reproducible "para-Hisian pacing" and stable right ventricular pacing

INTRODUCTION

Para-Hisian pacing, i.e., pacing the anteroseptal right ventricle (RV) with or without direct capture of the His bundle (HB), allows the differentiation of VA conduction over the AV node from conduction over an accessory pathway. Classically, it is performed by maneuvering a separate pacing catheter around the HB catheter, which may be difficult and time-consuming.

METHODS AND RESULTS

This study prospectively evaluated the use of a single standard octapolar HB catheter with 2-mm interelectrode spacing for simultaneous (para-Hisian) pacing from the distal bipole and recording from the three proximal bipoles in 148 consecutive patients. Para-Hisian pacing was successful in 146 of 148 patients, performed within a median of only 10 seconds, and easily repeated several times during the course of an electrophysiologic study. Retrograde HB activation could be recorded in 132 of 146 patients; a clearly different surface ECG configuration confirmed the presence or absence of HB capture in all other patients. Interestingly, stable RV pacing could be performed from the HB catheter for the rest of the electrophysiologic study in 138 of 142 patients in whom this was tried. RV pacing from this site also led to better interpretation of retrograde conduction, due to clear visualization of retrograde HB activation.

CONCLUSION

Pacing from the distal bipole of a regular diagnostic HB catheter provides a fast and reliable way to perform para-Hisian pacing. Therefore, it may be advocated as a routine diagnostic protocol during electrophysiologic procedures. Moreover, pacing from this site obviates the need for a separate RV pacing catheter in most patients.

Use of double ventricular extrastimulation to determine the preexcitation index in atrioventricular nodal reentrant tachycardia

The ability of single paced ventricular beats during tachycardia to penetrate the tachycardia circuit and reset the subsequent atrial depolarization (atrial preexcitation), enabling calculation of the "preexcitation index," can be helpful in analyzing supraventricular tachycardias. However, the ventricular refractory period often prevents ventricular capture of beats with the necessary prematurity to demonstrate atrial preexcitation, particularly in atrioventricular nodal reentrant tachycardia (AVNRT). We hypothesized that the use of double premature stimuli could overcome this limitation. In 25 consecutive patients with either AVNRT or atrioventricular reciprocating tachycardia (AVRT) we attempted to demonstrate atrial preexcitation with single and double ventricular extrastimuli. Whereas atrial preexcitation with a single extrastimulus could only be achieved in 3 of 11 patients with AVNRT, all but 1 patient demonstrated atrial preexcitation with the use of double ventricular extrastimuli. On the other hand, in all but 1 patient with AVRT, atrial preexcitation could be achieved with single and double extrastimuli. A formula was derived for obtaining a preexcitation index with double extrastimuli and shown to correspond closely with the preexcitation index obtained with a single extrastimulus in the 16 patients in whom atrial preexcitation could be achieved with single and double extrastimuli. Thus, this technique significantly enhances the ability to achieve atrial preexcitation and to calculate the preexcitation index in patients with AVNRT, and thus may be useful in deciphering tachycardia mechanism in some patients, as well as being a useful technique in studying the electrophysiological properties of the antegrade and retrograde limbs of AVNRT.