738

Automaticity in Mahaim FibersEDUARDO BACK STERNICK, M.D., EDUARDO A. SOSA, M.D.,∗ CARL TIMMERMANS, M.D.,†

FERNANDO E. CRUZ FILHO, M.D.,‡ LUZ-MARIA RODRIGUEZ, M.D.,†LUIZ M. GERKEN, M.D., MAURICIO I. SCANAVACCA, M.D.,∗ MARCIO L. FAGUNDES, M.D.,‡

SAVIA C. BUENO, M.D.,∗ MARIO O. VRANDECIC, M.D., and HEIN J.J. WELLENS, M.D.†From Biocor Instituto, Belo Horizonte, Brazil; ∗Instituto do Coracao da Faculdade de Medicina da Universidade de Sao Paulo, Brazil;

†University Hospital, Maastricht, The Netherlands; and ‡Instituto Nacional de Cardiologia Laranjeiras, Ministerio da Saude,Rio de Janeiro, Brazil

Automaticity in Mahaim Fibers. Introduction: Automatic rhythms associated with Mahaim fibersusually occur during radiofrequency catheter ablation. The incidence and significance of spontaneousautomaticity in Mahaim fibers are unknown.

Methods and Results: Spontaneous automatic rhythms were observed in 5 (12.5%) of 40 patients withMahaim fibers referred for nonpharmacologic therapy because of recurrent episodes of symptomatic tach-yarrhythmias, usually antidromic circus movement tachycardia (33/40 patients). Three were female and twowere male. Their mean age was 15 ± 7 years compared to 26 ± 13 years of the patients without automatic-ity (P = 0.09). Three patients had both antidromic tachycardia and asymptomatic spontaneous automaticrhythms recorded during ambulatory ECG (1 patient) or electrophysiologic study (2 patients). In 2 patients,the automatic rhythm triggered antidromic tachycardia. Two other patients had nonsustained repetitiveepisodes of wide QRS tachycardia due to automaticity arising in the Mahaim fiber, without antidromictachycardia. All automatic rhythms were abolished by successful catheter ablation of the Mahaim fibers.

Conclusion: Spontaneous automaticity occurred in 12.5% of our Mahaim patients and may triggerantidromic tachycardia. Spontaneous automaticity, which is not seen in rapidly conducting accessory path-ways, is another argument for the presence of an AV nodal-like structure in Mahaim fibers. (J CardiovascElectrophysiol, Vol. 15, pp. 738-744, July 2004)

Mahaim fiber, atriofascicular pathway, automaticity, accelerated junctional rhythm, slow and decrementalconduction, catheter ablation

Introduction

Automatic impulse formation originates in cells withspontaneous diastolic depolarization. Those cells are foundin the sinus node, AV node, coronary sinus ostium, andcrista terminalis. Despite the lack of conclusive anatomic-pathologic data,1 there are strong electrophysiologic argu-ments suggesting that Mahaim fibers contain accessory AVnodal tissue such as (1) slow and decremental anterogradeconduction,2 (2) a structure with a proximal AV nodal compo-nent and a distal bundle branch-like component, both with Hisbundle-like potentials,3,4 (3) conduction block in responseto adenosine,5 and (4) heat-induced automaticity during ra-diofrequency catheter ablation.6 According to the literature,spontaneous automaticity arising in rapidly conducting ac-cessory pathways is a rare finding.7,8 Spontaneous activityarising in Mahaim fiber has been reported only twice,9,10

and its significance is unknown. The aim of this study wasto describe spontaneous Mahaim automaticity in a cohort of40 patients with Mahaim fibers.

Address for correspondence: Eduardo Back Sternick, M.D., RuaCorreias 281/301, Zip 30315-340, Belo Horizonte, Minas Gerais, Brazil.Fax: 553132895273; E-mail: eduardosternick@aol.com

Manuscript received 5 November 2003; Accepted for publication 14 January2004.

doi: 10.1046/j.1540-8167.2004.03615.x

Methods

Definitions

Every patient showed a decrementally conducting AV by-pass tract with progressive AV and AH interval prolongationcoupled with a decreasing HV interval leading to a greaterdegree of ventricular preexcitation with a left bundle branchblock (LBBB)-like morphology during atrial pacing.2 TheHis-bundle deflection was inscribed after the right bundle de-flection during maximal preexcitation. At maximal ventricu-lar preexcitation, there was a constant QRS-His relationshipwithout further changes on shortening the atrial pacing cyclelength.

Study Population

We retrospectively studied 40 consecutive patients fromfour institutions who had accessory pathways with long an-terograde conduction times and decremental properties dur-ing sinus rhythm, with tachycardia with anterograde conduc-tion over the decremental pathway, and in whom ablation ofthe accessory pathway was performed.

There were 24 females and 16 males (mean age 24 ±12 years, range 8–80). Referral for electrophysiologic as-sessment was because of a preexcited tachycardia over theMahaim fiber in 33 patients (Table 1). Ebstein disease wasdiagnosed in 8 patients (20%). The atrial insertion of the fiberwas identified by localizing a discrete accessory pathwaypotential. In 32 patients and by assessing the shortest A-V

Sternick et al. Automaticity in Mahaim Fibers 739

TABLE 1

Clinical Data of Patients with Mahaim Fibers

Case No. Sex Age (years) Site Clinical Arrhythmia Therapy MAT WPW CBT

1 F 31 L Antidromic CMT/Mahaim S2 F 32 L Antidromic CMT/Mahaim; AVNRT RFp Yes3∗ F 19 PL Antidromic CMT/Mahaim RFp Yes4∗ F 21 PL Antidromic CMT/Mahaim RFp Yes RPS

RFp Orthodromic CMT5 M 13 PL Antidromic CMT/Mahaim RFp Yes RPS6 F 52 PL Preexcited atrial fibrillation RFp Yes7∗ F 19 PS Antidromic CMT/RAS AP RFp No RAS8 F 22 P Fast palpitations RFp Yes9 F 21 L Antidromic CMT/Mahaim RFp Yes10 F 23 L Antidromic CMT/Mahaim RFp Yes11 M 80 PL AVNRT + Mahaim bystander RFp Yes RPS12 F 19 A Antidromic CMT/Mahaim RFp Yes13 F 23 L Antidromic CMT/Mahaim RFp Yes14 F 25 PL Antidromic CMT/Mahaim RFp Yes15 M 35 L Antidromic CMT/Mahaim RFp Yes16 F 42 L Antidromic CMT/Mahaim RFp Yes17 F 23 L Antidromic CMT/Mahaim RFp Yes18 F 8 AL Mahaim automaticity RFp Yes19 M 30 PL Antidromic CMT/Mahaim RFp Yes20 F 27 L Antidromic CMT/Mahaim RFp Yes21 F 19 L Antidromic CMT/Mahaim RFp Yes22 M 12 MS Mahaim automaticity RFp No23 F 39 L Antidromic CMT/Mahaim RFp Yes24∗ F 12 L Antidromic CMT/Mahaim RFp No RL25 F 15 AL AVNRT + Mahaim bystander RFp Yes26 M 13 L Antidromic CMT/Mahaim RFp Yes27 M 15 A Antidromic CMT/Mahaim RFp Yes28 F 25 L Antidromic CMT/Mahaim RFp Yes29∗ M 17 L Antidromic CMT/Mahaim RFp Yes RPS RL

Orthodromic CMT30 M 18 L Antidromic CMT/Mahaim RFp Yes31 F 45 L Antidromic CMT/Mahaim RFp Yes32 M 24 L Antidromic CMT/Mahaim RFd33 M 11 L Antidromic CMT/Mahaim S34∗ M 28 L Antidromic CMT/Mahaim RFd LL35 M 25 A Antidromic CMT/Mahaim RFd36 F 26 P Antidromic CMT/Mahaim RFd37 F 31 L Antidromic CMT/Mahaim RFd38 F 22 L Antidromic CMT/Mahaim; AVNRT RFp Yes39 F 17 L Antidromic CMT/Mahaim RFp Yes40 M 17 P Antidromic CMT/Mahaim RFp Yes

Bold indicates patients with spontaneous automaticity.∗Ebstein disease.Antidromic CMT/Mahaim = antidromic circus movement tachycardia with anterograde conduction over the Mahaim fiber; CBT = concealed bypasstract; MAT = automatic tachycardia during radiofrequency ablation; RAS AP = right anteroseptal accessory pathway; RFd = targeting distal insertion;RFp = radiofrequency catheter ablation targeting proximal Mahaim potential at the annulus; S = surgical ablation; Site = tricuspid annulus (A—anterior,AL—anterolateral, L—lateral, MS—midseptal, P—posterior, PL—posterolateral; PS—posteroseptal); WPW = Wolff-Parkinson-White syndrome.

interval during atrial pacing at different sites in 8 patients.All patients underwent successful surgical (n = 2) or ra-diofrequency catheter ablation (n = 38). RF ablation wasperformed from the atrial end in 32 patients and from theventricular end after right ventricular pace mapping in 6 pa-tients. Five patients (cases 9, 18, 22, 33, and 36) showedspontaneous automaticity (Table 1).

Statistical Analysis

Values are given as mean ± SD. The significance of differ-ences (P < 0.05) between groups of clinical, electrocardio-graphic, or electrophysiologic parameters was assessed byStudent’s t-test or Fisher’s exact test.

Results

Spontaneous Automaticity

The five patients with spontaneous automaticity wereyounger than the 35 patients without spontaneous automatic-ity (15 ± 7 years vs 26 ± 13 years), but the difference didnot reach statistical significance (P = 0.09). Three of thesepatients were male, and one had Ebstein disease. None ofthe five patients had an associated rapidly conducting acces-sory pathway. Although the number of patients is too smallfor comparison, clinical presentation with antidromic tachy-cardia did not differ among Mahaim patients without spon-taneous automaticity (82%) and the 60% (3/5) of patientswith spontaneous automaticity. In the 5 patients (12.5%),

740 Journal of Cardiovascular Electrophysiology Vol. 15, No. 7, July 2004

TABLE 2

Electrophysiologic Data of Patients with Spontaneous Automaticity

Clinical AM Cycle Length Diagnosis a-CMT Isuprel VA Conductionn Case No. Arrhythmia Range (ms) At by AM Acceleration (Mahaim/AV Node) MAT

1 18 Automatic runs 550–450 ECG ∗ No/no Yes2 22 Automatic runs 360–400 ECG ∗ No/yes No3 9 a-CMT 720–580 EPS Yes ∗∗∗ No/yes Yes4 33 a-CMT 840–550 Holter∗/EPS Yes ∗∗∗ No/yes5 36 a-CMT 900–530 Holter∗/EPS ∗∗ No/yes

a-CMT = antidromic circus movement tachycardia; AM = automaticity in Mahaim fiber; MAT = Mahaim automatic tachycardia during radiofrequencyablation of the atrial insertion; EPS = electrophysiologic study; Holter∗ = accelerated idioventricular rhythm was suggested as the diagnosis before EPS.

spontaneous automatic rhythms were found to have the sameQRS complex configuration as observed during maximal pre-excitation. Mahaim automaticity rates ranged from a slowrhythm occurring during the night, indistinguishable froman accelerated idioventricular rhythm (cycle length 900 ms)to nonsustained repetitive tachycardia with a cycle length of360 ms (Table 2). Three patients referred because of an an-tidromic tachycardia with anterograde conduction over theMahaim fiber also showed spontaneous Mahaim automatic-ity. QRS complex configuration was identical during the reen-trant and the automatic rhythm (Fig. 1). In one patient (case36), episodes of asymptomatic slow automatic rhythm re-sembling accelerated idioventricular rhythm occurred at aslower rate during the nighttime and at a faster rate duringdaytime (Fig. 2). In two patients (cases 18 and 22), the clini-cal problem was automaticity. These patients sought medicaltreatment because of a long history of nonsustained palpita-tions. Both patients showed repetitive bursts of nonsustainedtachycardia with an LBBB configuration. There was VA dis-

Figure 1. Case 9. Left: Automatic rhythm from a right lateral Mahaim fiber with an LBBB-like morphology (QRS width 0.13 s) and cycle length of 680 mswith two sinus captures (beats 5 and 6) and a fusion beat (seventh QRS complex). Right: Antidromic circus movement tachycardia showing the same QRScomplex as during the automatic rhythm.

sociation during the majority of the episodes, but in case18 some tachycardias showed 2:1 retrograde VA conductionover the AV node.

Automatic rhythm was recorded in all patients duringelectrophysiologic study before the administration of isopro-terenol. During isoproterenol infusion, there was an increasein both the rate and the duration of the episodes. The ef-fect of isoproterenol was less pronounced in the two patientswith repetitive nonsustained automatic runs. In both of thesepatients, there was only a slight increase in automaticity rate.

Electrophysiologic validation of the site of origin of thisautomatic rhythm included the recording of a M potentialpreceding the preexcited QRS complex and retrograde de-polarization of the right bundle followed by the His bundle.In patient 9 (Fig. 3), there was VA dissociation during theautomatic rhythm.

During atrial overdrive stimulation, there was a transientsuppression without entrainment, acceleration, or persistenttermination of the tachycardia runs in cases 18 and 22.

Sternick et al. Automaticity in Mahaim Fibers 741

Figure 2. Case 36. Left: Twelve-lead ECG showing sinus rhythm and Mahaim automaticity at 60 beats/min (third QRS complex is a fusion beat). Right:During isoproterenol infusion, both sinus rhythm (580 ms) and Mahaim automaticity (640 ms) accelerate (fourth QRS complex is a fusion beat).

Following stimulation, tachycardia resumed almost imme-diately in cases 18 and 22.

Induction of Antidromic Circus MovementTachycardia by Automatic Rhythm

In two patients (cases 9 and 33), the automatic rhythm trig-gered episodes of antidromic tachycardia (Fig. 4). It occurred

Figure 3. Case 9. Registration during the electrophysiologic study at 100 mm/s showing an automatic rhythm with LBBB-like morphology. A Mahaimpotential (M) precedes each QRS (MV interval 50 ms). There is retrograde activation of the distal conduction system (right bundle followed by the His-bundlepotential) but without retrograde conduction to the atrial level, where the rhythm is clearly dissociated from that in the ventricle.

in both cases during isoproterenol infusion. Initiation of an-tidromic tachycardia was preceded by a long RR cycle dueto AV dissociation caused by automatic beats arising in theMahaim fiber. The preceding P wave, which was not con-ducted through the AV node, was conducted with a criticalslowing in the Mahaim fiber (Fig. 4). Isoproterenol improvedretrograde conduction over the AV node enabling antidromiccircus movement tachycardia to become sustained, but the

742 Journal of Cardiovascular Electrophysiology Vol. 15, No. 7, July 2004

Figure 4. Case 33. Twelve-lead ECG showing automatic beats among minimally preexcited sinus beats. Antidromic circus movement tachycardia is triggeredby an automatic beat with retrograde VA conduction through the AV node.

mode of tachycardia induction was not related to the VAconduction itself or to slowing of the sinus rate.

Heat-Induced Automaticity

Automaticity during ablation at the proximalinsertion of the Mahaim fiber

Mahaim automatic tachycardia was induced during ra-diofrequency catheter ablation in 30 (90%) of 33 patientswhen catheter ablation was targeted at the proximal Mahaimpotential at the tricuspid annulus. Mahaim automatic tachy-cardia usually started promptly after current delivery (usuallyin <5 seconds). Mahaim automatic tachycardia duration wasvariable, lasting from four beats to almost 2 minutes (Fig. 5).Its occurrence could not be prevented by atrial pacing duringcatheter ablation. In four patients with prolonged Mahaim au-tomatic tachycardia, successful ablation was only achievedafter complete elimination of that rhythm (Fig. 5B).

Automaticity During Ablation at the DistalInsertion of the Mahaim Fiber

We did not include in these results the five patients whowere ablated targeting the distal insertion (Table 1). These pa-tients did develop automaticity, but because there were slightdifferences in QRS configuration, we could not rule out thatthe rhythm was arising in the distal part of the right bundlebranch or ventricular myocardium.

Discussion

Early investigators suggested that Mahaim fibers could bea complete accessory AV conduction pathway with variable

length containing AV nodal-like tissue.1,3,4,11,12 Intracardiacelectrophysiologic testing has been the main tool to solvethe Mahaim puzzle. Transient accelerated automatic rhythmsarising from the Mahaim fiber during radiofrequency catheterablation6,13,14 have been recognized as one of the features thatsuch fibers share with AV nodal tissue. Those rhythms areanalogous to the accelerated junctional rhythm (AJR) aris-ing during slow AV nodal pathway ablation in patients withAV nodal reentrant tachycardia (AVNRT). AJR is consideredto be a sensitive marker of a successful ablation site.15 Itseems that heat-induced automaticity is as common duringablation of Mahaim fibers as during slow AV nodal pathwayablation; it was observed in 30 (91%) of 33 patients whenthe atrial insertion was targeted. Other investigators such asBraun et al.14 reported similar figures (15/15 cases [100%]),whereas Heald et al.13 reported 57% (12/21 patients).

We found that complete termination of heat-induced auto-maticity may be required as an ablation endpoint to achievelong-term success in four patients.6

Spontaneous automaticity arising in a Mahaim fiber wasfirst reported by Kanter et al.16 in a 7-year-old girl with anincessant bigeminy initially thought to be of ventricular ori-gin. She underwent electrophysiologic evaluation because24-hour Holter monitoring showed nonsustained runs sug-gestive of ventricular tachycardia. An atriofascicular path-way was diagnosed during electrophysiologic study. Sosaet al.9 reported a patient with symptomatic runs of an irregu-lar LBBB-like tachycardia with AV dissociation, most likelydue to spontaneous automaticity. Belhassen et al.10 reportedone patient with a likely automatic escape rhythm arising inan atriofascicular pathway. It was remarkable to find 12.5%spontaneous automaticity in this large cohort of 40 patients

Sternick et al. Automaticity in Mahaim Fibers 743

Figure 5. A: Case 13. A short episode of Mahaim automaticity consisting of four beats with LBBB-like morphology. Minimal ventricular preexcitation ispresent before (see “rS” pattern in lead III) and absent thereafter (“qR” in lead III). B: Case 9. The episode of Mahaim automaticity starts immediately afterRF current delivery (∗) lasting almost 1 minute. After Mahaim automaticity was completely terminated, both Mahaim conduction and Mahaim automaticitydid not recur.

with Mahaim fibers. These young patients presented with aspectrum of automatic rhythms ranging from asymptomaticslow rhythms to fast and repetitive bursts of tachycardia withLBBB-like morphology. This is another characteristic sharedwith the AV node.17 There are a few reports about young pa-tients with AVNRT showing spontaneous AJR, which canbe cured by slow pathway modification with radiofrequencycatheter ablation.18-20 Epstein et al.20 found that AJR servedas trigger for AVNRT, particularly with AJR at an acceler-ated rate, in 3 of 5 patients. We had the same observation in 2of 5 patients who had antidromic circus movement tachycar-dia triggered by automatic beats. They reported an incidenceof 17% (5/29 patients) of spontaneous episodic AJR in apopulation with a mean age of 12.9 ± 5.2 (range 4.2–25)years, which is similar to our study population with spon-taneous automaticity having a mean age of 15 ± 7 years(range 8–26). We also found that spontaneous automaticitywas terminated after successful catheter ablation, matchingthe observation by Epstein’s group that, in their patients withAJR and AVNRT, it was necessary to eliminate or drasticallymodulate AJR in order to achieve a long-term successfulresult.

The likely mechanism of spontaneous rhythms seen incases 9, 33, and 36 was normal automaticity. In these patients,Mahaim automaticity was dependent on slowing of the sinusrate (“Mahaim escape rhythm”) (Fig. 2). Isoproterenol in-fusion increased both sinus and Mahaim automaticity, but inthese patients a greater increase in the discharge rate was seenat the Mahaim fiber level (“Mahaim active rhythm”). Patients18 and 22 had an incessant repetitive nonsustained tachycar-dia that could not be terminated or induced by programmedelectrical stimulation. Isoproterenol infusion in these patients

caused only a slight increase in the discharge rate. The find-ings suggest an abnormal automaticity as the mechanism oftheir clinical tachycardia.

Conclusion

Mahaim automaticity occurring during ablation of itsatrial insertion is a common event, and its complete termina-tion may be required in some patients to achieve long-termsuccess.

Spontaneous automaticity arising in the Mahaim fiber canserve as a trigger for antidromic circus movement tachycardiaand can be a major clinical problem. It is another character-istic shared with the electrophysiologic behavior of the AVnode.

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