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Europace (2008) 10, 1428–1433
doi:10.1093/europace/eun283
Electrophysiology study and radiofrequency catheter
ablation of atriofascicular tracts with decremental
properties (Mahaim fibre) at the tricuspid annulus
Shomu Bohora, Santosh Kumar Dora*, Narayanan Namboodiri, Ajitkumar Valaparambil,
and Jaganmohan Tharakan
Department of Cardiology, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Trivandrum,
Kerala 695011, India
Received 5 April 2008; accepted after revision 21 September 2008; online publish-ahead-of-print 19 October 2008
KEYWORDS
Mahaim fibre;
Electrophysiology study;
Radiofrequency ablation
Aims The aim was to study the common mapping methods for Mahaim fibre and their role in radiofrequency (RF) ablation.
Methods and results Fifteen patients having Mahaim fibre tachycardia underwent electrophysiological
study. Mahaim fibre mapping methods like (i) Mahaim potential (M), (ii) shortest atrial stimulus-to-preexcitation (STP), and (iii) mechanical trauma induced loss of conduction were studied. Accessory
pathway mapping was performed by M potential in 10 patients (67%), shortest atrial STP in 3 patients
(20%), and mechanical trauma in 2 patients (13%). Mahaim fibre was localized at right atrial freewall
of tricuspid annulus (8–10 o’clock) in 13 patients (87%), at 6.30 o’clock in one patient, and at
5 o’clock in 1 patient. Fourteen patients underwent RF ablation. Thirteen patients had complete loss
of conduction over accessory pathway and one had partial modification with a conduction delay. Radiofrequency ablation was not performed in one patient (shortest STP group) due to its closeness to the
compact atrioventricular node. Mahaim junctional acceleration during RF ablation was observed in all
patients of M potential, 1 patient of mechanical trauma, and none of the atrial STP group. One
patient (M potential group) had tachycardia recurrence during follow-up.
Conclusion Mahaim fibre is commonly located between 8 and 10 o’clock at tricuspid annulus. M potential guides to successful RF ablation in most patients. Mahaim junctional acceleration is commonly seen
during RF ablation guided by M potential map.
Introduction
Methods
Mahaim fibre leads to wide QRS tachycardia and was first
described by Mahaim and Benatt.1 It is associated with the
characteristic
electrophysiological
(EP)
features.2–7
Although initially the Mahaim fibre was thought to be nodofascicular, later studies showed it to be atriofascicular8–17
pathway. Various mapping techniques have been described
in the literature to localize the Mahaim fibre. We tried to
evaluate the usefulness of these mapping techniques with
regard to the results of radiofrequency (RF) ablation.
All consecutive cases of Mahaim fibre tachycardia who underwent EP
study from January 2002 to June 2007 in our centre were included in
the study. Various EP properties of Mahaim fibre, techniques used to
localize it, results of catheter ablation, and follow-up data were
analysed.
Aim of the study
The aim was to study different mapping methods and their
role in RF ablation outcome in Mahaim fibre tachycardia.
* Corresponding author. Tel: þ91 471 2524466; fax: þ91 471 2446433.
E-mail address: [email protected]
EP study
During the study period, of the 510 patients who underwent EP
study with the intention of RF ablation, 15 were found to have
accessory pathway with the characteristics of Mahaim fibre. As
per the standard protocol of our institute, quadripolar catheters
were placed at right ventricular (RV) apex, His bundle (HB), and
right atrium. A decapolar catheter was positioned in the coronary
sinus via right internal jugular vein. The RV catheter was used to
record right bundle (RB) from either the proximal or distal poles.
Programmed atrial stimulation was performed to determine the
antegrade conduction pattern to ventricle. The refractory periods
of atrioventricular (AV) node and accessory pathway were noted.
Published on behalf of the European Society of Cardiology. All rights reserved. & The Author 2008.
For permissions please email: [email protected].
Mapping methods for Mahaim fibre and their role in RF ablation
Programmed stimulation was performed at RV apex to determine
the retrograde conduction pattern to the atrium. The relationship
of M potential, RB, and HB was studied during sinus rhythm and
tachycardia.
The criteria for the presence of Mahaim fibre conduction included
(i) baseline normal QRS or manifest pre-excitation with left bundle
branch block morphology, (ii) programmed atrial pacing leading to
manifest pre-excitation and increase in AV interval along with shortening of HV interval at shorter pacing cycle lengths, (iii) absence of
retrograde conduction over the accessory pathway during ventricular stimulation, and (iv) RB electrogram preceding HB activation
during antegrade pre-excitation and SVT.
Mapping and RF ablation
Once the presence of Mahaim accessory pathway was confirmed by
the EP properties described above, the mapping was performed to
localize it using a 4 mm deflectable tip ablation catheter (Stinger,
CR Bard Inc., MA, USA). Mapping was initiated at the lateral tricuspid annulus due to its common location at that region. The tricuspid
annulus was mapped during sinus rhythm in left anterior oblique and
right anterior oblique fluoroscopy projections. Three mapping strategies were adopted with the following order of preference: (i) M
potential, which is defined as a discrete deflection between atrial
and ventricular signal with the interval between the M potential
and ventricular electrogram remaining constant during the AV
delay produced by atrial pacing, (ii) shortest stimulus-to-preexcitation (STP) interval defined by the shortest interval from a
paced atrial site to the pre-excited QRS, and (iii) mechanical
trauma induced loss of conduction over Mahaim fibre. Long sheath
(SR-0, Daig, St Jude Medical, MN, USA) was routinely used in all
cases to facilitate the stability of the ablation catheter. A Bard EP
recording system was used for all the procedures (CR Bard Inc.).
After localizing M potential, RF energy was delivered at that site.
Other strategies of localizing the Mahaim fibre were attempted in
cases of failure to obtain the M potential or unsuccessful ablation
at the site of best presumed M potential despite the delivery of
optimal energy. The RF generator used was from either of the two
manufacturers (EPT-1000, Boston Scientific, MA, USA or
IBI-1500T11, St Jude Medical, MN, USA). A stimulation study was
performed after RF ablation to establish the absence of antegrade
conduction over the Mahaim fibre and non-inducibility of the
tachycardia.
Results
Of the 510 patients who underwent EP study during the
study period, 15 were found to have Mahaim fibre mediated
tachycardia. Baseline characteristics of these patients are
mentioned in Table 1. Of them, 11 patients had tachycardia
in spite of good compliance to medications. Fourteen
patients had a documented wide QRS tachycardia of left
bundle branch morphology. Two had associated Ebstein’s
anomaly of the tricuspid valve, whereas others had structurally normal heart. The QRS axis during the tachycardia
varied from þ45 to 260º.
Table 2 shows the EP characteristics of the patients
studied. Atrial pacing protocol showed manifest preexcitation with decremental AV conduction in all patients.
Ventricular pacing showed retrograde decremental conduction via the AV node. During absence of pre-excitation, the
HV interval was positive, whereas during pre-excitation it
was negative (Figure 1). The relationship of M potential,
HB and RB during sinus rhythm with preexcitation has been
shown (Figure 2). The tachycardia could be induced in 13
patients, and in all the cases, QRS morphology during tachycardia was same as pre-excited QRS during atrial pacing. The
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Table 1 Baseline characteristics
Number of patients
Age
Clinical features
Palpitation
Syncope
Duration of symptoms
Associated structural heart disease
Ebstein’s anomaly
Drugs
Calcium channel blocker
Betablocker
Amiodarone
Combination of two or more
No drugs
ECG
Minimal pre-excitation during sinus
rhythm
No pre-excitation
ECG documented tachycardia
Tachycardia rate
QRS axis during tachycardia
15:7 males and 8
females
12–44 years
15
2
0.2–20 years
2
11
4
3
4
1
9
6
14
183.53 + 18.53/min
(156–220)
þ458 to 2608
Table 2 Electrophysiological characteristics
Sinus cycle length
AH interval
HV interval
Antegrade refractory period
AV node
Mahaim fibre
SVT cycle length
Mahaim fibre localization
8–10 o’clock
5 o’clock
6.30 o’clock
Other associated tachycardias
AVNRT
Atrial flutter
818.8 + 130 (548–1032) ms
82.9 + 16.1 (58–120) ms
41.1 + 6.72 (30–54) ms
297 + 70.72 (210–420) ms
296.25 + 47.79 (230–360) ms
329.73 + 71.86 (262-440) ms
13
1
1
2
2
M potential could be mapped in 10 patients, shortest STP in
3 patients, and catheter trauma induced loss of preexcitation in 2 patients (Figure 2). The Mahaim fibre was
localized between 8 and 10 o’clock of tricuspid annulus in
13 patients and at 6.30 and 5 o’clock in one each
(Figure 3). Radiofrequency ablation was delivered in
temperature-controlled mode at a target temperature of
558C and power output of 30 W in all the cases. The ablation
time ranged from 120 to 500 (180 + 28) s.
All patients in the M potential group had successful RF
ablation. However, one patient had recurrence of tachycardia after 2 months, who underwent M-potential-guided successful ablation in another sitting. All other patients of this
group remained free of tachycardia at follow-up. In the
shortest STP group, one patient had successful RF ablation.
Another had partial modification of Mahaim fibre conduction
following RF ablation giving rise to prolonged antegrade conduction time. In this patient, the tachycardia was no more
inducible. Mahaim fibre was localized close to the compact
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S. Bohora et al.
Figure 1 (A) Burst atrial pacing shows pre-excited QRS. The right bundle (RB) precedes His bundle (HB). The last beat shows a sinus beat
with narrow QRS. Note that the HB precedes RB. (B) Wide QRS tachycardia with left bundle branch block morphology similar to pre-excited
QRS as seen in A. Note that the RB precedes HB. HRAD, high right atrium distal; HBED, His bundle electrogram distal; CSP, coronary sinus
proximal dipole; and CSD, coronary sinus distal dipole.
Figure 2 During sinus rhythm with wide QRS M precedes right bundle (RB), which in turn precedes His bundle (HB), indicating retrograde
activation of the proximal part of RB up to HB. RFD, radiofrequency distal. All other abbreviations are same as Figure 1.
Mapping methods for Mahaim fibre and their role in RF ablation
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Figure 3 Solid circles show the sites where Mahaim fibre was located at the tricuspid annulus.
Figure 4 Radiofrequency ablation at the site of Mahaim fibre location resulting transient Mahaim junctional acceleration. The QRS morphology is similar to Mahaim tachycardia. HRAP, high right atrium proximal. All other abbreviations are same as Figure 2.
AV node in one patient and thus ablation was not attempted.
Two patients had intermittent loss of pre-excitation on
mechanical trauma during catheter manipulation at the
lateral tricuspid annulus. Both had successful RF ablation
at the same site. Radiofrequency ablation resulted in
Mahaim junctional acceleration (MJA) in all the patients of
the M potential group and in one of the two patients who
had localization by ‘bump mapping’ (Figure 4). Time to
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S. Bohora et al.
junctional acceleration during RF ablation ranged from 3 to
25 (7 + 3.5) s. None of the shortest STP group had MJA
during RF ablation.
Programmed atrial stimulation showed dual AV node physiology and inducible AV re-entrant tachycardia (AVNRT) in
two patients. Both of them underwent successful slow
pathway ablation. Typical atrial flutter was inducible in
two patients, of whom one underwent successful cavotricuspid isthmus ablation. The other patient with atrial flutter,
had 2:1 AV conduction over the parahisian accessory
pathway with Mahaim fibre like properties, had Ebstein’s
anomaly and hence ablation of the isthmus or accessory
pathway was not attempted.
The patients had a mean follow-up of 18 + 5.4 months.
There was only one recurrence of tachycardia in the
M-potential-guided RF ablation group as mentioned above.
All other patients remained free of symptoms at follow-up.
the view that MJA during ablation is a dependable marker
of successful ablation.
Concomitant presence of other types of tachyarrhythmias
in patients with Mahaim fibre-mediated tachycardia has
been described in the literature.4,13,16–19 Our study
showed typical AVNRT and typical atrial flutter in two
patients each. No previous cases of co-existing atrial
flutter and Mahaim fibre have been reported.
Discussion
Conclusion
The EP property of the Mahaim fibre is characterized by
decremental antegrade conduction over the accessory
pathway.3,4,6,10 All our patients had EP properties consistent
with Mahaim fibre conduction. There are several reports on
ablation of Mahaim accessory pathway with a high success
rate.4,7,12–19
Various ablation strategies are described in the literature.
The recording of M Potential is associated with high degree
of acute procedural success.4,7,15–19 In our study, M potential
could be recorded in 67% of patients and RF ablation
resulted in abolition of conduction over accessory pathway
in all. M potential remains an easy and effective way of
mapping and ablating the Mahaim fibre.
Shortest STP as a mapping method for Mahaim fibre localization has remained inconsistent in the literature.12,17–19
This method involves stimulation at many locations of tricuspid annulus and is time consuming. Mahaim fibre was localized by this method in three of our patients one of whom
had an RF ablation resulting in partial modification, with
prolonged antegrade conduction. However, in the absence
of a proper M potential, this method remains helpful in localizing Mahaim fibre.
Mahaim fibre is located very close to the endocardium and
thus catheter movement-related mechanical trauma resulting in transient loss of conduction is not uncommon. Mapping
and RF ablation guided by this method has occasionally been
useful. Cappato et al.14 showed a very high immediate
success rate with this method but there was high recurrence
at follow-up. This can be due to delivery of RF ablation at a
wrong site leading to high recurrence at follow-up. Unintentional mechanical trauma during catheter positioning
sometimes results in abolition of conduction over the
pathway for minutes to hours.12,17 In our study, two patients
underwent successful RF ablation with this method with no
recurrence noted on follow-up. However, this should still
be the least preferred method of RF ablation in Mahaim
fibre.
Radiofrequency ablation of Mahaim fibre often gives rise
to MJA, which is considered as a marker of successful
ablation.4,13,18,19 Twelve of our patients had MJA, of which
only one had recurrence that was successfully ablated
again with the same approach. Our study also confirms to
M potential can be mapped at the lateral tricuspid annulus
in majority of patients having Mahaim fibre tachycardia,
which can guide to successful RF ablation. This approach
often results in MJA compared with other approaches for
ablation and reliably predicts long-term procedural
outcome. Coexistence of other tachyarrhythmias is not
uncommon.
Limitations
This is an observational and retrospective study. However,
we included all consecutive cases of Mahaim fibre mediated
tachycardia undergoing EP study since January 2002. This
study consists of only 15 patients which is a small number
but compares well with many other studies published in
the literature. This is a single centre study and Mahaim
fibre mediated tachycardia is relatively rare.
Acknowledgements
We are grateful to Mr K. Suji and Ms Setu, technical assistants, EP
lab, for their assistance during study of these cases.
Conflict of interest: none declared.
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