Diagnostic Electrophysiology & Ablation

Mahaim Accessory Pathways

Demosthenes G Katritsis, 1 Hein J Wellens, 2 Mark E Josephson 3

1. Athens Euroclinic, Athens, Greece, 2. Cardiovascular Research Institute, Maastricht, the Netherlands,
3. Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, USA

Abstract
The term Mahaim conduction is conventionally used to describe decrementally conducting connections between the right atrium or the
AV node and the right ventricle in or close to the right bundle branch. Although such pathways are rare, their unique properties make
their diagnosis and treatment cumbersome. In this article we review the published evidence, and discuss the electrocardiographic and
electrophysiological characteristics as well as the anatomy and origin of these fibres.

Keywords
Mahaim; accessory; pathway

Disclosure: The authors have no conflicts of interest to declare.

Received: 7 November 2016 Accepted: 2 March 2017 Citation: Arrhythmia & Electrophysiology Review 2017;6(1):29–32. DOI: 10.15420 /aer.2016:35:1
Correspondence: Dr DG Katritsis, Athens Euroclinic, 9 Ahanassiadou Street, Athens 11521, Greece. E: dkatrits@dgkatritsis.gr

In 1941, Mahaim and Winston described the histology of anomalous ECG during tachycardia
connections that arise from the AV node and insert into the right Although, typically, antidromic atrioventricular reentrant tachycardia
ventricle.1 This was the first description of nodoventricular or so-called over a Mahaim fibre has a left bundle branch (LBBB) morphology (see
Mahaim accessory pathways. Accessory pathways with decremental Figure 2), various QRS patterns and axis may occur (see Figure 3). 6,7 It
conduction properties that connected the atrium to the right bundle seems that these pathways insert into or near the RBB, and variations in
branch (RBB) were subsequently mapped mainly at the lateral aspect the frontal plane axis can be explained by the location of the exit of the
of the tricuspid annulus, and thus the term atriofascicular was also RBB and a variable degree of fusion of ventricular activation between
adopted.2-4 We know now that decrementally conducting connections anterograde conduction over the pathway and, following retrograde
can be between the right atrium or the AV node and the right ventricle invasion into the RBB, partial anterograde left ventricular activation
in or close to the RBB.5-7 Thus, although they are anatomically distinct over the left-sided conduction system, especially the anterior fascicle
from the initially described nodoventricular pathway, they present with (see Figure 4). The various QRS patterns and rate changes seen during
similar electrocardiographic and electrophysiological characteristics the change from short to long V-A tachycardia can be explained by
and the term ‘Mahaim’ has been adopted to describe pathways with the mode of retrograde conduction over the bundle branch system.6,7
the following features:
Electrophysiological Properties
• Baseline
 normal QRS or different degrees of manifest pre- Nodoventricular or Atriofascicular?
excitation with left bundle branch block morphology; Although the first case of this arrhythmia was studied
• Programmed
 atrial pacing leading to obvious manifest pre- electrophysiologically by Wellens and published in 1971, and considered
excitation following an increase in A-V interval along with to be based on a nodo-ventricular pathway,11 with the advent of surgical
shortening of H-V interval at shorter pacing cycle lengths; and and then catheter ablation in the 1980s, it was discovered that most
• Right
 bundle electrogram preceding His bundle activation during fibres with Mahaim conduction characteristics originated at the lateral
anterograde pre-excitation and supraventricular tachycardia (SVT). aspect of the tricuspid annulus, and the term atriofascicular pathway
was adopted.2-5,12-17 However, posteroseptal locations could also be
Although such pathways are rare, their unique properties as well as found,7,14 and true nodoventricular fibres have been identified (see
the still unanswered questions about their true nature, make them Figure 5).1,14,16 In addition, some of these pathways, so-called ‘short’
particularly interesting from an electrophysiological point of view.8 as opposed to ‘long’ Mahaims, may insert at the ventricle near rather
than in the RBB.5-7,18 Thus, pathways with Mahaim characteristics can
Electrocardiographic Features be atriofascicular, atrioventricular, nodofascicular and nodoventricular,
ECG During Sinus Rhythm depending on their variable proximal and distal insertions.
During sinus rhythm overt pre-excitation is usually absent (see
Figure 1). Only subtle ECG abnormalities, such as an rS pattern in Origin
lead III, absence of septal Q waves in leads I and V6, and terminal QRS The electrophysiological properties of Mahaim pathways are not
slurring or notching, suggest the presence of Mahaim conduction.9,10 uniform, and this may reflect the diversity of the limited histology
However, patients with short, rapidly conducting fibres may have findings.5,18 It seems that most, but not all, of these pathways represent
typical pre-excitation.10 duplications of the AV nodal conducting system and contain nodal

© RADCLIFFE CARDIOLOGY 2017 Access at: www.AERjournal.com 29

Diagnostic Electrophysiology & Ablation

Figure 1: ECG during Sinus Rhythm and Atrial Pacing Figure 2: Induction of Tachycardia by Atrial (left panel)
Demonstrating Characteristics of Mahaim Conduction and Ventricular Pacing (right panel)

16 17 18 19 32 33 34 35 36 37 21 22 0.4 0.5
I I
aVR
II
V1
III
V6
aVR
HRA
aVL
His5-6
aVF
His3-4
V1
V2 His1-2

V3 CS5-6

V4 CS3-4
CS1-2
V5 S1 S1 S2 S1 S1 S2
V6
100 mm/s
Arrows indicate retrograde His bundle potential. Reproduced from Giazitzoglou et al, 2008,24
25 mm/s
with kind permission.
Left panel: Absence of spontaneous pre-excitation during sinus rhythm. Right panel: Varying
degrees of pre-excitation during atrial pacing (right panel). Normal AV nodal conduction (first
Figure 4: Change in QRS Morphology from Short to Long
beat), fusion between nodal and Mahaim conduction (next two beats) and full left bundle
branch block appearance due to conduction over the Mahaim pathway (last beat) are shown. V-A Atrioventricular Re-entrant Tachycardia

Figure 3: 12-lead ECG of Three Different Patients with A I

Antidromic Tachycardia II
III
AVR
Site of ablation between 07:00 and 07:30
AVN AVL
AVF
His
V1
V2
LBB
V3
V4
AF V5
V6
RBB PF
1791 HRA
36276 2013Y01
I I I
II II II RB
III III III
HBE
AVR AVR AVR
AVL AVL
AVL CSp
AVF AVF
AVF
V1 I
V1 V1 B II
V2 III
V2
V2
V3 AVR
V3
V3 AVL
V4
V4
V4 AVF
V5
V5
AVN
V1
V6 V5
V6 His V2
10 mm/mV 25mm/s V6 V3
10 mm/mV 25mm/s
V4
superior horizontal normal LBB
V5
From left to right: superior, horizontal and normal frontal plane QRS axis ablated from the V6
same sector of the tricuspid annulus (site of ablation between 7 o’clock and 7:30).
AF
Reproduced from Sternick et al, 2014,7 with kind permission. RF

RBB PF RB
tissue, and their association with the R3-2Q mutation in PRKAG2
HBED
has been considered as an indication that this gene is involved in
the development of the cardiac conduction system.19 They may HBEP

display spontaneous or post-ablation automaticity,13,20 may respond to RVA

adenosine but not to verapamil,21 and their properties may depend on
their location and insertion site.5,13
A: During short V-A AVRT (tachycardia cycle length 300 ms), there is also anterograde activation
over the left anterior fascicle to produce a fused QRS complex with a normal axis. B: With
retrograde right bundle branch block, anterograde conduction over the left anterior fascicle is no
Catheter Ablation longer possible and conduction to the left ventricle proceeds only via the right free wall. Therefore,
Mahaim pathways are typically decremental and conducting only the long V-A AVRT (tachycardia cycle length 350ms) has a leftward axis. During the change from
short V-A AVRT to long V-A AVRT, the QRS width also increases from 120 to 150 ms. A: atrial
anterogradely. However, retrogradely conducting nodoventricular
electrogram; AF: anterior fascicle; AVN: atrioventricular node; AVRT: atrioventricular reentrant
pathways have been described.22,23 Whether such pathways are tachycardia; CSp: proximal coronary sinus catheter; HBE: His bundle catheter; HRA: high right
atrium catheter; LBB: left bundle branch catheter; M: Mahaim potential; PF: posterior fascicle; RB:
classified as ‘true’ Mahaims is a matter of terminology rather than
right bundle potential; RBB: right bundle branch catheter; RVA: right ventricular apex catheter; V-H:
essence. Catheter ablation is accomplished by identifying the proximal ventriculo-His interval. Reproduced from Gandhavadi et al, 2013,6 with kind permission.

30 ARRHYTHMIA & ELECTROPHYSIOLOGY REVIEW

 Mahaim Accessory Pathways

Figure 5: Site of Ablation of Each Atriofascicular Fibre Figure 6: Atrial Resetting of Tachycardia and Recording of
at the Tricuspid Annulus in 48 Different Antidromic Mahaim Potential
Tachycardias
I 55 I 46 47

Anteroseptal V1 V1
V6
V6
356 356 334 356
Antero- His bundle msec msec msec msec
His5-6 His5-6
lateral
His3-4
Mitral annulus His3-4
His1-2
His1-2
Abl1-2
CS5-6
Lateral
CS3-4 CS5-6
CS
Postero- CS1-2 CS3-4
lateral
CS1-2
Posterior/Posteroseptal
100 mm/s
Normal axis Horizontal axis Superior axis
Left panel. Resetting of tachycardia by an atrial extrastimulus. Please note advancement
The annulus was classified into regions: posterior and postero-septal sites (P), postero-
of preexcited ventricular, retrograde His, and atrial electrograms by 22 msec without
lateral sites (PL), lateral sites (L), anterolateral sites (AL) and anteroseptal sites (AS). The QRS
affecting the retrograde activation sequence. Right panel. Recording of a distal Mahaim
frontal plane axis was classified as superior (<-300), horizontal (≤+150 and ≥-300) and normal
potential (arrow) and earliest ventricular activation underneath the tricuspid annulus during
(>±150). Regardless of the frontal plane axis, most of the cases were ablated at the L and PL
atrial pacing and maximum pre-excitation. Reproduced from Giazitzoglou et al, 2008,24 with
regions. There was not a single case with normal frontal plane axis located at the AS region.
kind permission.
Reproduced from Sternick et al, 2014,7 with kind permission.

and distal insertions and, ideally, the recording of a proximal pathway

potential at the tricuspid annulus or a distal one on the right ventricular Clinical Perspective
free wall (see Figure 6).5,13-17 Pathway potential recording may be • Mahaim pathways are decrementally conducting connections
facilitated during atrial pacing. Since most of the Mahaims are mapped between the right atrium or the AV node and the right
on the lateral tricuspid annulus or right free wall underneath the valve, ventricle in or close to the right bundle branch.
the use of supportive long sheaths that stabilise the ablating catheter • The baseline QRS is normal or displays different degrees
may be very helpful.24 Rare true nodoventricular pathways may also be of manifest pre-excitation with left bundle branch block
ablated with preservation of AV nodal conduction.25 morphology.
• Programmed atrial pacing leads to obvious manifest pre-
Conclusion excitation following an increase in A-V interval along with
Mahaim pathways are decrementally conducting connections between shortening of H-V interval at shorter pacing cycle lengths,
the right atrium or the AV node and the right ventricle in or close to and right bundle electrogram preceding His bundle activation
the right bundle branch. They can be atriofascicular, atrioventricular, during antegrade pre-excitation and reentrant tachycardia.
nodofascicular and nodoventricular, depending on their variable • Catheter ablation is accomplished by identifying the proximal
proximal and distal insertions. Catheter ablation is accomplished by and distal insertions and, ideally, the recording of a proximal
identifying the proximal and distal insertions and, ideally, the recording pathway potential at the tricuspid annulus or a distal one on
of a proximal pathway potential at the tricuspid annulus or a distal one the right ventricular free wall.
on the right ventricular free wall. n

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