<s>
An	O
accessory	B-Algorithm
pathway	I-Algorithm
is	O
an	O
additional	O
electrical	O
connection	O
between	O
two	O
parts	O
of	O
the	O
heart	O
.	O
</s>
<s>
These	O
pathways	O
can	O
lead	O
to	O
abnormal	B-Application
heart	I-Application
rhythms	I-Application
or	O
arrhythmias	B-Application
associated	O
with	O
symptoms	O
of	O
palpitations	O
.	O
</s>
<s>
Some	O
pathways	O
may	O
activate	O
a	O
region	O
of	O
ventricular	O
muscle	O
earlier	O
than	O
would	O
normally	O
occur	O
,	O
referred	O
to	O
as	O
pre-excitation	O
,	O
and	O
this	O
may	O
be	O
seen	O
on	O
an	O
electrocardiogram	B-Application
.	O
</s>
<s>
The	O
combination	O
of	O
an	O
accessory	B-Algorithm
pathway	I-Algorithm
that	O
causes	O
pre-excitation	O
with	O
arrhythmias	B-Application
is	O
known	O
as	O
Wolff-Parkinson-White	O
syndrome	O
.	O
</s>
<s>
Accessory	B-Algorithm
pathways	I-Algorithm
are	O
often	O
diagnosed	O
using	O
an	O
electrocardiogram	B-Application
,	O
but	O
characterisation	O
and	O
location	O
of	O
the	O
pathway	O
may	O
require	O
an	O
electrophysiological	B-Application
study	I-Application
.	O
</s>
<s>
Accessory	B-Algorithm
pathways	I-Algorithm
may	O
not	O
require	O
any	O
treatment	O
,	O
but	O
those	O
causing	O
symptoms	O
may	O
be	O
treated	O
with	O
medication	O
including	O
calcium	O
channel	O
antagonists	O
,	O
beta	O
blockers	O
or	O
flecainide	O
.	O
</s>
<s>
Alternatively	O
,	O
the	O
electrical	O
conduction	B-Algorithm
through	O
an	O
accessory	B-Algorithm
pathways	I-Algorithm
can	O
be	O
abolished	O
using	O
catheter	O
ablation	O
,	O
potentially	O
offering	O
a	O
permanent	O
cure	O
.	O
</s>
<s>
The	O
most	O
common	O
sites	O
for	O
accessory	B-Algorithm
pathways	I-Algorithm
are	O
connections	O
between	O
muscle	O
tissue	O
in	O
the	O
atria	O
and	O
the	O
ventricles	O
(	O
atrio-ventricular	O
pathways	O
)	O
,	O
bypassing	O
the	O
atrioventricular	O
node	O
.	O
</s>
<s>
Rarer	O
sites	O
include	O
connections	O
between	O
atrial	O
muscle	O
and	O
the	O
conducting	B-Algorithm
tissue	I-Algorithm
within	O
the	O
ventricles	O
(	O
atrio-fascicular	O
pathways	O
)	O
,	O
between	O
the	O
atrioventricular	O
node	O
and	O
the	O
muscle	O
tissue	O
of	O
the	O
ventricle	O
(	O
nodo-ventricular	O
pathways	O
)	O
,	O
and	O
between	O
the	O
conducting	B-Algorithm
tissue	I-Algorithm
of	O
the	O
ventricle	O
and	O
the	O
ventricular	O
muscle	O
(	O
fasciculo-ventricular	O
pathways	O
)	O
.	O
</s>
<s>
These	O
rarer	O
accessory	B-Algorithm
pathways	I-Algorithm
are	O
sometimes	O
collectively	O
referred	O
to	O
as	O
Mahaim	O
pathways	O
or	O
Mahaim	O
fibres	O
.	O
</s>
<s>
The	O
fibres	O
often	O
conduct	O
slowly	O
and	O
in	O
one	O
direction	O
only	O
-	O
from	O
the	O
atria	O
to	O
the	O
ventricles	O
(	O
antegrade	O
conduction	B-Algorithm
)	O
;	O
not	O
from	O
the	O
ventricles	O
to	O
the	O
atria	O
(	O
retrograde	O
conduction	B-Algorithm
)	O
.	O
</s>
<s>
Unlike	O
most	O
atrio-ventricular	O
accessory	B-Algorithm
pathways	I-Algorithm
which	O
conduct	O
electrical	O
impulses	O
at	O
a	O
relatively	O
fixed	O
speed	O
,	O
conduction	B-Algorithm
through	O
a	O
Mahaim	O
pathway	O
varies	O
according	O
to	O
how	O
rapidly	O
it	O
is	O
stimulated	O
.	O
</s>
<s>
More	O
frequent	O
stimulation	O
leads	O
to	O
slower	O
conduction	B-Algorithm
,	O
known	O
as	O
decremental	O
conduction	B-Algorithm
.	O
</s>
<s>
If	O
conduction	B-Algorithm
to	O
the	O
ventricles	O
occurs	O
solely	O
through	O
the	O
pathway	O
(	O
maximal	O
pre-excitation	O
)	O
,	O
as	O
occurs	O
during	O
arrhythmias	B-Application
like	O
antidromic	O
atrioventricular	B-Algorithm
re-entrant	I-Algorithm
tachycardia	I-Algorithm
,	O
the	O
ECG	B-Application
appearance	O
is	O
of	O
QRS	B-Algorithm
complexes	I-Algorithm
with	O
a	O
left	O
bundle	O
branch	O
block	O
morphology	O
which	O
can	O
be	O
mistaken	O
for	O
ventricular	B-Application
tachycardia	I-Application
.	O
</s>
<s>
However	O
,	O
due	O
to	O
their	O
slow	O
decremental	O
conduction	B-Algorithm
,	O
during	O
sinus	O
rhythm	O
the	O
12-lead	B-Application
ECG	I-Application
will	O
often	O
show	O
little	O
pre-excitation	O
.	O
</s>
