<s>
A	O
bundle	B-Algorithm
branch	I-Algorithm
block	I-Algorithm
is	O
a	O
defect	O
in	O
one	O
of	O
the	O
bundle	O
branches	O
in	O
the	O
electrical	B-Algorithm
conduction	I-Algorithm
system	I-Algorithm
of	I-Algorithm
the	I-Algorithm
heart	I-Algorithm
.	O
</s>
<s>
The	O
heart	O
's	O
electrical	O
activity	O
begins	O
in	O
the	O
sinoatrial	O
node	O
(	O
the	O
heart	O
's	O
natural	B-Algorithm
pacemaker	I-Algorithm
)	O
,	O
which	O
is	O
situated	O
on	O
the	O
upper	O
right	O
atrium	O
.	O
</s>
<s>
From	O
the	O
AV	O
node	O
the	O
electrical	O
impulse	O
travels	O
down	O
the	O
bundle	B-Algorithm
of	I-Algorithm
His	I-Algorithm
and	O
divides	O
into	O
the	O
right	O
and	O
left	O
bundle	O
branches	O
.	O
</s>
<s>
Ultimately	O
,	O
the	O
fascicles	O
divide	O
into	O
millions	O
of	O
Purkinje	B-Algorithm
fibres	I-Algorithm
,	O
which	O
in	O
turn	O
interdigitate	O
with	O
individual	O
cardiac	O
myocytes	O
,	O
allowing	O
for	O
rapid	O
,	O
coordinated	O
,	O
and	O
synchronous	O
physiologic	O
depolarization	O
of	O
the	O
ventricles	O
.	O
</s>
<s>
This	O
results	O
in	O
altered	O
pathways	O
for	O
ventricular	B-Application
depolarization	I-Application
.	O
</s>
<s>
As	O
a	O
result	O
,	O
there	O
is	O
a	O
loss	O
of	O
ventricular	O
synchrony	O
,	O
ventricular	B-Application
depolarization	I-Application
is	O
prolonged	O
,	O
and	O
there	O
may	O
be	O
a	O
corresponding	O
drop	O
in	O
cardiac	O
output	O
.	O
</s>
<s>
When	O
heart	O
failure	O
is	O
present	O
,	O
a	O
specialized	O
pacemaker	B-Device
may	O
be	O
used	O
to	O
resynchronize	O
the	O
ventricles	O
.	O
</s>
<s>
In	O
theory	O
a	O
pacemaker	B-Device
like	O
this	O
will	O
shorten	O
the	O
QRS	B-Algorithm
interval	O
,	O
thus	O
bringing	O
the	O
timing	O
of	O
contraction	O
of	O
the	O
left	O
and	O
right	O
ventricles	O
closer	O
together	O
and	O
slightly	O
increasing	O
the	O
ejection	O
fraction	O
.	O
</s>
<s>
A	O
bundle	B-Algorithm
branch	I-Algorithm
block	I-Algorithm
can	O
be	O
diagnosed	O
when	O
the	O
duration	O
of	O
the	O
QRS	B-Algorithm
complex	I-Algorithm
on	O
the	O
ECG	B-Application
exceeds	O
120	O
ms.	O
A	O
right	B-Algorithm
bundle	I-Algorithm
branch	I-Algorithm
block	I-Algorithm
typically	O
causes	O
prolongation	O
of	O
the	O
last	O
part	O
of	O
the	O
QRS	B-Algorithm
complex	I-Algorithm
and	O
may	O
shift	O
the	O
heart	O
's	O
electrical	O
axis	O
slightly	O
to	O
the	O
right	O
.	O
</s>
<s>
The	O
ECG	B-Application
will	O
show	O
a	O
terminal	O
R	B-Algorithm
wave	I-Algorithm
in	O
lead	O
V1	O
and	O
a	O
slurred	O
S	O
wave	O
in	O
lead	O
I	O
.	O
</s>
<s>
Left	O
bundle	B-Algorithm
branch	I-Algorithm
block	I-Algorithm
widens	O
the	O
entire	O
QRS	B-Algorithm
,	O
and	O
in	O
most	O
cases	O
shifts	O
the	O
heart	O
's	O
electrical	O
axis	O
to	O
the	O
left	O
.	O
</s>
<s>
The	O
ECG	B-Application
will	O
show	O
a	O
QS	O
or	O
rS	O
complex	O
in	O
lead	O
V1	O
and	O
a	O
monophasic	O
R	B-Algorithm
wave	I-Algorithm
in	O
lead	O
I	O
.	O
</s>
<s>
Another	O
normal	O
finding	O
with	O
bundle	B-Algorithm
branch	I-Algorithm
block	I-Algorithm
is	O
appropriate	O
T	O
wave	O
discordance	O
.	O
</s>
<s>
In	O
other	O
words	O
,	O
the	O
T	O
wave	O
will	O
be	O
deflected	O
opposite	O
the	O
terminal	O
deflection	O
of	O
the	O
QRS	B-Algorithm
complex	I-Algorithm
.	O
</s>
<s>
Bundle	B-Algorithm
branch	I-Algorithm
block	I-Algorithm
,	O
especially	O
left	O
bundle	B-Algorithm
branch	I-Algorithm
block	I-Algorithm
,	O
can	O
lead	O
to	O
cardiac	O
dyssynchrony	O
.	O
</s>
<s>
The	O
simultaneous	O
occurrence	O
of	O
left	O
and	O
right	B-Algorithm
bundle	I-Algorithm
branch	I-Algorithm
block	I-Algorithm
leads	O
to	O
total	O
AV	O
block	O
.	O
</s>
<s>
Depending	O
on	O
the	O
anatomical	O
location	O
of	O
the	O
defect	O
which	O
leads	O
to	O
a	O
bundle	B-Algorithm
branch	I-Algorithm
block	I-Algorithm
,	O
the	O
blocks	O
are	O
further	O
classified	O
into	O
:	O
</s>
<s>
The	O
left	O
bundle	B-Algorithm
branch	I-Algorithm
block	I-Algorithm
can	O
be	O
further	O
sub	O
classified	O
into	O
:	O
</s>
<s>
Left	B-Algorithm
posterior	I-Algorithm
fascicular	I-Algorithm
block	I-Algorithm
.	O
</s>
<s>
Other	O
classifications	O
of	O
bundle	B-Algorithm
branch	I-Algorithm
blocks	I-Algorithm
are	O
;	O
</s>
<s>
Bifascicular	B-Algorithm
block	I-Algorithm
.	O
</s>
<s>
Trifascicular	B-Algorithm
block	I-Algorithm
.	O
</s>
<s>
This	O
is	O
a	O
combination	O
of	O
right	B-Algorithm
bundle	I-Algorithm
branch	I-Algorithm
block	I-Algorithm
with	O
either	O
left	O
anterior	O
fascicular	O
block	O
or	O
left	B-Algorithm
posterior	I-Algorithm
fascicular	I-Algorithm
block	I-Algorithm
together	O
with	O
a	O
first	B-Algorithm
degree	I-Algorithm
AV	I-Algorithm
block	I-Algorithm
.	O
</s>
<s>
A	O
simple	O
way	O
to	O
quickly	O
differentiate	O
between	O
the	O
two	O
types	O
is	O
to	O
note	O
the	O
deflection	O
of	O
the	O
QRS	B-Algorithm
complex	I-Algorithm
in	O
the	O
V1	O
lead	O
.	O
</s>
<s>
A	O
(	O
V1	O
)	O
QRS	B-Algorithm
segment	O
deflected	O
DOWN	O
,	O
its	O
LEFT	O
,	O
a	O
deflection	O
UP	O
indicates	O
RIGHT	O
(	O
RBBB	B-Algorithm
)	O
.	O
</s>
<s>
of	O
course	O
,	O
in	O
both	O
types	O
the	O
QRS	B-Algorithm
is	O
long	O
(	O
>.12	O
seconds	O
)	O
.	O
</s>
<s>
Some	O
people	O
with	O
bundle	B-Algorithm
branch	I-Algorithm
blocks	I-Algorithm
are	O
born	O
with	O
this	O
condition	O
.	O
</s>
<s>
People	O
with	O
bundle	B-Algorithm
branch	I-Algorithm
blocks	I-Algorithm
may	O
still	O
be	O
quite	O
active	O
,	O
and	O
may	O
have	O
nothing	O
more	O
remarkable	O
than	O
an	O
abnormal	O
appearance	O
to	O
their	O
ECG	B-Application
.	O
</s>
<s>
In	O
more	O
severe	O
cases	O
,	O
a	O
pacemaker	B-Device
may	O
be	O
required	O
to	O
restore	O
an	O
optimal	O
electrical	O
supply	O
to	O
the	O
heart	O
muscle	O
.	O
</s>
