<s>
Heart	B-Algorithm
block	I-Algorithm
(	O
HB	O
)	O
is	O
a	O
disorder	O
in	O
the	O
heart	O
's	O
rhythm	O
due	O
to	O
a	O
fault	O
in	O
the	O
natural	B-Algorithm
pacemaker	I-Algorithm
.	O
</s>
<s>
This	O
is	O
caused	O
by	O
an	O
obstruction	O
–	O
a	O
block	O
–	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>
Despite	O
the	O
severe-sounding	O
name	O
,	O
heart	B-Algorithm
block	I-Algorithm
may	O
cause	O
no	O
symptoms	O
at	O
all	O
in	O
some	O
cases	O
,	O
or	O
occasional	O
missed	O
heartbeats	O
in	O
other	O
cases	O
(	O
which	O
can	O
cause	O
light-headedness	O
,	O
syncope	O
(	O
fainting	O
)	O
,	O
and	O
palpitations	B-Algorithm
)	O
,	O
or	O
may	O
require	O
the	O
implantation	O
of	O
an	O
artificial	B-Device
pacemaker	I-Device
,	O
depending	O
upon	O
exactly	O
where	O
in	O
the	O
heart	B-Algorithm
conduction	I-Algorithm
is	O
being	O
impaired	O
and	O
how	O
significantly	O
it	O
is	O
affected	O
.	O
</s>
<s>
Heart	B-Algorithm
block	I-Algorithm
should	O
not	O
be	O
confused	O
with	O
other	O
conditions	O
,	O
which	O
may	O
or	O
may	O
not	O
be	O
co-occurring	O
,	O
relating	O
to	O
the	O
heart	O
and/or	O
other	O
nearby	O
organs	O
that	O
are	O
or	O
can	O
be	O
serious	O
,	O
including	O
angina	O
(	O
heart-related	O
chest	O
pain	O
)	O
,	O
heart	O
attack	O
(	O
myocardial	O
infarction	O
)	O
,	O
any	O
type	O
of	O
heart	O
failure	O
,	O
cardiogenic	O
shock	O
or	O
other	O
types	O
of	O
shock	O
,	O
different	O
types	O
of	O
abnormal	B-Application
heart	I-Application
rhythms	I-Application
(	O
arrhythmias	B-Application
)	O
,	O
cardiac	O
arrest	O
,	O
or	O
respiratory	O
arrest	O
.	O
</s>
<s>
In	O
severe	O
cases	O
where	O
the	O
heart	O
's	O
ability	O
to	O
control	O
and	O
trigger	O
heartbeats	O
may	O
be	O
completely	O
ineffective	O
or	O
unreliable	O
,	O
heart	B-Algorithm
block	I-Algorithm
can	O
usually	O
be	O
treated	O
by	O
inserting	O
an	O
artificial	B-Device
pacemaker	I-Device
,	O
a	O
medical	O
device	O
that	O
provides	O
correct	O
electrical	O
impulses	O
to	O
trigger	O
heart	O
beats	O
,	O
compensating	O
for	O
the	O
natural	B-Algorithm
pacemaker	I-Algorithm
's	O
unreliability	O
.	O
</s>
<s>
Therefore	O
,	O
heart	B-Algorithm
block	I-Algorithm
frequently	O
has	O
no	O
effects	O
,	O
or	O
mild	O
and	O
occasional	O
effects	O
,	O
and	O
is	O
not	O
life-threatening	O
in	O
the	O
vast	O
majority	O
of	O
cases	O
,	O
and	O
is	O
usually	O
treatable	O
in	O
more	O
serious	O
cases	O
.	O
</s>
<s>
The	O
human	O
heart	O
uses	O
electrical	O
signals	O
to	O
maintain	O
and	O
initiate	O
the	O
regular	O
heartbeat	O
in	O
a	O
living	O
person	O
;	O
incorrect	O
conduction	B-Algorithm
or	O
interference	O
from	O
external	O
sources	O
can	O
lead	O
to	O
mild	O
or	O
serious	O
symptoms	O
depending	O
upon	O
the	O
location	O
of	O
the	O
blockage	O
and	O
how	O
severely	O
conduction	B-Algorithm
is	O
being	O
blocked	O
.	O
</s>
<s>
Conduction	B-Algorithm
is	O
initiated	O
by	O
the	O
sinoatrial	O
node	O
(	O
"	O
sinus	O
node	O
"	O
or	O
"	O
SA	O
node	O
"	O
)	O
,	O
and	O
then	O
travels	O
to	O
the	O
atrioventricular	O
node	O
(	O
"	O
AV	O
node	O
"	O
)	O
which	O
also	O
contains	O
a	O
secondary	O
"	O
pacemaker	B-Device
"	O
that	O
acts	O
as	O
a	O
backup	O
for	O
the	O
SA	O
nodes	O
,	O
then	O
to	O
the	O
bundle	B-Algorithm
of	I-Algorithm
His	I-Algorithm
and	O
then	O
via	O
the	O
bundle	O
branches	O
to	O
the	O
point	O
of	O
the	O
apex	O
of	O
the	O
fascicular	O
branches	O
.	O
</s>
<s>
Blockages	O
are	O
therefore	O
classified	O
based	O
on	O
where	O
the	O
blockage	O
occurs	O
-	O
namely	O
the	O
SA	O
node	O
(	O
"	O
Sinoatrial	B-Algorithm
block	I-Algorithm
"	O
)	O
,	O
AV	O
node	O
(	O
"	O
AV	B-Algorithm
block	I-Algorithm
"	O
or	O
AVB	O
)	O
,	O
and	O
at	O
or	O
below	O
the	O
bundle	B-Algorithm
of	I-Algorithm
His	I-Algorithm
(	O
"	O
Intra-Hisian	O
"	O
or	O
"	O
Infra-Hisian	O
block	O
"	O
respectively	O
)	O
.	O
</s>
<s>
Infra-Hisian	O
blocks	O
may	O
occur	O
at	O
the	O
left	O
or	O
right	O
bundle	O
branches	O
(	O
"	O
bundle	B-Algorithm
branch	I-Algorithm
block	I-Algorithm
"	O
)	O
or	O
the	O
fascicles	O
of	O
the	O
left	O
bundle	O
branch	O
(	O
"	O
fascicular	O
block	O
"	O
or	O
"	O
Hemiblock	O
"	O
)	O
.	O
</s>
<s>
The	O
term	O
"	O
Wenckebach	O
block	O
"	O
is	O
also	O
used	O
for	O
second	O
degree	O
type	O
1	O
blocks	O
of	O
either	O
the	O
SA	O
or	O
AV	O
node	O
;	O
in	O
addition	O
second-degree	O
blocks	O
type	O
1	O
and	O
2	O
are	O
also	O
sometimes	O
known	O
as	O
"	O
Mobitz	B-Algorithm
1	I-Algorithm
"	O
and	O
"	O
Mobitz	O
2	O
"	O
.	O
</s>
<s>
Therefore	O
,	O
most	O
of	O
the	O
important	O
heart	B-Algorithm
blocks	I-Algorithm
are	O
AV	B-Algorithm
nodal	I-Algorithm
blocks	I-Algorithm
and	O
infra-Hisian	O
blocks	O
.	O
</s>
<s>
SA	B-Algorithm
blocks	I-Algorithm
are	O
usually	O
of	O
lesser	O
clinical	O
significance	O
,	O
since	O
in	O
the	O
event	O
of	O
SA	B-Algorithm
block	I-Algorithm
,	O
the	O
AV	O
node	O
contains	O
a	O
secondary	O
pacemaker	B-Device
which	O
would	O
still	O
maintain	O
a	O
heart	O
rate	O
of	O
around	O
40	O
-	O
60	O
beats	O
per	O
minute	O
,	O
sufficient	O
for	O
consciousness	O
and	O
much	O
of	O
daily	O
life	O
in	O
the	O
majority	O
of	O
individuals	O
.	O
</s>
<s>
Following	O
the	O
path	O
of	O
the	O
electrical	O
signals	O
,	O
the	O
places	O
where	O
conduction	B-Algorithm
can	O
be	O
blocked	O
give	O
rise	O
to	O
different	O
kinds	O
of	O
heart	B-Algorithm
blocks	I-Algorithm
:	O
</s>
<s>
Location	O
Name	O
Within	O
the	O
sinoatrial	O
node	O
(	O
SA	O
node	O
or	O
Sinus	O
node	O
)	O
,	O
where	O
the	O
heart	B-Algorithm
's	I-Algorithm
signals	I-Algorithm
originate	I-Algorithm
Sinoatrial	O
nodal	O
blocks	O
(	O
often	O
abbreviated	O
"	O
SA	O
nodal	O
block	O
"	O
or	O
"	O
SA	B-Algorithm
block	I-Algorithm
"	O
,	O
sometimes	O
written	O
"	O
Sinuatrial	O
block	O
"	O
)	O
Within	O
the	O
atrioventricular	O
node	O
(	O
AV	O
node	O
)	O
Atrioventricular	B-Algorithm
block	I-Algorithm
(	O
often	O
abbreviated	O
"	O
AV	B-Algorithm
nodal	I-Algorithm
block	I-Algorithm
"	O
,	O
"	O
AV	B-Algorithm
block	I-Algorithm
"	O
or	O
AVB	O
)	O
.	O
</s>
<s>
At	O
and	O
below	O
the	O
bundle	B-Algorithm
of	I-Algorithm
His	I-Algorithm
Intra-Hisian	O
blocks	O
and	O
Infra-Hisian	O
blocks	O
respectively	O
.	O
</s>
<s>
Within	O
the	O
left	O
or	O
right	O
bundle	O
branches	O
Bundle	B-Algorithm
branch	I-Algorithm
blocks	I-Algorithm
.	O
</s>
<s>
In	O
an	O
SA	B-Algorithm
block	I-Algorithm
,	O
the	O
electrical	O
impulse	O
is	O
delayed	O
or	O
blocked	O
on	O
the	O
way	O
to	O
the	O
atria	O
,	O
thus	O
delaying	O
atrial	O
depolarization	O
.	O
</s>
<s>
By	O
contrast	O
,	O
an	O
AV	B-Algorithm
block	I-Algorithm
occurs	O
in	O
the	O
AV	O
node	O
and	O
delays	O
ventricular	O
depolarization	O
.	O
</s>
<s>
The	O
term	O
"	O
Wenckebach	O
block	O
"	O
is	O
also	O
used	O
for	O
some	O
heart	B-Algorithm
blocks	I-Algorithm
,	O
and	O
can	O
refer	O
to	O
a	O
second	O
degree	O
type	O
I	O
block	O
in	O
either	O
the	O
SA	O
node	O
or	O
the	O
AV	O
node	O
,	O
however	O
the	O
ECG	O
features	O
of	O
the	O
two	O
are	O
quite	O
distinctly	O
different	O
.	O
</s>
<s>
SA	B-Algorithm
blocks	I-Algorithm
rarely	O
give	O
severe	O
symptoms	O
,	O
because	O
even	O
if	O
an	O
individual	O
had	O
complete	O
block	O
at	O
this	O
level	O
of	O
the	O
conduction	B-Algorithm
system	I-Algorithm
(	O
which	O
is	O
uncommon	O
)	O
,	O
the	O
secondary	O
pacemaker	B-Device
of	O
the	O
heart	O
would	O
be	O
at	O
the	O
AV	O
node	O
,	O
which	O
would	O
fire	O
at	O
40	O
to	O
60	O
beats	O
a	O
minute	O
,	O
which	O
is	O
enough	O
to	O
retain	O
consciousness	O
in	O
the	O
resting	O
state	O
.	O
</s>
<s>
However	O
SA	B-Algorithm
block	I-Algorithm
is	O
capable	O
of	O
causing	O
problematic	O
symptoms	O
even	O
so	O
,	O
and	O
may	O
also	O
hint	O
at	O
conduction	B-Algorithm
issues	O
elsewhere	O
in	O
the	O
heart	O
,	O
and	O
therefore	O
SA	B-Algorithm
blocks	I-Algorithm
are	O
-	O
despite	O
their	O
lower	O
level	O
of	O
life-threatening	O
risk	O
-	O
still	O
"	O
the	O
most	O
common	O
indication	O
for	O
pacemaker	B-Device
implantation	O
in	O
the	O
US	O
"	O
.	O
</s>
<s>
Types	O
of	O
SA	B-Algorithm
nodal	I-Algorithm
blocks	I-Algorithm
include	O
:	O
</s>
<s>
In	O
addition	O
to	O
the	O
above	O
blocks	O
,	O
the	O
SA	O
node	O
can	O
be	O
suppressed	O
by	O
any	O
other	O
arrhythmia	B-Application
that	O
reaches	O
it	O
.	O
</s>
<s>
This	O
includes	O
retrograde	O
conduction	B-Algorithm
from	O
the	O
ventricles	O
,	O
ectopic	B-Algorithm
atrial	O
beats	O
,	O
atrial	B-Application
fibrillation	I-Application
,	O
and	O
atrial	O
flutter	O
.	O
</s>
<s>
There	O
are	O
three	O
basic	O
types	O
of	O
AV	B-Algorithm
nodal	I-Algorithm
block	I-Algorithm
:	O
</s>
<s>
Infra-Hisian	O
block	O
is	O
that	O
of	O
the	O
distal	O
conduction	B-Algorithm
system	I-Algorithm
.	O
</s>
<s>
Of	O
these	O
types	O
of	O
infra-Hisian	O
block	O
,	O
Mobitz	B-Algorithm
II	I-Algorithm
heart	B-Algorithm
block	I-Algorithm
is	O
considered	O
most	O
important	O
because	O
of	O
the	O
possible	O
progression	O
to	O
complete	B-Architecture
heart	I-Architecture
block	I-Architecture
.	O
</s>
