<s>
Third-degree	B-Architecture
atrioventricular	I-Architecture
block	I-Architecture
(	O
AV	B-Algorithm
block	I-Algorithm
)	O
is	O
a	O
medical	O
condition	O
in	O
which	O
the	O
electrical	O
impulse	O
generated	O
in	O
the	O
sinoatrial	O
node	O
(	O
SA	O
node	O
)	O
in	O
the	O
atrium	O
of	O
the	O
heart	O
can	O
not	O
propagate	O
to	O
the	O
ventricles	O
.	O
</s>
<s>
Because	O
the	O
impulse	O
is	O
blocked	O
,	O
an	O
accessory	O
pacemaker	B-Device
in	O
the	O
lower	O
chambers	O
will	O
typically	O
activate	O
the	O
ventricles	O
.	O
</s>
<s>
This	O
is	O
known	O
as	O
an	O
escape	B-Algorithm
rhythm	I-Algorithm
.	O
</s>
<s>
Since	O
this	O
accessory	O
pacemaker	B-Device
also	O
activates	O
independently	O
of	O
the	O
impulse	O
generated	O
at	O
the	O
SA	O
node	O
,	O
two	O
independent	O
rhythms	O
can	O
be	O
noted	O
on	O
the	O
electrocardiogram	B-Application
(	O
ECG	B-Application
)	O
.	O
</s>
<s>
The	O
P	B-Algorithm
waves	I-Algorithm
with	O
a	O
regular	O
P-to-P	O
interval	O
(	O
in	O
other	O
words	O
,	O
a	O
sinus	O
rhythm	O
)	O
represent	O
the	O
first	O
rhythm	O
.	O
</s>
<s>
The	O
QRS	B-Algorithm
complexes	I-Algorithm
with	O
a	O
regular	O
R-to-R	O
interval	O
represent	O
the	O
second	O
rhythm	O
.	O
</s>
<s>
The	O
PR	B-Algorithm
interval	I-Algorithm
will	O
be	O
variable	O
,	O
as	O
the	O
hallmark	O
of	O
complete	B-Architecture
heart	I-Architecture
block	I-Architecture
is	O
the	O
lack	O
of	O
any	O
apparent	O
relationship	O
between	O
P	B-Algorithm
waves	I-Algorithm
and	O
QRS	B-Algorithm
complexes	I-Algorithm
.	O
</s>
<s>
People	O
with	O
third-degree	B-Architecture
AV	I-Architecture
block	I-Architecture
typically	O
experience	O
severe	O
bradycardia	B-Algorithm
(	O
an	O
abnormally	O
low	O
measured	O
heart	O
rate	O
)	O
,	O
hypotension	O
,	O
and	O
at	O
times	O
,	O
hemodynamic	O
instability	O
.	O
</s>
<s>
Many	O
conditions	O
can	O
cause	O
third-degree	B-Architecture
heart	I-Architecture
block	I-Architecture
,	O
but	O
the	O
most	O
common	O
cause	O
is	O
coronary	O
ischemia	O
.	O
</s>
<s>
Progressive	O
degeneration	O
of	O
the	O
electrical	B-Algorithm
conduction	I-Algorithm
system	I-Algorithm
of	I-Algorithm
the	I-Algorithm
heart	I-Algorithm
can	O
lead	O
to	O
third-degree	B-Architecture
heart	I-Architecture
block	I-Architecture
.	O
</s>
<s>
This	O
may	O
be	O
preceded	O
by	O
first-degree	B-Algorithm
AV	I-Algorithm
block	I-Algorithm
,	O
second-degree	B-Algorithm
AV	I-Algorithm
block	I-Algorithm
,	O
bundle	B-Algorithm
branch	I-Algorithm
block	I-Algorithm
,	O
or	O
bifascicular	B-Algorithm
block	I-Algorithm
.	O
</s>
<s>
In	O
addition	O
,	O
acute	O
myocardial	O
infarction	O
may	O
present	O
with	O
third-degree	B-Architecture
AV	I-Architecture
block	I-Architecture
.	O
</s>
<s>
An	O
inferior	O
wall	O
myocardial	O
infarction	O
may	O
cause	O
damage	O
to	O
the	O
AV	O
node	O
,	O
causing	O
third-degree	B-Architecture
heart	I-Architecture
block	I-Architecture
.	O
</s>
<s>
Studies	O
have	O
shown	O
that	O
third-degree	B-Architecture
heart	I-Architecture
block	I-Architecture
in	O
the	O
setting	O
of	O
an	O
inferior	O
wall	O
myocardial	O
infarction	O
typically	O
resolves	O
within	O
2	O
weeks	O
.	O
</s>
<s>
The	O
escape	B-Algorithm
rhythm	I-Algorithm
typically	O
originates	O
in	O
the	O
AV	O
junction	O
,	O
producing	O
a	O
narrow	O
complex	O
escape	B-Algorithm
rhythm	I-Algorithm
.	O
</s>
<s>
An	O
anterior	O
wall	O
myocardial	O
infarction	O
may	O
damage	O
the	O
distal	O
conduction	B-Algorithm
system	I-Algorithm
of	I-Algorithm
the	I-Algorithm
heart	I-Algorithm
,	O
causing	O
third-degree	B-Architecture
heart	I-Architecture
block	I-Architecture
.	O
</s>
<s>
Initially	O
demonstrated	O
by	O
animal	O
studies	O
,	O
this	O
is	O
due	O
to	O
a	O
stark	O
reduction	O
in	O
the	O
Kv	O
β-subunit	O
of	O
the	O
voltage-gated	O
K+	O
channels	O
in	O
the	O
pacemaker	B-Algorithm
cells	I-Algorithm
of	O
the	O
atrioventricular	O
junction	O
,	O
causing	O
significantly	O
decreased	O
propagation	O
of	O
ions	O
across	O
gap	O
junctions	O
between	O
cardiac	O
cells	O
and	O
thus	O
prolonging	O
the	O
PR	B-Algorithm
interval	I-Algorithm
.	O
</s>
<s>
This	O
is	O
typically	O
extensive	O
,	O
permanent	O
damage	O
to	O
the	O
conduction	B-Algorithm
system	I-Algorithm
,	O
eliciting	O
a	O
necessity	O
for	O
a	O
permanent	B-Device
pacemaker	I-Device
to	O
be	O
placed	O
.	O
</s>
<s>
The	O
escape	B-Algorithm
rhythm	I-Algorithm
typically	O
originates	O
in	O
the	O
ventricles	O
,	O
producing	O
a	O
wide	O
complex	O
escape	B-Algorithm
rhythm	I-Algorithm
.	O
</s>
<s>
Third-degree	B-Architecture
heart	I-Architecture
block	I-Architecture
may	O
also	O
be	O
congenital	O
and	O
has	O
been	O
linked	O
to	O
the	O
presence	O
of	O
lupus	O
in	O
the	O
mother	O
.	O
</s>
<s>
The	O
cause	O
of	O
congenital	O
third-degree	B-Architecture
heart	I-Architecture
block	I-Architecture
in	O
many	O
patients	O
is	O
unknown	O
.	O
</s>
<s>
Studies	O
suggest	O
that	O
the	O
prevalence	O
of	O
congenital	O
third-degree	B-Architecture
heart	I-Architecture
block	I-Architecture
is	O
between	O
1	O
in	O
15,000	O
and	O
1	O
in	O
22,000	O
live	O
births	O
.	O
</s>
<s>
Hyperkalemia	O
in	O
those	O
with	O
previous	O
cardiac	O
disease	O
and	O
Lyme	O
disease	O
can	O
also	O
result	O
in	O
third-degree	B-Architecture
heart	I-Architecture
block	I-Architecture
.	O
</s>
<s>
AV	B-Algorithm
block	I-Algorithm
may	O
be	O
observed	O
in	O
patients	O
with	O
hypermagnesemia	O
who	O
are	O
receiving	O
excessive	O
intravenous	O
doses	O
of	O
magnium	O
sulfate	O
.	O
</s>
<s>
Diagnosis	O
is	O
largely	O
focussed	O
on	O
analysis	O
of	O
the	O
patients	O
12-lead	B-Application
ECG	I-Application
.	O
</s>
<s>
A	O
patient	O
with	O
a	O
third-degree	B-Architecture
AV	I-Architecture
block	I-Architecture
will	O
likely	O
have	O
p-waves	O
not	O
corresponding	O
to	O
QRS	B-Algorithm
complexes	I-Algorithm
along	O
with	O
bradycardia	B-Algorithm
.	O
</s>
<s>
Atropine	O
is	O
often	O
used	O
as	O
a	O
first	O
line	O
treatment	O
of	O
a	O
third-degree	B-Architecture
heart	I-Architecture
block	I-Architecture
in	O
the	O
presence	O
of	O
a	O
narrow	O
QRS	B-Algorithm
which	O
indicates	O
a	O
nodal	O
block	O
,	O
but	O
,	O
may	O
have	O
little	O
to	O
no	O
effect	O
in	O
an	O
infra-nodal	O
block	O
.	O
</s>
<s>
Treatment	O
in	O
emergency	O
situations	O
can	O
involve	O
electrical	O
transcutaneous	B-Algorithm
pacing	I-Algorithm
in	O
those	O
who	O
are	O
acutely	O
hemodynamically	O
unstable	O
and	O
can	O
be	O
used	O
regardless	O
of	O
the	O
persons	O
level	O
of	O
consciousness	O
.	O
</s>
<s>
Sedative	O
agents	O
such	O
as	O
a	O
benzodiazepine	O
or	O
opiate	O
may	O
be	O
used	O
in	O
conjunction	O
with	O
transcutaneous	B-Algorithm
pacing	I-Algorithm
to	O
reduce	O
the	O
pain	O
caused	O
by	O
the	O
intervention	O
.	O
</s>
<s>
Third-degree	B-Architecture
AV	I-Architecture
block	I-Architecture
can	O
be	O
treated	O
more	O
permanently	O
with	O
the	O
use	O
of	O
a	O
dual-chamber	O
artificial	B-Device
pacemaker	I-Device
.	O
</s>
<s>
Pacemakers	B-Device
in	O
this	O
role	O
are	O
usually	O
programmed	O
to	O
enforce	O
a	O
minimum	O
heart	O
rate	O
and	O
to	O
record	O
instances	O
of	O
atrial	O
flutter	O
and	O
atrial	B-Application
fibrillation	I-Application
,	O
two	O
common	O
secondary	O
conditions	O
that	O
can	O
accompany	O
third-degree	B-Architecture
AV	I-Architecture
block	I-Architecture
.	O
</s>
<s>
Since	O
pacemaker	B-Device
correction	O
of	O
the	O
third-degree	B-Architecture
block	I-Architecture
requires	O
full-time	O
pacing	O
of	O
the	O
ventricles	O
,	O
a	O
potential	O
side	O
effect	O
is	O
pacemaker	B-Algorithm
syndrome	I-Algorithm
,	O
and	O
may	O
necessitate	O
the	O
use	O
of	O
a	O
biventricular	O
pacemaker	B-Device
,	O
which	O
has	O
an	O
additional	O
3rd	O
lead	O
placed	O
in	O
a	O
vein	O
in	O
the	O
left	O
ventricle	O
,	O
providing	O
more	O
coordinated	O
pacing	O
of	O
both	O
ventricles	O
.	O
</s>
<s>
The	O
risk	O
factors	O
for	O
asystole	O
include	O
1	O
)	O
previous	O
asystole	O
,	O
2	O
)	O
complete	B-Architecture
heart	I-Architecture
block	I-Architecture
with	O
wide	O
complexes	O
,	O
and	O
3	O
)	O
ventricular	O
pause	O
for	O
>	O
3	O
seconds	O
.	O
</s>
<s>
Mobitz	O
Type	O
2	O
AV	B-Algorithm
block	I-Algorithm
is	O
another	O
indication	O
for	O
pacing	O
.	O
</s>
<s>
As	O
with	O
other	O
forms	O
of	O
heart	O
block	O
,	O
secondary	O
prevention	O
may	O
also	O
include	O
medicines	O
to	O
control	O
blood	O
pressure	O
and	O
atrial	B-Application
fibrillation	I-Application
,	O
as	O
well	O
as	O
lifestyle	O
and	O
dietary	O
changes	O
to	O
reduce	O
risk	O
factors	O
associated	O
with	O
heart	O
attack	O
and	O
stroke	O
.	O
</s>
<s>
Early	O
treatment	O
of	O
atrioventricular	O
blockade	O
is	O
based	O
on	O
the	O
presence	O
and	O
severity	O
of	O
symptoms	O
and	O
signs	O
associated	O
with	O
ventricular	B-Algorithm
escape	I-Algorithm
rhythm	O
.	O
</s>
<s>
If	O
no	O
reversible	O
cause	O
is	O
identified	O
,	O
a	O
permanent	B-Device
pacemaker	I-Device
is	O
inserted	O
.	O
</s>
<s>
Most	O
stable	O
patients	O
have	O
persistent	O
bradycardia-related	O
symptoms	O
and	O
require	O
identification	O
and	O
treatment	O
of	O
any	O
reversible	O
cause	O
or	O
permanent	O
implantable	O
pacemaker	B-Device
.	O
</s>
<s>
Reversible	O
causes	O
of	O
complete	O
AV	B-Algorithm
block	I-Algorithm
should	O
be	O
ruled	O
out	O
before	O
the	O
insertion	O
of	O
a	O
permanent	B-Device
pacemaker	I-Device
,	O
such	O
as	O
drugs	O
that	O
slow	B-Algorithm
heart	I-Algorithm
rate	I-Algorithm
and	O
which	O
induce	O
hyperkalemia	O
.	O
</s>
<s>
Complete	O
atrioventricular	B-Algorithm
block	I-Algorithm
in	O
acute	O
myocardial	O
infarction	O
should	O
be	O
treated	O
with	O
temporary	O
pacing	O
and	O
revascularization	O
.	O
</s>
<s>
Complete	O
atrioventricular	B-Algorithm
block	I-Algorithm
caused	O
by	O
hyperkalemia	O
should	O
be	O
treated	O
to	O
lower	O
serum	O
potassium	O
levels	O
and	O
patients	O
with	O
hypothyroidism	O
should	O
also	O
receive	O
thyroid	O
hormone	O
.	O
</s>
<s>
If	O
there	O
is	O
no	O
reversible	O
cause	O
,	O
the	O
clear	O
treatment	O
of	O
complete	O
atrioventricular	B-Algorithm
block	I-Algorithm
is	O
mostly	O
permanent	B-Device
pacemaker	I-Device
placement	O
.	O
</s>
<s>
The	O
prognosis	O
of	O
patients	O
with	O
complete	B-Architecture
heart	I-Architecture
block	I-Architecture
is	O
generally	O
poor	O
without	O
therapy	O
.	O
</s>
