<s>
First-degree	B-Algorithm
atrioventricular	I-Algorithm
block	I-Algorithm
(	O
AV	B-Algorithm
block	I-Algorithm
)	O
is	O
a	O
disease	O
of	O
the	O
electrical	B-Algorithm
conduction	I-Algorithm
system	I-Algorithm
of	I-Algorithm
the	I-Algorithm
heart	I-Algorithm
in	O
which	O
electrical	O
impulses	O
conduct	O
from	O
the	O
cardiac	O
atria	O
to	O
the	O
ventricles	O
through	O
the	O
atrioventricular	O
node	O
(	O
AV	O
node	O
)	O
more	O
slowly	O
than	O
normal	O
.	O
</s>
<s>
First	B-Algorithm
degree	I-Algorithm
AV	I-Algorithm
block	I-Algorithm
does	O
not	O
generally	O
cause	O
any	O
symptoms	O
,	O
but	O
may	O
progress	O
to	O
more	O
severe	O
forms	O
of	O
heart	O
block	O
such	O
as	O
second	B-Algorithm
-	O
and	O
third-degree	B-Architecture
atrioventricular	I-Architecture
block	I-Architecture
.	O
</s>
<s>
It	O
is	O
diagnosed	O
using	O
an	O
electrocardiogram	B-Application
,	O
and	O
is	O
defined	O
as	O
a	O
PR	B-Algorithm
interval	I-Algorithm
greater	O
than	O
200	O
milliseconds	O
.	O
</s>
<s>
First	B-Algorithm
degree	I-Algorithm
AV	I-Algorithm
block	I-Algorithm
affects	O
0.65-1.1	O
%	O
of	O
the	O
population	O
with	O
0.13	O
new	O
cases	O
per	O
1000	O
persons	O
each	O
year	O
.	O
</s>
<s>
The	O
most	O
common	O
causes	O
of	O
first-degree	B-Algorithm
heart	I-Algorithm
block	I-Algorithm
are	O
AV	O
nodal	O
disease	O
,	O
enhanced	O
vagal	O
tone	O
(	O
for	O
example	O
in	O
athletes	O
)	O
,	O
myocarditis	O
,	O
acute	O
myocardial	O
infarction	O
(	O
especially	O
acute	O
inferior	O
MI	O
)	O
,	O
electrolyte	O
disturbances	O
and	O
medication	O
.	O
</s>
<s>
The	O
medications	O
that	O
most	O
commonly	O
cause	O
first-degree	B-Algorithm
heart	I-Algorithm
block	I-Algorithm
are	O
those	O
that	O
increase	O
the	O
refractory	O
time	O
of	O
the	O
AV	O
node	O
,	O
thereby	O
slowing	O
AV	O
conduction	B-Algorithm
.	O
</s>
<s>
In	O
normal	O
individuals	O
,	O
the	O
AV	O
node	O
slows	O
the	O
conduction	B-Algorithm
of	O
electrical	O
impulses	O
through	O
the	O
heart	O
.	O
</s>
<s>
This	O
is	O
manifest	O
on	O
a	O
surface	O
electrocardiogram	B-Application
(	O
ECG	B-Application
)	O
as	O
the	O
PR	B-Algorithm
interval	I-Algorithm
.	O
</s>
<s>
The	O
normal	O
PR	B-Algorithm
interval	I-Algorithm
is	O
from	O
120	O
ms	O
to	O
200	O
ms	O
in	O
length	O
.	O
</s>
<s>
This	O
is	O
measured	O
from	O
the	O
initial	O
deflection	O
of	O
the	O
P	O
wave	O
to	O
the	O
beginning	O
of	O
the	O
QRS	B-Algorithm
complex	I-Algorithm
.	O
</s>
<s>
In	O
first-degree	B-Algorithm
heart	I-Algorithm
block	I-Algorithm
,	O
the	O
AV	O
node	O
conducts	O
the	O
electrical	O
activity	O
more	O
slowly	O
.	O
</s>
<s>
This	O
is	O
seen	O
as	O
a	O
PR	B-Algorithm
interval	I-Algorithm
greater	O
than	O
200	O
ms	O
in	O
length	O
on	O
the	O
surface	O
ECG	B-Application
.	O
</s>
<s>
It	O
is	O
usually	O
an	O
incidental	O
finding	O
on	O
a	O
routine	O
ECG	B-Application
.	O
</s>
<s>
First-degree	B-Algorithm
heart	I-Algorithm
block	I-Algorithm
does	O
not	O
require	O
any	O
particular	O
investigations	O
except	O
for	O
electrolyte	O
and	O
drug	O
screens	O
,	O
especially	O
if	O
an	O
overdose	O
is	O
suspected	O
.	O
</s>
<s>
Even	O
though	O
it	O
usually	O
does	O
not	O
progress	O
to	O
higher	O
forms	O
of	O
heart	O
block	O
,	O
it	O
may	O
require	O
outpatient	O
follow-up	O
and	O
monitoring	O
of	O
the	O
ECG	B-Application
,	O
especially	O
if	O
there	O
is	O
a	O
comorbid	O
bundle	B-Algorithm
branch	I-Algorithm
block	I-Algorithm
.	O
</s>
<s>
If	O
there	O
is	O
a	O
need	O
for	O
treatment	O
of	O
an	O
unrelated	O
condition	O
,	O
care	O
should	O
be	O
taken	O
not	O
to	O
introduce	O
any	O
medication	O
that	O
may	O
slow	O
AV	O
conduction	B-Algorithm
.	O
</s>
<s>
If	O
this	O
is	O
not	O
feasible	O
,	O
clinicians	O
should	O
be	O
very	O
cautious	O
when	O
introducing	O
any	O
drug	O
that	O
may	O
slow	O
conduction	B-Algorithm
;	O
and	O
regular	O
monitoring	O
of	O
the	O
ECG	B-Application
is	O
indicated	O
.	O
</s>
<s>
Isolated	O
first-degree	B-Algorithm
heart	I-Algorithm
block	I-Algorithm
has	O
no	O
direct	O
clinical	O
consequences	O
.	O
</s>
<s>
In	O
the	O
Framingham	O
Heart	O
Study	O
,	O
however	O
,	O
the	O
presence	O
of	O
a	O
prolonged	O
PR	B-Algorithm
interval	I-Algorithm
or	O
first	B-Algorithm
degree	I-Algorithm
AV	I-Algorithm
block	I-Algorithm
doubled	O
the	O
risk	O
of	O
developing	O
atrial	B-Application
fibrillation	I-Application
,	O
tripled	O
the	O
risk	O
of	O
requiring	O
an	O
artificial	B-Device
pacemaker	I-Device
,	O
and	O
was	O
associated	O
with	O
a	O
small	O
increase	O
in	O
mortality	O
.	O
</s>
<s>
A	O
subset	O
of	O
individuals	O
with	O
the	O
triad	O
of	O
first-degree	B-Algorithm
heart	I-Algorithm
block	I-Algorithm
,	O
right	B-Algorithm
bundle	I-Algorithm
branch	I-Algorithm
block	I-Algorithm
,	O
and	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
(	O
known	O
as	O
trifascicular	B-Algorithm
block	I-Algorithm
)	O
may	O
be	O
at	O
an	O
increased	O
risk	O
of	O
progression	O
to	O
complete	B-Architecture
heart	I-Architecture
block	I-Architecture
.	O
</s>
