<s>
In	O
electrocardiography	B-Application
,	O
the	O
PR	B-Algorithm
interval	I-Algorithm
is	O
the	O
period	O
,	O
measured	O
in	O
milliseconds	O
,	O
that	O
extends	O
from	O
the	O
beginning	O
of	O
the	O
P	B-Algorithm
wave	I-Algorithm
(	O
the	O
onset	O
of	O
atrial	B-Application
depolarization	I-Application
)	O
until	O
the	O
beginning	O
of	O
the	O
QRS	B-Algorithm
complex	I-Algorithm
(	O
the	O
onset	O
of	O
ventricular	B-Application
depolarization	I-Application
)	O
;	O
it	O
is	O
normally	O
between	O
120	O
and	O
200	O
ms	O
in	O
duration	O
.	O
</s>
<s>
The	O
PR	B-Algorithm
interval	I-Algorithm
is	O
sometimes	O
termed	O
the	O
PQ	B-Algorithm
interval	I-Algorithm
.	O
</s>
<s>
Variations	O
in	O
the	O
PQ	B-Algorithm
interval	I-Algorithm
can	O
be	O
associated	O
with	O
certain	O
medical	O
conditions	O
:	O
</s>
<s>
A	O
long	O
PR	B-Algorithm
interval	I-Algorithm
(	O
of	O
over	O
200	O
ms	O
)	O
indicates	O
a	O
slowing	O
of	O
conduction	B-Algorithm
between	O
the	O
atria	O
and	O
ventricles	O
,	O
usually	O
due	O
to	O
slow	O
conduction	B-Algorithm
through	O
the	O
atrioventricular	O
node	O
(	O
AV	O
node	O
)	O
.	O
</s>
<s>
This	O
is	O
known	O
as	O
first	B-Algorithm
degree	I-Algorithm
heart	I-Algorithm
block	I-Algorithm
.	O
</s>
<s>
A	O
short	B-Algorithm
PR	I-Algorithm
interval	O
(	O
of	O
less	O
than	O
120ms	O
)	O
may	O
be	O
associated	O
with	O
a	O
Pre-excitation	O
syndromes	O
such	O
as	O
Wolff	O
–	O
Parkinson	O
–	O
White	O
syndrome	O
or	O
Lown	B-Algorithm
–	I-Algorithm
Ganong	I-Algorithm
–	I-Algorithm
Levine	I-Algorithm
syndrome	I-Algorithm
,	O
and	O
also	O
junctional	O
arrhythmia	O
like	O
atrioventricular	B-Algorithm
reentrant	I-Algorithm
tachycardia	I-Algorithm
or	O
junctional	B-Algorithm
rhythm	I-Algorithm
.	O
</s>
<s>
A	O
variable	O
PR	B-Algorithm
interval	I-Algorithm
may	O
indicate	O
other	O
types	O
of	O
heart	B-Algorithm
block	I-Algorithm
.	O
</s>
