<s>
A	O
sinoatrial	B-Algorithm
block	I-Algorithm
(	O
also	O
spelled	O
sinuatrial	O
block	O
)	O
is	O
a	O
disorder	O
in	O
the	O
normal	O
rhythm	O
of	O
the	O
heart	O
,	O
known	O
as	O
a	O
heart	B-Algorithm
block	I-Algorithm
,	O
that	O
is	O
initiated	O
in	O
the	O
sinoatrial	O
node	O
.	O
</s>
<s>
The	O
initial	O
action	B-Algorithm
impulse	I-Algorithm
in	O
a	O
heart	O
is	O
usually	O
formed	O
in	O
the	O
sinoatrial	O
node	O
(	O
SA	O
node	O
)	O
and	O
carried	O
through	O
the	O
atria	O
,	O
down	O
the	O
internodal	O
atrial	O
pathways	O
to	O
the	O
atrioventricular	O
node	O
(	O
AV	O
)	O
node	O
.	O
</s>
<s>
In	O
normal	O
conduction	O
,	O
the	O
impulse	O
would	O
travel	O
across	O
the	O
bundle	B-Algorithm
of	I-Algorithm
His	I-Algorithm
(	O
AV	B-Algorithm
bundle	I-Algorithm
)	O
,	O
down	O
the	O
bundle	O
branches	O
,	O
and	O
into	O
the	O
Purkinje	B-Algorithm
fibers	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
the	O
atrial	O
beat	O
.	O
</s>
<s>
(	O
An	O
AV	B-Algorithm
block	I-Algorithm
,	O
occurs	O
in	O
the	O
AV	O
node	O
and	O
delays	O
ventricular	O
depolarisation	O
)	O
.	O
</s>
<s>
SA	B-Algorithm
blocks	I-Algorithm
are	O
categorized	O
into	O
three	O
classes	O
based	O
on	O
the	O
length	O
of	O
the	O
delay	O
.	O
</s>
<s>
Sinoatrial	B-Algorithm
blocks	I-Algorithm
are	O
typically	O
well	O
tolerated	O
.	O
</s>
<s>
They	O
are	O
not	O
as	O
serious	O
as	O
an	O
AV	B-Algorithm
block	I-Algorithm
and	O
most	O
often	O
do	O
not	O
require	O
treatment	O
.	O
</s>
<s>
They	O
can	O
also	O
lead	O
to	O
cessation	B-Algorithm
of	I-Algorithm
the	I-Algorithm
SA	I-Algorithm
node	I-Algorithm
and	O
more	O
serious	O
dysrhythmias	O
.	O
</s>
<s>
In	O
a	O
first	O
degree	O
sinoatrial	B-Algorithm
block	I-Algorithm
,	O
there	O
is	O
a	O
lag	O
between	O
the	O
time	O
that	O
the	O
SA	O
node	O
fires	O
and	O
actual	O
depolarization	O
of	O
the	O
atria	O
.	O
</s>
<s>
It	O
can	O
be	O
detected	O
only	O
during	O
an	O
electrophysiology	B-Algorithm
study	I-Algorithm
when	O
a	O
small	O
wire	O
is	O
placed	O
against	O
the	O
SA	O
node	O
from	O
within	O
the	O
heart	O
and	O
the	O
electrical	O
impulses	O
can	O
be	O
recorded	O
as	O
they	O
leave	O
the	O
p-cells	O
in	O
the	O
centre	O
of	O
the	O
node	O
[	O
see	O
pacemaker	B-Algorithm
potential	I-Algorithm
]	O
,	O
followed	O
by	O
observing	O
a	O
delay	O
in	O
the	O
onset	O
of	O
the	O
p	O
wave	O
on	O
the	O
ECG	O
.	O
</s>
<s>
Second	O
degree	O
SA	B-Algorithm
blocks	I-Algorithm
are	O
broken	O
down	O
into	O
two	O
subcategories	O
just	O
like	O
AV	B-Algorithm
blocks	I-Algorithm
are	O
:	O
</s>
<s>
Note	O
that	O
this	O
is	O
quite	O
different	O
from	O
the	O
Wenckebach	O
AV	B-Algorithm
block	I-Algorithm
,	O
in	O
which	O
the	O
PR	O
interval	O
gets	O
progressively	O
longer	O
,	O
before	O
the	O
dropped	O
QRS	O
segment	O
.	O
</s>
<s>
A	O
third	O
degree	O
sinoatrial	B-Algorithm
block	I-Algorithm
looks	O
very	O
similar	O
to	O
a	O
sinus	B-Algorithm
arrest	I-Algorithm
.	O
</s>
<s>
However	O
,	O
a	O
sinus	B-Algorithm
arrest	I-Algorithm
is	O
caused	O
by	O
a	O
failure	O
to	O
form	O
impulses	O
.	O
</s>
<s>
The	O
pause	O
ends	O
with	O
a	O
P	O
wave	O
,	O
instead	O
of	O
a	O
junctional	B-Algorithm
escape	I-Algorithm
beat	I-Algorithm
the	O
way	O
a	O
sinus	B-Algorithm
arrest	I-Algorithm
would	O
.	O
</s>
<s>
Emergency	O
treatment	O
consists	O
of	O
administration	O
of	O
atropine	O
sulfate	O
or	O
transcutaneous	B-Algorithm
pacing	I-Algorithm
.	O
</s>
