<s>
An	O
ectopic	O
pacemaker	O
,	O
also	O
known	O
as	O
ectopic	B-Algorithm
focus	I-Algorithm
or	O
ectopic	O
foci	O
,	O
is	O
an	O
excitable	O
group	O
of	O
cells	O
that	O
causes	O
a	O
premature	B-Algorithm
heart	I-Algorithm
beat	I-Algorithm
outside	O
the	O
normally	O
functioning	O
SA	O
node	O
of	O
the	O
heart	O
.	O
</s>
<s>
It	O
is	O
thus	O
a	O
cardiac	B-Algorithm
pacemaker	I-Algorithm
that	O
is	O
ectopic	O
,	O
producing	O
an	O
ectopic	B-Algorithm
beat	I-Algorithm
.	O
</s>
<s>
Acute	O
occurrence	O
is	O
usually	O
non-life-threatening	O
,	O
but	O
chronic	O
occurrence	O
can	O
progress	O
into	O
tachycardia	B-Application
,	O
bradycardia	B-Algorithm
or	O
ventricular	O
fibrillation	O
.	O
</s>
<s>
However	O
,	O
in	O
the	O
instance	O
of	O
either	O
a	O
malfunctioning	O
SA	O
node	O
or	O
an	O
ectopic	B-Algorithm
focus	I-Algorithm
bearing	O
an	O
intrinsic	O
rate	O
superior	O
to	O
SA	O
node	O
rate	O
,	O
ectopic	O
pacemaker	O
activity	O
may	O
take	O
over	O
the	O
natural	O
heart	B-Algorithm
rhythm	I-Algorithm
.	O
</s>
<s>
As	O
a	O
rule	O
,	O
premature	O
ectopic	B-Algorithm
beats	I-Algorithm
(	O
i.e.	O
</s>
<s>
with	O
a	O
shorter	O
than	O
the	O
prevailing	O
preceding	O
R-R	O
'	O
interval	O
)	O
indicate	O
increased	O
myocyte	O
or	O
conducting	O
tissue	O
excitability	O
,	O
whereas	O
late	O
ectopic	B-Algorithm
beats	I-Algorithm
(	O
i.e.	O
</s>
<s>
with	O
a	O
prolonged	O
preceding	O
R-R	O
'	O
interval	O
)	O
indicate	O
proximal	O
pacemaker	O
or	O
conduction	B-Algorithm
failure	O
with	O
an	O
escape	O
'	O
ectopic	O
 '	O
beat	O
.	O
</s>
<s>
Isolated	O
ectopic	B-Algorithm
beats	I-Algorithm
frequently	O
cause	O
no	O
symptoms	O
,	O
although	O
the	O
most	O
common	O
symptom	O
is	O
the	O
perception	O
of	O
a	O
'	O
missed	O
beat	O
 '	O
.	O
</s>
<s>
An	O
ectopic	O
pacemaker	O
can	O
reside	O
within	O
a	O
part	O
of	O
the	O
electrical	B-Algorithm
conduction	I-Algorithm
system	I-Algorithm
of	I-Algorithm
the	I-Algorithm
heart	I-Algorithm
,	O
or	O
within	O
the	O
muscle	O
cells	O
of	O
the	O
atria	O
or	O
ventricles	O
.	O
</s>
<s>
On	O
an	O
ECG	B-Application
,	O
the	O
QRS	B-Algorithm
complex	I-Algorithm
will	O
be	O
abnormally	O
shaped	O
when	O
looking	O
at	O
ventricular	O
ectopic	O
activity	O
,	O
often	O
it	O
occurs	O
earlier	O
with	O
an	O
absent	O
P	O
wave	O
.	O
</s>
<s>
It	O
can	O
be	O
perceived	O
as	O
a	O
skipped	O
beat	O
on	O
both	O
the	O
ECG	B-Application
and	O
through	O
normal	O
pulse-taking	O
.	O
</s>
<s>
However	O
the	O
QRS	B-Algorithm
complex	I-Algorithm
and	O
T	O
waves	O
appear	O
relatively	O
normal	O
.	O
</s>
<s>
Conversely	O
,	O
during	O
junctional	O
ectopic	O
activity	O
the	O
P	O
wave	O
is	O
frequently	O
absent	O
or	O
can	O
be	O
hidden	O
in	O
the	O
QRS	B-Algorithm
complex	I-Algorithm
.	O
</s>
