<s>
A	O
fusion	B-Algorithm
beat	I-Algorithm
occurs	O
when	O
electrical	O
impulses	O
from	O
different	O
sources	O
act	O
upon	O
the	O
same	O
region	O
of	O
the	O
heart	O
at	O
the	O
same	O
time	O
.	O
</s>
<s>
If	O
it	O
acts	O
upon	O
the	O
ventricular	O
chambers	O
it	O
is	O
called	O
a	O
ventricular	O
fusion	B-Algorithm
beat	I-Algorithm
,	O
whereas	O
colliding	O
currents	O
in	O
the	O
atrial	O
chambers	O
produce	O
atrial	O
fusion	B-Algorithm
beats	I-Algorithm
.	O
</s>
<s>
Ventricular	O
fusion	B-Algorithm
beats	I-Algorithm
can	O
occur	O
when	O
the	O
heart	O
's	O
natural	O
rhythm	O
and	O
the	O
impulse	O
from	O
a	O
pacemaker	B-Device
coincide	O
to	O
activate	O
the	O
same	O
part	O
of	O
a	O
ventricle	O
at	O
the	O
same	O
time	O
,	O
causing	O
visible	O
variation	O
in	O
configuration	O
and	O
height	O
of	O
the	O
QRS	B-Algorithm
complex	I-Algorithm
of	O
an	O
electrocardiogram	B-Application
reading	O
of	O
the	O
heart	O
's	O
activity	O
.	O
</s>
<s>
This	O
contrasts	O
with	O
the	O
pseudofusion	O
beat	O
wherein	O
the	O
pacemaker	B-Device
impulse	O
does	O
not	O
affect	O
the	O
complex	O
of	O
the	O
natural	O
beat	O
of	O
the	O
heart	O
.	O
</s>
<s>
Rare	O
or	O
isolated	O
fusion	B-Algorithm
beats	I-Algorithm
caused	O
by	O
pacemakers	B-Device
are	O
normal	O
as	O
well	O
,	O
but	O
if	O
they	O
occur	O
too	O
frequently	O
may	O
reduce	O
cardiac	O
output	O
and	O
so	O
can	O
require	O
adjustment	O
of	O
the	O
pacemaker	B-Device
.	O
</s>
