<s>
Parasystole	B-Algorithm
is	O
a	O
kind	O
of	O
arrhythmia	B-Application
caused	O
by	O
the	O
presence	O
and	O
function	O
of	O
a	O
secondary	O
pacemaker	B-Algorithm
in	O
the	O
heart	O
,	O
which	O
works	O
in	O
parallel	O
with	O
the	O
SA	O
node	O
.	O
</s>
<s>
Parasystolic	B-Algorithm
pacemakers	O
are	O
protected	O
from	O
depolarization	O
by	O
the	O
SA	O
node	O
by	O
some	O
kind	O
of	O
entrance	B-Algorithm
block	I-Algorithm
.	O
</s>
<s>
Parasystolic	B-Algorithm
pacemakers	O
can	O
exist	O
in	O
both	O
the	O
atrium	O
or	O
the	O
ventricle	O
.	O
</s>
<s>
Two	O
forms	O
of	O
ventricular	O
parasystole	B-Algorithm
have	O
been	O
described	O
in	O
the	O
literature	O
,	O
fixed	O
parasystole	B-Algorithm
and	O
modulated	O
parasystole	B-Algorithm
.	O
</s>
<s>
Fixed	O
ventricular	O
parasystole	B-Algorithm
occurs	O
when	O
an	O
ectopic	B-Algorithm
pacemaker	I-Algorithm
is	O
protected	O
by	O
entrance	B-Algorithm
block	I-Algorithm
,	O
and	O
thus	O
its	O
activity	O
is	O
completely	O
independent	O
from	O
the	O
sinus	O
pacemaker	B-Algorithm
activity	O
.	O
</s>
<s>
Hence	O
,	O
the	O
ectopic	B-Algorithm
pacemaker	I-Algorithm
is	O
expected	O
to	O
fire	O
at	O
a	O
fixed	O
rate	O
.	O
</s>
<s>
Therefore	O
,	O
on	O
ECG	B-Application
,	O
the	O
coupling	O
intervals	O
of	O
the	O
manifest	O
ectopic	B-Algorithm
beats	I-Algorithm
will	O
wander	O
through	O
the	O
basic	O
cycle	O
of	O
the	O
sinus	O
rhythm	O
.	O
</s>
<s>
Accordingly	O
,	O
the	O
traditional	O
electrocardiographic	B-Application
criteria	O
used	O
to	O
recognize	O
the	O
fixed	O
form	O
of	O
parasystole	B-Algorithm
are	O
:	O
</s>
<s>
the	O
presence	O
of	O
variable	O
coupling	O
intervals	O
of	O
the	O
manifest	O
ectopic	B-Algorithm
beats	I-Algorithm
;	O
</s>
<s>
fusion	B-Algorithm
beats	I-Algorithm
.	O
</s>
<s>
According	O
to	O
the	O
modulated	O
parasystole	B-Algorithm
hypothesis	O
,	O
rigid	O
constancy	O
of	O
a	O
pacemaker	B-Algorithm
might	O
be	O
expected	O
if	O
the	O
entrance	B-Algorithm
block	I-Algorithm
were	O
complete	O
,	O
but	O
if	O
there	O
is	O
an	O
escape	O
route	O
available	O
for	O
the	O
emergence	O
of	O
ectopic	O
activity	O
,	O
then	O
clearly	O
there	O
must	O
be	O
an	O
effective	O
ionic	O
communication	O
,	O
not	O
complete	O
insulation	O
,	O
between	O
the	O
two	O
tissues	O
.	O
</s>
<s>
communication	O
between	O
the	O
two	O
,	O
then	O
the	O
depolarization	O
of	O
the	O
surrounding	O
ventricle	O
may	O
influence	O
the	O
ectopic	B-Algorithm
pacemaker	I-Algorithm
.	O
</s>
<s>
of	O
the	O
pacemaker	B-Algorithm
cells	I-Algorithm
.	O
</s>
<s>
Therefore	O
,	O
appropriate	O
diagnosis	O
of	O
modulated	O
parasystole	B-Algorithm
relies	O
upon	O
the	O
construction	O
of	O
a	O
“	O
phase	O
response	O
curve	O
”	O
as	O
theoretical	O
evidence	O
of	O
modulation	O
of	O
the	O
ectopic	B-Algorithm
pacemaker	I-Algorithm
cycle	O
length	O
by	O
the	O
electrotonic	O
activity	O
generated	O
by	O
the	O
sinus	O
discharges	O
across	O
the	O
area	O
of	O
protection	O
.	O
</s>
<s>
In	O
this	O
case	O
,	O
the	O
timing	O
of	O
the	O
arrival	O
of	O
the	O
electronic	O
stimulus	O
will	O
serve	O
to	O
delay	O
or	O
advance	O
the	O
subsequent	O
pacemaker	B-Algorithm
activation	O
.	O
</s>
