<s>
Quatrefoil	B-Algorithm
reentry	I-Algorithm
is	O
a	O
type	O
of	O
cardiac	B-Application
arrhythmia	I-Application
that	O
consists	O
of	O
two	O
adjacent	O
figure-of-eight	O
reentrant	O
circuits	O
.	O
</s>
<s>
Quatrefoil	B-Algorithm
reentry	I-Algorithm
can	O
be	O
induced	O
by	O
stimulating	O
the	O
heart	O
through	O
a	O
single	O
electrode	O
twice	O
,	O
with	O
the	O
second	O
stimulus	O
timed	O
near	O
the	O
end	O
of	O
the	O
refractory	O
period	O
of	O
the	O
first	O
action	O
potential	O
.	O
</s>
<s>
Quatrefoil	B-Algorithm
reentry	I-Algorithm
can	O
be	O
understood	O
qualitatively	O
using	O
a	O
simple	O
cellular	O
automaton	O
.	O
</s>
