
Biomedical
Signal
Processing
and
Control
21
(2015)
49–57
Contents
lists
available
at
ScienceDirect
Biomedical
Signal
Processing
and
Control
jo
ur
nal
homepage:
www.elsevier.com/locate/bspc
Measuring
synchronization
in
coupled
simulation
and
coupled
cardiovascular
time
series:
A
comparison
of
different
cross
entropy
measures
Chengyu
Liu
a,b,∗,1
,
Chengqiu
Zhang
c,1
,
Li
Zhang
d
,
Lina
Zhao
b
,
Changchun
Liu
b
,
Hongjun
Wang
a,∗
a
School
of
Information
Science
and
Engineering,
Shandong
University,
Jinan
250100,
China
b
School
of
Control
Science
and
Engineering,
Shandong
University,
Jinan
250061,
China
c
Department
of
Cardiology,
School
Hospital
of
Shandong
University,
Jinan
250061,
China
d
Department
of
Computing
Science
and
Digital
Technologies,
University
of
Northumbria,
Newcastle
upon
Tyne
NE1
8ST,
UK
a
r
t
i
c
l
e
i
n
f
o
Article
history:
Received
1
January
2015
Received
in
revised
form
16
April
2015
Accepted
5
May
2015
Keywords:
Cross
entropy
measure
Cardiovascular
time
series
Synchronization
Fuzzy
measure
entropy
RR
interval
Pulse
transit
time
a
b
s
t
r
a
c
t
Synchronization
provides
an
insight
into
underlying
the
interaction
mechanisms
among
the
bivariate
time
series
and
has
recently
become
an
increasing
focus
of
interest.
In
this
study,
we
proposed
a
new
cross
entropy
measure,
named
cross
fuzzy
measure
entropy
(C-FuzzyMEn),
to
detect
the
synchroniza-
tion
of
the
bivariate
time
series.
The
performances
of
C-FuzzyMEn,
as
well
as
two
existing
cross
entropy
measures,
i.e.,
cross
sample
entropy
(C-SampEn)
and
cross
fuzzy
entropy
(C-FuzzyEn),
were
first
tested
and
compared
using
three
coupled
simulation
models
(i.e.,
coupled
Gaussian
noise,
coupled
MIX(p)
and
coupled
Henon
model)
by
changing
the
time
series
length,
the
threshold
value
for
entropy
and
the
cou-
pling
degree.
The
results
from
the
simulation
models
showed
that
compared
with
C-SampEn,
C-FuzzyEn
and
C-FuzzyMEn
had
better
statistical
stability
and
compared
with
C-FuzzyEn,
C-FuzzyMEn
had
better
discrimination
ability.
These
three
measures
were
then
applied
to
a
cardiovascular
coupling
problem,
synchronization
analysis
for
RR
and
pulse
transit
time
(PTT)
series
in
both
the
normal
subjects
and
heart
failure
patients.
The
results
showed
that
the
heart
failure
group
had
lower
cross
entropy
values
than
the
normal
group
for
all
three
cross
entropy
measures,
indicating
that
the
synchronization
between
RR
and
PTT
time
series
increases
in
the
heart
failure
group.
Further
analysis
showed
that
there
was
no
signifi-
cant
difference
between
the
normal
and
heart
failure
groups
for
C-SampEn
(normal
2.13
±
0.37
vs.
heart
failure
2.07
±
0.16,
P
=
0.36).
However,
C-FuzzyEn
had
significant
difference
between
two
groups
(normal
1.42
±
0.25
vs.
heart
failure
1.31
±
0.12,
P
<
0.05).
The
statistical
difference
was
larger
for
two
groups
when
performing
C-FuzzyMEn
analysis
(normal
2.40
±
0.26
vs.
heart
failure
2.15
±
0.13,
P
<
0.01).
©
2015
Elsevier
Ltd.
All
rights
reserved.
1.
Introduction
Measuring
the
coupling
relationship
between
two
cardiovascu-
lar
time
series,
as
well
as
named
synchronization
measurement,
has
been
an
increasing
focus
of
interest
in
clinical
research
[1–3].
It
is
a
prerequisite
for
the
understanding
of
the
complexity
of
underlying
signal
generating
mechanisms
and
thus
for
the
detection
of
car-
diovascular
disorders
and
ongoing
perturbations
to
the
circulation
∗
Corresponding
authors
at:
Shandong
University,
Institute
of
Biomedical
Engi-
neering,
School
of
Control
Science
and
Engineering,
Jingshi
Road
17923,
Jinan
250061,
Shandong,
China.
Tel.:
+86
159
53148364;
fax:
+86
531
88393578.
E-mail
addresses:
(C.
Liu),
(H.
Wang).
1
Joint
first
authors:
these
authors
contributed
equally
to
this
work.
system
[4].
Traditionally,
the
cross-correlation
in
the
time
domain
as
well
as
the
cross-spectrum
or
coherency
in
the
frequency
domain
has
been
used
for
synchronization
measurement
[5].
These
tech-
niques
are
able
to
give
the
linear
relationship
between
two
systems.
However,
they
are
not
suitable
for
characterizing
the
real
cardio-
vascular
signals,
which
are
non-stationary
and
inherently
nonlinear
[6]
.
In
recent
years,
entropy-based
measures,
such
as
the
typical
approximate
entropy
(ApEn)
and
sample
entropy
(SampEn),
have
been
widely
used
for
the
physiological
time
series
analysis
to
explore
their
inherent
complexity.
And
their
generalized
forms,
cross-approximate
entropy
(C-ApEn)
[7]
and
cross-sample
entropy
(C-SampEn)
[8],
were
used
for
the
synchronization
test
[9–12].
For
fixed
bivariate
time
series
x(i)
and
y(i)(1
≤
i
≤
N),
C-ApEn
measures
the
conditional
regularity
or
frequency
of
y-patterns
similar
to
a
http://dx.doi.org/10.1016/j.bspc.2015.05.005
1746-8094/©
2015
Elsevier
Ltd.
All
rights
reserved.