Transcript Measurement of Intracardiac Bioimpedance in Rate Adaptive

ICE2004, Kyoto, June 27–July 1, 2004
Measurement of Intracardiac Bioimpedance
in Rate Adaptive Pacemakers
Alar Kuusik, Raul Land, Mart Min, Toomas Parve, Gustav Poola
Tallinn University of Technology, Tallinn, Estonia
ABSTRACT:
A method of highly accurate measurement of intracardiac bioimpedance
usable in implantable rate adaptive pacemakers and portative cardiomonitors
based on lock-in signal processing and bipolar pulse waveform signals
is proposed
ICE2004, Kyoto,
June 27-July 1, 2004
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Measurement of Intracardiac Bioimpedance in Rate Adaptive Pacemaker
A. Kuusik, R. Land, M. Min, T. Parve, G. Poola
(Tallinn University of Technology, Tallinn, Estonia)
EQUIVALENT CIRCUITS AND PHASOR DIAGRAM
FOR THE ELECTRICAL BIOIMPEDANCE
Ż
time

rext
a)
a)
electrical
bioimpedance
Time
variant
bioimpedance Ż of
a biological object
rint
time
time
C
time

Ż=R+ jX
R
time
X
time
b)
c)
b)
c)
electrical equiserial equivalent
valent
circuit
The
three-element
The two-component
equivalent circuit of
(R=ReŻ and X=ImŻ)
the bioimpedance Ż serial equivalent of Ż
- jX
Ż(t)
X(t)
X0
Ż(t)
Ż(t)
 (t)
Ż0 
R0 R(t)
R
d)
Phasor diagram of
the time variant
bioimpedance Ż=R+jX
In the multiple-element equivalent circuit (b) variations of resistive elements rext and rint
can cause changes in the imaginary part X=ImŻ, and variations of the capacitive element
C can cause changes in the real part R=ReŻ.
Depending on frequency, these contradictory changes in ReŻ and ImŻ can be more or less
significant. But because of this phenomenon unexpectedly high accuracy of measurement of R and X is needed to allow to calculate by them the values for rext , rint , and C.
ICE2004, Kyoto,
June 27-July 1, 2004
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Measurement of Intracardiac Bioimpedance in Rate Adaptive Pacemaker
A. Kuusik, R. Land, M. Min, T. Parve, G. Poola
(Tallinn University of Technology, Tallinn, Estonia)
a) Conventional solution
SHAPES OF SIGNALS
(a)
Conventional solution of pulse wave excitation
signal for using in intracardiac impedance
measurement.
Excitation current waveform
10 μA
0 1
1.15
5
10 11
15
11.15
20
Time,
ms
b) Novel solution
Excitation current waveform
10 μA
(b)
Novel solution of pulse wave excitation and
reference signals for lock-in signal conversion in
measurement of the intracardiac EBI.
0°
90°
180°
270°
360°
Phase,
deg
Reference signal for lock-in demodulation
0°
90°
180°
270°
360°
Phase,
deg
ICE2004, Kyoto,
June 27-July 1, 2004
Measurement of Intracardiac Bioimpedance in Rate Adaptive Pacemaker
A. Kuusik, R. Land, M. Min, T. Parve, G. Poola
4
(Tallinn University of Technology, Tallinn, Estonia)
THE NOVEL LOCK-IN EBI MEASUREMENT SYSTEM
Block diagram of the novel lock-in EBI measurement system
based on application of the shortened pulse signals.
Note: In comparison with the common solution modifications are introduced in SDs, Formator, and
Sequencer.
ICE2004, Kyoto,
June 27-July 1, 2004
Measurement of Intracardiac Bioimpedance in Rate Adaptive Pacemaker
A. Kuusik, R. Land, M. Min, T. Parve, G. Poola
5
(Tallinn University of Technology, Tallinn, Estonia)
THE LOCK-IN EBI MEASUREMENT SYSTEM (continued)
SD
+G
VZ
Mux
VOUT
−G
{Vref }
Circuit diagram of the synchronous detector (SD)
operating with shortened pulse.
Very little changes are needed - in the Mux the third position with grounded input is introduced.
ICE2004, Kyoto,
June 27-July 1, 2004
Measurement of Intracardiac Bioimpedance in Rate Adaptive Pacemaker
A. Kuusik, R. Land, M. Min, T. Parve, G. Poola
6
(Tallinn University of Technology, Tallinn, Estonia)
THE SHAPES AND SPECTRA OF THE SIGNALS
1.0
Rectangular waveforms
with pulse shortening
by 18° and 30°
Spectra of the
rectangular waveforms
with shortened pulses
0.5
The 1st harmonic
0.0
18º
30º
-0.5
-1.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
t/T
Relative magnitude of harmonic
1.0
0.8
0.6
f ( x) 

4a  cosb
cos3b
cos5b

sin
x

sin
3
x

sin
5
x

...

  1
3
5


cos(2n  1)b
 sin(2n  1) x
 n  0 2n  1
4a

The coinciding
harmonics
0.4
0.2
0.0
01
3
5
7
9
11 13 15 17 19 21 23 25
Order of harmonics
ICE2004, Kyoto,
June 27-July 1, 2004
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Measurement of Intracardiac Bioimpedance in Rate Adaptive Pacemaker
A. Kuusik, R. Land, M. Min, T. Parve, G. Poola
(Tallinn University of Technology, Tallinn, Estonia)
THE PHASOR ERRORS
1.4
1.4
X
max  = ± 4.1
max Z = 23%
1.3
1.2
corresponds to rectangular wave
1.1
corresponds to sine wave
1.0
0.9
0.9
0.8
0.8
0.6
0.5
X
0.7
true vector
0.6
Z
^
X
measured vector
0.4
corresponds to sine wave
Z
^
X
0.5
0.4

0.3

0.3
R
0.2

0.1
0.0
0.0
corresponds to the SDC wave
1.1
1.0
0.7
max  = ± 1
max Z = ±2.4%
1.3 X
1.2
0.2
0.2
R
0.4
0.6
0.8

0.1
^
1.0
a) ordinary rectangular waveforms
1.2 R 1.4
0.0
0.0
^
R
0.2
0.4
0.6
0.8
R
1.0
1.2
1.4
b) rectangular waveforms with shortened pulses
Trajectories of the tip of impedance phasor Ż = R + jX
for the two cases of estimating the phasor from the results of measurement R and X
using rectangular waveform signals instead of the sine-waves (giving an arc of a cycle)
in the case of a purely resistive reference and variable time delay used as the phase shift
(the systematic magnitude error Z and phase error  are shown).
ICE2004, Kyoto,
June 27-July 1, 2004
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Measurement of Intracardiac Bioimpedance in Rate Adaptive Pacemaker
A. Kuusik, R. Land, M. Min, T. Parve, G. Poola
(Tallinn University of Technology, Tallinn, Estonia)
THE PHASOR ERRORS (Cont.1)
Relative
magnitude
errorerror
ΔZ/Z,
% %
Relative
magnitude
ΔZ/Z,
25
20
20
15
15
10
10
5
5
0
0
-5
-5
-10
-10
-15
0
10
20
30
40
50
60
70
80
, °
90
-15
0
2
0
0
-2
-2
0
10
20
30
40
50
60
70
80
, °
a) ordinary rectangular waveforms
20
30
40
50
60
70
80
90
Phase error ΔΦ, °
4
2
-4
10
, °
Phase error ΔΦ, °
4
Relative magnitude error ΔZ/Z, %
25
90
-4
0
10
20
30
40
50
60
70
80
90
, °
b) rectangular waveforms with shortened pulses
Systematic magnitude error Z and phase error 
in case of applying rectangular waveform signals instead of the sine-wave signals
to a purely resisitive element.
ICE2004, Kyoto,
June 27-July 1, 2004
9
Measurement of Intracardiac Bioimpedance in Rate Adaptive Pacemaker
A. Kuusik, R. Land, M. Min, T. Parve, G. Poola
(Tallinn University of Technology, Tallinn, Estonia)
THE PHASOR ERRORS (cont.2)
Z, R0
2.0
1.0
1.0
0.1
0.5
1
0.0
10
100
1000
10000
10
100
1000
10000
0
-0.5
-10
500ohm
200ohm
-1.0
The 1st harmonic
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
-30
1.75nF
Relative magnitude of harmonic
1.0
t/T
All the harmonics
are coinciding
0.6
0.4
f,
0.000
ΔZ
R0
-0.060
01
3
5
7
9 11 13 15 17 19 21 23 25
Order of harmonics
The frequency response characteristics of
the typical equivalent circuit achieved
using ordinary rectangular waveforms
as excitation and reference signals.
The phasor errors can reach 5 deg and 10%.
kHz
-0.020
-0.040
0.2
0.0
-40
1
0.8
Equivalent of a
tissue segment
( myocardium )
used.
Φ
-20
-0.080
-0.100
1
10
100
1000
10000
10
100
1000
10000
5.000
ΔΦ
4.000
3.000
2.000
1.000
0.000
1
f,
kHz
ICE2004, Kyoto,
June 27-July 1, 2004
10
Measurement of Intracardiac Bioimpedance in Rate Adaptive Pacemaker
A. Kuusik, R. Land, M. Min, T. Parve, G. Poola
(Tallinn University of Technology, Tallinn, Estonia)
THE PHASOR ERRORS (cont.3)
Z , R0
2.0
1.0
1.0
0.1
0.5
0.0
18º
30º
-0.5
500ohm
200ohm
1
The 1st harmonic
100
1000
10000
10
100
1000
10000
0
-1.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
t/T
1.75nF
Relative magnitude of harmonic
1.0
10
-10
Φ
-20
-30
-40
1
0.8
Equivalent of a
tissue segment
( myocardium )
used.
f,
0.6
The coinciding
harmonics
0.4
0.2
0.0
kHz
0.010
ΔZ
R0
0.008
0.006
0.004
01
3
5
7
9 11 13 15 17 19 21 23 25
Order of harmonics
The frequency response characteristics of the
typical equivalent circuit achieved using
rectangular waveforms with shortened
pulses as excitation and reference signals.
0.002
0.000
1
100
1000
10000
10
100
1000
10000
0.500
ΔΦ
0.400
0.300
0.200
0.100
0.000
1
The phasor errors do net exceed 0.2 deg and 0.3%.
10
f,
kHz
ICE2004, Kyoto,
June 27-July 1, 2004
Measurement of Intracardiac Bioimpedance in Rate Adaptive Pacemaker
A. Kuusik, R. Land, M. Min, T. Parve, G. Poola
11
(Tallinn University of Technology, Tallinn, Estonia)
CONCLUSIONS
Proposed lock-in signal conversion techniques on the basis
of rectangular waveforms with shortened pulses is sufficiently
simple and power efficient to be used in implantable biomedical
devices.
Despite its simplicity, it ensures acceptable estimates of the real
(Re) and imaginary (Im) parts of the electrical bioimpedance.
Obtained estimations are trustable for determination of the beatto-beat stroke volume and duration of systolic and diastolic
intervals, which are playing an important role in the adaptive
adjustment of pacing rate, and in maintaining of the required level
of myocardium's energy supply.
ICE2004, Kyoto,
June 27-July 1, 2004
Measurement of Intracardiac Bioimpedance in Rate Adaptive Pacemaker
A. Kuusik, R. Land, M. Min, T. Parve, G. Poola
12
(Tallinn University of Technology, Tallinn, Estonia)
Acknowledgments
This work is supported by Estonian Science Foundation, grants 5892, 5897, and 5902,
and partially by Japan Society for the Promotion of Science (JSPS) 2003 postdoctoral
fellowship program.
REFERENCES
J.G. Webster (Ed.), Design of Cardiac Pacemakers. 1995.
A rate adpative pacemaker. Internat. patents PCT WO 0057953 and PCT WO 00/57954, M. Min, A. Kink and T. Parve. 2000.
S. Grimnes and Ų.G. Martinsen, Bioimpedance and Bioelectricity Basics. 2000.
M. Min, O. Märtens and T. Parve, - Measurement. 27, no.1, 21 (2000).
M. Min, T. Parve, V. Kukk and A. Kuhlberg, - IEEE Trans. Instrum. & Meas., 51, 674 (2002).
A. Kuusik, R. Land, M. Min and T. Parve. - Internat. Journ. of BioElectroMagnetism, 5, 1, 23 (2003).
A method and a device for measuring electrical bioimpedance. International patent application PCT/EE03/00006, filed 28.11.2003, M. Min, A. Kink, R. Land, T. Parve.
Thank you for your attention!
Resuscitation of an animal (pig) heart
in the Laboratory of Bionics at the Chair of Electronic Measurements,
Department of Electronics, Tallinn University of Technology, ESTONIA
ICE2004, Kyoto,
June 27-July 1, 2004
Measurement of Intracardiac Bioimpedance in Rate Adaptive Pacemaker
A. Kuusik, R. Land, M. Min, T. Parve, G. Poola
13
(Tallinn University of Technology, Tallinn, Estonia)
Using of intracardiac electrical bioimpedance (EBI) for pacing rate control requires trustable measurements. Usually, the short (<1ms) and low level (10mA) excitation pulses are used to get the response characterizing the
impedance. Unfortunately, the response is weak and spread over the frequency range, and it is difficult to interpret the measurement results. In our novel approach, the excitation energy is concentrated at the frequency of interest,
and reliable determination of both, the real (Re) and imaginary (Im) parts of the impedance is achieved at selected frequencies. Different bipolar pulse waveforms are used for excitation and for lock-in demodulator. Obtained
EBI-based information is trustable for determination of the beat-to-beat stroke volume and duration of systolic and diastolic intervals, used for adaptive adjustment of the pacing rate, and for maintaining required myocardium’s
energy supply level.
1. Introduction
Pacing rate control, based on information extracted from the measurement of intracardiac electrical bioimpedance (EBI) is safe only when the measurement results are trustable [1]. This is not an easy task, particularly in case of
implantable pacemakers, which have to operate for years without any service. Usually, therefore the simplest methods of EBI measurement are exploited, which work properly for determining the parameters of breathing activity
and the cardiac activity [1].
In general, the EBI comprises more information, e.g., on the status of cardiac muscle (myocard) [1, 2]. This information can not be easily obtained from the results of pulse based EBI measurement, as it usually is based on
analysis of the transient response processes. Typically, a short (<1ms) and low level (10μA) excitation pulses (Fig.1a) are used to get the response, which is used to determine the impedance [1]. Unfortunately, the response to a
short pulse is spread over a wide frequency range and reflection of the certain components of the equivalent circuit in the response signal is weak. So it is difficult to interpret the measurement results, even if the simplest threeelement equivalent circuit is used [3].
For three-element equivalent circuit, both the transient response and the frequency response measurement can be used. But in the case of real EBI, which in fact has a much more complicated equivalent circuit, it is quite
complicated or even impossible to perform, because only limited computing resources are available in the implanted devices.
2. Method
As the pulse form signals are very suitable for the implantable devices, it is of the interest to obtain the EBI measurement method, where the pulse form signals are used, though the term of complex impedance has been defined
for the sine wave signals. But it is still possible to measure directly only the active and reactive components R and X of the complex impedance Ż = R + jX (or G and B of the complex admittance Y = G + jB), which are mutually in
quadrature [3, 4, 5].
To avoid excessive mathematical conversion errors, R and X (or G and B) must be measured with required uncertainty, which is hardly achievable in the implantable devices.
In our novel approach, the pulse waves are successfully used for high precision EBI measurements thanks to using of the lock-in approach, where the excitation energy as well as measurement sensitivity are concentrated at the
frequency of interest, and reliable determination of both, the real (Re) and imaginary (Im) parts of the impedance is achieved at selected frequencies [6].
The block diagram of the lock-in EBI measurement system based on application of the novel pulse waveform signals in Fig. 2, where traditional lock-in system is modified without introducing significant complexity.
Essential is to reduce the higher odd harmonic content of the excitation signal, and to decrease the sensitivity of the switching-type synchronous detectors to the lower order of harmonics of the excitation signal. The simplest
appropriate approximation of the sine wave is shortening of the rectangular signal pulses and introducing zero-level intervals, yielding spectrum, given by:
(1)
where - a is the magnitude value of the pulse signal, b characterises the shortening of pulses , and is equal to the duration of the signal’s zero value segment within half period (b = 0…/2).
According to Eq.(1), from all of the easy-to-generate waveform pairs with maximally different harmonic content, the best one is consisting of waveforms having 30° (/6) and 18° (/10) pulse shortening, which removes the
harmonics 3(2n+1) and 5(2n+1) correspondingly from the signal spectra.
As different bipolar pulse waveforms are used for excitation, and for lock-in demodulation (Fig.1b), the errors caused by higher odd harmonics are reduced significantly [6]. In the case of typical three-element equivalent circuit
the systematic error of determining the frequency response of impedance is reduced to a level not exceeding 0.3% (against 10% in the case of using common rectangular waveforms).
In Fig. 3 a block diagram of the modified synchronous detector is shown, which is modified in comparison with the conventional switching type SD. The operating mode with shortened pulses is achieved by introducing the
third, zero-gain phase of the synchronous detector [7].