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DIFFERENT APPROACHES MEASUREMENT OF HEMODYNAMIC BY
ELECTRICAL IMPEDANCE PLETHYSMOGRAPHY METHOD
Albinas Stankus
KMU, Institute of Psychophysiology and Rehabilitation, Palanga,
Lithuania
[email protected]
Abstract: The goal of presented study is an elaboration of
methodology for direct measurement ofcirculation by means of volume
units. Particular hardware has been elaborated for this reason. A
newprinciple in measurement of relative changes of the pulse wave
amplitude allows registering byelectroplethysmogram. It enabled
directly to measure changes in the volume of tissues and to
assesshemodynamics of various body parts by relative (ml/100ml)
volume units and do this method more precise.This study enabled to
improve this method for measurement of the blood circulation in
legs. This methodcould be used for diagnostic purposes in vascular
pathology.
Introduction
Popularity of the impedance plethysmography method is based on
its simplicity. However, somebiophysical and methodological
problems occur in the impedance cardiography, performing
measurementsof the stroke volume. During the last 30 years, main
attention of the investigators was focused on thecomparison of the
absolute stroke volume values, measured by this method and found by
other methods.Among investigated persons correlation values in 23
studies were from 0.49 to 0.97. Because of physicalreasons, results
from earlier mentioned dependences might not be absolutely
accurate. Usage of thismethod for investigations of the blood
circulation in lower limbs is not doubtful. Values used in
limbsvolume computing, are the following: specific tissue impedance
(, *cm), distance between electrodes (l,cm) and constant impedance
(Z0).Specific impedance of tissues is considered constant and equal
to 135-150*cm. However experimentshowed that it depends on blood
hematocryte changes from 22 to 66%. Adapting the specific impedance
inthe concrete instances makes this method more difficult. In real
conditions it is very difficult to keep
parallelism of the placed band electrodes to precisely measure
them and then to keep a distance. Constantimpedance in many cases
is measured incorrectly, also from indicator. Generally, there are
too manyapproximate calculations and not enough precise
measurements. Measurement of the specific impedance,dependent on
tissues and blood properties (in the other words homogeneity of the
object to electricalconductance), is most difficult.
Materials and Methods
The new approaches measurement of hemodynamic is based on the
premise, that we must measuretetrapolar means directly the ratio,
as it is shown on equation: V/V0=- ko (Z/Z0). It is easy to realize
it byusing a computer or/and electronic method [1]. Supposing, that
the specific impedance of the measuredarea is constant, a direct
ratio is found between the alternating volume changes and the total
volumemeasured by the electrical method [1]. The blood filling of
the measured segment, whose pulse curveindicates how it changes in
time, averages the result, measured by this way. V/V0= ml/ml *100
%. Thisallows one to come to the measurement units, used in
physiology ml/100ml. The device is calibrated by0.1 % change of the
main impedance by a parallel connection of the resistor. The
maximum amplitudevalues found, by using this method, indicate how
many ml volume increases in each 100 ml segmentsbetween electrodes.
The amplitude changes without additional calculations that are
found relative changesof the volume in time. Circumferential
electrodes are placed over a segment of the thigh, knee-joint,
shinand foot.
Results
I investigated 43 healthy subjects (HSs) 30 - 69 years old and
42 patients (Pts) with differently stage ofendarteritis obliterans,
from 40 to 69 year age. Table 1 shows the results of measuring the
magnitude of
ratio resistances. Magnitude of maximal pulse waves amplitude
ratio to constant impedance was atThe maximume in knee-joint. In
other region this ratio was lesser. The statistic analysis of the
mainparameters shows very distinct and reliable differences between
the HSs and the Pts (p
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Table 1: Distribution of ratio resistance in leg
Subjects Region Thigh Knee-joint Shin Foot
HealthyLeft, % 0,115 0,00
60,165 0,010
0,115 0,073
0,103 0,010
Right, % 0,117 0,007
0,169 0,011 0,116 0,083
0,106 0,08
PatientsLeft, % 0.099 0.007* 0.121 0.008* 0.079 0.006* 0.073
0.006*
Right, % 0.095 0.007* 0.124 0.008* 0.086 0.007* 0.074 0.008*
Discussion
Applying of this method, had solved several serious problems. 1.
Registering amplitude changes withoutadditional recalculations
depict volumetric changes. The changes of the amplitude are
valuated in percentvalues. For example, registered in the leg area
0.1 % the impedance changes mean, that for each 100 mlof the leg
volume during the systole, comes 0.1 ml of blood. Its allow
directly evaluate circulation of thevarious regions of the organism
by measurement units, which are acceptable in medicine, e.g.
ml/100ml of
material. 2. The direct Z/Zmeasurement is performed more
precisely, than in other methods, where theZvalue was found from
the electropletysmographic sensor. 3. There is no need for
calculations of thecross section area or the length of the
conductor, because obtained results are not dependent on
thesevalues. Pulse wave volume changes are measured in each 100 ml.
volume as much as in big or in smallvolumes. There is another
dependence: physiological and anatomical. When changes in length or
in thecross section area of the conductor occur because of changes
in electrode position, and in the spacebetween electrodes from
another tissue type with different blood circulation can get in.
The distancebetween the electrodes influences other physiological
dependence, e.g. blood flow measurements isaveraged more in higher
volume because of the diversity of the tissues. 4. There is no need
for themeasurement of the specific impedance of the tissues,
because during calculations the ratio of theimpedances
disappears.This method of measurement reveals some discussable
problems, but does not avoid conflicts becausenon-homogeneity of
the tissues to the electrical conductance. This problem may be
solved only by
searching for tissues homogeneity, e.g. finding the best places
for generating and measuring electrodes.The problem of homogeneity
is related to one problem coefficients ko show, that absolute
compliancedoes not always exists between impedance and volume.
Current flow could occupy only a part of thevolume seen or measured
by physical methods. Adapting the practice values of the specific
impedance [2]for different organs is difficult and inaccurate. It
is better to define this value as specific impedance,corresponding
to the body area and to measure it every time. That would allow
closer a measurement ofthe blood circulation, not calculation. This
theme has many unanswered questions and needs
furtherinvestigating.
Conclusions.
The method of impedance plethysmogrephy with direct Z/Z
measurement of hemodynamic is moreprecisely. Relative amplitude of
the leg artery pulse wave was decreased in patients with
endarteritis
obliterans of leg.
References
[1] STANKUS A., LAURINAVIIUS A.(1995): Elektopletizmografas.
Patentas Nr. LT3906 BA61B 5/02:6[2] FAES T. J., VAN DER MEIJ H. A.,
DE MUNCK J. C., HEETHAAR R. M. (1999): The electric resistivityof
human tissues (100 Hz-10 MHz): a meta-analysis of review studies,
Physiol. Meas., 20(4): pp. R1-10.
