Peripheral Vascular Measurement Using Electrical Impedance ...

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This system combines the advanced analog amplifier with the calculation power of digital signal processing to acquire real time monitoring. Our bioimpedance ...
Peripheral Vascular Measurement Using Electrical Impedance Plethysmography C. Corciova1, R. Ciorap1, R. Matei2, and A. Salceanu2 1

"Gr. T. Popa" University of Medicine and Pharmacy Iasi, Romania 2 “Gheorghe Asachi” Technical University of Iasi, Romania

Abstract— Impedance plethysmography (IPG) is a safe and noninvasive method for measuring peripheral hemodynamics. This paper’s aim is to develop a medical device system for a continuous monitoring of this parameter using the impedance technique. This system combines the advanced analog amplifier with the calculation power of digital signal processing to acquire real time monitoring. Our bioimpedance measuring system uses a generator which is under microprocessor control. Experimental investigations are conducted in order to determine the optimum interrelation between current injector output characteristics, power supply and electrode spacing. Keywords— impedance plethysmography, microcontroller, resistivity, algoritm, blood flow.

I. INTRODUCTION Numerous attempts and methods have been developed in order to measure pathologic and functional vascular change in the extremities. Little attention has been given to the quantitative changes recordable by various methods of electrical plethysmography, which until recently have been difficult to formulate. The electrical conductivity method gives a physical measure of the ionic conduction of a given body segment in contrast with electronic conduction characteristic of metallic substances. An attempt will be made to restate and formulate the laws of electrical conduction as they appear applicable to changes within a body segment which is being studied by the passage of a radio frequency current. As has been shown in literature, transient and static values of electrical conductivity are associated, respectively, with dynamic and balanced conditions of arterio - venous blood volume differences within a given segment. [3] The electrical impedance pulsation represents a changing number of ions brought to the segment by the arterial stream at a rate exceeding the venous outflow during the cycle. The overall change in volume of a segment is the differential effect of expansion and emptying of the vascular components of the entire segment. It may be possible to account segment ally for the volumetric shift of blood by considering its effect as a variable parallel electrical shunt. [5]

Equations for the effect of parallel resistance are well known. The total electrical conductance of the extremity segment is probably equal to the sum of the paralleled conductances of the blood and the corresponding segment. Each additional pulse of blood represents another path through which electrical current will flow. The effective parallel resistive value of the added or displaced blood may be derived by substitution of measured values in the expression: 2

Rblood =

Rtotal R0 R or Rblood ≈ 0 ΔR R0 − Rtotal

(1)

in which R0 represents the original resistance of the segment, Rtotal, the new total resistance. R0- Rtotal, which is equal to ΔR, represents the change in resistance incurred by the change in blood volume of the segment, by pulsation or otherwise. When small volume and resistive changes occur, then R02 expresses essentially value of the product RtotalR0. The volume of blood within the segment is a direct and linear function of electrical conductivity. This is true within wide limits of expansion of elastic cylinders such as arteries, veins, intestines, and rubber tubes. The inclusion of ground meat, long bone, or ground bone changes the slope of the relationship but does not destroy the lineal effect. The change in volume of blood uniformly distributed within a segment may then be calculated from the derived expression of the volume of a cylindrical conductor: Vb = ρ

l2 Rb

(2)

where (ρ) represents the specific resistivity of the segmental blood, (l) the length of the segment being measured, and Rb the calculated effective parallel resistance of the blood related to the change. The volume of blood pulsed into a peripheral segment is usually equal to the volume of blood leaving during the cycle. If one had an accurate measure of either the input or output, or of both volumes, it is probable that valuable data covering vascular responses could be scientifically expressed. If the measured value is closely proportional to the

S. Vlad and R.V. Ciupa (Eds.): MEDITECH 2011, IFMBE Proceedings 36, pp. 136–139, 2011. www.springerlink.com

Peripheral Vascular Measurement Using Electrical Impedance Plethysmography

absolute pulse volume, it would not be necessary to know either phase of the volume change. [7]

II. MATERIAL AND METHODS Impedance plethysmography is the measurement of volume changes through the measurement of the electrical impedance of the body tissues of interest. When measuring in limbs, these volume changes are mainly due to the blood flow. Common plethysmography measurements in a limb use four band electrodes placed around it to obtain a homogeneous electric field in the target volume. The signal detected was more affected by electrode placement, which may be important to estimate blood flow, but it is not essential for heart rate estimation. The distance (D) between the current injection electrodes should be as large as feasible, so as to cover as much area as possible of the leg segment being measured. The optimal separation distance between voltages - detection electrodes is not known in advance.

Fig. 1 The tetrapolar arrangement. The inner electrodes are designated E1, E2, and the outer electrodes I1, I2. The segment D (cm) in length between E1 and E2 is the effective resistance (R0).

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Essentially, it represents the bioimpedance measuring system device for the purposes of the present paper. Surface electrodes applied to peripheral vasculature form the interconnection to the patient. The measurement method involves providing of a periodic current excitation signal to the body part via a pair of current injecting electrodes. The flow through the body electrical current develops a potential corresponding to the pulsatile impedance variation of the tissues in accordance with the blood circulation. A second electrode pair, positioned between the first electrodes, senses the pulse wave related voltage changes. The bioimpedance measuring system includes a microprocessor controlled current generator with variable output. The microprocessor system drives the generator’s output stage under program control. We used microcontroller MSP430F169 because the total power consumption is reduced. The implemented microprocessor program control provides easy frequency adjustment in the range between 10 kHz to about 200 kHz. The MSP430 has 16 registers, and the ability to perform arithmetic directly on values in the memory. C compilers can be from this and produce more compact, efficient code. Texas Instruments issued a competitive benchmarking document that contains a comparison of the MSP430 with a range of other microcontrollers. Their performance was measured by compiling and executing the source code of a bunch of frequently used applications. [20]

Artery expansion during heart systole increases its cross section area from S0 to S0+ΔS and the arterial impedance of this segment decreases correspondingly from Z0 to Z0 −ΔZ. The fractional variation is thus

ΔZ ΔS . =− Z0 S0

The functional diagram of the implemented experimental embodiment is illustrated in Figure 2

Fig. 3 Circuit diagram of the current generator with microcontroller An amplitude detector follows and rectifies the modulated signal. After removing the DC component from the signal, an amplification of the resulting pulse wave takes place. The output signal thus obtained, which is indicative of the bioimpedance being measured and hence of the blood flow in the body segment, is visualized by a computer. [12] Fig. 2 System for bioimpedance measurement

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R1

D1 1

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Fig. 4 Schematic diagram for demodulator The algorithm for calculating the hemodynamic variables was implemented in MATLAB. The majority of the data processing and analysis was done in MATLAB to simplify software development. The system prompts for data inputs and then calculates hemodynamic parameters from the impedance signal. A measurement was made on 20 volunteers, clinically normal. Limb blood flow was calculated as flow / min, flow / beat and as a percentage of the stroke volume from the contemporaneously measured cardiac output. Every calculation was performed as an average over 6 heart beats.

In practice, one obtains the mean height of the pulse volume by planimetric integration over the entire pulse distance. The measurement of several pulses serves to reduce the error. This value is multiplied by two, since the recorded volume increase served both as a measure of input and of output volumes during the entire pulse cycle. The four electrode method basically eliminates the reactive skin and leaves one with a better measure of the internal tissues, including the blood, which appear predominantly resistive to alternating current. The pulsatile volume should probably be calculated on the resistive values obtained by tetrapolar leads, if a closer approximation to the true proportional pulse volume is desired. The figures below present plethysmography waveforms obtained for different values of distance (D) between the collecting electrodes E1 and E2 from the same volunteer.

Fig. 6 Plethysmography wave function of the distance (D/2), (D) and (2D) between the E1 and E2

Fig. 5 User interface in MATLAB

III. RESULTS AND DISCUSSION The recorded pulse volume is directly proportional, but not necessarily equal to the sum of the true arterial inflow and the venous outflow from a given segment. It follows that the mean height measurement of the pulse wave should be a valid index of this proportional volume. In our study, twice the mean height for the area under the curve is chosen to represent both input and output volume. In effect, this represents a sequestration of the total segmental input without occlusion or run - off of the venous return.

Fig. 7 Power spectral density of the plethysmographic wave: in figure a) for distance (D) and in figure b) for distance (2D)

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Peripheral Vascular Measurement Using Electrical Impedance Plethysmography

Table 1 Limb blood flow measurements in normal limbs % stroke volume Flow / beat (ml) Flow / minute (ml) Z0 (ohm)

Arm 1.10 – 5.00 0.95 – 4.30 92 - 283 42.0 - 81

Leg 1.70 – 6.56 1.58 – 7.10 103 - 480 37 - 70

The table 1 shows some calculated values of blood flow. The system needs to be calibrated in order to be able to calculate meaningful hemodynamic parameters from this data. In order to calibrate this system, a fixed resistor with known impedance would be measured by the system.

IV. CONCLUSIONS In this paper, an impedance plethysmography system for real time noninvasive cardiac output monitoring is developed and evaluated. This system combines the advance analog amplifier with the calculation power of digital signal processing; it is capable of detecting events in the incoming signal under different circumstances. Measurements of limb blood flow may be expressed as flow/min, flow/beat, as proportion of the flow in a control limb. The relative ease of impedance plethysmography facilitates the measurement of limb blood flow relative to the stroke volume or to flow in a control contra lateral limb, when changes in blood flow due to local factors are being monitored.

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Autor: Calin Corciova Institute: “Gr. T. Popa” University of Medicine and Pharmacy, Faculty of Medical Bioengineering Street: M. Kogalniceanu, No 9-13 City: Iasi

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