RU2251387C1 - Method for bioimpedance detecting the volumes of body liquid - Google Patents

Abstract

FIELD: medicine.

SUBSTANCE: the method deals with measuring geometric body size and electric impedances of patient's hands, body and legs at their probing with low- and high-frequency current due to current and potential electrodes applied onto distal parts of limbs, and, thus, detecting extracellular, cellular and total volumes of liquid in patient's hands, body and legs. While implementing the method one should additionally apply current electrodes onto left-hand and right-hand parts of neck, and potential electrodes - onto distal femoral parts. Body impedance (Z_b) should be measured due to successive measuring the impedance of its right-hand Z_rb and left-hand Z_lb parts at probing current coming between electrodes of similar sides of patient's neck and legs to detect Z_b, as Z_b = Ѕ x (Z_rb + Z_lb), impedance of legs Z_l should be detected due to measuring femoral impedance Z_f and that of shins Z_s, as Z_l = Z_f + Z_s. At detecting the volumes of liquid in body and legs one should apply measured values of Z_b and Z_l, moreover, as geometric body size one should apply the distance against the plane coming through the upper brachial surface up to the middle of radiocarpal articulation in case of patient's hand being along the body.

EFFECT: higher accuracy of detection.

5 dwg, 2 ex, 3 tbl

Description

The invention relates to medicine, namely to non-invasive methods for determining fluid volumes in parts of the body in which an alternating electric current is used as a probing signal.

Modern diagnostic methods make it possible to conduct comprehensive studies without violating the integrity of the skin of the patient. This is achieved through the use of probe radiation having high penetrating power, for example, x-ray, ultrasound or magnetic, as well as electric current. As a result of interaction with body tissues, the signals of the probe radiation change their parameters, on the basis of which it is possible to obtain a number of characteristics that reflect the structure of the body and the state of organs and tissues.

In addition to diagnostic capabilities, methods are evaluated according to the degree of safety, i.e. ability not to cause complications and residual effects in the body during and after research.

Diagnostic methods that use alternating electric current as the probing signal are the most sparing. This is achieved through the use of a probe signal of lower intensity compared to other influences. For example, studies of the liquid-fat composition of the body are carried out by means of an alternating probe current with a force of tenths of a milliampere. Sounding the body with alternating current allows you to monitor the dynamic state of the circulatory system and respiratory system [1]. An alternating current containing two or more harmonic components separated in the frequency range allows obtaining information on the qualitative state of tissues and organs, for example, on the water balance and fat mass of the body and its parts, on the state of tissues, grafts and their suitability for implantation [2].

A known method for determining the volume of body fluid during hemodialysis is carried out by measuring the impedance of the legs at low and high frequencies and determining the volume of extracellular, cellular and total body fluid based on the relationship of impedance with the volume of the electrically conductive fluid in the measured part of the body [3]. The disadvantage of this method is the low accuracy due to the fact that the fluid volumes of the whole organism are estimated indirectly, additionally using the parameters of the change in blood parameters (hematocrit).

A known method for determining the volume of liquid sectors of the body, carried out by measuring the growth (geometric size of the body) and the impedance of the body when sensing it with a current of low and high frequency, which is passed from hand to foot [4]. The disadvantage of this method is the low accuracy of determining the total volume of body fluid, due to the fact that with successive passage of current along the path: arms - trunk - legs in the measured impedance, the component of the arms is mainly reflected, because they have its largest value relative to the trunk and legs. Another disadvantage of this method is that the volume of fluid is determined for the whole body without dividing it by its component parts: arms, trunk, legs.

The closest analogue is the method of bioimpedance determination of fluid volumes in parts of the body, which consists in measuring growth (geometric size of the body), applying potential and current electrodes to the distal parts of the limbs, measuring the impedance of the arms, trunk and legs when probing tissues with low and high frequency current and determining on based on the measured parameters of the volumes of extracellular, cellular and total fluids [5]. The disadvantage of this method is the low accuracy of determining the volume of body fluid, due to the high uneven distribution of the probe current in the body and the change in the value of the impedance of the body due to breathing. Measurement of the impedance of the torso in this method is performed by passing a probe current between the right (left) hand and the left (right) leg and measuring the voltage drop across the torso between the left (right) arm and the right (left) leg. Moreover, in the shoulder region, the probing current changes its direction almost to the opposite, as a result of which its distribution in the volume of the upper body is uneven and leads to a decrease in the accuracy of measuring the impedance of the entire body [6]. A change in lung volume during respiration also affects the distribution of the probe current in the upper body and is reflected in the measured body impedance in the form of a periodic change in its value. Using in this method the impedance values of the entire leg to find the volume of fluid in it also reduces the accuracy of this method. This is due to the fact that the main part of the leg fluid is contained in the thigh, but it has approximately two times less impedance than the lower leg, due to which the measured impedance of the entire leg largely reflects the volume of fluid contained in the lower leg, and not in the whole leg.

The technical result of the invention is to increase the accuracy of determining body fluid volumes by increasing the uniformity of the distribution of the probe current in the body, taking into account the asymmetric arrangement of organs in it and a differentiated anatomical approach when measuring the impedance of the legs.

The technical result is achieved by the fact that in the method of bio-impedance determination of body fluid volumes, which consists in measuring the geometric size of the body and the electrical impedance of the arms, trunk and legs when they are probed by low and high frequency currents using current and potential electrodes superimposed on the distal parts of the limbs, and determining based on the obtained results of measurements of extracellular, cellular and total volumes of fluid in the arms, trunk and legs, additionally apply current electrodes to the lion the left and right parts of the neck and potential electrodes on the distal parts of the thighs, the measurement of the impedance of the body Z _{T is} carried out by sequentially measuring the impedance of its right Z _PT and left Z _LT parts when the probing current passes between the electrodes of the same sides of the neck and legs and find Z _T as Z _T = 1/2 · (Z _PT + Z _LT ), the impedance of the legs Z _N is determined by measuring the impedance of the hips Z _B and lower legs Z _G as Z _N = Z _B + Z _G , and the measured values are used to determine the volume of fluid in the trunk and legs Z _T and Z _H , and as a geometric dimension m ate use the distance from the plane passing through the upper surface of the shoulder to the middle of the wrist joint with the arm located along the body.

The invention is illustrated by drawings, reflecting the equivalent spatial scheme of the spatial conjugation of the impedance of body parts, where:

figure 1 shows the equivalent circuit depicting the impedance of the analyzed parts of the body;

figure 2 shows a block diagram for measuring the impedance of body parts;

figure 3 shows the equivalent circuit diagram of the replacement of body tissues when sensing it with alternating electric current;

figure 4 shows the result of monitoring the dynamics of the volumes of extracellular fluid;

figure 5 shows the result of monitoring the dynamics of the volume of cell fluid.

The method of bioimpedance determination of body fluid volumes is as follows.

The volume of the electrically conductive physiological fluid located in the test volume of the body is determined based on the theory of electricity. An alternating probing current of constant magnitude is passed through the studied part of the body and the voltage drop arising on it is measured. The value of the measured voltage is proportional to the impedance of the investigated area. If the cross-sectional areas of the investigated area along the path of the probing current are approximately the same value, then its volume of the electrically conductive liquid is found from the following expression:

V _AND = ρ $\genfrac{}{}{0ex}{}{\text{2}}{\text{AND}}$ / Z _And ,

where: ρ is the resistivity of physiological fluid;

l _And is the distance between the potential electrodes;

Z _And - electrical impedance of the investigated area.

The fulfillment of the requirement regarding the constancy of the cross-sectional area of the investigated body volume is carried out due to the fact that the measurement of body fluid volumes is carried out discretely in the following parts: hands with the exception of the volume of the hands, trunk and legs with the exception of the volume of the feet.

To measure the impedance of the body parts, the tetrapolar method is used, in which pairs of current and potential electrodes are applied to the distal parts of the forearms and legs, additional current electrodes are applied to the left and right parts of the neck, and additional potential electrodes are applied to the distal parts of the hips. The impedance in the selected leads is measured by means of a probing current of low and high frequencies, the switching of which between current electrodes is carried out by means of a current switch controlled through an interface by a controller (Fig. 2). Signals from potential electrodes through a potential switch, a voltage meter and an interface are sent to the controller, which also controls the potential switch, which ensures registration in the selected leads. The impedance measured in the leads contains the impedance of the following parts of the body (figure 1):

Z _P = Z _PR + Z _PT + Z _PB + Z _PG ,

where: Z _P - the impedance of the right side of the body, which is measured with the passage of current between the right hand and the right foot by measuring between them the voltage leads for the right side of the body;

Z _PB - impedance of the right thigh;

Z _PG - impedance of the right lower leg;

Z _PT - the impedance of the right side of the body;

Z _PR - impedance of the right hand.

Z _Л = Z _ЛР + Z _ЛТ + Z _ЛБ + Z _ЛГ ,

where Z _L is the impedance of the left side of the body, which was measured during the passage of current between the left hand and left foot by measuring the lead voltage between the left side of the body;

Z _LB - impedance of the left thigh;

Z _LH - impedance of the left tibia;

Z _LT - the impedance of the left side of the body;

Z _LR - impedance of the left hand.

Z _N = Z _PG + Z _PB + Z _PG ,

where: Z _N - the impedance of the legs, which is measured by the passage of current between the legs by measuring the voltage between them.

Z _PTN = Z _PT + Z _PB + Z _PG ,

where: Z _PTN is the impedance of the right side of the body and the right foot, which is measured by the passage of current between the right side of the neck and the right foot by measuring the voltage between the right hand and the right foot.

Z _LTN = Z _LT + Z _LB + Z _LH ,

where: Z _LTN is the impedance of the left side of the body and the left foot, which is measured by the passage of current between the left side of the neck and the left foot by measuring the voltage between the left hand and the left foot.

Z _B = Z _PB + Z _LB ,

where: Z _B is the impedance of the hips, which contains the impedance of the right thigh (Z _PB ) and the impedance of the left thigh (Z _LB ) and is measured with the passage of current between the legs by measuring the voltage between the hips.

During the respiratory cycle, a change in lung volume occurs, reflected in the value of the impedance of the body (Z _T ) in the form of a deviation of its value from 2 to 10%, depending on the depth of inspiration. This error is significant, because the volume of fluid contained in the body is at least half the total volume of body fluid. Therefore, when determining the total volume of body fluid by measuring impedance, the error caused by breathing can reach 5%. Compensation for this type of error is made by averaging the measured values over the course of one or more respiratory cycles. The main requirement when using the averaging method is to minimize the measurement time of the parameter values so that its values during the measurement would change insignificantly.

Also, to measure Z _T, it is necessary that the probing current (I ₃ ) be uniformly distributed throughout the entire electrically conductive volume of the body. This condition is met to the greatest extent when the current electrodes are located on the neck and leg. With this arrangement of electrodes, the uniformity of the distribution of I ₃ over the cross-sectional area of the trunk is increased and a longitudinal orientation of the direction of I ₃ along the entire length of the body is achieved. The value of Z _{T is} obtained as a result of two successive measurements of the impedance: Z _PTN and Z _LTN , as well as measurements of Z _N , which can be performed before or after the sequential measurement of the impedance Z _PTN and Z _LTN :

Z _T = 1/2 / (Z _PTN + Z _LTN- Z _Н ) = 1/2 (Z _PT + Z _LT ).

Sequential time measurements of the impedance Z _PTN and Z _LTN allow you to get the value of Z _T for a minimum time and thereby fulfill one of the requirements necessary to compensate for the error caused by the breathing cycle.

The peculiarity of measuring Z _T lies in the fact that it is made between two imaginary "electric boundaries": the first passes at the level of the hip joints along the lower streamline as it passes through the legs, the second - along the line passing in the upper plane between the shoulders, i.e. . on the highest equipotential surface in the shoulder area. Together with a uniform distribution of I ₃ in the body due to the fixation of current electrodes on the neck and legs and taking into account the successive measurement of the impedance of the right and left parts of the body, it is possible to obtain the value of Z _T , which reliably displays the impedance of the body.

The impedance of the right hand Z _PR is determined from the measurement results of the impedance Z _P and the impedance Z _PTN :

Z _P -Z _PTN = Z _PR.

The impedance of the left hand Z _LR is determined from the measurement results of the impedance Z _L and the impedance Z _LTN :

Z _L -Z _LTN = Z _LR.

The impedance of the legs Z _G is determined from the measurement results of the impedance Z _N and the impedance Z _B :

Z _N -Z _B = Z _PG + Z _LG = Z _G.

The magnitude of the tissue electrical impedance decreases with increasing frequency of the probing current, which is equivalent to the presence of a capacitive component in the tissue impedance. Tissue impedance (Z _LF ), measured at a low frequency (5 kHz), is due to the passage of current through a space filled with extracellular fluid. The tissue impedance, measured at a high frequency (500 kHz), is additionally characterized by the presence of a capacitive component due to the capacitive properties of cell membranes, which in the equivalent circuit (Fig. 3) is represented by a resistance (Z _C ) connected in parallel with a resistance (Z _LF ).

The volumes of extracellular (V _BH ), cellular (V _CL ) and total (V _AB ) fluids for the arms, trunk, thighs and lower legs are determined according to the well-known biophysical model [7] based on their impedance (Z _BH , Z _CL ), resistivity body fluids (c) and body lengths (l) according to the following formulas:

V _BH = ρ · l ² / Z _BH ; V _CR = ρ · l ² / Z _CR ; V _AB = V _BH + V _KL ;

Z _BH = Z _LF ; Z _CL = k · Z _C ; Z _С = Z _LF · Z _HF / (Z _LF -Z _HF ),

where: Z _LF - tissue impedance measured at a low frequency (LF) of 5 kHz;

Z _HF - tissue impedance measured at high frequency (HF) 500 kHz;

Z _C is the value of the capacitive component of tissue impedance measured at a high frequency (HF) of 500 kHz;

k = 0.42 - coefficient taking into account the presence of a membrane between the cellular and extracellular fluid.

As the base length (l), the distance from the plane passing through the upper surface of the shoulder to the middle of the wrist joint is measured, and based on its size and known anthropometric ratios, the values used as the length for the studied parts of the body are determined:

l _p = 0.9 l; l _T = l; l _B = 1.1 l; l _G = 1.1 · l;

where: l _P is the value used to determine the volume of the hands, which takes into account that the electrodes are fixed in front of the wrist joint;

l _T - the value used to determine the volume of the body;

l _B - the value used to determine the volume of the hips;

l _G - the value used to determine the volume of the legs.

The value of resistivity (ρ) used to determine the volume of fluid was clarified in the process of bioimpedance control of medical procedures that carry out a deficiency of fluid in the body: hemodialysis, plasmapheresis. For the probing current frequencies of 5 and 500 kHz and the impedance measurement by the tetrapolar method: c = 70 Ohm · cm.

The impedance values Z _P , Z _L , Z _N , Z _PTN , Z _LTN , Z _B are obtained using the analyzer ABC-01 (Medass) or a similar device, for example, made according to the above block diagram (Fig. 2).

The metrological reliability of the method is clearly demonstrated when monitoring the volume of fluid in a patient during the plasmapheresis (PF) procedure. During PF, blood is taken from a vein, it is divided into plasma, which is removed, and the erythrocyte mass, which is periodically returned to the body. With this procedure, a consistent dynamics of fluid volumes is cyclically created in the body: 500 ml of blood are taken, after which 300 ml of red blood cell is returned. In the apparatus for PF, the volume of blood taken is monitored continuously. By comparing simultaneously the measured volume of blood taken by the apparatus for PF and the volume of fluid in the body, measured by the bioimpedance method, one can judge the reliability of the latter.

Example 1. Patient K ..., 23 years old, weight 57 kg, l = 52 cm. Bioimpedance control of fluid volumes in the body was carried out before the PF procedure and at the time of the patient's intake of 670 ml of fluid, including 200 ml of plasma and 470 ml of blood. The impedance values measured in the leads before blood sampling and after sampling 670 ml are shown in Table 1, and the calculated impedance values for body parts are shown in Table 2:

Table 1 Current frequency Impedance in leads (before PF / during PF), Ohm Z _P Z _L Z _N Z _PTH Z _LTN Z _B LF 525.8 / 538.6 579.0 / 572.7 518.1 / 519.8 292.5 / 297.4 303.1 / 306.3 130.0 / 130.0 Treble 400.0 / 405.6 446.9 / 445.6 390.3 / 386.0 223.0 / 219.4 232.2 / 233.8 91.7 / 90.6 Table 2 Current frequency Impedance of body parts (before PF / during PF), Ohm hands torso hips lower legs LF 126.4 / 126.6 38.8 / 42.0 130.0 / 130.0 388.1 / 389.8 Treble 97.0 / 99.1 32.5 / 33.6 91.7 / 90.6 298.6 / 295.4

The values of the volumes of V _BH , V _CL and V _{AB of} liquids calculated according to the formulas of the biophysical model [6] are shown in Table 3:

Table 3 Body parts Liquid volumes (before PF / during PF), l extracellular cell common Hands (P) 1.22 / 1.22 2.43 / 2.32 3.65 / 3.54 Torso (T) 4.88 / 4.51 8.09 / 7.89 12.97 / 12.40 Hips (B) 1.76 / 1.76 4.34 / 4.40 6.10 / 6.16 Drumsticks (G) 0.59 / 0.59 1.18 / 1.22 1.77 / 1.81 (P + T + B + D) 8.45 / 8.08 16.04 / 15.83 24.49 / 23.91 Dynamics:
(P + T + B + D) 0.37 0.21 0.58

Analysis of the obtained values of the volume of liquid (table 3) shows:

- the amount of decrease in the total fluid volume in the patient (580 ml) is in good agreement with the amount of fluid volume (plasma + blood: 670 ml) taken from the patient at the time of the bioimpedance measurements;

- during blood sampling bioimpedance measurements show that in a patient a decrease in fluid volumes occurs almost exclusively in the body;

- according to bioimpedance measurements, a decrease in extracellular fluid volume (0.37 L) is greater than a decrease in cell volume (0.21 L), which qualitatively coincides with the clinical nature of the procedure: plasma, which is an extracellular fluid, is removed from the body.

Example 2. Patient G ..., 35 years old, the first month is peritoneal dialysis, the volume of injected solution into the abdominal cavity: 2 liters.

Determined by the proposed method, changes in fluid volumes in parts of the body with a solution poured into the stomach in an interval equal to 6 days and during one procedure for replacing the solution in the stomach: "full stomach> empty stomach> full stomach". The dynamics of the volume of extracellular fluid shown in figure 4, and the volume of cell fluid in figure 5.

During the period between the first and sixth day of observation, the volumes of extracellular and cellular fluid in all parts of the body significantly decreased. According to the results of measuring the impedance, the total fluid volume in the patient for this period decreased by 4.77 l.

During the procedure for replacing the solution in the abdomen, the volumes of extracellular fluid significantly decreased only in the body by 1.34 l and remained at the same level in the arms and legs. The volumes of cellular fluid in the trunk and legs during the procedure changed in a larger range than the volumes of extracellular fluid. The volumes of total fluid in the patient before and after the procedure were 28.2 liters and 28.05 liters, respectively.

Analysis of the results of example 2.

The results obtained in the form of a synchronous decrease in the values of the volumes of cellular and extracellular fluid in the interval of six days in all parts of the body and the volumetric response of the extracellular fluid of the body during the procedure demonstrate the model's ability to display the dynamics of fluid volumes in parts of the body over long and short time intervals.

During the procedure, the calculation of the volume of cell fluid should be carried out with a correction that takes into account the shunting effect of the solution on the tissues of the abdominal cavity. The shunting effect of the solution is confirmed by the approximate equality of the total fluid values in the patient before and after the procedure.

Compared with the prototype, which determines the volume of fluid in parts of the body by sensing current low and high frequencies, the claimed method has the following advantages:

- the accuracy of determining the impedance of the body, and therefore the volume of its fluid, is increased due to a more uniform distribution of the probe current in the thoracic region, not covered by the prototype [5, 6] and constitutes ~ 20% of the total body volume;

- the accuracy of determining the volume of leg fluid is increased by separately measuring the impedance of the legs and hips, in which the ratio of fluid volumes is ~ 1/3, and therefore their impedances as 3/1, which previously led to the need for mathematical compensation> 100% of the error, reflecting the impedance of the legs, used to calculate the volume of their fluid, in the claimed method, this error is eliminated methodically.

Thus, the proposed method allows to increase the accuracy of non-invasive determination of the volumes of extracellular, cellular and total fluids in parts of the body and for the whole organism by measuring the parameters of the trunk and legs in a more complete volume and compensating for the error that occurs during breathing.

SOURCES OF INFORMATION

1. L.Z. Polonetskiy, L.G. Gelis, A.V. Frolov Impedance plethysmography. Instrumental research methods in cardiology. (Leadership) Minsk, 1994, 81-119 .;

2. Ivanov G.G., Nikolaev D.V., Baluev E.P., Zaks I.O., Ivleva V.V., Meshcheryakov G.N., Kravchenko N.R. The method of bioimpedance spectroscopy in the assessment of total water and extracellular fluid. M .: News of the science of technology, MEDICINE series, No. 3, 1997, p. 28-33.

3. K.Sakamoto, H. Kanai, K.Sakurai. Estimation of the fluid distribution change during hemodialysis by the electrical admittance method. Oslo, Proceedings of the XI international conference on electrical bio-impedance, 2001, 377-380.

4. Patent RU No. 20930069, class A 61 B 5/05, 1991.

5. Patent SU No. 1826864, cl. A 61 B 5/05, 1990.

6. Nikolaev D., Smirnov A., Tarnakin A. Bioimpedance analysis with automatically electrode commutation in equipment for intensive care unit. Oslo, Proceedings of the XI international conference on electrical bio-impedance, 2001, 381-384.

7. Kapitanov E.N. Biophysical model for determining the volume of fluid in the body when it is probed by alternating electric current, M., Materials of the fifth scientific and practical conference: "Diagnosis and treatment of dysregulation of the cardiovascular system", March 2003, p.196-203.

Claims (1)

The method of bio-impedance determination of body fluid volumes, which consists in measuring the geometric size of the body and the electrical impedance of the arms, trunk and legs when they are probed with low and high frequency currents using current and potential electrodes superimposed on the distal parts of the limbs, and determining, based on the obtained measurement results, extracellular, cell and total volumes of fluid in the hands, trunk and legs, characterized in that they additionally impose current electrodes on the left and right parts of the neck and potentially electrodes on the distal portion of the hips, the measurement of the impedance of the trunk Z _T is performed by sequentially measuring impedance his right Z _Fr and left Z _LT parts when passing the probe current between the electrodes of the same side of the neck and the legs, and are Z _T as Z _T = 1/2 * (Z _PT + Z _LT ), the impedance of the legs Z _N is determined by measuring the impedance of the hips Z _B and lower legs Z _G as Z _H = Z _B + Z _G , and when determining the volume of fluid in the trunk and legs, the measured values of Z _T and Z are used _H, wherein as used geometric body size rasstoyatsya e of the plane passing through the upper surface of the shoulder up to the middle of the wrist joint at hand, situated along the body.

Images