De Lorenzo, A., A. Andreoli, J. Matthie, and P. Withers.Predicting body cell mass with bioimpedance by using theoretical methods: a technological review. J. Appl. Physiol. 82(5): 1542–1558, 1997.—T...

Predicting body cell mass with bioimpedance by using theoretical methods: a technological review

Applied potential tomography (APT) or electrical impedance imaging has received considerable attention during the past few years and some in vivo images have been produced. This paper reviews the current situation in terms of what in vivo results have been and are likely to be obtained in the near future. Both static and dynamic imaging are possible and these two areas are dealt with separately. Features of the existing in vivo imaging system are good tissue contrast, high-speed data collection, good sensitivity to resistivity changes, low spatial resolution, low cost and no known hazard. It is concluded that the most promising way forward to clinical application in the short term is to use dynamic as opposed to static imaging. An example of lung imaging is shown and the application to measuring regional ventilation and pulmonary oedema is discussed. Use of APT for the detection of intraventricular bleeding in neonates is discussed as is the proven ability to study gastric physiology by imaging resistivity distribution changes following the ingestion of conducting or insulating fluids. Other areas of possible application which are considered are blood flow measurement, cell counting, measurement of lean-fat ratios and the detection of soft tissue lesions.

https://iopscience.iop.org/article/10.1088/0143-0815/6/2/002/pdf

Applied potential tomography: possible clinical applications

. Fluid balance is an integral component of hemodialysis treatments to prevent under- or overhydration, both of which have been demonstrated to have significant effects on intradialytic morbidity and long-term cardiovascular complications. Fluid removal is usually achieved by ultrafiltration to achieve a clinically derived value for “dry weight.” Unfortunately, there is no standard measure of dry weight and as a consequence it is difficult to ascertain adequacy of fluid removal for an individual patient. Additionally, there is a lack of information on the effect of ultrafiltration on fluid shifts in the extracellular and intracellular fluid spaces. It is evident that a better understanding of both interdialytic fluid status and fluid changes during hemodialysis is required to develop a precise measure of fluid balance. This article describes the current status of dry weight estimation and reviews emerging techniques for evaluation of fluid shifts. Additionally, it explores the need for a marker of adequacy for fluid removal.

Assessment of Dry Weight in Hemodialysis An Overview

The 1994 National Institutes of Health Technology Conference on bioelectrical impedance analysis (BIA) did not support the use of BIA under conditions that alter the normal relationship between the...

Single- and multifrequency models for bioelectrical impedance analysis of body water compartments.

Methods and apparatus for obtaining a representation of the distribution of electrical impedance within a multiphase flow with an electrically continuous or discontinuous principle flow (3) contained within an electrically conductive solid ring electrode (1), comprising providing a plurality of mutually spaced electrical contacts (2) mounted at the outside wall of the ring and electrically contacted with the ring, applying currents or voltages (4) to the ring from the electrical contacts (2), generating a more homogeneous electric field distribution within the material (3), measuring voltage or current (5) distribution along the ring from other electrical contacts (2), relatively intensifying the imaging sensitivity at the central area of the sensing domain using a π/2 angle sensing strategy and reconstructing the representation of the impedance distribution using CG method with an error processing strategy.

Electrical impedance tomography

A method to measure intra- and extracellular conductivity is evaluated. In vitro experiments show that these two variables can be measured separately. The conductivity appears to depend on the concentration and fluid volume of the compartment concerned. In vitro variation of the intracellular volume of blood by dilution and by shrinking and swelling of blood cells is detectable by this method. In vivo too the method seems to detect intra- and extracellular fluid volumes separately. The course of these two variables during different strategies of haemodialysis is determined.

Measurement of transcellular fluid shift during haemodialysis. Part 2. In vitro and clinical evaluation.

A method is presented to measure transcellular fluid shifts during haemodialysis based on a simplified model of the electrical admittance of biological tissues. It allows for the measurement of intracellular and extracellular conductivities and their ratios. The method is noninvasive, clean and harmless, and can be easily computerised in order to be performed continuously. A typical example is given of a recording during haemodialysis.

Measurement of transcellular fluid shift during haemodialysis. Part 1. Method.

Haemodialysis has a profound effect on fluid balance. Since fluid is initially withdrawn from the intravascular compartment, hypovolaemia is a frequent complication. A fluid shift from the overhydrated interstitium towards the intravascular compartment can counteract hypovolaemia. However, a fast decline in extracellular osmolality may cause an increase in the intracellular volume, reducing the available amount of fluid to compensate for the hypovolaemia. To overcome this problem, the use of alternating high and low sodium dialysate is advocated. In this study six patients were studied during standard haemodialysis (HD) and during dialysis with alternating high and low sodium dialysate (HLSD). Changes in intracellular fluid volume (IFV) and extracellular fluid volume (EFV) of tissue and blood were measured by means of a non-invasive electrical conductivity method. Changes in blood volume (BV) were studied by serial erythrocyte counts. Plasma sodium concentration was determined at regular intervals. The distribution volume of sodium during the high and low sodium episodes of HLSD was calculated according to a mathematical model. HLSD led to fluctations in plasma sodium concentration that induced changes in red cell volume, but not in IFV. Distribution of sodium was largely confined to blood. BV was better preserved during HLSD than during HD, probably due to a higher mean plasma sodium concentration. Postdialysis sodium concentration however, was not significantly different between HLSD and HD. These data suggest that the better BV preservation during HLSD results from an induced osmotic gradient across the capillary wall, rather than from an osmotic gradient across the cell membrane.

Influence of High and Low Sodium Dialysis on Blood Volume Preservation

The paper describes an instrument designed for accurate measurement of the mean specific electrical impedance of tissue. Two basic measurements are possible. One implementation called tetrapolar impedance plethysmography is employed for investigation with respect to transcellular fluid shifts during haemodialysis. The other measuring configuration called differential tetrapolar impedance plethysmography, deals with measurement of intrathoracic fluid content, which is important in monitoring treatment and prevention of lung oedema. An alternating current of 1 mA is passed through the tissue at different frequencies. Two measuring channels are available for differential tetrapolar measurement. Only one channel is required for straightforward tetrapolar measurement. Recovery of the impedance signals is achieved by synchronous demodulation. The impedance signal is passed through an analogue-to-digital convertor for digital processing to estimate the specific impedance in case of a differential tetrapolar measurement. Output display is possible via an xy recorder. The design of the instrument is discussed and some results shown.

Microprocessor-based system for measurement of electrical impedances during haemodialysis and in postoperative care.

A critical evaluation of the measurement of the transthoracic basic impedance Zo according to Kubicek's method revealed inhomogeneous electrical field-distributions in the region of the neck and the shoulders and in the region of the mammae in female subjects. These inhornogeneties were found to introduce uncontrollable errors, like position dependency into the measurements. Also the sensitivity is deteriorated and the comparability between subjects is lost. A method for measuring thoracic impedances that avoids those disturbances is proposed, and some examples of registrations are presented. The measurements seemed to depend on thoracic fluid content, blood pressure, physiological variations and age. It was concluded that differential impedance plethysmography can be a useful tool in evaluating intra-thoracic processes, both in clinical care and physiological studies. The increased reliability might provide new insight into the determination of cardiac output by impedance cardiography.

J. H. Meijer

Papers

Measurement of transcellular fluid shift during haemodialysis. Part 2. In vitro and clinical evaluation.

Measurement of transcellular fluid shift during haemodialysis. Part 1. Method.

Influence of High and Low Sodium Dialysis on Blood Volume Preservation

Microprocessor-based system for measurement of electrical impedances during haemodialysis and in postoperative care.

Differential impedance plethysmography for measuring thoracic impedances.