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A Review Of Impedance Analyzers

Electrical Impedance Tomography for Cardio-Pulmonary Monitoring

Abstract

Electrical Impedance Tomography (EIT) is a bedside monitoring device that noninvasively visualizes local ventilation as well as conceivably lung perfusion distribution. This article reviews and analyzes the methodological and clinical aspects of thoracic EIT. Initially, researchers focused on the possibility of using EIT to determine regional ventilation. Recent studies concentrate on its clinical applications to measure lung collapse, TIDAL recruitment, as well as lung overdistension to measure positive end-expiratory pressure (PEEP) and tidal volume. In addition, EIT may help to detect pneumothorax. Recent studies examined EIT as a means to gauge regional lung perfusion. Indicator-free EIT measurements might be sufficient for continuous monitoring of stroke volume. Utilizing a contrast agent like saline could be necessary to evaluate regional lung perfusion. Thus, EIT-based monitoring of regional respiration and lung perfusion might reveal local ventilation and perfusion matching and can prove helpful in treating patients with acute respiratory distress syndrome (ARDS).

Keywords: electrical impedance tmography bioimpedance; reconstruction of images Thorax; regional ventilation; regional perfusion; monitoring

1. Introduction

Electronic impedance transmission (EIT) is a non-radiation functional imaging method that permits the non-invasive monitoring of bedside regional lung ventilation and arguably perfusion. Commercially accessible EIT devices were introduced for clinical application of this technique and the thoracic EIT is safe in both adult and pediatric patients 2, 2.

2. Basics of Impedance Spectroscopy

Impedance Spectroscopy can be defined as the variation in the voltage of biological tissue to externally applied alternating electronic current (AC). It is commonly obtained using four electrodes, where two are employed for AC injection and the other two electrodes are used to measure voltage 3.,4. Thoracic EIT measures the regional variability of Impedance Spectroscopy in the thoracic area and can be viewed as an extension of the principle of four electrodes to the image plane that is spanned by the electrode belt [ 11. In terms of dimension, electrical impedance (Z) is the same as resistance and its equivalent International System of Units (SI) unit is Ohm (O). It can be expressed as a complex number where the real portion is resistance, and the imaginary component is called the reactance, which measures the effects of resistance or capacitance. Capacitance depends on the biomembranes’ features of the tissue , including ion channels, fatty acids, and gap junctions. While resistance is mainly determined by the content and quantity of extracellular fluid 1, 2]. At frequencies less than 5 Kilohertz (kHz) electricity moves through extracellular fluid, and is mostly dependent on the resistivity characteristics of tissues. At higher frequencies of up to 50 kHz, electrical currents are a little deflected by cell membranes , which results in an increase in tissue capacitive properties. When frequencies exceed 100kHz electrical current can flow through cell membranes and reduce the capacitive component [ 21. So, the results that determine the tissue’s impedance depend on the utilized stimulation frequency. Impedance Spectroscopy typically refers to conductivity or resistance. Both is a measure of conductance or resistance to unit length and area. The SI units used consist of Ohm-meter (O*m) for resistivity and Siemens per meters (S/m) as for conductivity. The resistance of lung tissue can range between 150 O*cm for blood up to 700 O*cm for lung tissue that is deflated, all the way up to 2400 O*cm in tissues that have been inflated ( Table 1). In general, tissue resistivity or conductivity depends on the amount of fluid and the ion concentration. In the case of respiratory lungs it also depends on the amount of air present in the alveoli. Although most tissues exhibit isotropic behavior, heart and skeletal muscle behave anisotropic, in which the degree of resistance depends on the direction from which you measure it.

Table 1. The electrical resistivity of the thoracic muscles.

3. EIT Measurements and Image Reconstruction

For EIT measurements electrodes are placed on the thorax in a transverse plan generally in the 4th-5th intercostal spaces (ICS) near an angle called the parasternal line5. In turn, the variations in the impedance of the lungs can be measured within the lower lobes of the right and left lungs, as well as in the heart region ,21. To place the electrodes below the 6th ICS could be difficult since the diaphragm and abdominal content are frequently inserted into the measurement plane.

Electrodes are either single self-adhesive electrodes (e.g. electrocardiogram ECG,) that are placed in a similar spacing between electrodes or are incorporated into electrode belts [ ,21. Also, self-adhesive stripes are designed to be more comfortable for application [ ,2[ 1,2]. Chest wounds, chest tubes and non-conductive bandages as well as conductive wire sutures could block or negatively impact EIT measurements. Commercially available EIT devices typically use 16 electrodes, but EIT systems with 8 (or 32) electrodes are also available (please read Table 2 to get information) For more information, refer to Table 2. ,2[ 1,2.

Table 2. The commercially-available electrical impedance tomography (EIT) equipment.

In an EIT measurements, small AC (e.g. the smallest value of 5 1 mA at 100 kHz) are applied through different pairs of electrodes and the generated voltages are measured with the remaining electrodes 6. Bioelectrical impedance that is measured between the injecting and the electrode pairs used to measure the voltage can be calculated by using the applied current and the measured voltages. Most often the electrodes adjacent to each other are utilized to allow AC application in a 16-elektrode setup, while 32-elektrode systems often utilize a skip-pattern (see Table 2) which increases the distance of electrodes that inject the current. The resulting voltages are measured by using the remaining electrodes. In the present, there is an ongoing debate on the different electrical stimulation techniques and their specific advantages and disadvantages [77. For a complete EIT data set of bioelectrical measurements that are injected and electrode pairs used for measuring are constantly rotationally positioned around the entire chest .

1. Voltage measurements and current application around the thorax using an EIT system consisting of 16 electrodes. Within milliseconds, two electrodes measuring current and activated voltage electrodes will be repeatedly rotating around the thorax.

The AC employed during EIT tests is safe for body surface applications and is not detectable by the patient. For safety reasons, the use of EIT in patients with electrically active devices (e.g., cardiac pacemakers or cardioverter-defibrillators) is not recommended.

A EIT data set captured during a single cycle that is recorded during one cycle of AC Applications is called a frame and contains the voltage measurements to generate what is known as the Raw EIT image. The term “frame rate” refers to the amount of EIT frames captured per second. Frame rates of at minimum 10 images/s are essential to monitor ventilation , and 25 images/s to check the cardiac function or perfusion. Commercially accessible EIT devices have frames with a frame rate between 40 and 50 images/s [2], as depicted in

In order to create EIT images from recorded frames, the technique known as image reconstruction procedure is utilized. Reconstruction algorithms aim to solve the opposite problem of EIT that is determination of the conductivity distribution within the thorax based upon the voltage measurements acquired at the electrodes on the thorax surface. At first, EIT reconstruction assumed that electrodes were placed on an ellipsoid plane, however, more modern algorithms use information about anatomy of the thorax. Today, we use using the Sheffield back-projection algorithm [ and the finite element algorithm (FEM) based linearized Newton-Raphson algorithm [ ] and the Graz consensus reconstruction algorithm for EIT (GREIT) [10are often employed.

The majority of EIT images can be compared to a two-dimensional computed-tomography (CT) image. These images are rendered conventionally so that the operator is looking at the cranial and caudal regions when taking a look at the picture. In contrast to a CT image the EIT image doesn’t display an actual “slice” but an “EIT sensitivity region” [1111. The EIT sensitization region is a lens-shaped intrathoracic region with impedance-related changes that contribute to the EIT creation of images11. The size and shape of the EIT sensitive region are determined by the dimensions, the bioelectrical properties, and also the anatomy of the Thorax, as well in the used current injection and voltage measurement pattern [12The shape and thickness of the EIT sensitivity region is determined by the voltage measurement pattern [.

Time-difference imaging is a method that is used for EIT reconstruction to show changes in conductivity rather than relative conductivity of the levels. Time-difference EIT image compares the variation in impedance to a base frame. This affords the opportunity to monitor the changes in physiological activity over time like lung ventilation and perfusion [22. The color-coding used in EIT images is not unicoded but typically shows the change in impedance in relation to a reference level (2). EIT images are generally colored using a rainbow color scheme with red representing the high value of relative imperf (e.g., during inspiration) with green being a medium relative impedance, and blue the lowest relative impedance (e.g., during expiration). In clinical settings the best option is using color scales which range from black (no impedance changes) through blue (intermediate impedance changes), and white (strong impedance changes) to code ventilation . from black, to white, and red towards mirror perfusion.

2. Different color codings for EIT images in comparison to CT scan. The rainbow-color scheme uses red for the highest relative impedance (e.g., during inspiration), green for a low relative impedance and blue as the one with the lowest impedance (e.g. during expiration). Modern color scales make use of instead black for no impedance change), blue for an intermediate change in impedance, while white is the one with the strongest impedance changes.

4. Functional Imaging and EIT Waveform Analysis

Analysis of Impedance Analyzers data is based on EIT waveforms that form in individual image pixels in a series of raw EIT images over length of (Figure 3.). A region of interest (ROI) is a term used to describe activity in the individual pixels in the image. Within the ROIs, each waveform shows changes in regional conductivity over time resulting from the process of ventilation (ventilation-related signal, VRS) or heart activity (cardiac-related signal, CRS). Additionally, electrically conducting contrast agents such as hypertonic saline can be used to produce the EIT pattern (indicator-based signal IBS) and could be connected to lung perfusion. The CRS could come from both the heart and lung region and may also be due to lung perfusion. Its exact origin and composition isn’t fully understood 13]. Frequency spectrum analysis can be used to distinguish between ventilator- and cardiac-related Impedance Analyzers changes. Impedance changes that are not periodic could be caused by changes in the setting of the ventilator.

Figure 3. EIT forms and the functions of EIT (fEIT) pictures are derived from Raw EIT images. EIT waves can be described pixels-wise or based on a area to be studied (ROI). Conductivity changes occur naturally as a result of the process of ventilation (VRS) or cardiac activity (CRS) but can also be generated artificially e.g. with bolus injection (IBS) for perfusion measurements. fEIT images display regional physiological parameters like ventilation (V) and blood flow (Q), extracted from the raw EIT images by using an equation over time.

Functional EIT (fEIT) images are produced by applying a mathematical operation on a sequence of raw images together with the appropriate pixel EIT waves [14]. Since the mathematical procedure is used to determine an appropriate physiological parameter for each pixel, physiological regional aspects like regional ventilation (V), respiratory system compliance, as well as region-wide perfusion (Q) can be measured to be displayed (Figure 3.). The data from EIT waveforms as well as simultaneously recorded pressures of the airways can be utilized to determine the lung’s compliance, as well as the lung’s opening and closing times for each pixel by calculating changes of impedance and pressure (volume). The comparable EIT measurements of the deflation and inflation of lung volume allow for the display of curves representing volume and pressure at one pixel. The mathematical operations used to calculate different types of fEIT photos could reflect different functional characteristics in the cardiopulmonary system.

Stephen Young

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