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Not known Details About Impedance Analyzers

Eugene Hill

Electrical Impedance Tomography for Cardio-Pulmonary Monitoring

Abstract

Electrical Impedance Tomography (EIT) is an instrument for monitoring bedside that is non-invasively able to assess local ventilation and , possibly, lung perfusion distribution. This article reviews and analyzes the methodological and clinical aspects of the thoracic EIT. Initially, investigators addressed the possibility of using EIT to measure regional ventilation. Current studies focus mainly on clinical applications of EIT for assessing lung collapse an increase in tidal volume, and overdistension, in order to determine positive end-expiratory pressure (PEEP) and the volume of tidal. In addition, EIT may help to detect pneumothorax. Recent studies have evaluated EIT as a way to measure regional lung perfusion. Indicator-free EIT measurements could be adequate to continuously measure cardiac stroke volume. A contrast agent, such as saline, may be necessary for assessing the regional lung perfusion. Therefore, EIT-based surveillance of regional airflow and lung perfusion might reveal the perfusion match and local ventilation, which can be helpful in the treatment of patients suffering from acute respiratory distress syndrome (ARDS).

Keywords: electrical impedance tomography bioimpedance; image reconstruction; thorax; regional ventilation Regional perfusion; monitoring

1. Introduction

Electric impedance tomography (EIT) is one of the non-radiation functional imaging technology that permits non-invasive bedside monitoring of respiratory ventilation in the region and possibly perfusion. Commercially accessible EIT devices were introduced to allow clinical applications of this method and the thoracic EIT has been successfully used for both pediatric and adult patients [ 1., 1.

2. Basics of Impedance Spectroscopy

Impedance Spectroscopy is the electrical response of biological tissue to externally applied alternating electric current (AC). It is usually measured with four electrodes, of which two are utilized for AC injection and the other two are used for measuring voltage 3,,]. Thoracic EIT measures the regional Impedance Spectroscopy of the thoracic region and can be viewed by extending the four electrode principle to the imaging plane, which is divided by the electrode belt [ 1]. Dimensionallyspeaking, electrical impedance (Z) is identical to resistance and the related International System of Units (SI) unit is Ohm (O). It can be conveniently expressed as a complicated number, in which the real part is resistance, while the imaginary portion is called reactance, which evaluates the effects that result from either inductance or capacitance. Capacitance is a function of biomembranes’ features of the tissue , including ion channels and fatty acids as well as gap junctions. However, resistance is mostly determined by composition and quantity of extracellular fluid [ 1, 22. In frequencies that are less than 5 kilohertz (kHz), electrical current circulates through extracellular fluids and is in a major way dependent on the resistivity characteristics of tissues. When frequencies are higher, up to 50 kHz electrical currents are slightly slowed down at the cell membranes resulting in an increase in the tissue’s capacitive properties. At frequencies above 100 kHz electrical current can flow through cell membranes, and diminish the capacitive component [ 22. Therefore, the effects which determine the tissue’s impedance depend on the stimulation frequency. Impedance Spectroscopy is typically described as conductivity or resistivity, which equalizes conductance and resistance to units’ area and length. The SI units that correspond to it can be described as Ohm-meter (O*m) for resistivity, and Siemens per meters (S/m) to measure conductivity. The tissue’s resistance varies from 150 O*cm for blood as high as 700 O*cm with lung tissue that has been deflated, and between 2400 and 2400 O*cm of tissues that have been inflated ( Table 1). In general, the tissue’s resistance or conductivity varies based on volume of the fluid and the amount of ions. In terms of breathing, it also is dependent on the quantity of air that is present in the alveoli. While the majority of tissues exhibit isotropic behaviour, the heart and muscle skeleton exhibit anisotropy, meaning that the resistance is strongly dependent on the direction from which they are measured.

Table 1. The electrical resistivity of the thoracic tissues.

3. EIT Measurements and Image Reconstruction

To conduct EIT measurements electrodes are positioned around the Thorax in a horizontal plane generally in the 4th to the 5th intercostal space (ICS) near that line called parasternal [55. As a result, changes in impedance can be measured in the lower lobes in the left and right lungs and also in the heart region [ ,21. To place the electrodes below the 6th ICS could be difficult because the abdominal contents and diaphragm periodically enter the measurement plane.

Electrodes are either single self-adhesive electrodes (e.g. electrocardiogram, ECG) that are placed individually with equal spacing between the electrodes, or they are integrated into electrode belts [ ,21 2. Self-adhesive lines are designed to be more comfortable for application [ ,21,2. Chest wounds, chest tubes non-conductive bandages, or conductive sutures made of wire can greatly affect EIT measurements. Commercially available EIT devices usually use 16 electrodes. However, EIT systems with 8 and 32 electrodes are available (please consult Table 2 for information) For more information, refer to Table 2. ,2[ 1,2].

Table 2. Electric impedance tomography (EIT) gadgets.

In an EIT measurement sequence, small AC (e.g. approximately 5 mgA at a rate of 100 kHz) are applied to various pairs of electrodes and the produced voltages are measured using the remaining other electrodes [ 6. Bioelectrical impedance that is measured between the injecting and electrode pairs measuring the electrodes is determined by analyzing the applied current and measured voltages. Most often connected electrode pairs are used for AC application in a 16 electrode system in 32-elektrode devices, whereas 16-elektrode employ a skip pattern (see the table 2) so that the electrodes are closer to electrodes that inject the current. The resulting voltages are then measured with the remaining electrodes. There is currently an ongoing debate on the different current stimulation patterns , and their unique advantages and disadvantages [7]. To acquire a complete EIT data set of bioelectrical tests The injecting and electrode pairs used for measuring are constantly rotated around the entire thorax .

1. Current measurement and voltage measurements in the thorax using an EIT system that includes 16 electrodes. Within milliseconds each of the electrodes for current and these active electrodes are continuously turned within the thorax.

The AC employed during EIT measurements are safe for a body surface application that is undetectable by the individual patient. For safety reasons, the use of EIT in patients with electrically active devices (e.g., cardiac pacemakers or cardioverter-defibrillators) is not recommended.

The EIT data set that is stored during one cycle from AC programs is termed frame. It contains voltage measurements used to create what is known as the raw EIT image. The term frame rate refers to the amount of EIT frames recorded each second. Frame rates at least 10 images/s are required to monitor ventilation and 25 images/s in order to monitor the cardiac function or perfusion. Commercially available EIT devices employ frames of between 40 and 50 images/s [2], as demonstrated in

To create EIT images from the captured frames, a process known as image reconstruction process is employed. Reconstruction algorithms aim to solve the problem that is the reverse of EIT that is recovery of the conductivity distribution within the thorax based upon the voltage measurements that have been taken at the electrodes on the thorax surface. At first, EIT reconstruction assumed that electrodes were placed on an ellipsoid, circular or circular plane. However, newer techniques make use of information regarding anatomy of the thorax. In the present, there are three main algorithms used for EIT: the Sheffield back-projection algorithm , the finite element method (FEM) built on a linearized Newton and Raffson algorithm ], and the Graz consensus reconstruction algorithm for EIT (GREIT) [10are often employed.

A lot of the time, EIT pictures are similar with a two-dimensional computed (CT) image: these images are rendered conventionally so that the operator looks from caudal to cranial when analysing the image. In contrast to CT images, unlike a CT image EIT images are not a two-dimensional image. EIT image does not display an image “slice” but an “EIT sensitivity region” [1111. The EIT sensitive region is a lens-shaped intra-thoracic area in which changes in impedance contribute to EIT creation of images11. The shape and the thickness of the EIT sensitivity region depend on the dimensions, the bioelectric propertiesand form of the thorax, as well with the type of current injection and voltage measurement pattern [1212.

Time-difference Imaging is a method which is utilized for EIT reconstruction in order to display changes in conductivity rather than absolute conductivity levels. It is a technique that uses time to show the change in conductivity. EIT image compares the changes in impedance with a baseline frame. This is a great way to monitor the changes in physiological activity over time such as lung ventilation and perfusion [22. Color coding of EIT images isn’t unified but typically shows the change in intensity to a baseline level (2). EIT images are typically created using a spectrum of colors with red indicating the greatest proportional impedance (e.g., during inspiration) and green for a middle relative impedance, and blue the smallest relative impedance (e.g. when expiration is in progress). In clinical settings, an interesting option is using color scales which range from black (no impedance change) or blue (intermediate impedance changes) and white (strong impedance changes) for coded ventilation. from black, to white, up to mirror-perfusion.

2. There are a variety of color codes available for EIT images as compared to CT scan. The rainbow-color scheme employs red for the highest relative impedance (e.g. when inspiration occurs) and green for a intermediate relative impedance, and blue for the lowest relative impedance (e.g. at expiration). Modern color scales make use of instead of black (which has no impedance changes), blue for an intermediate change in impedance and white for the highest changing of the impedance.

4. Functional Imaging and EIT Waveform Analysis

Analyzing Impedance Analyzers data is performed using EIT waves that are generated by individual image pixels within the form of a sequence of raw EIT images that are scanned over the course of time (Figure 3.). An area of concern (ROI) can be defined to describe activity in the individual pixels in the image. Within all ROIs, the waveform displays fluctuations in regional conductivity in time , resulting from ventilation (ventilation-related signal, VRS) and cardiac activities (cardiac-related signal CRS). Additionally, electrically conductive contrast-agents such as hypertonic salinity can be utilized to create the EIT pattern (indicator-based signal, IBS) which may be related to perfusion in the lung. The CRS could be a result of both the lung as well as the cardiac region and may be partly attributable to lung perfusion. The exact nature and origin isn’t fully understood 1313. Frequency spectrum analysis has been used to identify ventilationor cardiac-related changes in the impedance. Impedance changes that do not occur regularly could result from changes in the settings of the ventilator.

Figure 3. EIT waveforms , as well as the functional EIT (fEIT) Images originate from the raw EIT images. EIT waves can be described as pixel-wise, or by using a region of interest (ROI). Conductivity fluctuations are the result of ventilation (VRS) or heart activity (CRS) but may also be generated artificially e.g., by IBS (IBS) to determine perfusion. FEIT images show the some of the regional physiological parameters including ventilation (V) as well as perfusion (Q) which are extracted from the raw EIT images using an algorithmic operation over time.

Functional EIT (fEIT) images are created by applying a mathematical function on the raw images as well as the corresponding pixel EIT signal waveforms. Since the mathematical operation is applied to calculate an appropriate physiological parameter for each pixel. Regional physiological features like regional ventilation (V), respiratory system compliance, as along with regional perfusion (Q) can be determined and visualized (Figure 3.). The information derived taken from EIT waveforms and simultaneously registered pressures of the airways can be utilized to determine the lung’s compliance as also lung closing and opening times for each pixel using changes of impedance and pressure (volume). The comparable EIT measurements taken during the deflation and inflation of the lungs permit the display of pressure-volume curves at the pixel level. The mathematical operations used to calculate various types of fEIT pictures may address different functional characteristics within the cardio-pulmonary systems.