Electrical Impedance Tomography for Cardio-Pulmonary Monitoring

Abstract

Electrical Impedance Tomography (EIT) is a bedside monitor that visually examines the local environment and perhaps lung perfusion. The paper summarizes and analyzes the methodological and clinical aspects of the thoracic EIT. Initially, researchers addressed the validity of EIT to assess regional ventilation. Current studies focus mainly on clinical applications of EIT to quantify lung collapse, the tidal response, and lung overdistension in order to regulate positive end-expiratory pressure (PEEP) and tidal volume. In addition, EIT may help to detect pneumothorax. Recent studies evaluated EIT as a means to measure lung perfusion in the region. Indicate-free EIT measurements might be sufficient to continuously measure cardiac stroke volume. A contrast agent such as saline might be required to assess the regional lung perfusion. Because of this, EIT-based monitors of regional ventilation and lung perfusion can reveal local perfusion and oxygenation that could prove useful in the treatment of patients with chronic respiratory distress syndrome (ARDS).

Keywords: Electrical impedance tomography and bioimpedance. Image reconstruction Thorax; regional circulation Regional perfusion; monitoring

1. Introduction

Electrical impedance tomography (EIT) can be described as a non-radiation functional imaging technique that permits the non-invasive monitoring of bedside regional lung ventilation , and possibly perfusion. Commercially-available EIT devices were introduced to allow clinical application of this technique, and thoracic EIT has been successfully used in both pediatric and adult patients 1, 2.

2. Basics of Impedance Spectroscopy

Impedance Spectroscopy is the range of the biological tissue’s voltage to an externally applied electrical current (AC). It is normally measured using four electrodes, of which two are employed to inject AC injection, and the remaining two are used to measure voltage 3,,[ 3, 4]. Thoracic EIT measures the regional variation of the intra-thoracic bioimpedance. It is seen as an expansion of the principle of four electrodes into the image-plane spanned by an electrode belt [ 1]. Dimensionally, electrical impedance (Z) is the same as resistance and its equivalent International System of Units (SI) unit is Ohm (O). It can be conveniently expressed as a complicated number, in which the real portion is resistance while the imaginary part is called reactance. This measures the effects of capacitance or inductance. The amount of capacitance is determined by biomembranes’ specifics of the tissues, such as ion channels, fatty acids, and gap junctions, whereas resistance is mainly determined by composition and quantity of extracellular fluid 1, 22. Below 5 kilohertz (kHz) electricity circulates through extracellular fluids and is in a major way dependent on the characteristics of resistivity of tissues. At higher frequencies up to 50 kHz the electrical currents are slightly diverted at cell membranes , leading to an increase in capacitive tissue properties. At frequencies above 100 kHz electrical currents can travel through cell membranes, and diminish the capacitive portion 22. So, the results which determine the tissue’s impedance depend on the stimulation frequency. Impedance Spectroscopy usually refers to conductivity and resistivity. They is a measure of conductance or resistance to units’ area and length. The SI units used include Ohm-meter (O*m) for resistivity and Siemens per meters (S/m) as for conductivity. The resistance of lung tissue can range from 150 O*cm when blood is present to 700 O*cm when it comes to tissues that have been deflated and inflated, to as high as 2400 O*cm when dealing with ballooned lung tissue ( Table 1). In general, the tissue’s resistance or conductivity is determined by the quantity of fluid in the tissue and the concentration of ions. In terms of the lungs, it depends on the volume of air inside the alveoli. While most tissues show isotropic characteristic, the heart and the muscle fibers in the skeletal system exhibit anisotropic behavior, meaning that the resistance is strongly dependent on the direction that the measurement is made.

Table 1. Electrical resistivity of thoracic tissues.

3. EIT Measurements and Image Reconstruction

To carry out EIT measurements electrodes are positioned around the thorax in a transverse plane that is usually located in the 4th through 5th intercostal spaces (ICS) near the parasternal line [5]. Following that, changes in impedance can be observed in the lower lobes and lobes of the left and right lungs and also in the heart area ,2[ 1,2]. To position the electrodes above the 6th ICS could be difficult since abdominal content and the diaphragm occasionally enter the measurement area.

Electrodes are self-adhesive electrodes (e.g. electrocardiogram, ECG) that are placed individually with equal spacing in-between the electrodes or integrated into electrode belts [ ,2(1). Also, self-adhesive stripes are accessible for a more convenient application ,2]. Chest tubes, chest wounds (non-conductive) bandages or sutures for wires can severely affect EIT measurements. Commercially available EIT equipment typically uses 16 electrodes. However, EIT devices with 8 and 32 electrodes are also available (please read Table 2 to get specifics) The following table shows the electrodes available. ,2[ 1,2].

Table 2. Electronic impedance (EIT) equipment.

In an EIT measuring sequence, very small AC (e.g., <5 milliamps at a frequency of 100 kHz) are applied to several electrode pairs, and the results are then measured using the remaining other electrodes [ 6. The bioelectrical resistance between the injecting and electrodes that are measuring is calculated using the applied current as well as the observed voltages. The majority of the time the electrodes adjacent to each other are utilized to allow AC application in a 16-elektrode device however 32-elektrode systems usually employ a skip pattern (see the table 2) that increases the distance between electrodes used for injecting current. The resulting voltages are measured using all the electrodes. Currently, there is an ongoing debate about the various kinds of current stimulation, as well as their unique advantages and disadvantages [77. To obtain a full EIT data set that includes bioelectrical tests The injecting and electrode pairs measuring are continuously rotationally positioned around the entire chest .

1. The measurements of voltage and current are made around the thorax with an EIT system that has 16 electrodes. In a matter of milliseconds each of the electrodes for current as well as these active electrodes get moved about the chest.

The AC used during the EIT tests is safe to use to use on the body and is not detectable 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.

This EIT data set that is recorded over a single cycle that is recorded during one cycle of AC programs is known as frames. They contain the voltage measurements that create this original EIT image. The term frame rate reflects the amount of EIT frames recorded per second. Frame rates of at minimum 10 images/s are needed to monitor ventilation , and 25 images/s to monitor the cardiac function or perfusion. Commercially available EIT equipment uses frames that have a frame rate of between 40 and 50 images/s [2], is shown in

In order to create EIT images from recorded frames, the so-called image reconstruction method is used. Reconstruction algorithms strive to resolve the opposite problem of EIT, which is the determination of the conductivity distribution within the thorax, based on the voltage measurements that have been obtained at the electrodes located on the thorax’s surface. At first, EIT reconstruction assumed that electrodes were placed on an ellipsoid plane, while newer algorithms employ information on anatomy of the thorax. At present, the Sheffield back-projection algorithm as well as the finite-element method (FEM) using a linearized Newton–Raphson algorithm [ ], and the Graz consensus reconstruction algorithm for EIT (GREIT) [10are commonly used.

A lot of the time, EIT images have a similarity with a two-dimensional computed (CT) image: these images are rendered conventionally so that the user is able to look from cranial to caudal when analyzing the picture. Contrary to the CT image EIT images are not a two-dimensional image. EIT image does not show the appearance of a “slice” but an “EIT sensitivity region” [1111. The EIT sensitive region is a thoracic-specific lens and is where the impedance change contributes to the EIT image generation [11(11, 11). The dimensions and shape of the EIT sensitization region is determined by the dimensions, bioelectric properties, as well as the appearance of the Thorax as and the applied current injection and voltage measurement pattern [1212.

Time-difference imaging is a technique that is used in EIT reconstruction to display changes in conductivity instead of relative conductivity of the levels. The time-difference EIT image compares the variation in impedance with a baseline frame. This is a great way to track the time-dependent physiological changes such as respiratory ventilation and perfusion [22. Color coded EIT images isn’t uniform, but typically shows the change in intensity to a baseline level (2). EIT images are generally coded using a rainbow-color scheme with red indicating the highest relative impedance (e.g., during inspiration) with green being a medium relative impedance and blue the smallest relative impedance (e.g. when expiration is in progress). For clinical applications it is possible to employ color scales that vary from black (no change in impedance) and blue (intermediate impedance change) as well as white (strong impedance changes) to code ventilation , or from black, to white, then up to mirror-perfusion.

2. Different color codings for EIT images when compared with the CT scan. The rainbow-color scheme makes use of red to indicate the highest relative impedance (e.g. in the time of inspiration) while green is used for moderate relative impedance, and blue for the lowest relative impedance (e.g. during expiration). Newer color scales utilize instead black for no impedance change), blue for an intermediate change in impedance and white for the most powerful changing of the impedance.

4. Functional Imaging and EIT Waveform Analysis

Analysis of Impedance Analyzers data is done using EIT waveforms , which are generated inside individual image pixels within a series of raw EIT images over length of (Figure 3). An area of concern (ROI) is a term used to summarize activity in individual pixels of the image. In all ROIs, the image shows the changes in conductivity of the region over the course of time that result from respiration (ventilation-related signal, also known as VRS) or cardiac activity (cardiac-related signal, CRS). In addition, electrically conductive contrast agents like hypertonic saltsaline may be used in the production of the EIT pattern (indicator-based signal IBS) and can be linked to perfusion in the lung. The CRS could come from both the heart and lung region and may be partly attributed to lung perfusion. The exact cause and the composition is not fully understood 13]. Frequency spectrum analysis is frequently used to discriminate between ventilationand cardiac-related impedance variations. Impedance changes that aren’t periodic may be caused by changes in ventilator settings.

Figure 3. EIT form and function EIT (fEIT) photos are extracted from raw EIT images. EIT waveforms may be defined either pixel-wise or in a region that is of particular interest (ROI). Conductivity variations are caused by the process of ventilation (VRS) or cardiac activity (CRS) but may be artificially induced, e.g. or through bolus injection (IBS) for measuring perfusion. Images of fEIT show specific physiological parameters of the region, such as ventilation (V) or perfusion (Q), extracted from the raw EIT images by using a mathematical procedure over time.

Functional EIT (fEIT) images are generated by applying a mathematical procedure on the sequence of raw images and the corresponding pixel EIT waves [14]. Because the mathematical process is applied to determine the physiologically relevant parameters for each pixel. The regional physiological features like regional ventilation (V), respiratory system compliance as well as the regional flow (Q) can be determined and visualized (Figure 3). The information derived generated from EIT waveforms and concurrently recorded pressures of the airways can be used to determine the lung’s compliance as well as lung closing and opening at each pixel, using variations of impedance and pressure (volume). Comparable EIT measurements taken during gradual inflation and deflation of lung volume allow for the display of volume-pressure curves at the pixel level. Based on the mathematical procedure, various types of fEIT pictures could be used to analyze different functions of the cardio-pulmonary system.