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Electrical Impedance Tomography for Cardio-Pulmonary Monitoring

Electrical Impedance Tomography (EIT) is an instrument for monitoring bedside that provides non-invasive visualisation of local ventilation and possibly lung perfusion distribution. In this article, we review and discusses the clinical and methodological aspects of thoracic EIT. Initially, researchers focused on the validation of EIT to determine regional ventilation. Research is currently focused on clinical applications of EIT for assessing lung collapse increased tidal flow, and lung overdistension to titrate positive end-expiratory pressure (PEEP) and Tidal volume. In addition, EIT may help to detect pneumothorax. Recent research has evaluated EIT as a way for measuring regional lung perfusion. Indicator-free EIT tests could be enough for continuous measurement of cardiac stroke volume. The use of a contrast agent like saline might be required in order to determine regional perfusion of the lungs. In the end, EIT-based monitoring of regional respiration and lung perfusion may visualize the perfusion match and local ventilation that can prove beneficial in treating patients with chronic respiratory distress syndrome (ARDS).

Keywords: electrical impedance tomography bioimpedance; reconstruction of images Thorax; regional ventilation and regional perfusion monitoring.

1. Introduction

Electronic impedance transmission (EIT) can be described as a non-radiation functional imaging modality that allows the non-invasive monitoring of bedside regional lung ventilation as well as arguably 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, [ 1, 2].

2. Basics of Impedance Spectroscopy

Impedance Spectroscopy is the range of the biological tissue’s voltage to externally applied alternating voltage (AC). It is usually achieved using four electrodes. Two are utilized to inject AC injection, and the remaining two are used to measure voltage 3,4. Thoracic EIT measures the regional range of intra-thoracic bioimpedance. This could be seen in the same way as applying the four electrode principle to the image plane that is spanned by the electrode belt 11. Dimensionally, electrical resistance (Z) is identical to resistance and the 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, and the imaginary component is known as reactance, which quantifies effects resulting from resistance or capacitance. The capacitance of a cell is determined by the biomembranes’ characteristics of the tissue , for example, ion channels, fatty acids, and gap junctions. The resistance is determined by the structure and the amount of extracellular fluid [ 1, 2[ 1, 2]. When frequencies are below 5 kilohertz (kHz) electricity moves through extracellular fluid, and is primarily dependent on the characteristics of resistivity of tissues. At higher frequencies of up to 50 kHz, electrical impulses are slightly redirected at cell membranes which leads to an increase in capacitive tissue properties. For frequencies higher than 100 kHz electric currents are able to travel through cell membranes and decrease the capacitive portion 2]. Therefore, the effects that determine the impedance of tissue depend on the utilized stimulation frequency. Impedance Spectroscopy is often described as conductivity or resistivity. These will normalize conductance or resistance in relation to the area of the unit and the length. The SI units of equivalent can be described as Ohm-meter (O*m) for resistivity, and Siemens per meter (S/m) to measure conductivity. The thoracic tissue’s resistance ranges from 150 O*cm in blood up to 700 O*cm for deflated lung tissue, up to 2400 O*cm for ballooned lung tissue ( Table 1). In general, the tissue’s resistance or conductivity is a function of amount of fluid and the ion concentration. In the case of respiratory lungs it also depends on the volume of air present in the alveoli. Although most tissues exhibit isotropic response, heart and muscle in particular exhibit anisotropic properties, meaning that the resistance is strongly dependent on the direction that it’s measured.

Table 1. The electrical resistivity of the thoracic muscles.

3. EIT Measurements and Image Reconstruction

To conduct EIT measurements, electrodes are placed around the chest in a transverse line, usually within the 4th to 5th intercostal space (ICS) in the line between parasternal and lateral [5]. Following that, changes in impedance can be measured in the lower lobes and lobes of the left and right lungs and also in the heart region ,2]. Placing the electrodes beneath the 6th ICS may be difficult, as the diaphragm and abdominal contents frequently enter the measurement plane.

Electrodes are either single self-adhesive electrodes (e.g., electrocardiogram ECG) that are positioned individually in a similar spacing between electrodes, or are integrated into electrode belts [ ,2]. Additionally, self-adhesive strips are offered for a more user-friendly application [ ,2[ 1,2]. Chest tubes, chest wounds (non-conductive) bandages or sutures for wires can severely affect EIT measurements. Commercially available EIT systems typically employ 16 electrodes. However, EIT devices with 8 and 32 electrodes are available (please see Table 2 for information) It is recommended to consult Table 2 for more details. ,21.

Table 2. Electronic impedance (EIT) devices.

During an EIT measure sequence, small AC (e.g. 5, milliamps at a frequency of 100 kHz) are applied to different electrode pairs, and the resultant voltages are recorded using the remaining electrodes ]. Bioelectrical impedance that is measured between the injecting and the electrodes that are measuring is determined from the applied current as well as the observed voltages. Most commonly nearby electrode pairs are used to allow AC application in a 16-elektrode system in 32-elektrode devices, whereas 16-elektrode utilize a skip-pattern (see Table 2) which increases the distance of electrodes for current injection. The resulting voltages are measured using those remaining electrodes. In the present, there is a debate ongoing about various current stimulation patterns , and their distinct advantages and disadvantages [77. In order to obtain an complete EIT data set that includes bioelectrical tests that are injected and electrode pairs used for measuring are constantly rotated around the entire thorax .

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 measurements is safe for use on body surfaces 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.

A EIT data set which is recorded in a single phase that is recorded during one cycle of AC software is known as an image frame. It includes voltage measurements needed to produce an unprocessed EIT image. The term “frame rate” refers to the amount of EIT frames recorded in a second. Frame rates of at least 10 frames/s are required in order to monitor ventilation and 25 images/s to check perfusion or cardiac function. Commercially accessible EIT devices employ frame rates of 40 to 50 images/s [2], as depicted in

To produce EIT images using the recorded frames, the process of image reconstruction process is employed. Reconstruction algorithms try to solve the inverse problem of EIT that is the restoration of the conductivity pattern 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) [10is frequently employed.

The majority of EIT pictures are similar to a two-dimensional computed-tomography (CT) image: these images are conventionally rendered so that the operator is looking across the entire cranial region when reviewing the image. Contrary to CT images, unlike a CT image however, an EIT image does not display the form of a “slice” but an “EIT sensitivity region” [1111. The EIT sensitivity region is a tubular intra-thoracic structure and is where the impedance change contributes to the EIT imaging process [11]. The dimensions and shape of the EIT sensitization region is determined by the dimensions, the bioelectric properties, as well as the structure of the chest 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 to show changes in conductivity, not actual conductivity level. In a time-difference EIT image shows the difference in impedance against a baseline frame. It is an opportunity to study the underlying physiological phenomenon that changes over time such as lung respiration and perfusion [22. The color coding of EIT images is not uniform but generally displays the change in impedance in relation to a reference level (2). EIT images are usually coded using a rainbow-colored scheme with red indicating the most significant value of relative imperf (e.g. in the time of inspiration) with green being a medium relative impedance and blue the lowest impedance (e.g. when expiration is in progress). For clinical applications it is possible to employ color scales that vary from black (no impedance change) to blue (intermediate impedance changes) and white (strong impedance changes) to code ventilation or between black and white and then red towards mirror perfusion.

2. There are a variety of color codes available 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) as well as green for a moderate relative impedance, and blue to indicate the least relative imperceptibility (e.g. at expiration). A more recent color scale uses instead of black (which has no impedance changes) and blue for an intermediate impedance variation, while white is the one with the strongest impedance change.

4. Functional Imaging and EIT Waveform Analysis

Analysis of Impedance Analyzers data is performed using EIT waveforms that form in individual image pixels in an array of raw EIT images that are scanned over time (Figure 3). In a region of focus (ROI) is a term used to represent activity within individual pixels in the image. Within each ROI, the waveform displays changes in the region’s conductivity over time resulting from breathing (ventilation-related signal, VRS) as well as cardiac activity (cardiac-related signal CRS). Additionally, electrically conductive contrast-agents such as hypertonic salinity can be used to get the EIT Waveform (indicator-based signal IBS) and is linked to the perfusion of 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 isn’t fully understood 13]. Frequency spectrum analysis is often utilized to differentiate between ventilationand cardiac-related impedance fluctuations. Impedance changes that do not occur regularly could be caused by adjustments in the ventilation settings.

Figure 3. EIT waves and Functional EIT (fEIT) images can be derived from Raw EIT images. EIT waveforms are defined pixel-wise or on a region of 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 the purpose of measuring perfusion. Images of fEIT show variables of regional physiological activity like perfusion (Q) and ventilation (V) along with perfusion (Q) which are extracted from raw EIT images by using a mathematical operation over time.

Functional EIT (fEIT) images are created by applying a mathematical function on a sequence of raw images and the corresponding EIT form [14]. Since the mathematical procedure is used to determine a physiologically relevant parameter for each pixel, physiological regional characteristics like regional ventilation (V) and respiratory system compliance, as in addition to regional perfusion (Q) can be measured as well as displayed (Figure 3). Data from EIT waveforms , as well as concurrently registered airway pressure measurements can be utilized to calculate the lung compliance as well as the lung’s opening and closing times for each pixel based on changes of pressure and impedance (volume). The comparable EIT measurements taken during increments of inflation and deflation in lung volume allow for the display of volume-pressure curves at the pixel level. Based on the mathematical procedure, different kinds of fEIT images can be used to examine different functional aspects in the cardiopulmonary system.

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