Electrical Impedance Tomography for Cardio-Pulmonary Monitoring

Electrical Impedance Tomography (EIT) is an instrument that monitors the bedside and noninvasively visualizes local ventilation and arguably lung perfusion distribution. The article discusses and discusses both methodological and clinical aspects of thoracic EIT. Initially, investigators addressed the possibility of using EIT for measuring regional airflow. Current studies focus mainly on clinical applications of EIT to quantify lung collapse, tidal recruitment, 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 tests could be enough to measure continuously the cardiac stroke volume. Utilizing a contrast agent, such as saline, could be necessary to evaluate the regional lung perfusion. As a result, EIT-based assessment of regional respiratory and lung perfusion may reveal the local perfusion and ventilation that could prove useful in the treatment of patients with chronic respiratory distress syndrome (ARDS).

Keywords: Electrical impedance tomography; bioimpedance; image reconstruction 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 , 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.

  1. Basics of Impedance Spectroscopy

Impedance Spectroscopy is the range of the biological tissue’s voltage to an externally applied alternating electrical current (AC). It is normally measured using four electrodes, of which two are utilized to inject AC injection, and the remaining two are used for measuring voltage 3,4. Thoracic EIT measures the regional variation of the intra-thoracic bioimpedance. It could be seen in the same way as applying the four electrode principle to the image plane that is spanned through the electro belt 11. In terms of dimensions, electrical impedance (Z) is identical to resistance and the corresponding International System of Units (SI) unit is Ohm (O). It can be conveniently expressed as a complicated number, in which the real component is resistance, and the imaginary component is called reactance, which describes the effects that are caused by resistance or capacitance. The capacitance of a cell is determined by the biomembranes’ characteristics of the tissue , which includes ion channels, fatty acids, and gap junctions. The resistance is mostly determined by the structure and the amount of extracellular fluid [ 1., 2]. In frequencies that are less than 5 kilohertz (kHz) the electrical current runs through extracellular fluid and is mostly dependent on the resistivity characteristics of tissues. At higher frequencies up to 50 kHz electrical currents can be slightly deflected through cell membranes , which results in an increase in capacitive tissues properties. When frequencies exceed 100kHz the electrical current is able to pass through cell membranes, reducing the capacitive component [ 2[ 1, 2]. Thus, the factors which determine the level of impedance in the tissue depend on the used stimulation frequency. Impedance Spectroscopy typically refers to resistivity or conductivity, which is a measure of conductance or resistance to the unit’s area and length. The SI units used consist of Ohm-meter (Om) for resistivity, and Siemens per meters (S/m) for conductivity. The thoracic tissue’s resistance ranges from 150 Ocm in blood up to 700 Ocm for lung tissue that has been deflated, and as high as 2400 Ocm when dealing with ballooned lung tissue ( Table 1). In general, tissue resistivity or conductivity is determined by the amount of fluid and the ion concentration. Regarding respiratory lungs it depends on the volume of air inside the alveoli. While most tissues show isotropic behavior, the heart and muscle skeleton exhibit anisotropy, meaning that the resistance is strongly dependent on the direction from which it is measured.

Table 1. Electrical resistivity of thoracic tissues.

  1. EIT Measurements and Image Reconstruction

To carry out EIT measurements electrodes are put around the chest in a transverse line typically in the 4th through 5th intercostal space (ICS) in that line called parasternal [5]. Subsequently, the changes of the impedance of the lungs can be measured within those lobes that are lower in the right and left lungs, as well as within the heart region ,22. It is possible to position the electrodes below the 6th ICS might be challenging as the diaphragm as well as abdominal content regularly enter the measurement plan.

Electrodes are either single self-adhesive electrodes (e.g., electrocardiogram ECG) that are placed individually in a similar spacing between electrodes, or they are integrated into electrode belts ,2]. Self-adhesive stripes are also made available for more user-friendly application ,21. Chest tubes, chest wounds (non-conductive) bandages or wire sutures could block or negatively impact EIT measurements. Commercially available EIT equipment typically uses 16 electrodes. However, EIT systems with eight to 32 electrodes may be available (please see Table 2 for details) The following table shows the electrodes available. ,21.

Table 2. Electric impedance (EIT) gadgets.

In an EIT measurement , small AC (e.g. approximately 5 mA at a frequency of 100 kHz) is applied to different electrode pairs and the resultant voltages are recorded using the remaining other electrodes [ ]. The bioelectrical impedance of the injecting and electrode pairs that measure is calculated based on the applied current as well as the observed voltages. Most commonly electrode pairs that are adjacent to each other are utilized to allow AC application in a 16-elektrode device, while 32-elektrode systems often utilize a skip-pattern (see the table 2.) that increases the distance between current injecting electrodes. The resulting voltages can be measured by using all the electrodes. In the present, there is a debate ongoing about various current stimulation patterns , and the advantages and disadvantages of each [7]. For a complete EIT data set that includes bioelectrical measurements, the injecting and the measuring electrode pairs are continuously rotated throughout the entire thorax .

  1. The measurements of voltage and current are made around the thorax using an EIT system that includes 16 electrodes. Within milliseconds two electrodes measuring current and the active voltage electrodes are rotating in the area of the thorax.

The AC used during the EIT measurements is safe for body surface applications and remains undetected 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 which is recorded during one cycle in AC applications is technically called frame. It contains voltage measurements to generate EIT’s image. EIT image. The term frame rate refers to the amount of EIT frames recorded in a second. Frame rates at least 10 frames/s are required in order to monitor ventilation , and 25 images/s to monitor perfusion or cardiac function. Commercially accessible EIT devices utilize frame rates between 40 and 50 images/s [2], shown in

To create EIT images using recorded frames, the so-called image reconstruction method is used. Reconstruction algorithms aim to solve the problem that is the reverse of EIT that is recuperation of the conductivity distribution inside the thorax on the basis of the voltage measurements taken at the electrodes on the thorax surface. Initially, EIT reconstruction assumed that electrodes were placed on an ellipsoid, circular or circular plane, but more recent algorithms employ information on the anatomical structure of the thorax. Presently, there is an algorithm called the Sheffield back-projection algorithm [ as well as the finite-element method (FEM) that is a linearized Newton-Raphson algorithm ] as well as the Graz consensus reconstruction algorithm for EIT (GREIT) [10typically used.

It is generally true that EIT images can be compared to a two-dimensional computed (CT) image. These images are traditionally rendered so that the operator looks from caudal to cranial when studying the image. In contrast to CT images, unlike a CT image EIT images are not a two-dimensional image. EIT image does not show an image “slice” but an “EIT sensitivity region” [1111. The EIT sensitization region is a lens-shaped intra-thoracic area that is the source of impedance variations which contribute to the EIT picture generation]. The size and shape of EIT sensitivity region depend on the dimensions, the bioelectric propertiesas well as the shape of the thorax as well in the used current injection and voltage measurement pattern [1212.

Time-difference imaging is a method that is employed in EIT reconstruction to show changes in conductivity instead of relative conductivity of the levels. Time-difference EIT image compares the changes in impedance against a baseline frame. It is an opportunity to study the underlying physiological phenomenon that changes over time such as lung ventilation and perfusion [22. The color coding of EIT images isn’t uniform, but typically shows the change in intensity to a baseline level (2). EIT images are typically colored using a rainbow color scheme with red indicating the greatest in relative intensity (e.g. when inspiration occurs) while green is a moderate relative impedance, and blue the lowest impedance (e.g., during expiration). For clinical purposes it is possible to use color scales ranging from black (no impedance changes) or blue (intermediate impedance change) as well as white (strong impedance change) to code ventilation , or between black and white, and red to mirror perfusion.

  1. There are a variety of color codes available for EIT images in comparison with the CT scan. The rainbow-color scheme utilizes red for the most powerful percentage of the relative imperceptibility (e.g. in the time of inspiration), green for a moderate relative impedance, and blue when the relative resistance is lowest (e.g. during expiration). A newer color scheme uses instead black (no impedance change) as well as blue for an intermediate impedance variation, and white for the most powerful impedance variation.
  2. Functional Imaging and EIT Waveform Analysis

Analysis of Impedance Analyzers data is based on EIT waveforms which are created by individual image pixels within an array of raw EIT images that are scanned over time (Figure 3.). Region of Interest (ROI) can be defined as a summary of activity in specific pixels of the image. Within the ROIs, each waveform displays variations in the conductivity of the region over time due to respiration (ventilation-related signal, VRS) as well as cardiac activity (cardiac-related signal CRS). Additionally, electrically conducting contrast agents such as hypertonic sodium can be utilized to create the EIT signal (indicator-based signal IBS) and could be connected to the perfusion of the lung. The CRS may originate from both the heart and lung region, and is possibly related to lung perfusion. Its exact origin and composition isn’t fully understood 13]. Frequency spectrum analysis can be used to discriminate between ventilationand cardiac-related impedance fluctuations. Non-periodic changes in impedance may be caused by changes in the settings of the ventilator.

Figure 3. EIT Waveforms as well as functional EIT (fEIT) photographs are extracted from EIT raw EIT images. EIT waveforms are defined pixel-wise or on a region to be studied (ROI). Conductivity changes result naturally from ventilatory (VRS) (or cardiac activity (CRS) but they can be produced artificially e.g. or through the injection of bolus (IBS) for perfusion measurement. Images of fEIT show local physiological parameters including ventilation (V) along with perfusion (Q) and perfusion (Q) that are extracted from the raw EIT images by applying the mathematical process of time over.

Functional EIT (fEIT) images are produced by applying a mathematical function on the raw images together with the appropriate pixel EIT Waveforms. Because the mathematical process is used to calculate the physiologically relevant parameters for every pixel, the regional physiological characteristics like regional ventilation (V), respiratory system compliance as along with regions perfusion (Q) can be assessed and displayed (Figure 3). The data obtained from EIT waveforms along with simultaneously registered airway pressure values can be used to calculate the lung’s compliance as also lung opening and closing at each pixel, using variations of impedance and pressure (volume). The comparable EIT measurements taken during stepwise inflation and deflation of the lungs permit the display of curves representing volume and pressure at the pixel level. Based on the mathematical operation, different types of fEIT photos may address different functional characteristics that are associated with the cardiovascular system.

By Master Henry

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