Electrical Impedance Tomography for Cardio-Pulmonary Monitoring

Electrical Impedance Tomography (EIT) is an instrument used to monitor the bed that can be used to visualize the local airflow and , possibly, lung perfusion distribution. The article discusses and analyzes the methodological and clinical aspects of the thoracic EIT. Initially, researchers addressed the possibility of using EIT to assess regional ventilation. Research is currently focused on its clinical applications to measure lung collapse TIDAL recruitment, as well as lung 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 assessed EIT as a means to measure regional lung perfusion. The absence of indicators in EIT tests could be enough for continuous measurement of cardiac stroke volume. The use of a contrast agent, such as saline, could be necessary to evaluate the regional perfusion of the lungs. In the end, EIT-based monitoring of respiratory ventilation and lung perfusion could reveal local ventilation and perfusion matching which may be useful in the treatment of patients suffering from acute respiratory distress syndrome (ARDS).

Keywords: electrical impedance tomography and bioimpedance. Image reconstruction Thorax; regional airflow; regional perfusion; monitoring

1. Introduction

Electric impedance tomography (EIT) is one of the radiation-free functional imaging modality that permits non-invasive monitoring at bedside of both regional lung ventilation , and possibly perfusion. Commercially accessible EIT devices were introduced to allow clinical application of this technique, and the thoracic EIT can be used with safety in both adult and pediatric patients [ 1, 2.

2. Basics of Impedance Spectroscopy

Impedance Spectroscopy may be described as the resistance of biological tissues to externally applied alternating electric current (AC). It is normally measured using four electrodes. Two are utilized for AC injection and the other two are used for measuring voltage 3,4. Thoracic EIT measures the regional variability of Impedance Spectroscopy in the thoracic area and can be considered in the same way as applying the four electrode principle to the image plane that is spanned by the electrode belt 11. Dimensionally, electrical inductance (Z) is exactly the same as resistance. the related International System of Units (SI) unit is Ohm (O). It can be easily expressed as a complex figure where the actual part is resistance and the imaginary part is called the reactance, which determines the effect of either inductance or capacitance. Capacitance is a function of biomembranes’ characteristics of a tissue , including ion channels, fatty acids, and gap junctions. In contrast, resistance is determined by the composition and the amount of extracellular fluid [ 1., 2]. In frequencies that are less than 5 kilohertz (kHz) electricity travels through extracellular fluids and is heavily dependent on its resistive properties of tissues. In higher frequencies above 50 kHz. electrical currents are slightly slowed down at cells’ membranes which causes an increase of capacitive tissue properties. If frequencies are higher than 100 kHz electricity can pass through cell membranes and lower the capacitive portion 2]. Therefore, the effects 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 the unit’s area and length. The SI units used include Ohm-meter (O*m) for resistivity and Siemens per meters (S/m) (S/m) for conductivity. The resistance of lung tissue can range between 150 O*cm of blood and up to 700 o*cm for air-filled lung tissue, and all the way to 2400O*cm for air-filled lung tissue ( Table 1). In general, tissue resistivity or conductivity varies based on volume of the fluid and the amount of ions. For respiratory lungs it is dependent on the quantity of air inside the alveoli. Although most tissues exhibit isotropic behavior, heart and muscles of the skeletal are anisotropic. in which the degree of resistance depends on the direction in which it’s measured.

Table 1. The electrical resistance of the thoracic tissue.

3. EIT Measurements and Image Reconstruction

In order to conduct EIT measurements electrodes are positioned around the Thorax in a horizontal plane generally in the 4th through 5th intercostal spaces (ICS) near Parasternal Line [5]. In turn, the variations in impedance can also be measured in the lower lobes of both the right and left lungs, as well as in the region of the heart ,22. The placement of the electrodes below the 6th ICS is not easy as the diaphragm as well as abdominal content periodically enter the measurement plane.

Electrodes are either single self-adhesive electrodes (e.g., electrocardiogram, ECG) which are placed with equal spacing between the electrodes, or are integrated into electrode belts [ ,21 2. Self-adhesive stripes are also readily available for a user-friendly application [ ,21. Chest wounds, chest tubes bandsages that are not conductive or wire sutures could block or severely affect EIT measurements. Commercially available EIT devices typically utilize 16 electrodes. However, EIT devices that use 8 or 32 electrodes is also available (please see Table 2 for information) It is recommended to consult Table 2 for more details. ,21.

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

During an EIT measurement sequence, small AC (e.g. approximately 5 microamps at 100 kHz) is applied to different electrodes, and the produced voltages are measured using the remaining other electrodes [ 6. Bioelectrical impedance that is measured between the injecting and the electrodes that are measuring is calculated using the applied current as well as the observed voltages. The majority of the time connected electrode pairs are used to allow AC application in a 16-elektrode system in 32-elektrode devices, whereas 16-elektrode apply a skip pattern (see Table 2) so that the electrodes are closer to the electrodes used for injecting current. The resulting voltages are measured using those remaining electrodes. Presently, there’s a constant debate regarding different current stimulation patterns , and their advantages and disadvantages [77. In order to obtain an complete EIT data set that includes bioelectrical tests both the injecting and electrodes that measure are constantly rotated throughout the entire thorax .

1. Measurements of voltage and current around the thorax by using an EIT system consisting of 16 electrodes. Within a few milliseconds, all the active voltage electrodes and activated voltage electrodes will be repeatedly turned within the thorax.

The AC utilized during EIT tests are 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.

This EIT data set that is captured during a single cycle in AC applications is technically called a frame . It is comprised of the voltage measurements necessary to create that Raw EIT image. The term frame rate reflects the amount of EIT frames recorded per second. Frame rates that exceed 10 images/s are necessary for monitoring ventilation and 25 images/s to check perfusion or cardiac function. Commercially accessible EIT devices employ frame rates ranging from 40 to 50 images/s [2], as described in

To generate EIT images from the captured frames, the so-called image reconstruction technique is used. Reconstruction algorithms are designed to address the other aspect of EIT, which is the determination of the conductivity distribution within the thorax based upon the voltage measurements that have been recorded at the electrodes that are on the thorax surface. In the beginning, EIT reconstruction assumed that electrodes were placed on an ellipsoid plane, while newer algorithms incorporate information about anatomy of the thorax. At present, the Sheffield back-projection algorithm [ as well as the finite element technique (FEM) with a linearized Newton–Raphson algorithm ], and the Graz consensus reconstruction algorithm for EIT (GREIT) [10is frequently employed.

The majority of EIT images can be compared to a two-dimensional computed-tomography (CT) image: these images are typically rendered in a way that the user is able to look from cranial to caudal when analysing the image. Contrary to CT images, unlike a CT image the EIT image doesn’t display an actual “slice” but an “EIT sensitivity region” [11]. The EIT sensitive region is a thoracic-specific lens in which changes in impedance contribute to the EIT imaging process [1111. Shape and thickness of the EIT area of sensitivity are dependent upon the dimensions, the bioelectric properties, and the shape of the thorax depending on the voltage measurement and current injection pattern [1212.

Time-difference imaging is a method which is employed for EIT reconstruction to show changes in conductivity rather than relative conductivity of the levels. It is a technique that uses time to show the change in conductivity. EIT image compares the changes in impedance with the baseline frame. This allows you to monitor the changes in physiological activity over time such as lung ventilation and perfusion [22. Color coding of EIT images isn’t unified however it usually shows the shift in impedance to a reference level (2). EIT images are usually colored using a rainbow color scheme with red representing the greatest percentage of impedance (e.g. during inspiration), green a medium relative impedance and blue being the lowest relative impedance (e.g. for expiration). In clinical settings there is a good option to use color-scales that range from black (no changes in impedance) to blue (intermediate impedance changes) as well as white (strong impedance changes) to code ventilation , or from black, to red, and white to mirror perfusion.

2. Different color codes that are available for EIT images in comparison to the CT scan. The rainbow-color scheme is based on red for the most powerful value of impedance relative (e.g. when inspiration occurs), green for a low relative impedance and blue, for the lowest ratio of impedance (e.g. during expiration). The newer color scales employ instead of black, which has no impedance change) Blue for an intermediate change in impedance and white for the highest changing of the impedance.

4. Functional Imaging and EIT Waveform Analysis

Analysis of Impedance Analyzers data is performed using EIT waveforms that form inside individual image pixels within an array of raw EIT images that are scanned over time (Figure 3). A region of interest (ROI) is a term used to describe activity in the individual pixels in the image. Within each ROI, the waveform shows fluctuations in regional conductivity in time , resulting from ventilation (ventilation-related signal, VRS) or heart activity (cardiac-related signal, CRS). Additionally, electrically conducting contrast agents such as hypertonic salinity can be used to get the EIT pattern (indicator-based signal IBS) which may be related to perfusion in the lung. The CRS may originate from both the lung as well as the cardiac region, and is possibly linked to lung perfusion. Its precise source and composition are incompletely understood [ 1313. Frequency spectrum analysis is frequently employed to distinguish between ventilationas well as cardiac-related changes in impedance. Impedance changes that do not occur regularly could be caused by changes in the settings of the ventilator.

Figure 3. EIT waves and Functional EIT (fEIT) image can be derived from original EIT images. EIT waveforms are defined pixel-wise or on a region to be studied (ROI). Conductivity changes occur naturally as a result of 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. FEIT images show the regional physiological parameters like perfusion (Q) and ventilation (V) and perfusion (Q) that are extracted from raw EIT images by applying an algorithmic process over time.

Functional EIT (fEIT) images are produced using a mathematical process on an array of raw images together with the appropriate pixel EIT waves [14]. Because the mathematical process is used to determine an appropriate physiological parameter for every pixel, the regional physiological parameters like regional respiration (V), respiratory system compliance as in addition to respiratory system compliance as well as regional perfusion (Q) can be measured and visualized (Figure 3). The data generated from EIT waveforms and simultaneously registered airway pressure values can be utilized to determine the lung’s compliance, as well as lung closing and opening times in each pixel using the changes in pressure and impedance (volume). Similar EIT measurements during the inflation and deflation steps of the lungs can be used to display of curves representing volume and pressure at an individual pixel. Based on the mathematical process, various types of fEIT pictures could be used to analyze different functions that are associated with the cardiovascular system.

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