Electrical Impedance Tomography for Cardio-Pulmonary Monitoring

Abstract

Electrical Impedance Tomography (EIT) is a bedside monitoring tool that provides non-invasive visualisation of local ventilation and arguably lung perfusion distribution. This paper reviews and analyzes the clinical and methodological aspects of the thoracic EIT. Initially, researchers were concerned about the validation of EIT to measure regional ventilation. These studies focus on its clinical applications to measure lung collapse TIDAL recruitment, as well as lung overdistension. This allows for the titration of positive end-expir pressure (PEEP) and the volume of tidal. In addition, EIT may help to detect pneumothorax. Recent studies have evaluated EIT as a tool to measure regional lung perfusion. Indicator-free EIT tests could be enough to continuously measure cardiac stroke volume. The use of a contrast agent like saline might be required to assess the regional perfusion of the lungs. This is why EIT-based monitoring of respiratory ventilation and lung perfusion could reveal the perfusion match and local ventilation and can prove helpful in the treatment of patients with chronic respiratory distress syndrome (ARDS).

Keywords: electrical impedance tomography; bioimpedance; image reconstruction Thorax; regional circulation Regional perfusion; monitoring

1. Introduction

Electrical impedance tomography (EIT) is a non-radiation functional imaging modality that permits non-invasive monitoring of bedside regional lung ventilation and , possibly perfusion. Commercially accessible EIT devices were developed for the clinical use of this technique and thoracic EIT has been used safely in both adult and pediatric patients [ 2, ].

2. Basics of Impedance Spectroscopy

Impedance Spectroscopy may be described as the biomaterial’s voltage response to an externally applied electric current (AC). It is typically achieved by using four electrodes, where two are used to inject AC injection and the other two are used for measuring voltage 3.,]. Thoracic EIT measures the regional range of intra-thoracic bioimpedance. This can be seen to extend the four electrode principle onto the image plane which is defined through the electro belt 1]. Dimensionally, electrical inductance (Z) is identical to resistance and the appropriate International System of Units (SI) unit is Ohm (O). It is often expressed as a complicated number, in which the real component is resistance and the imaginary part is called the reactance, which evaluates the effects that result from capacitors or the effect of inductance. The amount of capacitance is determined by biomembranes’ specifics of the tissue such as ion channels, fatty acids, and gap junctions. In contrast, resistance is determined by the structure and the amount of extracellular fluid [ 1., 22. At frequencies below 5 kilohertz (kHz) (kHz), electrical current circulates through extracellular fluids and is primarily dependent upon the characteristics of resistivity of tissues. When frequencies are higher, up to 50 kHz, currents are a little deflected by cells’ membranes which causes an increase of capacitive tissue properties. At frequencies above 100 kHz electrical currents can travel through cell membranes, and diminish the capacitive portion 22. Thus, the factors that determine the amount of tissue impedance depend on the used stimulation frequency. Impedance Spectroscopy typically refers to conductivity or resistivity. These will normalize conductance or resistance in relation to unit size and length. The SI units that correspond to it are Ohm-meter (O*m) for resistivity and Siemens per meter (S/m) as for conductivity. The resistance of the thoracic tissues ranges between 150 O*cm of blood and 700 O*cm in collapsed lung tissue and up to 2400 O*cm in ballooned lung tissue ( Table 1). In general, tissue resistance or conductivity varies based on level of fluids and ions. Regarding lung function, this is dependent on the quantity of air inside the alveoli. While the majority of tissues exhibit isotropic characteristic, the heart and the muscle fibers in the skeletal system exhibit anisotropic behavior, in which the degree of resistance depends on the direction from which it is measured.

Table 1. The electrical resistance of the thoracic tissue.

3. EIT Measurements and Image Reconstruction

For EIT measurements electrodes are placed on the thorax in a transverse plane typically in the 4th through 5th intercostal areas (ICS) at that line called parasternal [5]. In turn, the variations in impedance can also be measured in the lower lobes in the right and left lungs, and also in the area of the heart ,2[ 1,2]. It is possible to position the electrodes below the 6th ICS may be difficult, as the abdominal and diaphragm often enter the measurement plane.

Electrodes are self-adhesive electrodes (e.g. electrocardiogram, ECG) that are placed with equal spacing between electrodes or are incorporated into electrode belts ,2[ 1,2]. Additionally, self-adhesive stripe are available for a more user-friendly application ,21. Chest tubes, chest wounds non-conductive bandages, or conductive wire sutures can block or greatly affect EIT measurements. Commercially available EIT equipment typically uses 16 electrodes. However, EIT systems with eight (or 32) electrodes are available (please read Table 2 to get information) There are also 32 electrodes (please refer to Table ,2].

Table 2. Electric impedance tomography (EIT) instruments.

In an EIT measurements, small AC (e.g. five microamps at 100 kHz) are applied through different electrodes, and the generated voltages are measured with the remaining electrodes ]. The bioelectrical impedance between the injecting and the electrode pairs used to measure the voltage can be calculated by using the applied current and the measured voltages. The majority of the time connected electrode pairs are utilized to allow AC application in a 16-elektrode system in 32-elektrode devices, whereas 16-elektrode utilize a skip-pattern (see Table 2.) so that the electrodes are closer to the electrodes for current injection. The resultant voltages are measured by using one of the other electrodes. There is currently an ongoing discussion about different current stimulation patterns , and their unique advantages and disadvantages [7]. For a complete EIT data set that includes bioelectrical tests as well as the injecting and electrodes used to measure the electrodes are continuously rotated throughout the entire thorax .

1. Current measurement and voltage measurements around the thorax by using an EIT system with 16 electrodes. In just a few milliseconds all the active voltage electrodes and these active electrodes get turned within the thorax.

The AC employed during EIT tests is safe to use to use on the body and remains undetected by the 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 recorded in a single phase of AC apps is called frame. It contains voltage measurements necessary to create the raw EIT image. The term frame rate refers to the amount of EIT frames recorded per second. Frame rates of no less than 10 images/s are needed to monitor ventilation , and 25 images/s to check the perfusion or cardiac function. Commercially available EIT devices utilize frame rates of 40 to 50 images/s [2], shown in

In order to create EIT images from the captured frames, the process of image reconstruction process is employed. Reconstruction algorithms strive to resolve the inverse problem of EIT, which is the determination of the conductivity distribution within the thorax, based on the voltage measurements taken at the electrodes on the thorax surface. In the beginning, EIT reconstruction assumed that electrodes were placed on a circular or ellipsoid plane, while newer algorithms incorporate information about the anatomical structure of the thorax. Currently, using the Sheffield back-projection algorithm as well as the finite-element method (FEM) which is a linearized Newton-Raphson algorithm ] and the Graz consensus reconstruction algorithm for EIT (GREIT) [10typically used.

It is generally true that EIT image are basically similar with a two-dimensional computed (CT) image: these images are rendered conventionally so that the operator looks at the cranial and caudal regions when analyzing the picture. Contrary to a CT image An EIT image doesn’t display the form of a “slice” but an “EIT sensitivity region” [11]. The EIT sensitivity region is a thoracic-specific lens where impedance fluctuations contribute to EIT creation of imagesIt is a lens-shaped intra-thoracic volume that contributes to the generation. The shape and the thickness of the EIT sensitive region are determined by the dimensions, the bioelectric propertiesas well as the form of the thorax as well depending on the voltage measurement and current injection pattern [12It is important to note that the shape of the thoracic thorax can.

Time-difference imaging is a method that is employed in EIT reconstruction to show variations in conductivity, rather than pure conductivity amounts. The time-difference EIT image compares the change in impedance with the baseline frame. This affords the opportunity to examine the effects of time on physiological events such as lung ventilation and perfusion [22. The color-coding used in EIT images isn’t uniform, however, it typically displays the change in impedance to a reference level (2). EIT images are usually coded using a rainbow-color scheme with red representing the highest relative impedance (e.g. when inspiration occurs) as well as green, which is a medium relative impedance, and blue the lowest impedance (e.g. during expiration). For clinical applications An interesting approach is to use color scales ranging from black (no change in impedance) to blue (intermediate impedance changes) and white (strong impedance changes) to code ventilation . between black and red, and white towards mirror perfusion.

2. Different color codings for EIT images as compared to CT scan. The rainbow color scheme uses red for the highest in terms of relative intensity (e.g., during inspiration) Green for a moderate relative impedance, blue is the color that has the lowest relative intensity (e.g. when expiration is in progress). The newer color scales employ instead of black, which has no impedance change) and blue for the intermediate impedance change and white for the highest impedance shift.

4. Functional Imaging and EIT Waveform Analysis

Analyzing Impedance Analyzers data is based on EIT waveforms that form in individual image pixels in a series of raw EIT images that are scanned over time (Figure 3). An area of concern (ROI) can be defined to represent activity within individual pixels of the image. Within any ROI, the waveform shows changes in the region’s conductivity over time resulting from ventilatory activity (ventilation-related signal, VRS) (or cardiac activity (cardiac-related signal CRS). Additionally, electrically conductive contrast agents like hypertonic saltsaline may be used to produce the EIT waveshape (indicator-based signal, IBS) and could be connected to perfusion in the lung. The CRS could originate from both the cardiac and lung region and may also be related to lung perfusion. The exact cause and the composition are incompletely understood [ 13]. Frequency spectrum analysis is commonly used to identify ventilation- and cardiac-related Impedance Analyzers changes. Impedance changes that do not occur regularly could be caused by modifications in the settings of the ventilator.

Figure 3. EIT forms and the functions of EIT (fEIT) photographs are extracted from original EIT images. EIT waveforms are defined as pixel-wise, or by using a region to be studied (ROI). Conductivity changes occur naturally as a result of the process of ventilation (VRS) as well as cardiac activity (CRS) however, they can also be created artificially, e.g. with bolus injection (IBS) to measure perfusion. FEIT images depict various physiological parameters in the region such as ventilation (V) or perfusion (Q), extracted from raw EIT images using an algorithmic process over time.

Functional EIT (fEIT) images are produced through the application of a mathematical algorithm on the sequence of raw pictures together with the appropriate pixel EIT signal waveforms. Since the mathematical operation is used to calculate a physiologically relevant parameter for each pixel. Regional physiological aspects like regional ventilation (V) and respiratory system compliance, as along with regions perfusion (Q) can be assessed and visualized (Figure 3). The data obtained from EIT waveforms , as well as concurrently registered airway pressure measurements can be used to calculate lung compliance and the lung’s opening and closing times for each pixel using changes of impedance and pressure (volume). Comparable EIT measurements taken during the deflation and inflation of the lungs allow the displaying of volume-pressure curves at one pixel. Depending on the mathematical operation different kinds of fEIT photographs can be used to examine different functional aspects that are associated with the cardiovascular system.