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Electrical Impedance Tomography for Cardio-Pulmonary Monitoring

Abstract

Electrical Impedance Tomography (EIT) is an instrument for monitoring bedside that does not require any surgery to see the local airflow and perhaps lung perfusion. The present article reviews and discusses the clinical and methodological aspects of thoracic EIT. Initially, researchers focused on the validation of EIT to determine regional ventilation. Recent studies concentrate on its clinical applications to measure lung collapse tidal recruitment, 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 method to measure lung perfusion in the region. Indicator-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 perfusion of the lungs. Therefore, EIT-based monitoring of regional respiration as well as lung perfusion can be used to assess the local perfusion and ventilation which may be useful in the treatment of patients suffering from chronic respiratory distress syndrome (ARDS).

Keywords: electrical impedance tmography; bioimpedance; image reconstruction Thorax; regional circulation as well as monitoring regional perfusion.

1. Introduction

Electric impedance tomography (EIT) can be described as a non-radiation functional imaging modality that provides the non-invasive monitoring of bedside regional lung ventilatory and perhaps perfusion. Commercially-available EIT devices were introduced to allow clinical application of this technique, and the thoracic EIT has been used safely for both pediatric and adult patients [ 2, 1.

2. Basics of Impedance Spectroscopy

Impedance Spectroscopy is the range of the biological tissue’s voltage to an externally applied alternating electron current (AC). It is typically measured using four electrodes, where two are employed to inject AC injection and the other two are used to measure voltage 3,,3. Thoracic EIT measures the regional variability of Impedance Spectroscopy in the thoracic area and can be considered to extend the principle of four electrodes to the imaging plane, which is divided through the electro belt 11. Dimensionally, electrical inductance (Z) is equivalent to resistance, as is the related International System of Units (SI) unit is Ohm (O). It can be expressed as a complicated number, in which the actual component is resistance, while the imaginary portion is called reactance. It is the measurement of effects caused by an inductance, capacitance, or. Capacitance is dependent on biomembranes’ features of the tissue , which includes ion channels, fatty acids, and gap junctions, whereas resistance is primarily determined by content and quantity of extracellular fluid [ 1., 2]. For frequencies lower than 5 kilohertz (kHz) the electrical current is carried by extracellular fluid and is predominantly dependent on the resistance characteristics of tissues. For higher frequencies that exceed 50 kHz, currents are slightly redirected at cell membranes , which results in an increase in the tissue’s capacitive properties. For frequencies higher than 100 kHz, electrical current can pass through cell membranes and decrease the capacitive component 22. So, the results which determine the level of impedance in the tissue depend on the used stimulation frequency. Impedance Spectroscopy is usually given as conductivity or resistance. Both regulates conductance or resistance according to unit area and length. The SI units that correspond to it consist of Ohm-meter (O*m) for resistivity, and Siemens per meters (S/m) to measure conductivity. The resistance of lung tissue can range from 150 O*cm when blood is present and 700 O*cm in tissues that have been deflated and inflated, to up to 2400 o*cm for ballooned lung tissue ( Table 1). In general, tissue resistivity or conductivity is determined by the fluid content and ion concentration. In the case of the lungs, it is dependent on the quantity of air inside the alveoli. While the majority of tissues exhibit isotropic characteristic, the heart and the skeletal muscle behave anisotropic, in which the degree of resistance depends on the direction that you measure it.

Table 1. Thoracic tissues have electrical resistance.

3. EIT Measurements and Image Reconstruction

To carry out EIT measurements electrodes are positioned around the chest in a transverse plane typically between the 4th and 5th intercostal spaces (ICS) in Parasternal Line [5]. As a result, changes in the impedance of the lungs can be measured within the lower lobes in the right and left lungs, and also in the heart region ,22. To position the electrodes above the 6th ICS could be difficult since the diaphragm and abdominal contents are frequently inserted into the measurement plane.

Electrodes are either single self-adhesive electrodes (e.g., electrocardiogram ECG,) that are positioned individually with equal spacing between electrodes or integrated into electrode belts ,2]. Additionally, self-adhesive strips are readily available for a user-friendly application [ ,21. Chest wounds, chest tubes Non-conductive bandages and conductive sutures for wires can greatly affect EIT measurements. Commercially available EIT devices typically utilize 16 electrodes, but EIT systems with eight or 32 electrodes is available (please see Table 2 for specifics) The following table shows the electrodes available. ,21.

Table 2. Electronic impedance tomography (EIT) devices.

During an EIT measurement sequence, small AC (e.g., <5 million mA with a frequency of 100 kHz) are applied through different pairs of electrodes and the generated voltages are measured with the remaining electrodes ]. The bioelectrical resistance between the injecting and the electrode pairs that measure is determined by analyzing the applied current and measured voltages. Most often electrode pairs that are adjacent to each other are utilized to allow AC application within a 16-elektrode configuration in 32-elektrode devices, whereas 16-elektrode utilize a skip-pattern (see the table 2) which increases the distance of the current injecting electrodes. The resultant voltages are measured with all the electrodes. There is currently an ongoing discussion about different types of current stimulation and their distinct advantages and disadvantages [7]. For a complete EIT data set that includes bioelectrical tests as well as the injecting and measuring electrode pairs are continuously rotated throughout the entire thorax .

1. Current application and voltage measurements around the thorax utilizing an EIT system consisting of 16 electrodes. In a matter of milliseconds simultaneously, the current electrode and an active voltage electrode get rotating about the chest.

The AC that is used in EIT measurements are safe for use on body surfaces and are not detected 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 one cycle within AC application is referred to as an image frame. It includes voltage measurements to generate the Raw EIT image. Frame rate refers to the amount of EIT frames captured per second. Frame rates that exceed 10 images/s are necessary for monitoring ventilation and 25 images/s to check cardiac function or perfusion. Commercially available EIT devices employ frames that have a frame rate of between 40 and 50 images/s [2], as illustrated in

To produce EIT images using the recorded frames, so-called image reconstruction process is employed. Reconstruction algorithms seek to solve the issue that causes EIT that is recovering the conductivity distribution within the thorax, based on the voltage measurements that have been made at the electrodes of the thorax surface. Initially, EIT reconstruction assumed that electrodes were placed in an ellipsoid or circular plane. However, newer techniques use information about the anatomical contour of the thorax. At present, it is the Sheffield back-projection algorithm and the finite element method (FEM) based linearized Newton-Raphson algorithm ] as well as the Graz consensus reconstruction algorithm for EIT (GREIT) [10are commonly used.

A lot of the time, EIT photographs are similar with a two-dimensional computed (CT) image. These images are typically rendered in a way that the viewer is looking from cranial to caudal while analysing the image. Contrary to the CT image the EIT image does not show the appearance of a “slice” but an “EIT sensitivity region” [11]. The EIT sensitivity region is a lens-shaped intrathoracic region where impedance fluctuations contribute to EIT imaging process [1111. The dimensions and shape of the EIT sensitization region is determined by the dimensions, the bioelectrical properties, and also the contour of the thorax as well depending on the current injection and voltage measurement pattern [12(13, 14).

Time-difference imaging can be described as a technique that is used in EIT reconstruction in order to display changes in conductivity rather than the Absolute conductivity values. In a time-difference EIT image compares the changes in impedance with the baseline frame. This gives the possibility to observe time-dependent physiological processes such as lung respiration and perfusion [22. Color coded EIT images isn’t unified but commonly displays the change in the impedance of the patient to a standard (2). EIT images are usually created using a spectrum of colors with red representing the highest in relative intensity (e.g., during inspiration) with green being a medium relative impedance and blue being the lowest relative impedance (e.g. when the expiration time is). For clinical applications An interesting approach is to use color-scales that range from black (no impedance changes) to blue (intermediate impedance changes) as well as white (strong impedance shift) to code ventilation , or from black to white and then red up to mirror-perfusion.

2. Different color codes are available for EIT images as compared to CT scan. The rainbow-color scheme is based on red to indicate the highest relative impedance (e.g. in the time of inspiration) while green is used for low relative impedance and blue, for the lowest ratio of impedance (e.g. when expiration is in progress). Newer color scales utilize instead of black (which has no impedance changes), blue for an intermediate impedance shift, and white for the most powerful impedance change.

4. Functional Imaging and EIT Waveform Analysis

Analysis of Impedance Analyzers data is based on EIT waveforms , which are generated in individual image pixels in the form of a sequence of raw EIT images that are scanned over length of (Figure 3.). In a region of focus (ROI) can be defined to represent activity within individual pixels of the image. Within any ROI, the image shows fluctuations in regional conductivity in time as a result of ventilation (ventilation-related signal, VRS) as well as cardiac activity (cardiac-related signal, CRS). Additionally, electrically conductive contrast-agents such as hypertonic saline can be used to get the EIT pattern (indicator-based signal IBS) and may be linked to perfusion in the lung. The CRS can originate from both the lung and cardiac region, and could be partially associated with lung perfusion. Its exact origin and composition isn’t fully understood 1313. Frequency spectrum analysis has been used to discriminate between ventilation- and cardiac-related Impedance Analyzers changes. Impedance changes that aren’t periodic may be caused by adjustments in the ventilation settings.

Figure 3. EIT Waveforms as well as functional EIT (fEIT) image are created from initial EIT images. EIT waves can be described pixels-wise or based on a area which is of concern (ROI). Conductivity changes result naturally from breathing (VRS) as well as cardiac activity (CRS) but can be caused artificially, e.g., by injection of bolus (IBS) to measure perfusion. Images of fEIT show local physiological parameters such as ventilation (V) or perfusion (Q) as extracted from the raw EIT images using an algorithmic operation over time.

Functional EIT (fEIT) images are generated through the application of a mathematical algorithm on the raw images together with the appropriate pixel EIT spectrums. Because the mathematical process is used to determine a physiologically relevant parameter for each pixelof the image, regional physiological features like regional ventilation (V), respiratory system compliance as in addition to local perfusion (Q) can be measured as well as displayed (Figure 3). Information taken from EIT waveforms along with simultaneously registered airway pressure measurements can be utilized to calculate lung’s compliance and the rate of lung opening and closing at each pixel, using variations of impedance and pressure (volume). Comparable EIT measurements taken during stepwise inflation and deflation of lung volumes allow for the display of pressure-volume curves at an individual level. The mathematical operations used to calculate various types of fEIT pictures might address distinct functional characteristics that are associated with the cardiovascular system.