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Electrical Impedance Tomography for Cardio-Pulmonary Monitoring

Abstract

Electrical Impedance Tomography (EIT) is a bedside monitoring device that noninvasively visualizes local ventilation and arguably lung perfusion distribution. The article discusses and analyzes the methodological and clinical aspects of the thoracic EIT. Initially, researchers addressed the validation of EIT to assess regional ventilation. Research is currently focused on its clinical applications to assess lung collapse, increased tidal flow, and lung overdistension to titrate positive end-expiratory pressure (PEEP) and the volume of tidal. In addition, EIT may help to detect pneumothorax. Recent research has evaluated EIT as a means to determine regional lung perfusion. Indicate-free EIT tests could be enough for continuous measurement of cardiac stroke volume. A contrast agent, such as saline, might be required in order to determine regional lung perfusion. Thus, EIT-based assessment of regional respiratory and lung perfusion can reveal local perfusion and oxygenation, which can be helpful in the treatment of patients with acute respiratory distress syndrome (ARDS).

Keywords: electrical impedance imaging bioimpedance, image reconstruction Thorax; regional circulation; regional perfusion; monitoring

1. Introduction

Electric impedance tomography (EIT) is a radiation-free functional imaging modality that permits the non-invasive monitoring of bedside respiratory ventilation in the region and possibly perfusion. Commercially-available EIT devices were introduced for clinical application of this technique and thoracic EIT is widely used for both pediatric and adult patients 1., 1.

2. Basics of Impedance Spectroscopy

Impedance Spectroscopy can be defined as the variation in the voltage of biological tissue to an externally applied alternating electronic current (AC). It is typically measured using four electrodes. Two are employed for AC injection and the other two are for voltage measurement 3.,[ 3, 4]. Thoracic EIT measures the regional variation of the intra-thoracic bioimpedance. It can be seen to extend the four electrode principle onto the image plane which is defined by an electrode belt 1]. Dimensionally, electrical resistance (Z) is equivalent to resistance, as is the equivalent International System of Units (SI) unit is Ohm (O). It can be easily expressed as a complex number in which it is the actual portion of resistance and the imaginary is called reactance, which is the measurement of effects caused by capacitance or inductance. Capacitance is a function of biomembranes’ characteristics of a tissues such as ion channel and fatty acids as well as gap junctions. Resistance is determined by the composition and the amount of extracellular fluid 1, 22. For frequencies lower than 5 kilohertz (kHz), electrical current circulates through extracellular fluids and is primarily dependent on the resistive characteristics of the tissues. In higher frequencies above 50 kHz. electrical currents are slightly diverted at cell membranes , leading to an increase in capacitive tissues properties. In frequencies that exceed 100 kHz electrical currents can travel through cell membranes and reduce the capacitive component [ 21 2. Therefore, the effects that determine the amount of tissue impedance depend on the used stimulation frequency. Impedance Spectroscopy is often described as conductivity or resistance, which equalizes conductance and resistance to unit size and length. The SI units of equivalent can be described as Ohm-meter (O*m) for resistivity and Siemens per meter (S/m) for conductivity. The resistance of lung tissue can range from 150 O*cm for blood and up to 700 o*cm for lung tissue that is deflated, all the way between 2400 and 2400 O*cm of the lung tissue that has been inflated ( Table 1). In general, tissue resistivity or conductivity is dependent on volume of the fluid and the amount of ions. In terms of the lungs, it is dependent on the quantity of air that is present in the alveoli. While most tissues exhibit anisotropic behaviour, the heart and muscle skeleton exhibit anisotropy, in which the degree of resistance depends on the direction in which they are measured.

Table 1. The electrical resistance of the thoracic tissue.

3. EIT Measurements and Image Reconstruction

In order to perform EIT measurements electrodes are put around the thorax in a transverse plane that is usually located within the 4th to 5th intercostal areas (ICS) in the line between parasternal and lateral [55. As a result, changes in impedance can be observed in areas of the lower part of the right and left lungs, as well as within the heart region ,21. To place the electrodes below the 6th ICS could be difficult because the diaphragm and abdominal contents frequently enter the measurement plane.

Electrodes are self-adhesive electrodes (e.g., electrocardiogram, ECG) that are positioned individually in a similar spacing between electrodes or are integrated in electrode belts ,2]. Self-adhesive lines are offered for a more user-friendly application ,2]. Chest wounds, chest tubes bandsages that are not conductive or wire sutures could block or negatively impact EIT measurements. Commercially available EIT devices typically utilize 16 electrodes. However, EIT devices that use 8 to 32 electrodes may be also available (please refer to Table 2 for details) The following table shows the electrodes available. ,2].

Table 2. Electrical impedance devices that are commercially accessible. (EIT) devices.

In an EIT measuring sequence, very small AC (e.g., <5 million mA with a frequency of 100 kHz) are applied to different electrode pairs and the results are then measured using the other electrodes 6. Bioelectrical impedance between the injecting and electrodes used for measuring is determined by analyzing the applied current and measured voltages. Most commonly adjacent electrode pairs are used for AC application in a 16-elektrode setup in 32-elektrode devices, whereas 16-elektrode use a skip pattern (see the table 2) in order to extend the space between electrodes that inject current. The voltages generated are measured by using all the electrodes. In the present, there is a constant debate regarding different current stimulation patterns and their unique advantages and disadvantages [77. To collect a complete EIT data set of bioelectrical tests in the injecting as well as the electrodes that measure are constantly rotated around the entire thorax .

1. Measurements of voltage and current around the thorax using an EIT system that has 16 electrodes. Within a few milliseconds, simultaneously, the current electrode as well as those with active voltage electrodes are continuously turned across the upper thorax.

The AC used during EIT measurements is safe for use on the body that is undetectable 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 that is recorded in a single phase in AC application is referred to as a frame and contains the voltage measurements that create that Raw EIT image. The term frame rate reflects the amount of EIT frames recorded in a second. Frame rates that are at least 10 images/s is required to monitor ventilation , and 25 images/s in order to monitor heart function or perfusion. Commercially available EIT equipment uses frame rates between 40 and 50 images/s [2], depicted in

To create EIT images from the recorded frames, a process known as image reconstruction is applied. Reconstruction algorithms strive to resolve the reverse problem of EIT, which is the recuperation of the conductivity distribution inside the thorax based on the voltage measurements that have been recorded at the electrodes that are on the thorax surface. Initially, EIT reconstruction assumed that electrodes were placed in an ellipsoid or circular plane, whereas newer algorithms take into account the anatomical shape of the thorax. In the present, it is 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) [10often used.

As a rule, EIT photographs are similar with a two-dimensional computed (CT) image. These images are usually rendered so that the viewer looks from caudal to cranial when analysing the image. In contrast to a CT image one can observe that an EIT image doesn’t display an actual “slice” but an “EIT sensitivity region” [1111. The EIT sensitive region is a lens-shaped intra-thoracic area from which impedance changes contribute to EIT image generation [1111. The size and shape of EIT sensitivity region are dependent on the dimensions, bioelectric properties, and the appearance 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 used in EIT reconstruction to show variations in conductivity rather than Absolute conductivity values. In a time-difference EIT image compares the change in impedance to the baseline frame. This affords the opportunity to study the underlying physiological phenomenon that changes over time like lung ventilation and perfusion [22. Color coded EIT images is not unified but commonly displays the change in impedance in relation to a reference level (2). EIT images are typically encoded using a color scheme that is rainbow-like with red representing the most significant absolute impedance (e.g. when inspiration occurs) and green representing a medium relative impedance and blue being the lowest relative impedance (e.g. when expiration is in progress). For clinical applications the best option is to employ color scales that vary from black (no changes in impedance) up to blue (intermediate impedance change) and white (strong impedance shift) to code ventilation or between black and white, and red up to mirror-perfusion.

2. Different available color codings of EIT images in comparison with the CT scan. The rainbow-color scheme makes use of red for the most powerful value of impedance relative (e.g. when inspiration occurs) Green for a low relative impedance and blue to indicate the least relative imperceptibility (e.g., during expiration). The newer color scales employ instead black for no impedance change), blue for an intermediate impedance shift, and white for the most powerful impedance changes.

4. Functional Imaging and EIT Waveform Analysis

Analyzing Impedance Analyzers data is based on EIT waveforms which are created by individual image pixels within a series of raw EIT images over duration (Figure 3.). A “region of study” (ROI) can be defined to describe activity in the individual pixels in the image. In all ROIs, the waveform displays changes in the region’s conductivity over time resulting from ventilatory activity (ventilation-related signal, VRS) or activity in the heart (cardiac-related signal CRS). Additionally, electrically conductive contrast agents such as hypertonic saline can be used to get the EIT waveshape (indicator-based signal, IBS) and can be linked to lung perfusion. The CRS could originate from both the lung as well as the cardiac region, and can be attributed to lung perfusion. The exact cause and the composition are not understood fully 1313. Frequency Spectrum Analysis is typically used to distinguish between ventilationand cardiac-related impedance fluctuations. Impedance changes that aren’t periodic may be caused by changes in the settings of the ventilator.

Figure 3. EIT waveforms , as well as the functional EIT (fEIT) photos can be derived from Raw EIT images. EIT waveforms can be defined pixel-wise or on a region which is of concern (ROI). Conductivity changes result naturally from ventilatory (VRS) as well as cardiac activity (CRS) but can be produced artificially e.g. or through the injection of bolus (IBS) to measure perfusion. FEIT images show the local physiological parameters, such as ventilation (V) (V) and perfusion (Q) that are extracted from the raw EIT images by applying a mathematical procedure over time.

Functional EIT (fEIT) images are produced by applying a mathematical procedure on the raw images along with the associated pixel EIT spectrums. Since the mathematical operation is applied to determine a physiologically relevant parameter for every pixel, the regional physiological characteristics such as regional ventilation (V) and respiratory system compliance, as and regional perfusion (Q) can be determined and visualized (Figure 3.). Data from EIT waveforms and concurrently recorded pressures of the airways can be used to calculate the lung compliance and the rate of lung opening and closing for each pixel based on changes in pressure and impedance (volume). The comparable EIT measurements during the inflation and deflation steps of the lungs enable the display of pressure-volume curves at scales of pixel. Based on the mathematical process, different types of fEIT scans can be used to examine different functional aspects from the cardio-pulmonary apparatus.

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