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Electrical Impedance Tomography for Cardio-Pulmonary Monitoring

Abstract

Electrical Impedance Tomography (EIT) is an instrument used to monitor the bed that visually examines the local environment as well as conceivably lung perfusion distribution. The article discusses and discusses both methodological and clinical aspects of the thoracic EIT. Initially, researchers focused on the possibility of using EIT for measuring regional airflow. Research is currently focused on its clinical applications for assessing lung collapse the tidal response, and 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 examined EIT as a way to measure lung perfusion in the region. Indicator-free EIT measurements might be sufficient to measure continuously the cardiac stroke volume. Utilizing a contrast agent such as saline might be required to assess regional perfusion of the lungs. In the end, EIT-based surveillance of regional airflow and lung perfusion can 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 Bioimpedance; image reconstruction; thorax; regional ventilation as well as monitoring regional perfusion.

1. Introduction

Electronic impedance transmission (EIT) is one of the non-radiation functional imaging modality that permits the non-invasive monitoring of bedside regional lung ventilation and arguably perfusion. Commercially available EIT devices were developed for clinical applications of this method and the thoracic EIT is used in a safe manner in both adult and pediatric patients 2, 2.

2. Basics of Impedance Spectroscopy

Impedance Spectroscopy can be defined as the variation in the voltage of biological tissue to an externally applied electronic current (AC). It is typically measured using four electrodes, of which two are used for AC injection and the other two for voltage measurement 3,,4. Thoracic EIT measures the regional variability of Impedance Spectroscopy in the thoracic area and can be viewed in the same way as applying the four electrode principle into the image-plane spanned by an electrode belt [ 11. In terms of dimension, electrical impedance (Z) is equivalent to resistance, as is the appropriate International System of Units (SI) unit is Ohm (O). It is often expressed as a complicated number, in which the real part is resistance, while the imaginary portion is called the reactance, which determines the effect of either inductance or capacitance. Capacitance is dependent on biomembranes’ properties of the tissues such as ion channel and fatty acids as well as gap junctions. In contrast, resistance is determined by the nature and amount of extracellular fluid 1., 2[ 1, 2]. When frequencies are below 5 kilohertz (kHz) that is, electrical energy runs through extracellular fluid and is predominantly dependent on the resistance characteristics of tissues. Higher frequencies, as high as 50 kHz, currents are slightly redirected at cell membranes . This leads to an increase in tissue capacitive properties. For frequencies higher than 100 kHz electric currents are able to travel through cell membranes, reducing the capacitive portion 21. So, the results that determine the tissue’s impedance depend on the stimulation frequency. Impedance Spectroscopy can be described in terms of resistivity or conductivity, which equalizes conductance and resistance to unit size and length. The SI units of equivalent is Ohm-meter (O*m) for resistivity and Siemens per meters (S/m) in the case of conductivity. The resistance of thoracic tissue varies between 150 O*cm for blood as high as 700 O*cm with tissues that have been deflated and inflated, to between 2400 and 2400 O*cm of the lung tissue that has been inflated ( Table 1). In general, tissue resistivity or conductivity will vary based on levels of ion and fluid content. In terms of breathing, it is dependent on the quantity of air present in the alveoli. Though most tissues exhibit an isotropic behavior, the heart and skeletal muscle behave anisotropic, in which the degree of resistance depends on the direction that they are measured.

Table 1. The electrical resistivity of the thoracic tissues.

3. EIT Measurements and Image Reconstruction

In order to conduct EIT measurements electrodes are positioned around the thorax in a transverse plan generally in the 4th to the 5th intercostal space (ICS) near the line between parasternal and lateral [5]. As a result, changes in impedance can be assessed in those lobes that are lower in the right and left lungs, and also in the heart area ,2[ 1,2]. To position the electrodes above the 6th ICS may be difficult, as the diaphragm and abdominal content frequently enter the measurement plane.

Electrodes are self-adhesive electrodes (e.g. electrocardiogram ECG) that are placed with equal spacing in-between the electrodes or integrated into electrode belts [ ,21. Also, self-adhesive stripes are made available for more user-friendly application [ ,2]. Chest tubes, chest wounds (non-conductive) bandages or sutures for wires can substantially affect EIT measurements. Commercially available EIT devices typically utilize 16 electrodes, but EIT systems with 8 as well as 32 electrodes are also available (please see Table 2 for more details) For more information, refer to Table 2. ,21.

Table 2. Electronic impedance (EIT) technology.

During an EIT measurements, small AC (e.g., <5 mgA at a rate of 100 kHz) are applied to various electrode pairs. The resultant voltages are recorded using the remaining electrodes ]. The bioelectrical impedance between the injecting and the electrode pairs measuring the electrodes is calculated from the known applied current and the measured voltages. The majority of the time adjacent electrode pairs are used for AC application in a 16-elektrode system for example, while 32-elektrode systems generally utilize a skip-pattern (see Table 2) that increases the distance between electrodes for current injection. The resulting voltages are measured using the remaining electrodes. In the present, there is an ongoing discussion about different current stimulation patterns and their advantages and disadvantages [77. To acquire a complete EIT data set that includes bioelectrical measurements that are injected and electrodes used to measure the electrodes are continuously rotationally positioned around the entire chest .

1. Measurements of voltage and current within the thorax, using an EIT system consisting of 16 electrodes. Within milliseconds, as well as the voltage and current electrodes and those with active voltage electrodes get turned across the upper thorax.

The AC used during EIT measurements are safe to use on the body and remain 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.

A EIT data set that is captured during a single cycle of AC Applications is termed a frame . It is comprised of the voltage measurements that create EIT’s original EIT image. The term frame rate reflects the number of EIT frames that are recorded every second. Frame rates of at minimum 10 images/s are required to monitor ventilation , and 25 images/s to track perfusion or cardiac function. Commercially available EIT equipment uses frames with a frame rate between 40 and 50 images/s, as described in

In order to create EIT images using recorded frames, the technique known as reconstructing of images is carried out. Reconstruction algorithms seek to solve the opposite problem of EIT that is the reconstruction of the conductivity distribution in the thorax using the voltage measurements recorded at the electrodes that are on the thorax surface. In the beginning, EIT reconstruction assumed that electrodes were placed on an ellipsoid, circular or circular plane. Newer methods take into account the anatomical form of the thorax. The current algorithms include it is the Sheffield back-projection algorithm , the finite element method (FEM) which is a linearized Newton-Raphson algorithm ], and the Graz consensus reconstruction algorithm for EIT (GREIT) [10are often employed.

In general, EIT photographs are similar to a 2-dimensional computed (CT) image: these images are normally rendered so that the operator looks from cranial towards caudal when taking a look at the picture. Contrary to an CT image however, an EIT image does not show an actual “slice” but an “EIT sensitivity region” [1111. The EIT sensitization region is a lens-shaped intrathoracic space that is the source of impedance variations which contribute to the EIT image generation [11The EIT image is generated by impedance changes. The size and shape of the EIT sensitivity region depend on the dimensions, bioelectrical properties, and also the appearance of the Thorax and the applied voltage measurement and current injection pattern [12It is important to note that the shape of the thoracic thorax can.

Time-difference image is a technique which is employed for EIT reconstruction, which displays changes in conductivity rather than actual conductivity level. It is a technique that uses time to show the change in conductivity. EIT image compares changes in impedance to a base frame. It is an opportunity to track the time-dependent physiological changes such as lung ventilation and perfusion [2]. Color coded EIT images is not unicoded but commonly displays the change in intensity to a baseline level (2). EIT images are typically encoded using a color scheme that is rainbow-like with red representing the high relative impedance (e.g. in the time of inspiration), green a medium relative impedance, and blue the least relative impedance (e.g., during expiration). For clinical applications An interesting approach is to use color scales ranging from black (no change in impedance) or blue (intermediate impedance change), and white (strong impedance changes) to code ventilation , or from black to white and then red to mirror perfusion.

2. Different color codes that are available for EIT images when compared with the CT scan. The rainbow-color scheme uses red for the greatest percentage of the relative imperceptibility (e.g. when inspiration occurs) while green is used for medium relative impedance, and blue when the relative resistance is lowest (e.g., during expiration). A newer color scheme uses instead of black, which has no impedance change) Blue for the intermediate impedance change as well as white for the greatest impedance shift.

4. Functional Imaging and EIT Waveform Analysis

Analyzing Impedance Analyzers data is done using EIT waveforms that form inside individual image pixels within an array of raw EIT images over the course of time (Figure 3.). A “region of study” (ROI) can be defined for a summation of activity within the individual pixels of the image. Within each ROI the waveform shows changes in the regional conductivity over time , resulting from respiration (ventilation-related signal, also known as VRS) as well as cardiac activity (cardiac-related signal, CRS). Additionally, electrically conductive contrast-agents such as hypertonic Saline can be used to get an EIT form (indicator-based signal, IBS) and is linked to the perfusion of the lung. The CRS could originate from both the heart and lung region, and could be partially linked to lung perfusion. The exact nature and origin are not well understood. 13]. Frequency spectrum analysis is frequently used to distinguish between ventilatorand cardiac-related impedance fluctuations. Non-periodic changes in impedance may be caused by changes in ventilator settings.

Figure 3. EIT waves and Functional EIT (fEIT) Images are created from raw EIT images. EIT waves can be described either pixel-wise or in a region that is of particular interest (ROI). Conductivity changes result naturally from breathing (VRS) as well as cardiac activity (CRS) but can be artificially induced, e.g. via injection of bolus (IBS) for perfusion measurement. FEIT images present various physiological parameters in the region, such as ventilation (V) and perfusion (Q) and perfusion (Q) that are extracted from raw EIT images using an algorithmic operation over time.

Functional EIT (fEIT) images are produced using a mathematical process on a sequence of raw images together with the appropriate pixel EIT signal waveforms. Because the mathematical process is used to determine an appropriate physiological parameter for every pixel, the regional physiological parameters like regional respiration (V) and respiratory system compliance, as in addition to the regional flow (Q) are measured in a visual display (Figure 3). Information drawn from EIT waveforms , as well as concurrently registered pressures of the airways can be utilized to calculate lung’s compliance as well as lung opening and closing at each pixel, using variations of impedance and pressure (volume). The comparable EIT measurements taken during the deflation and inflation of the lungs allow the displaying of curves representing volume and pressure at an individual pixel. Based on the mathematical operation, different types of fEIT scans might address distinct functional characteristics within the cardio-pulmonary systems.

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