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Impedance Analyzers Fundamentals Explained

Electrical Impedance Tomography for Cardio-Pulmonary Monitoring

Abstract

Electrical Impedance Tomography (EIT) is a bedside monitor that can be used to visualize the local airflow as well as conceivably lung perfusion distribution. The paper summarizes and discusses the clinical and methodological aspects of the thoracic EIT. Initially, researchers were concerned about the validity of EIT for measuring regional airflow. Recent studies concentrate on clinical applications of EIT to assess lung collapse, TIDAL recruitment, as well as lung overdistension to measure positive end-expiratory pressure (PEEP) and Tidal volume. In addition, EIT may help to detect pneumothorax. Recent studies looked at EIT as a tool to measure regional lung perfusion. The absence of indicators in EIT tests could be enough to measure continuously the cardiac stroke volume. The use of a contrast agent such as saline might be required in order to determine the regional lung perfusion. In the end, EIT-based assessment of regional respiratory and lung perfusion might reveal the local perfusion and ventilation which could be beneficial in treating patients suffering from chronic respiratory distress syndrome (ARDS).

Keywords: Electrical impedance tomography bioimpedance, image reconstruction Thorax; regional ventilation Monitoring regional perfusion

1. Introduction

Electronic impedance transmission (EIT) is an radiation-free functional imaging modality that permits the non-invasive monitoring of bedside regional lung ventilation and , possibly perfusion. Commercially accessible EIT devices were developed for clinical applications of this method, and the thoracic EIT can be used with safety in both adult and pediatric patients [ 1, ].

2. Basics of Impedance Spectroscopy

Impedance Spectroscopy may be described as the biomaterial’s voltage response to an externally applied electricity (AC). It is usually achieved using four electrodes. Two are employed to inject AC injection, and the remaining two are used for measuring voltage 3.,]. Thoracic EIT measures the regional variability of Impedance Spectroscopy in the thoracic area and can be viewed as an expansion of the four electrode principle onto the image plane which is defined by an electrode belt [ 11. Dimensionallyspeaking, electrical impedance (Z) is exactly the same as resistance. the corresponding International System of Units (SI) unit is Ohm (O). It can be expressed as a complex number , where the real part is resistance, while the imaginary portion is called reactance. This determines the effect of an inductance, capacitance, or. Capacitance varies based on biomembranes’ characteristics of the tissue , which includes ion channels and fatty acids as well as gap junctions. The resistance is mainly determined by content and quantity of extracellular fluid [ 1., 22. At frequencies below 5 kilohertz (kHz) that is, electrical energy is carried by extracellular fluid and is predominantly dependent on its resistive properties of tissues. At higher frequencies up to 50 kHz the electrical currents are slightly diverted at cell membranes , leading to an increase in tissue capacitive properties. If frequencies are higher than 100 kHz electrical current can flow through cell membranes and lower the capacitive component 22. Therefore, the effects that determine the tissue’s impedance depend on the used stimulation frequency. Impedance Spectroscopy is usually given as resistivity or conductivity, which normalize resistance or conductance to unit length and area. The SI units for the same are Ohm-meter (O*m) for resistivity and Siemens per meters (S/m) as for conductivity. The thoracic tissue’s resistance ranges from 150 O*cm for blood and 700 O*cm in air-filled lung tissue, and up to 2400 O*cm in air-filled lung tissue ( Table 1). In general, the tissue’s resistance or conductivity is a function of volume of the fluid and the amount of ions. In the case of those in the lungs, it also depends on the amount of air inside the alveoli. While most tissues exhibit isotropic behavior, the heart as well as muscle in particular exhibit anisotropic properties, meaning that resistivity strongly depends on the direction in which you measure it.

Table 1. Electrical resistance of thoracic tissues.

3. EIT Measurements and Image Reconstruction

To perform EIT measurements electrodes are set around the Thorax in a horizontal plane which is typically located in the 4th to 5th intercostal spaces (ICS) in the parasternal line [5]. The changes in impedance can be measured in the lower lobes and lobes of the left and right lungs as well as in the area of the heart ,2[ 1,2]. To place the electrodes below the 6th ICS could be difficult because the diaphragm and abdominal content frequently enter the measurement plane.

Electrodes can be self-adhesive or single electrodes (e.g. electrocardiogram ECG) that are placed individually with equal spacing between the electrodes or integrated into electrode belts ,2]. Also, self-adhesive electrodes are readily available for a user-friendly application ,21. Chest wounds, chest tubes Non-conductive bandages and conductive wire sutures could block or greatly affect EIT measurements. Commercially available EIT systems typically employ 16 electrodes, but EIT systems with eight to 32 electrodes may be also available (please refer to Table 2 for specifics) The following table shows the electrodes available. ,2[ 1,2].

Table 2. Electric impedance (EIT) equipment.

In an EIT measure sequence, small AC (e.g. approximately 5 mgA at a rate of 100 kHz) are applied to different pairs of electrodes and the produced voltages are measured using the remaining other electrodes [ 6. Bioelectrical Impedance between the injecting and electrodes used for measuring is calculated using the applied current as well as the measured voltages. Most often the electrodes adjacent to each other are utilized for AC application in a 16-elektrode system, while 32-elektrode systems often utilize a skip-pattern (see the table 2) that increases the distance between current injecting electrodes. The resulting voltages can be measured using one of the other electrodes. Presently, there’s a constant debate regarding different types of current stimulation and their specific advantages and disadvantages [77. To obtain a full EIT data set that includes bioelectrical tests, the injecting and the electrode pairs that measure are continually moved around the entire thorax .

1. Current measurements and voltage measurements within the thorax, using an EIT system featuring 16 electrodes. In only a few milliseconds each of the electrodes for current as well as an active voltage electrode can be rotated within the thorax.

The AC used during the EIT measurements are safe for body surface applications 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.

This EIT data set which is recorded in a single phase during AC applications is technically called a frame and contains the voltage measurements required to create the raw EIT image. Frame rate refers to the number of EIT frames recorded in a second. Frame rates at least 10 images/s are necessary to monitor ventilation and 25 images/s to track perfusion and cardiac function. Commercially accessible EIT devices utilize frame rates between 40 and 50 images/s as described in

To produce EIT images using the recorded frames, the process of image reconstruction process is employed. Reconstruction algorithms seek to solve the problem that is the reverse of EIT that is the recuperation of the conductivity distribution within the thorax, based on the voltage measurements collected at electrodes on the thorax’s surface. In the beginning, EIT reconstruction assumed that electrodes were placed on a circular or ellipsoid plane. Newer methods take into account the anatomical form of the thorax. The current algorithms include EIT reconstruction algorithms such as the Sheffield back-projection algorithm [ , the finite element method (FEM) using a linearized Newton–Raphson algorithm ] and the Graz consensus reconstruction algorithm for EIT (GREIT) [10is frequently employed.

The majority of EIT images have a similarity to a two-dimensional computed-tomography (CT) image: these images are usually rendered so that the viewer is looking from caudal to cranial when studying the image. Contrary to CT images, unlike a CT image one can observe that an EIT image does not display an actual “slice” but an “EIT sensitivity region” [11]. The EIT sensitivity region is a lens-shaped intrathoracic region where impedance fluctuations contribute to the EIT image generation [11The EIT image is generated by impedance changes. The dimensions and shape of the EIT sensitivity region depend on the dimensions, bioelectric properties, as well as the form 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 which is employed for EIT reconstruction, which displays changes in conductivity, not Absolute conductivity values. In a time-difference EIT image displays the change in impedance with the baseline frame. This provides the chance to examine the effects of time on physiological events such as respiratory ventilation and perfusion [2]. The color code of EIT images isn’t unified however it usually shows the shift in impedance to an appropriate level (2). EIT images are generally coded using a rainbow-colored scheme with red indicating the highest relative impedance (e.g., during inspiration) with green being a medium relative impedance, and blue the smallest relative impedance (e.g., during expiration). In clinical settings one option to consider is to use color scales ranging from black (no impedance changes) up to blue (intermediate impedance change) and white (strong impedance shift) to code ventilation . between black and white, to mirror perfusion.

2. Different color codes that are available for EIT images in comparison to CT scan. The rainbow-color scheme employs red for the greatest relative impedance (e.g. in the time of inspiration), green for a medium relative impedance, and blue when the relative resistance is lowest (e.g., during expiration). A newer color scales use instead of black for no impedance change) Blue for an intermediate change in impedance, as well as white for the greatest impedance changes.

4. Functional Imaging and EIT Waveform Analysis

Analyzing Impedance Analyzers data is based on EIT waveforms , which are generated inside individual image pixels within a series of raw EIT images over the course of time (Figure 3). Region of Interest (ROI) can be defined to show the activity of individual pixels in the image. Within every ROI, the waveform shows the changes in conductivity of the region over time due to breathing (ventilation-related signal, also known as VRS) (or cardiac activity (cardiac-related signal CRS). Additionally, electrically conductive contrast-agents such as hypertonic saline can be used to produce the EIT waveshape (indicator-based signal IBS) and may be linked to the perfusion of the lung. The CRS can be traced to both the lung and cardiac region, and could be partially related to lung perfusion. The precise origins and components aren’t understood completely 1313. Frequency spectrum analysis has been employed to distinguish between ventilationand cardiac-related changes in impedance. Impedance changes that are not periodic could result from modifications in the settings of the ventilator.

Figure 3. EIT Waveforms as well as functional EIT (fEIT) photographs are created from Raw EIT images. EIT waves can be defined by pixel or on a particular region in interest (ROI). Conductivity changes occur naturally as a result of ventilatory (VRS) as well as cardiac activity (CRS) but may also be induced artificially, e.g. via IBS (IBS) for perfusion measurement. FEIT images show the local physiological parameters, such as perfusion (Q) and ventilation (V) (V) and perfusion (Q) taken from raw EIT images by using an equation over time.

Functional EIT (fEIT) images are created by applying a mathematical procedure on the raw images together with the appropriate pixel EIT Waveforms. Since the mathematical procedure is applied to calculate the physiologically relevant parameters for each pixel, physiological regional features like regional ventilation (V) and respiratory system compliance as and local perfusion (Q) can be determined as well as displayed (Figure 3). Information generated from EIT waveforms and simultaneously registered airway pressure values can be utilized to calculate lung’s compliance and the lung’s opening and closing times at each pixel, using variations of pressure and impedance (volume). Comparable EIT measurements taken during increments of inflation and deflation in lung volumes allow for the display of curves representing volume and pressure at an individual pixel. Depending on the mathematical method used, different types of fEIT scans can be used to examine different functional aspects that are associated with the cardiovascular system.

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