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Detailed Notes on Impedance Spectroscopy

Electrical Impedance Tomography for Cardio-Pulmonary Monitoring

Abstract

Electrical Impedance Tomography (EIT) is an instrument that monitors the bedside and does not require any surgery to see the local airflow and arguably lung perfusion distribution. In this article, we review and discusses the methodological and clinical aspects of thoracic EIT. Initially, researchers focused on the validity of EIT to measure regional ventilation. These studies focus on its clinical applications to assess lung collapse, TIDAL recruitment, as well as lung overdistension, in order to determine 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 gauge regional lung perfusion. The absence of indicators in EIT measurements may be sufficient to continuously measure the heart stroke volume. The use of a contrast agent, such as saline, could be necessary to check regional perfusion of the lungs. Because of this, EIT-based assessment of regional respiratory and lung perfusion may reveal the perfusion match and local ventilation, which can be helpful in the treatment of patients with chronic respiratory distress syndrome (ARDS).

Keywords: electrical impedance tmography bioimpedance, image reconstruction Thorax; regional vent, regional perfusion; monitoring

1. Introduction

Electronic impedance transmission (EIT) is an non-radiation functional imaging modality that permits non-invasive bedside monitoring of respiratory ventilation in the region and possibly perfusion. Commercially available EIT devices were developed for the clinical use of this technique and the thoracic EIT is safe in both adult and pediatric patients [ 2, 1 2.

2. Basics of Impedance Spectroscopy

Impedance Spectroscopy is the electrical response of biological tissue to an externally applied electron current (AC). It is commonly obtained using four electrodes, of which two are utilized to inject AC injection, and the remaining two are for voltage measurement 3.,3. Thoracic EIT measures the regional variation of the intra-thoracic bioimpedance. It can be considered by extending the principle of four electrodes onto the image plane which is defined by an electrode belt [ 11. In terms of dimensions, electrical impedance (Z) is exactly the same as resistance. its equivalent International System of Units (SI) unit is Ohm (O). It can be described as a complex figure where the real part is resistance, while the imaginary portion is called the reactance, which determines the effect of capacitance or inductance. Capacitance varies based on biomembranes’ properties of the tissue , which includes ion channels and fatty acids as well as gap junctions. The resistance is primarily determined by composition and quantity of extracellular fluid [ 1., 2]. At frequencies below 5 kilohertz (kHz) (kHz), electrical current travels through extracellular fluids and is mostly dependent on the resistive characteristics of the tissues. At higher frequencies of up to 50 kHz, electrical impulses are a little deflected by cell membranes . This leads to an increase of capacitive tissue properties. When frequencies exceed 100kHz electric currents are able to travel through cell membranes and lower the capacitive component [ 22. Therefore, the effects that determine the impedance of tissue depend on the stimulation frequency. Impedance Spectroscopy is often described as resistivity or conductivity, which equalizes conductance and resistance to unit size and length. The SI units used include Ohm-meter (O*m) for resistivity, and Siemens per meter (S/m) for conductivity. The thoracic tissue’s resistance ranges between 150 O*cm of blood to 700 O*cm when it comes to air-filled lung tissue, and up to 2400 o*cm for the lung tissue that has been inflated ( Table 1). In general, tissue resistance or conductivity depends on the level of fluids and ions. Regarding breathing, it also is dependent on the quantity of air in the alveoli. Though most tissues exhibit an isotropic behaviour, the heart and muscle in particular exhibit anisotropic properties, meaning that the resistance is strongly dependent on the direction that they are measured.

Table 1. Thoracic tissues have electrical resistance.

3. EIT Measurements and Image Reconstruction

To conduct EIT measurements electrodes are positioned around the Thorax in a horizontal plane typically between the 4th and 5th intercostal spaces (ICS) near Parasternal Line [5]. After that, the changes in impedance are measured in the lower lobes of both the left and right lungs and also in the heart area ,2[ 1,2]. To position the electrodes above the 6th ICS could be difficult because the abdominal contents and diaphragm often enter the measurement plane.

Electrodes are self-adhesive electrodes (e.g., electrocardiogram, ECG) that are placed with equal spacing between the electrodes, or are integrated into electrode belts ,21. Self-adhesive lines are available for a more user-friendly application [ ,21,2. Chest tubes, chest wounds non-conductive bandages, or conductive sutures for wires can substantially affect EIT measurements. Commercially available EIT devices typically utilize 16 electrodes. However, EIT devices with 8 (or 32) electrodes are also available (please refer to Table 2 for details) (see Table 2 for details). ,2[ 1,2.

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

During an EIT measure sequence, small AC (e.g., <5 mgA at a rate of 100 kHz) are applied to different electrode pairs, and the resulting voltages are measured using the remaining other electrodes [ 6. The bioelectrical resistance between the injecting and the electrode pairs that measure is calculated using the applied current and the measured voltages. Most commonly connected electrode pairs are used to allow AC application in a 16-elektrode system for example, while 32-elektrode systems generally utilize a skip-pattern (see the table 2) for increasing the spacing between electrodes for current injection. The voltages generated are measured by using one of the other electrodes. There is currently a constant debate regarding different current stimulation patterns , and their advantages and disadvantages [77. For a complete EIT data set of bioelectrical tests The injecting and electrode pairs that measure are continually rotationally positioned around the entire chest .

1. Current measurement and voltage measurements around the thorax with an EIT system that includes 16 electrodes. In just a few milliseconds two electrodes measuring current and their active voltage electrodes get moved across the upper thorax.

The AC used during EIT measurements is safe for use on body surfaces and will not be 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.

The EIT data set captured during a single cycle during AC programs is called a frame and contains the voltage measurements required to create the image. EIT image. The term “frame rate” reflects the amount of EIT frames captured per second. Frame rates that are at least 10 images/s are required to monitor ventilation and 25 images/s in order to monitor perfusion or cardiac function. Commercially accessible EIT devices have frame rates between 40 and 50 images/s as depicted in

In order to create EIT images using recorded frames, the process of image reconstruction is applied. Reconstruction algorithms are designed to address the problem that is the reverse of EIT which is the recuperation of the conductivity distribution in the thorax using the voltage measurements that have been acquired at the electrodes on the thorax surface. Initially, EIT reconstruction assumed that electrodes were placed in an ellipsoid or circular plane, but more recent algorithms utilize information about the anatomical shape of the thorax. Currently, it is the Sheffield back-projection algorithm and the finite element algorithm (FEM) with a linearized Newton–Raphson algorithm ], and the Graz consensus reconstruction algorithm for EIT (GREIT) [10is frequently employed.

As a rule, EIT images have a similarity with a two-dimensional computed (CT) image: these images are usually rendered so that the operator is looking from caudal to cranial when studying the image. In contrast to a CT image An EIT image doesn’t display an image “slice” but an “EIT sensitivity region” [1111. The EIT sensitive region is a lens-shaped intra-thoracic area with impedance-related changes that contribute to EIT imaging process [11(11, 11). The dimensions and shape of the EIT sensitization region is determined by the dimensions, bioelectrical properties, and also the anatomy of the Thorax as well according to the particular voltage measurement and current injection pattern [1212.

Time-difference image is a technique that is employed in EIT reconstruction in order to display changes in conductivity and not the total conductivity. The time-difference EIT image shows the difference in impedance against a baseline frame. This affords the opportunity to track the time-dependent physiological changes like lung ventilation and perfusion [22. Color coded EIT images isn’t unified but typically shows the change in intensity to a baseline level (2). EIT images are usually coded using a rainbow-color scheme with red representing the highest value of relative imperf (e.g., during inspiration) and green for a middle relative impedance and blue the least relative impedance (e.g. when expiration is in progress). In clinical settings the best option is to employ color scales that vary from black (no impedance change) and blue (intermediate impedance changes) and white (strong impedance shift) for coded ventilation. from black to white and then red to mirror perfusion.

2. Different color codes that are available for EIT images in comparison to CT scan. The rainbow-color scheme is based on red to indicate the highest absolute impedance (e.g., during inspiration) with green for low relative impedance and blue when the relative resistance is lowest (e.g. when expiration is in progress). Modern color scales make use of instead of black to avoid any impedance change) Blue for an intermediate change in impedance, and white for the highest impedance alteration.

4. Functional Imaging and EIT Waveform Analysis

Analysis of Impedance Analyzers data is done using EIT waveforms , which are generated in the individual pixels of an array of raw EIT images that are scanned over period of (Figure 3). An area of concern (ROI) is a term used to summarize activity in individual pixels in the image. In each ROI the waveform shows changes in regional conductivity over time as a result of either ventilation (ventilation-related signal, also known as VRS) or cardiac activity (cardiac-related signal, CRS). Additionally, electrically conducting contrast agents such as hypertonic Saline can be used to generate an EIT Waveform (indicator-based signal IBS) which may be related to the perfusion of the lung. The CRS could come from both the lung as well as the cardiac region and may also be associated with lung perfusion. The exact nature and origin is not fully understood 1313. Frequency spectrum analysis can be employed to distinguish between ventilationand cardiac-related impedance variations. Impedance changes that do not occur regularly could result from changes in ventilator settings.

Figure 3. EIT waveforms and functional EIT (fEIT) photographs can be derived from raw EIT images. EIT waves can be described pixel-wise or on a region of interest (ROI). Conductivity changes are a natural result of ventilatory (VRS) and cardiac activities (CRS) however, they can be caused artificially, e.g. or through bolus injection (IBS) for perfusion measurements. FEIT images show the various physiological parameters in the region such as perfusion (Q) and ventilation (V) and blood flow (Q), extracted from raw EIT images by using an algorithmic operation over time.

Functional EIT (fEIT) images are generated by applying a mathematical procedure on a sequence of raw images and the corresponding pixel EIT waveforms [14]. Because the mathematical process is applied to calculate an appropriate physiological parameter for each pixel, physiological regional characteristics such as regional ventilation (V) and respiratory system compliance, as along with regions perfusion (Q) can be assessed to be displayed (Figure 3.). The information derived from EIT waves and simultaneously registered airway pressure values can be utilized to calculate lung’s compliance, as well as the opening and closing of the lungs for each pixel by calculating changes in pressure and impedance (volume). Comparable EIT measurements of increments of inflation and deflation in the lungs permit the display of pressure-volume curves on scales of pixel. Depending on the mathematical operation different types of fEIT scans may address different functional characteristics of the cardio-pulmonary system.

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