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Electrical Impedance Tomography for Cardio-Pulmonary Monitoring

Abstract

Electrical Impedance Tomography (EIT) is a bedside monitor that noninvasively visualizes local ventilation and , possibly, lung perfusion distribution. The paper summarizes and analyzes the methodological and clinical aspects of thoracic EIT. Initially, investigators addressed the validity of EIT to assess regional ventilation. Recent studies concentrate on its clinical applications to assess lung collapse, an increase in tidal volume, and overdistension. The goal is to monitor positive end expiratory pressure (PEEP) and the volume of tidal. In addition, EIT may help to detect pneumothorax. Recent studies looked at EIT as a method to determine regional lung perfusion. Indicator-free EIT measurements may be sufficient for continuous measurement of cardiac stroke volume. The use of a contrast agent like saline might be required to assess the regional lung perfusion. Thus, EIT-based surveillance of regional airflow and lung perfusion might reveal the local perfusion and 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 as well as monitoring regional perfusion.

1. Introduction

The electrical impedance imaging (EIT) is one of the radiation-free functional imaging technique that permits non-invasive monitoring of bedside regional lung ventilation and arguably perfusion. Commercially-available EIT devices were introduced for the clinical use of this technique and the thoracic EIT has been used safely for both pediatric and adult patients [ 1., 1.

2. Basics of Impedance Spectroscopy

Impedance Spectroscopy can be defined as the range of the biological tissue’s voltage to externally applied alternating voltage (AC). It is commonly obtained using four electrodes. Two are employed for AC injection and the other two are used to measure voltage 3,,3. 4. Thoracic EIT measures the regional Impedance Spectroscopy of the thoracic region and is seen as an expansion of the four electrode principle to the image plane spanned through the electro belt 11. Dimensionally, electrical resistance (Z) is identical to resistance and the equivalent International System of Units (SI) unit is Ohm (O). It is easily expressed as a complex number where it is the actual portion of resistance while the imaginary part is called reactance. This evaluates the effects that result from either inductance or capacitance. Capacitance is a function of biomembranes’ characteristics of a tissue , including ion channels, fatty acids, and gap junctions, whereas resistance is mainly determined by composition of the tissue and the quantity of extracellular fluid 1, 22. At frequencies below 5 kilohertz (kHz) electricity moves through extracellular fluid, and is primarily dependent on the characteristics of resistivity of tissues. At higher frequencies up to 50 kHz the electrical currents are slightly slowed down at the cell membranes resulting in an increase in tissue capacitive properties. When frequencies exceed 100 kHz electrical current can flow through cell membranes, and diminish the capacitive portion 21. So, the results that determine the impedance of tissue depend on the stimulation frequency. Impedance Spectroscopy typically refers to conductivity or resistivity, which regulates conductance or resistance according to unit length and area. The SI equivalent units comprise Ohm-meter (O*m) for resistivity and Siemens per meter (S/m) on conductivity. Resistivity of thoracic tissue ranges from 150 o*cm for blood as high as 700 O*cm with lung tissue that is deflated, all the way up to 2400 o*cm for the lung tissue that has been inflated ( Table 1). In general, tissue resistance or conductivity is dependent on volume of the fluid and the amount of ions. For the lungs, it depends on the volume of air that is present in the alveoli. While the majority of tissues exhibit isotropic behavior, the heart and skeletal muscle behave anisotropic, meaning that the resistance is strongly dependent on the direction that they are measured.

Table 1. The electrical resistivity of the thoracic tissues.

3. EIT Measurements and Image Reconstruction

To carry out EIT measurements electrodes are placed on the Thorax in a transverse typically between the 4th and 5th intercostal areas (ICS) at that line called parasternal [55. The changes in impedance can be assessed in the lower lobes of the right and left lungs, as well as in the heart area ,2[ 1,2]. To position the electrodes above the 6th ICS could be difficult since abdominal content and the diaphragm occasionally enter the measurement area.

Electrodes can be self-adhesive or single electrodes (e.g., electrocardiogram, ECG) that are placed individually with equal spacing in-between the electrodes or integrated into electrode belts [ ,21 2. Self-adhesive stripes are also available for a more user-friendly application [ ,2]. Chest tubes, chest wounds and non-conductive bandages as well as conductive sutures made of wire can negatively impact EIT measurements. Commercially available EIT devices typically use 16 electrodes, but EIT systems with eight (or 32) electrodes are available (please see Table 2 for information) It is recommended to consult Table 2 for more details. ,21.

Table 2. The commercially-available electrical impedance (EIT) devices.

In an EIT measurement , small AC (e.g., <5 million mA with a frequency of 100 kHz) are applied to several electrode pairs and the output voltages are analyzed using the other electrodes 6. The bioelectrical resistance between the injecting and electrodes that are measuring is calculated using the applied current as well as the observed voltages. Most commonly connected electrode pairs are utilized to allow AC application in a 16-elektrode device, while 32-elektrode systems often employ a skip pattern (see the table 2) for increasing the spacing between electrodes that inject current. The voltages generated are measured with other electrodes. At present, there is an ongoing discussion on different current stimulation patterns and their advantages and disadvantages [7]. To collect a complete EIT data set that includes bioelectrical tests in the injecting as well as the electrode pairs measuring are continuously rotated throughout the entire thorax .

1. Current measurement and voltage measurements in the thorax using an EIT system consisting of 16 electrodes. Within milliseconds simultaneously, the current electrode and their active voltage electrodes are continuously rotating across the upper thorax.

The AC utilized during EIT tests are safe for body surface applications and are not detected 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 that is recorded in one cycle from AC applications is technically termed a frame . It is comprised of the voltage measurements used to create the image. EIT image. Frame rate is the amount of EIT frames that are recorded every second. Frame rates at least 10 frames/s are required in order to monitor ventilation , and 25 images/s to monitor cardiac function or perfusion. Commercially available EIT devices run frames with a frame rate between 40 and 50 images/s (see Figure 2), as described in

To create EIT images from the recorded frames, so-called image reconstruction is applied. Reconstruction algorithms attempt to solve the issue that causes EIT, which is the recovering the conductivity distribution inside the thorax based on the voltage measurements made at the electrodes of the thorax surface. Initially, EIT reconstruction assumed that electrodes were placed on an ellipsoid, circular or circular plane. However, newer techniques utilize information about the anatomical form of the thorax. At present, the Sheffield back-projection algorithm [ and the finite element method (FEM) built on a linearized Newton and Raffson algorithm [ ], and the Graz consensus reconstruction algorithm for EIT (GREIT) [10is frequently employed.

It is generally true that EIT images can be compared to a two-dimensional computed-tomography (CT) image: these images are conventionally rendered so that the operator looks from caudal to cranial when analyzing the picture. In contrast to an CT image the EIT image doesn’t display a “slice” but an “EIT sensitivity region” [1111. The EIT sensitization region is a lens-shaped intra-thoracic area that is the source of impedance variations which contribute to EIT imaging process [1111. The shape and the thickness of the EIT sensitive region are determined by the dimensions, the bioelectrical properties, and also the structure of the chest depending on the voltage measurement and current injection pattern [12The shape and thickness of the EIT sensitivity region is determined by the voltage measurement pattern [.

Time-difference imaging can be described as a technique that is used in EIT reconstruction to display changes in conductivity instead of relative conductivity of the levels. The time-difference EIT image compares the variation in impedance with a baseline frame. This is a great way to track the time-dependent physiological changes such as respiratory ventilation and perfusion [22. Color coding of EIT images isn’t unified however it usually shows the shift in impedance to a reference level (2). EIT images are typically created using a spectrum of colors with red representing the high relative impedance (e.g., during inspiration) while green is a moderate relative impedance, and blue the lowest impedance (e.g. when expiration is in progress). For clinical applications it is possible to employ color scales that vary from black (no impedance changes) up to blue (intermediate impedance change) as well as white (strong impedance changes) to code ventilation , or from black, to red, and white to mirror perfusion.

2. Different available color codings of EIT images in comparison to CT scan. The rainbow-color scheme utilizes red for the highest percentage of the relative imperceptibility (e.g. during inspiration) while green is used for moderate relative impedance, and blue for the lowest relative impedance (e.g. during expiration). Newer color scales utilize instead black for no impedance changes) and blue for an intermediate impedance variation, and white for the largest impedance changes.

4. Functional Imaging and EIT Waveform Analysis

Analysis of Impedance Analyzers data is done using EIT waveforms that are formed in the individual pixels of the raw EIT images over period of (Figure 3.). The term “region of interest” (ROI) is a term used to summarize activity in individual pixels of the image. In all ROIs, the image shows variations in the conductivity of the region over time , resulting from ventilation (ventilation-related signal, VRS) and cardiac activities (cardiac-related signal CRS). Additionally, electrically conducting contrast agents such as hypertonic saline can be used to produce the EIT waveshape (indicator-based signal IBS) and is linked to the perfusion of the lung. The CRS could come from both the heart and lung region and may also be attributed to lung perfusion. The exact source and composition are not understood fully 13]. Frequency spectrum analysis is commonly used to distinguish between ventilationor cardiac-related changes in the impedance. Non-periodic changes in impedance may result from changes in the setting of the ventilator.

Figure 3. EIT Waveforms as well as functional EIT (fEIT) pictures are derived from the EIT raw EIT images. EIT waveforms can be defined pixels-wise or based on a area in interest (ROI). Conductivity fluctuations are the result of ventilatory (VRS) or heart activity (CRS) however, they can be produced artificially e.g. through injection of bolus (IBS) to determine perfusion. FEIT images present some of the regional physiological parameters including ventilation (V) or perfusion (Q), extracted from raw EIT images by applying an algorithmic operation over time.

Functional EIT (fEIT) images are created by applying a mathematical calculation on the raw images along with the associated pixel EIT spectrums. Since the mathematical operation is applied to calculate an appropriate physiological parameter for each pixelof the image, regional physiological features like regional ventilation (V), respiratory system compliance as and local perfusion (Q) are measured to be displayed (Figure 3.). Information taken from EIT waveforms and simultaneously registered airway pressure values can be used to calculate lung’s compliance and the opening and closing of the lungs for each pixel using changes in pressure and impedance (volume). The comparable EIT measurements of increments of inflation and deflation in lung volume allow for the display of pressure-volume curves on scales of pixel. The mathematical operations used to calculate different types of fEIT scans could reflect different functional characteristics from the cardio-pulmonary apparatus.

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