Electrical Impedance Tomography for Cardio-Pulmonary Monitoring

Abstract

Electrical Impedance Tomography (EIT) is a bedside monitor that provides non-invasive visualisation of local ventilation and perhaps lung perfusion. This article reviews and analyzes the clinical and methodological aspects of thoracic EIT. Initially, researchers focused on the validation of EIT to determine regional ventilation. Research is currently focused on its clinical applications to measure lung collapse tidal recruitment, and lung overdistension. This allows for the titration of positive end-expir pressure (PEEP) and tidal volume. In addition, EIT may help to detect pneumothorax. Recent studies have evaluated EIT as a tool to measure regional lung perfusion. The absence of indicators in EIT measurements could be adequate to measure continuously the cardiac stroke volume. The use of a contrast agent, such as saline, may be necessary for assessing the regional lung perfusion. In the end, EIT-based surveillance of regional airflow as well as lung perfusion can be used to assess the local perfusion and ventilation which may be useful in treating patients suffering from chronic respiratory distress syndrome (ARDS).

Keywords: electrical impedance tmography and bioimpedance. Image reconstruction Thorax; regional vent as well as monitoring regional perfusion.

1. Introduction

EI tomography (EIT) can be described as a non-radiation functional imaging technology that permits non-invasive monitoring of bedside regional lung ventilation as well as arguably perfusion. Commercially accessible EIT devices were first introduced for the clinical use of this technique, and thoracic EIT can be used with safety for both pediatric and adult patients 2, 2.

2. Basics of Impedance Spectroscopy

Impedance Spectroscopy can be defined as the electrical response of biological tissue to an externally applied voltage (AC). It is typically achieved by using four electrodes, of which two are utilized for AC injection and the other two are used for measuring voltage 3.,3. Thoracic EIT measures the regional range of intra-thoracic bioimpedance. This could be seen by extending the four electrode principle to the imaging plane, which is divided by an electrode belt 1]. In terms of dimension, electrical impedance (Z) is the same as resistance , and its equivalent International System of Units (SI) unit is Ohm (O). It is easily expressed as a complicated number, in which the actual component is resistance, while the imaginary portion is called the reactance, which quantifies effects resulting from capacitance or inductance. Capacitance depends on the biomembranes’ characteristics of the tissue such as ion channels, fatty acids, and gap junctions, whereas resistance is determined by the composition and quantity of extracellular fluid 1, 22. At frequencies less than 5 Kilohertz (kHz) (kHz), electrical current travels through extracellular fluids and is in a major way dependent on the resistivity characteristics of tissues. Higher frequencies, as high as 50 kHz electrical currents are a little deflected by cell membranes , which results in an increase in tissue capacitive properties. For frequencies higher than 100 kHz the electrical current is able to pass through cell membranes, and diminish the capacitive component [ 22. Thus, the factors that determine the tissue’s impedance depend on the utilized stimulation frequency. Impedance Spectroscopy usually refers to conductivity or resistance, which compares conductance or resistance units’ area and length. The SI equivalent units is Ohm-meter (O*m) for resistivity and Siemens per meter (S/m) to measure conductivity. The tissue’s resistance varies between 150 O*cm of blood as high as 700 O*cm with air-filled lung tissue, and all the way to 2400O*cm for air-filled lung tissue ( Table 1). In general, tissue resistivity or conductivity will vary based on amount of fluid and the ion concentration. When it comes to lung function, this also depends on the amount of air that is present in the alveoli. While the majority of tissues exhibit isotropic behavior, heart and muscle fibers in the skeletal system exhibit anisotropic behavior, meaning that resistivity strongly depends on the direction from which it is measured.

Table 1. Thoracic tissues have electrical resistance.

3. EIT Measurements and Image Reconstruction

To conduct EIT measurements electrodes are positioned around the chest in a transverse plane that is usually located in the 4th-5th intercostal areas (ICS) in an angle called the parasternal line]. The changes in impedance can be observed in the lower lobes of the left and right lungs as well as in the heart area ,21 2. To place the electrodes below the 6th ICS may be difficult, as the diaphragm and abdominal contents periodically enter the measurement plane.

Electrodes are self-adhesive electrodes (e.g. electrocardiogram ECG,) that are placed individually in a similar spacing between electrodes or integrated into electrode belts [ ,2[ 1,2]. Also, self-adhesive electrodes are designed to be more comfortable for application ,21,2. Chest wounds, chest tubes Non-conductive bandages and conductive wire sutures can block or severely affect EIT measurements. Commercially available EIT equipment typically uses 16 electrodes. However, EIT systems that have 8 and 32 electrodes are also available (please refer to Table 2 for more details) The following table shows the electrodes available. ,21 2.

Table 2. Commercially available electrical impedance tomography (EIT) equipment.

In an EIT measurements, small AC (e.g. 5 microamps at 100 kHz) are applied to various electrode pairs, and the resultant voltages are recorded using the remaining electrodes ]. The bioelectrical resistance between the injecting and the electrode pairs used to measure the voltage can be calculated by using the applied current as well as the measured voltages. Most often adjacent electrode pairs are used to allow AC application in a 16-elektrode setup, while 32-elektrode systems often employ a skip pattern (see the table 2) so that the electrodes are closer to electrodes that inject current. The resulting voltages are measured by using one of the other electrodes. Presently, there’s an ongoing discussion about different current stimulation patterns and their advantages and disadvantages [77. To acquire a complete EIT data set that includes bioelectrical tests The injecting and electrode pairs measuring are continuously rotationally positioned around the entire chest .

1. Voltage measurements and current application around the thorax using an EIT system featuring 16 electrodes. In just a few milliseconds as well as the voltage and current electrodes and activated voltage electrodes are rotating within the thorax.

The AC used during the EIT tests is safe to use on the body 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 stored during one cycle from AC apps is called an image frame. It includes voltage measurements necessary to create that initial EIT image. Frame rate is the number of EIT frames captured per second. Frame rates of at least 10 images/s are needed for monitoring ventilation and 25 images/s to monitor heart function or perfusion. Commercially accessible EIT devices utilize frame rates between 40 and 50 images/s (see Figure 2), as illustrated in

To create EIT images using the recorded frames, the technique known as image reconstruction technique is used. Reconstruction algorithms strive to resolve the other aspect of EIT that is the reconstruction of the conductivity distribution in the thorax using the voltage measurements made at the electrodes of the thorax surface. Initially, EIT reconstruction assumed that electrodes were placed on an ellipsoid plane. However, newer techniques utilize information about anatomy of the thorax. At present, the Sheffield back-projection algorithm [ as well as the finite-element method (FEM) which is a linearized Newton-Raphson algorithm ] as well as the Graz consensus reconstruction algorithm for EIT (GREIT) [10often used.

In general, EIT pictures are similar to a two-dimensional computed tomography (CT) image. These images are typically rendered in a way that the user is able to look across the entire cranial region when looking at the image. Contrary to a CT image An EIT image does not display the form of a “slice” but an “EIT sensitivity region” [1111. The EIT sensitive region is a lens-shaped intra-thoracic volume where impedance fluctuations contribute to the EIT production of the image [11It is a lens-shaped intra-thoracic volume that contributes to the generation. The size and shape of EIT area of sensitivity are dependent upon the dimensions, the bioelectric characteristics, and the anatomy of the Thorax as according to the particular current injection and voltage measurement pattern [1212.

Time-difference Imaging is a method that is used in EIT reconstruction in order to display changes in conductivity rather than the absolute conductivity levels. An time-difference EIT image compares the change in impedance with a baseline frame. It is an opportunity to monitor the changes in physiological activity over time such as lung ventilation and perfusion [22. Color-coding for EIT images is not unicoded but commonly displays the change in intensity to a baseline level (2). EIT images are typically colored using a rainbow color scheme with red representing the most significant absolute impedance (e.g., during inspiration) while green is a moderate relative impedance, while blue is the lowest impedance (e.g. during expiration). For clinical purposes the best option is to use color scales ranging from black (no change in impedance) to blue (intermediate impedance changes) and white (strong impedance changes) to code ventilation . from black, to red, and white and white to reflect perfusion.

2. Different color codings for EIT images as compared to CT scan. The rainbow color scheme uses red to indicate the highest absolute impedance (e.g. during inspiration) as well as green for a middle relative impedance and blue, for the lowest ratio of impedance (e.g. during expiration). A newer color scheme uses instead of black (which has no impedance change) while blue is used for an intermediate impedance change, while white is the one with the strongest impedance changes.

4. Functional Imaging and EIT Waveform Analysis

Analyzing Impedance Analyzers data is based on EIT waves that are generated in individual image pixels in a series of raw EIT images that are scanned over the course of time (Figure 3). In a region of focus (ROI) can be defined to summarize activity in individual pixels in the image. In the ROIs, each image shows the changes in conductivity of the region over time , resulting from ventilation (ventilation-related signal, also known as VRS) or heart activity (cardiac-related signal CRS). In addition, electrically conductive contrast agents like hypertonic saline could be used to generate the EIT waveshape (indicator-based signal, IBS) and can be linked to the perfusion of the lung. The CRS could come from both the cardiac and lung region and could be attributable to lung perfusion. Its precise source and composition aren’t understood completely 13]. Frequency spectrum analysis can be used to distinguish between ventilation- and cardiac-related Impedance Analyzers changes. Impedance changes outside of the periodic cycle could be caused by modifications in the settings of the ventilator.

Figure 3. EIT waveforms , as well as the functional EIT (fEIT) photos are derived from raw EIT images. EIT waveforms are defined by pixel or on a particular region of interest (ROI). Conductivity fluctuations are the result of ventilation (VRS) or cardiac activity (CRS) however they could be produced artificially e.g., by the injection of bolus (IBS) to measure perfusion. FEIT images are a visual representation of regional physiological parameters, such as perfusion (Q) and ventilation (V) or perfusion (Q) which are extracted from the raw EIT images by applying an algorithmic operation over time.

Functional EIT (fEIT) images are created by applying a mathematical calculation on the sequence of raw images together with the appropriate pixel EIT waveforms [14]. Since the mathematical procedure is used to calculate an appropriate physiological parameter for each pixelof the image, regional physiological characteristics such as regional ventilation (V) and respiratory system compliance as well as respiratory system compliance as well as regional perfusion (Q) can be assessed and display (Figure 3). Information drawn from EIT waves and simultaneously registered airway pressure values can be used to determine the lung compliance and the lung’s opening and closing times for each pixel through changes in pressure and impedance (volume). Similar EIT measurements during inflating and deflating the lungs enable the display of pressure-volume curves at one pixel. Based on the mathematical operation, different types of fEIT images could be used to analyze different functions that are associated with the cardiovascular system.