Electrical Impedance Tomography for Cardio-Pulmonary Monitoring
Electrical Impedance Tomography (EIT) is a bedside monitor that visually examines the local environment and even lung perfusion. The paper summarizes and analyzes the methodological and clinical aspects of the thoracic EIT. Initially, researchers addressed the validity of EIT to determine regional ventilation. These studies focus on its clinical applications to measure lung collapse, TIDAL recruitment, as well as lung overdistension. The goal is to monitor positive end expiratory pressure (PEEP) and tidal volume. In addition, EIT may help to detect pneumothorax. Recent studies evaluated EIT as a means to determine regional lung perfusion. The absence of indicators in EIT measurements could be adequate to continuously measure cardiac stroke volume. Utilizing a contrast agent like saline may be required to measure regional perfusion of the lungs. Therefore, EIT-based monitoring of regional ventilation as well as lung perfusion can be used to assess the perfusion match and local ventilation which could be beneficial in treating patients with acute respiratory distress syndrome (ARDS).
Keywords: electrical impedance tomography bioimpedance; reconstruction of images Thorax; regional vent as well as monitoring regional perfusion.
EI tomography (EIT) is one of the radiation-free functional imaging technology that permits an uninvasive monitoring of respiratory ventilation in the region and possibly perfusion. Commercially accessible EIT devices were first introduced for clinical applications of this method, and thoracic EIT is widely used for both pediatric and adult patients 1, [ 1, 2].
2. Basics of Impedance Spectroscopy
Impedance Spectroscopy is the range of the biological tissue’s voltage to an externally applied alternating electricity (AC). It is usually achieved using four electrodes. Two are utilized to inject AC injection, and the remaining two are used to measure voltage 3,,3. Thoracic EIT measures the regional Impedance Spectroscopy of the thoracic region and could be seen like an extension of four electrode principle to the image plane that is spanned by the electrode belt 11. Dimensionally, electrical resistance (Z) is the same as resistance and its equivalent International System of Units (SI) unit is Ohm (O). It can be described as a complex number where the actual component is resistance while the imaginary part is called reactance, which measures the effects of the inductance of capacitance. Capacitance varies based on biomembranes’ particulars of the tissue , which includes ion channels, fatty acids, and gap junctions. However, resistance is mostly determined by the composition and quantity of extracellular fluid [ 1., 22. For frequencies lower than 5 kilohertz (kHz) an electrical current is carried by extracellular fluid and is primarily dependent upon its resistive properties of tissues. In higher frequencies above 50 kHz, electrical impulses can be slightly deflected through cell membranes , which results in an increase in the capacitive properties. At frequencies above 100 kHz electrical current can flow through cell membranes and reduce the capacitive component 22. Thus, the factors that determine tissue impedance strongly depend on the utilized stimulation frequency. Impedance Spectroscopy is typically described as conductivity or resistivity, which will normalize conductance or resistance in relation to unit length and area. The SI equivalent units can be described as Ohm-meter (O*m) for resistivity, and Siemens per meters (S/m) (S/m) for conductivity. The thoracic tissue’s resistance ranges between 150 O*cm for blood and up to 700 o*cm for deflated lung tissue, up to 2400 O*cm for the lung tissue that has been inflated ( Table 1). In general, tissue resistivity or conductivity depends on the fluid content and ion concentration. In terms of respiratory lungs it is dependent on the quantity of air present in the alveoli. While the majority of tissues exhibit isotropic behaviour, the heart and muscle skeleton exhibit anisotropy, meaning that the resistance is strongly dependent on the direction that it’s measured.
Table 1. Electrical resistance of thoracic tissues.
3. EIT Measurements and Image Reconstruction
To conduct EIT measurements electrodes are placed on the Thorax in a horizontal plane which is typically located within the 4th to 5th intercostal areas (ICS) near an angle called the parasternal line5. Following that, changes in impedance can be observed in areas of the lower part of the right and left lungs, as well as in the heart region ,21 2. It is possible to position the electrodes below the 6th ICS could be difficult since the diaphragm as well as 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 are embedded in electrode belts ,21 2. Additionally, self-adhesive stripe are made available for more user-friendly application ,2]. Chest wounds, chest tubes Non-conductive bandages and conductive sutures for wires can significantly affect EIT measurements. Commercially available EIT devices typically use 16 electrodes, but EIT systems with 8 or 32 electrodes are available (please refer to Table 2 for details) (see Table 2 for details). ,2[ 1,2].
Table 2. Electric impedance (EIT) gadgets.
In an EIT measuring sequence, very small AC (e.g. five million mA with a frequency of 100 kHz) is applied through various electrode pairs, and the generated voltages are measured with the remaining other electrodes [ 6. The bioelectrical resistance between the injecting and the measuring electrode pairs is determined by analyzing the applied current as well as the measured voltages. Most often nearby electrode pairs are used to allow AC application in a 16-elektrode system for example, while 32-elektrode systems generally employ a skip pattern (see Table 2) to increase the distance between the electrodes that inject the current. The resulting voltages are then measured with those remaining electrodes. In the present, there is an ongoing debate about the various electrical stimulation techniques and their specific advantages and disadvantages . In order to obtain an complete EIT data set that includes bioelectrical tests The injecting and electrode pairs measuring are continuously rotated around the entire thorax .
1. Voltage measurements and current application within the thorax, using an EIT system consisting of 16 electrodes. In only a few milliseconds each of the electrodes for current as well as these active electrodes are repeatedly rotating across the upper thorax.
The AC used during EIT tests is safe to use on the body 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.
It is the EIT data set captured during a single cycle in AC software is termed a frame and contains the voltage measurements necessary to create what is known as the initial EIT image. Frame rate is the amount of EIT frames recorded each second. Frame rates that are at least 10 images/s are needed for monitoring ventilation and 25 images/s are required to monitor perfusion or cardiac function. Commercially accessible EIT devices utilize frames that have a frame rate of between 40 and 50 images/s as described in
To produce EIT images using the recorded frames, an algorithm known as image reconstruction procedure is utilized. Reconstruction algorithms are designed to address the issue that causes EIT that is the restoration of the conductivity pattern in the thorax by analyzing the voltage measurements collected at electrodes on the thorax’s surface. Initially, EIT reconstruction assumed that electrodes were placed on an ellipsoid, circular or circular plane, whereas newer algorithms employ information on how the anatomical shape of thorax. Presently, there is there are three main algorithms used for EIT: the Sheffield back-projection algorithm and the finite element algorithm (FEM) that is a linearized Newton-Raphson algorithm [ ] as well as the Graz consensus reconstruction algorithm for EIT (GREIT) [10typically used.
As a rule, EIT image are basically similar to a two-dimensional computed tomography (CT) image: these images are normally rendered so that the user looks from cranial towards caudal when analysing the image. In contrast to an CT image, an EIT image does not show the appearance of a “slice” but an “EIT sensitivity region” . The EIT sensitive region is a thoracic-specific lens and is where the impedance change contributes to the EIT images [11The EIT image is generated by impedance changes. The shape and the thickness of the EIT sensitization region is determined by the dimensions, bioelectric propertiesand structure of the chest as in the used voltage measurement and current injection pattern [12(13, 14).
Time-difference imaging is a method which is employed for EIT reconstruction to show variations in conductivity rather than the pure conductivity amounts. Time-difference EIT image compares the variation in impedance to a baseline frame. This allows you to monitor the changes in physiological activity over time such as lung breathing and perfusion [22. Color-coding for EIT images is not unicoded however, it typically displays the change in intensity to a baseline level (2). EIT images are generally coded using a rainbow-colored scheme with red representing the greatest proportional impedance (e.g., during inspiration) while green is a moderate relative impedance, and blue being the lowest relative impedance (e.g. when the expiration time is). For clinical applications the best option is using color scales which range from black (no change in impedance) from blue (intermediate impedance change) as well as white (strong impedance shift) to code ventilation . from black to white, towards mirror perfusion.
2. Different color codings for EIT images in comparison with the CT scan. The rainbow-color scheme is based on red for the most powerful ratio of resistance (e.g. when inspiration occurs) while green is used for moderate relative impedance, and blue to indicate the least relative imperceptibility (e.g. at expiration). A newer color scheme uses instead of black for no impedance change) or blue to indicate the intermediate impedance change and white for the highest impedance change.
4. Functional Imaging and EIT Waveform Analysis
Analysis of Impedance Analyzers data is performed using EIT waveforms , which are generated within individual image pixels of the form of a sequence of raw EIT images over period of (Figure 3.). An area of concern (ROI) is a term used to represent activity within individual pixels in the image. In all ROIs, the image shows changes in the regional conductivity over time resulting from breathing (ventilation-related signal, also known as VRS) (or cardiac activity (cardiac-related signal, CRS). In addition, electrically conductive contrast agents such as hypertonic saline can be utilized to create an EIT waveshape (indicator-based signal IBS) and is linked to perfusion in the lung. The CRS can be traced to both the lung and cardiac region, and can be attributable to lung perfusion. The precise origins and components are not well understood. 1313. Frequency spectrum analysis is frequently employed to distinguish between ventilationand cardiac-related impedance fluctuations. Impedance changes outside of the periodic cycle could result from adjustments in the ventilation settings.
Figure 3. EIT Waveforms as well as functional EIT (fEIT) pictures are derived from the unprocessed EIT images. EIT waveforms may be defined as pixel-wise, or by using a region or region of interest (ROI). Conductivity fluctuations are the result of ventilation (VRS) or the activity of cardiac muscles (CRS) but they may be artificially induced, e.g. via injection of bolus (IBS) to determine perfusion. fEIT images display specific physiological parameters of the region, such as perfusion (Q) and ventilation (V) and perfusion (Q) that are extracted from raw EIT images using a mathematical operation over time.
Functional EIT (fEIT) images are created by applying a mathematical operation on an array of raw images together with the appropriate pixel EIT waves . Because the mathematical process is used to determine an appropriate physiological parameter for each pixel, physiological regional traits like regional airflow (V) and respiratory system compliance, as well as region-wide perfusion (Q) are measured and display (Figure 3.). Information drawn from EIT waves and simultaneously registered pressures of the airways can be used to determine the lung’s compliance as also 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 the lungs enable the display of pressure-volume curves at an individual level. Based on the mathematical operation, different types of fEIT photos could reflect different functional characteristics from the cardio-pulmonary apparatus.