Electrical Impedance Tomography for Cardio-Pulmonary Monitoring
Electrical Impedance Tomography (EIT) is an instrument for monitoring bedside that provides non-invasive visualisation of local ventilation and possibly lung perfusion distribution. In this article, we review and discusses the clinical and methodological aspects of thoracic EIT. Initially, investigators addressed the validation of EIT to measure regional ventilation. The current research focuses on its clinical applications to quantify lung collapse, the tidal response, and lung overdistension, in order to determine positive end-expiratory pressure (PEEP) and tidal volume. In addition, EIT may help to detect pneumothorax. Recent studies evaluated EIT as a means to gauge regional lung perfusion. Indicate-free EIT tests could be enough to measure continuously the cardiac stroke volume. Utilizing a contrast agent, such as saline, could be necessary to evaluate the regional lung perfusion. As a result, EIT-based assessment of regional respiratory and lung perfusion might reveal the local perfusion and ventilation and can prove helpful in the treatment of patients suffering from acute respiratory distress syndrome (ARDS).
Keywords: electrical impedance imaging and bioimpedance. Image reconstruction Thorax; regional circulation Regional perfusion; monitoring
Electric impedance tomography (EIT) is a radiation-free functional imaging technology that permits non-invasive bedside monitoring of regional lung ventilation and arguably perfusion. Commercially available EIT devices were first introduced for clinical applications of this method and thoracic EIT is safe in both pediatric and adult patients 2, ].
2. Basics of Impedance Spectroscopy
Impedance Spectroscopy can be defined as the variation in the voltage of biological tissue to an externally applied electric current (AC). It is usually measured with four electrodes, of which two are employed to inject AC injection, and the remaining two electrodes are used to measure voltage 3,,3. 4. Thoracic EIT measures the regional range of intra-thoracic bioimpedance. This can be viewed like an extension of principle of four electrodes into the image-plane spanned with the belt of electrodes [ 11. Dimensionallyspeaking, electrical impedance (Z) is equivalent to resistance, as is the related International System of Units (SI) unit is Ohm (O). It can be expressed as a complex number , where the real part is resistance and the imaginary is called reactance. This is the measurement of effects caused by the inductance of capacitance. Capacitance varies based on biomembranes’ specifics of the tissues, such as ion channels and fatty acids as well as gap junctions. Resistance is mainly determined by composition of the tissue and the quantity of extracellular fluid 1, 22. At frequencies below 5 kilohertz (kHz) that is, electrical energy is carried by extracellular fluid and is primarily dependent upon the resistive characteristics of the tissues. For higher frequencies that exceed 50 kHz, currents are a little deflected by cell membranes , which results in an increase in the capacitive properties. At frequencies above 100 kHz electrical currents can travel through cell membranes, and diminish the capacitive portion 21. Therefore, the effects that determine the impedance of tissue depend on the utilized stimulation frequency. Impedance Spectroscopy is often described as conductivity or resistivity. These will normalize conductance or resistance in relation to unit size and length. The SI units for the same can be described as Ohm-meter (O*m) for resistivity and Siemens per meter (S/m) for conductivity. Resistivity of thoracic tissue ranges from 150 O*cm for blood as high as 700 O*cm with collapsed lung tissue and all the way to 2400O*cm for air-filled lung tissue ( Table 1). In general, tissue resistivity or conductivity depends on the levels of ion and fluid content. Regarding lung function, this also depends on the volume of air in the alveoli. Though most tissues exhibit an isotropic characteristic, the heart and the muscle skeleton exhibit anisotropy, which means that resistance strongly depends on the direction from which it is measured.
Table 1. The electrical resistivity of the thoracic muscles.
3. EIT Measurements and Image Reconstruction
In order to perform EIT measurements electrodes are put around the chest in a transverse plane that is usually located in the 4th to 5th intercostal spaces (ICS) near Parasternal Line [55. The changes in impedance can be assessed in areas of the lower part of the right and left lungs, as well as in the heart area ,21. It is possible to position the electrodes below the 6th ICS might be difficult as the abdominal contents and diaphragm regularly enter the measurement plan.
Electrodes can be self-adhesive or single electrodes (e.g., electrocardiogram, ECG) that are placed individually with equal spacing in-between the electrodes, or are integrated into electrode belts [ ,2]. Additionally, self-adhesive stripe are made available for more user-friendly application ,21,2. Chest wounds, chest tubes bandsages that are not conductive or sutures made of wire can negatively impact EIT measurements. Commercially available EIT devices typically use 16 electrodes. However, EIT devices that use 8 or 32 electrodes is also available (please read Table 2 to get specifics) The following table shows the electrodes available. ,2].
Table 2. Commercially available electrical impedance (EIT) technology.
During an EIT measure sequence, small AC (e.g. five milliamps at a frequency of 100 kHz) are applied to different electrode pairs, and the resultant voltages are recorded using the remaining electrodes ]. Bioelectrical impedance that is measured between the injecting and the electrode pairs used to measure the voltage is calculated based on the applied current and measured voltages. Most often electrode pairs that are adjacent to each other are utilized to allow AC application in a 16-elektrode device, while 32-elektrode systems often employ a skip pattern (see the table 2) that increases the distance 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 . For a complete EIT data set of bioelectrical measurements The injecting and electrode pairs that measure are continually turned around the entire thorax .
1. Current measurements and voltage measurements around the thorax utilizing an EIT system that includes 16 electrodes. Within milliseconds as well as the voltage and current electrodes and their active voltage electrodes are continuously rotating within the thorax.
The AC that is used in EIT tests is safe for a body surface application that is undetectable 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 that is captured during a single cycle of AC Applications is referred to as a frame and contains the voltage measurements required to create an unprocessed EIT image. The term “frame rate” refers to the number of EIT frames recorded per second. Frame rates of at minimum 10 images/s are needed to monitor ventilation , and 25 images/s for monitoring the cardiac function or perfusion. Commercially available EIT equipment uses frame rates between 40 and 50 images/s , is shown in
To generate EIT images from recorded frames, the so-called image reconstruction method is used. Reconstruction algorithms strive to resolve the opposite problem of EIT which is the recovery of the conductivity distribution in the thorax using the voltage measurements that have been collected at electrodes on the thorax’s surface. At first, EIT reconstruction assumed that electrodes were placed on a circular or ellipsoid plane, while newer algorithms incorporate information about anatomy of the thorax. At present, the Sheffield back-projection algorithm and the finite element algorithm (FEM) using a linearized Newton–Raphson algorithm ], and the Graz consensus reconstruction algorithm for EIT (GREIT)  are frequently used.
In general, EIT images are comparable to a two-dimensional computed tomography (CT) image. These images are usually rendered so that the viewer looks from cranial towards caudal when studying the image. In contrast to an CT image An EIT image doesn’t display a “slice” but an “EIT sensitivity region” [1111. The EIT sensitivity region is a lens-shaped intra-thoracic volume and is where the impedance change contributes to the EIT imaging process . The dimensions and shape of the EIT sensitization region is determined by the dimensions, bioelectric properties, as well as the anatomy of the Thorax as according to the particular current injection and voltage measurement pattern [12It is important to note that the shape of the thoracic thorax can.
Time-difference image is a technique that is used for EIT reconstruction in order to display changes in conductivity rather than total conductivity. It is a technique that uses time to show the change in conductivity. EIT image compares the changes in impedance to a base frame. This affords the opportunity to examine the effects of time on physiological events such as lung ventilation and perfusion . Color-coding for EIT images is not uniform but generally displays the change in impedance in relation to a reference level (2). EIT images are usually colored using a rainbow color scheme with red representing the most significant value of relative imperf (e.g. in the time of inspiration) with green being a medium relative impedance and blue the smallest relative impedance (e.g. during expiration). For clinical applications it is possible to employ color scales that vary from black (no impedance change) to blue (intermediate impedance changes) and white (strong impedance changes) to code ventilation or between black and white and then red towards mirror perfusion.
2. There are a variety of color codes available for EIT images as compared to CT scan. The rainbow-color scheme utilizes red for the highest relative impedance (e.g. during inspiration) while green is used for moderate relative impedance, blue as the one with the lowest impedance (e.g., during expiration). A newer color scheme uses instead of black to avoid any impedance changes) while blue is used for an intermediate impedance change, and white for the largest impedance shift.
4. Functional Imaging and EIT Waveform Analysis
Analyzing Impedance Analyzers data is based on EIT waveforms , which are generated in individual image pixels in an array of raw EIT images that are scanned over time (Figure 3). In a region of focus (ROI) is a term used to represent activity within individual pixels in the image. Within any ROI, the waveform displays changes in the regional conductivity over time resulting from breathing (ventilation-related signal, VRS) or activity in the heart (cardiac-related signal CRS). Additionally, electrically conducting contrast agents such as hypertonic saline can be utilized to create an EIT signal (indicator-based signal, IBS) and could be connected to lung perfusion. The CRS can originate from both the lung and the cardiac region and could be attributable to lung perfusion. The precise origins and components aren’t understood completely 1313. Frequency spectrum analysis can be used to distinguish between ventilatoror 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 originate from the Raw EIT images. EIT waveforms can be identified in a pixel-wise manner or based on a specific region to be studied (ROI). Conductivity changes occur naturally as a result of ventilation (VRS) or the activity of cardiac muscles (CRS) but they may also be induced artificially, e.g. through injection of bolus (IBS) to determine perfusion. fEIT images display some of the regional physiological parameters including ventilation (V) or perfusion (Q), extracted from the raw EIT images by using a mathematical procedure over time.
Functional EIT (fEIT) images are generated through the application of a mathematical algorithm on the raw images and the corresponding EIT form . Since the mathematical procedure is applied to calculate a physiologically relevant parameter for each pixelof the image, regional physiological traits like regional airflow (V) and respiratory system compliance, as also regional perfusion (Q) can be measured as well as displayed (Figure 3). Data from EIT waveforms , as well as concurrently registered airway pressure measurements can be utilized to calculate the lung compliance as well as the lung’s opening and closing times for each pixel based on changes of pressure and impedance (volume). The comparable EIT measurements of inflating and deflating lung volumes allow for the display of curves representing volume and pressure at an individual pixel. Based on the mathematical process, various types of fEIT pictures could be used to analyze different functions of the cardio-pulmonary system.