Electrical Impedance Tomography for Cardio-Pulmonary Monitoring
Electrical Impedance Tomography (EIT) is an instrument that monitors the bedside and noninvasively visualizes local ventilation and , possibly, lung perfusion distribution. The present article reviews and analyzes the clinical and methodological aspects of thoracic EIT. Initially, researchers focused on the validation of EIT to assess regional ventilation. Present research is focused on its clinical applications to assess lung collapse, Tidal Recruitment, and lung overdistension. This allows for the titration of positive end-expir pressure (PEEP) and the volume of tidal. In addition, EIT may help to detect pneumothorax. Recent studies examined EIT as a method to determine regional lung perfusion. The absence of indicators in EIT measurements could be adequate for continuous measurement of cardiac stroke volume. The use of a contrast agent like saline might be required in order to determine regional lung perfusion. Thus, EIT-based surveillance of regional airflow and lung perfusion might reveal local ventilation and perfusion matching and can prove helpful in the treatment of patients suffering from acute respiratory distress syndrome (ARDS).
Keywords: electrical impedance imaging bioimpedance; image reconstruction Thorax; regional adrenergic Monitoring regional perfusion
Electrical impedance tomography (EIT) can be described as a non-radiation functional imaging technique that permits the non-invasive monitoring of bedside regional lung ventilation , and possibly perfusion. Commercially-available EIT devices were introduced to allow clinical application of this technique, and thoracic EIT is widely used in both pediatric and adult patients 1, [ 1, 2].
2. Basics of Impedance Spectroscopy
Impedance Spectroscopy is the biomaterial’s voltage response to externally applied alternating electron current (AC). It is typically achieved by using four electrodes, where two are used for AC injection and the other two for voltage measurement 3.,[ 3, 4]. Thoracic EIT measures the regional variability of Impedance Spectroscopy in the thoracic area and can be viewed like an extension of principle of four electrodes into the image-plane spanned by an electrode belt [ 11. Dimensionallyspeaking, electrical impedance (Z) is equivalent to resistance, as is the International System of Units (SI) unit is Ohm (O). It is often expressed in a complex form, where the actual part is resistance and the imaginary part is called the reactance, which describes the effects that are caused by resistance or capacitance. The capacitance of a cell is determined by the biomembranes’ characteristics of the tissue , for example, ion channels and fatty acids as well as gap junctions. Resistance is mostly determined by composition and quantity of extracellular fluid 1, 22. When frequencies are below 5 kilohertz (kHz) electricity travels through extracellular fluids and is primarily dependent upon the properties of the resistive tissues. At higher frequencies of up to 50 kHz, electrical impulses are slightly redirected at cell membranes . This leads to an increase in capacitive tissue properties. For frequencies higher than 100 kHz electric currents are able to travel through cell membranes and decrease the capacitive component [ 2[ 1, 2]. Thus, the factors which determine the level of impedance in the tissue depend on the used 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 consist of Ohm-meter (O*m) for resistivity and Siemens per meter (S/m) for conductivity. The resistance of thoracic tissue varies between 150 O*cm of blood and up to 700 o*cm for air-filled lung tissue, and up to 2400 O*cm for inflated lung tissue ( Table 1). In general, tissue resistance or conductivity is dependent on level of fluids and ions. For lung function, this is dependent on the quantity of air present in the alveoli. While most tissues show isotropic response, heart and muscle in particular exhibit anisotropic properties, meaning that the resistance is strongly dependent on the direction that the measurement is made.
Table 1. Electrical resistivity of thoracic tissues.
3. EIT Measurements and Image Reconstruction
To conduct EIT measurements, electrodes are placed around the chest in a transverse line, usually in the 4th to the 5th intercostal areas (ICS) in that line called parasternal [55. In turn, the variations in the impedance of the lungs can be measured within the lower lobes in the left and right lungs and also in the heart region ,2]. To position the electrodes above the 6th ICS might be difficult as the abdominal and diaphragm are frequently inserted into the measurement plane.
Electrodes are self-adhesive electrodes (e.g. electrocardiogram ECG) that are positioned individually in a similar spacing between electrodes or are integrated in electrode belts ,2]. Additionally, self-adhesive strips are offered for a more user-friendly application [ ,2]. Chest tubes, chest wounds (non-conductive) bandages or sutures for wires can significantly impact EIT measurements. Commercially available EIT equipment typically uses 16 electrodes, but EIT devices with 8 and 32 electrodes are available (please read Table 2 to get specifics) The following table shows the electrodes available. ,2[ 1,2].
Table 2. Electronic impedance (EIT) tools.
During an EIT measurement sequence, small AC (e.g. approximately 5 mgA at a rate of 100 kHz) is applied to different electrodes, and the output voltages are analyzed using the remaining other electrodes [ 6. The bioelectrical resistance between the injecting and the electrode pairs measuring the electrodes is calculated using the applied current and the measured voltages. The majority of the time the electrodes adjacent to each other are utilized for AC application in a 16-elektrode set-up and 32-elektrode systems typically use a skip pattern (see the table 2) that increases the distance between electrodes used for injecting current. The resulting voltages can be measured using those remaining electrodes. In the present, there is a debate ongoing about various current stimulation patterns , and their distinct advantages and disadvantages [77. In order to obtain an complete EIT data set that includes bioelectrical tests as well as the injecting and measuring electrode pairs are continuously rotating around the entire thorax .
1. Current measurement and voltage measurements within the thorax, using an EIT device with 16 electrodes. In just a few milliseconds two electrodes measuring current and activated voltage electrodes are rotated around the thorax.
The AC used during EIT measurements is safe for use on body surfaces and is not detectable 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.
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 EIT’s raw EIT image. Frame rate refers to the number of EIT frames recorded each second. Frame rates of no less than 10 images/s are essential to monitor ventilation , and 25 images/s in order to monitor perfusion and cardiac function. Commercially accessible EIT devices utilize frame rates between 40 and 50 images/s , is shown in
To generate EIT images from the captured frames, the so-called image reconstruction method is used. Reconstruction algorithms aim to solve the opposite problem of EIT which is the recovery of the conductivity distribution in the thorax by analyzing the voltage measurements recorded at the electrodes that are on the thorax’s surface. In the beginning, EIT reconstruction assumed that electrodes were placed on an ellipsoid plane, whereas newer algorithms employ information on how the anatomical shape of thorax. Currently, it is the Sheffield back-projection algorithm [ as well as the finite element technique (FEM) with 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 traditionally rendered so that the viewer looks from cranial towards caudal when taking a look at the picture. In contrast to a CT image one can observe that an EIT image doesn’t show an actual “slice” but an “EIT sensitivity region” . The EIT sensitive region is a lens-shaped intrathoracic region with impedance-related changes that 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 the EIT sensitive region are determined by the dimensions, bioelectric properties, and the structure of the chest as well and the applied current injection and voltage measurement pattern [1212.
Time-difference image is a technique that is used for EIT reconstruction in order to display changes in conductivity rather than total conductivity. A time-difference EIT image compares the changes in impedance to a base frame. This allows you to examine the effects of time on physiological events like lung ventilation and perfusion . The color code of EIT images is not unicoded but commonly displays the change in the impedance of the patient to a standard (2). EIT images are typically coded using a rainbow-color scheme with red indicating the highest absolute impedance (e.g., during inspiration) while green is a moderate relative impedance, and blue the smallest relative 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 changes) as well as white (strong impedance changes) to code ventilation , or from black, to white, then up to mirror-perfusion.
2. Different available color codings of EIT images in comparison with the CT scan. The rainbow color scheme uses red to indicate the highest in terms of relative intensity (e.g., during inspiration) Green for a middle relative impedance and blue as the one with the lowest impedance (e.g., during expiration). Modern color scales make use of instead of black for no impedance changes) as well as blue for an intermediate change in impedance, as well as white for the greatest impedance shift.
4. Functional Imaging and EIT Waveform Analysis
Analyzing Impedance Analyzers data is based on EIT waveforms created within individual image pixels of the form of a sequence of raw EIT images that are scanned over the course of time (Figure 3.). A “region of study” (ROI) can be defined for a summation of activity within the individual pixels in the image. In every ROI, the waveform shows changes in regional conductivity over time as a result of ventilatory activity (ventilation-related signal, also known as VRS) or cardiac activity (cardiac-related signal, CRS). In addition, electrically conductive contrast agents like hypertonic saltsaline may be used in the production of the EIT waveform (indicator-based signal IBS) which may be related to perfusion in the lung. The CRS may originate from both the lung and the cardiac region and could be attributable to lung perfusion. Its precise source and composition are not well understood. 1313. Frequency Spectrum Analysis is typically used to identify ventilationand cardiac-related changes in impedance. Impedance fluctuations that are not frequent can result from modifications in the settings of the ventilator.
Figure 3. EIT forms and the functions of EIT (fEIT) photos are created from raw EIT images. EIT waveforms may be defined either pixel-wise or in a region that is of particular interest (ROI). Conductivity changes result naturally from breathing (VRS) as well as cardiac activity (CRS) but they can be caused artificially, e.g., by the injection of bolus (IBS) for perfusion measurement. FEIT images depict local physiological parameters such as perfusion (Q) and ventilation (V) as well as perfusion (Q) as extracted from raw EIT images using the mathematical process of time over.
Functional EIT (fEIT) images are produced using a mathematical process on an array of raw images together with the appropriate pixel EIT waves . Because the mathematical process is applied to calculate the physiologically relevant parameters for each pixel, regional physiological aspects like regional ventilation (V), respiratory system compliance as and local perfusion (Q) are measured and display (Figure 3.). The information derived collected 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 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.