Electrical Impedance Tomography for Cardio-Pulmonary Monitoring
Electrical Impedance Tomography (EIT) is a bedside monitoring device that does not require any surgery to see the local airflow and even lung perfusion. In this article, we review and analyzes both methodological and clinical aspects of the thoracic EIT. Initially, researchers addressed the possibility of using EIT to assess regional ventilation. These studies focus on its clinical applications to measure lung collapse the tidal response, and lung overdistension in order to regulate positive end-expiratory pressure (PEEP) and tidal volume. In addition, EIT may help to detect pneumothorax. Recent studies assessed EIT as a method for measuring regional lung perfusion. The absence of indicators in EIT measurements might be sufficient for continuous monitoring of stroke volume. Utilizing a contrast agent, such as saline, might be required to assess the regional lung perfusion. Therefore, EIT-based monitoring of respiratory ventilation and lung perfusion could reveal local perfusion and oxygenation which may be useful in treating patients suffering from chronic respiratory distress syndrome (ARDS).
Keywords: Electrical impedance tomography and bioimpedance. Image reconstruction Thorax; regional ventilation; regional perfusion; monitoring
Electric impedance tomography (EIT) is a radiation-free functional imaging modality that allows non-invasive bedside monitoring of both regional lung ventilation , and possibly perfusion. Commercially available EIT devices were first introduced for the clinical use of this technique and thoracic EIT has been successfully used in both adult and pediatric patients [ 2, [ 1, 2].
2. Basics of Impedance Spectroscopy
Impedance Spectroscopy may be described as the electrical response of biological tissue to an externally applied electrical current (AC). It is typically achieved by using four electrodes, where two are utilized for AC injection and the other two for voltage measurement [ 3,,3. Thoracic EIT measures the regional distribution of intra-thoracic Impedance Spectroscopyand can be viewed like an extension of four electrode principle onto the image plane which is defined with the belt of electrodes 11. Dimensionallyspeaking, electrical impedance (Z) is the same as resistance and the appropriate International System of Units (SI) unit is Ohm (O). It is easily expressed as a complicated number, in which the real component is resistance and the imaginary part is called reaction, which evaluates the effects that result from capacitors or the effect of inductance. The amount of capacitance is determined by biomembranes’ specifics of the tissue , which includes ion channels, fatty acids, and gap junctions. However, resistance is mostly determined by the composition of the tissue and the quantity of extracellular fluid [ 1., 2[ 1, 2]. When frequencies are below 5 kilohertz (kHz) that is, electrical energy is carried by extracellular fluid and is in a major way dependent on the resistivity characteristics of tissues. Higher frequencies, as high as 50 kHz electrical currents are slightly deflected at cell membranes . This leads to an increase of capacitive tissue properties. When frequencies exceed 100 kHz electricity can pass through cell membranes, and diminish the capacitive portion 22. Therefore, the effects that determine the impedance of tissue depend on the stimulation frequency. Impedance Spectroscopy can be described in terms of resistivity or conductivity, which regulates conductance or resistance according to the unit’s area and length. The SI units used include Ohm-meter (O*m) for resistivity, and Siemens per meters (S/m) for conductivity. The resistance of the thoracic tissues ranges from 150 O*cm when blood is present as high as 700 O*cm with collapsed lung tissue and up to 2400 o*cm for the lung tissue that has been inflated ( Table 1). In general, the tissue’s resistance or conductivity is dependent on level of fluids and ions. For breathing, it depends on the amount of air in the alveoli. While most tissues exhibit anisotropic behavior, the heart as well as skeletal muscle behave anisotropic, meaning that resistivity strongly depends on the direction that it is measured.
Table 1. Thoracic tissues have electrical resistance.
3. EIT Measurements and Image Reconstruction
For EIT measurements, electrodes are placed around the Thorax in a horizontal plane typically between the 4th and 5th intercostal spaces (ICS) in the parasternal line [55. After that, the changes in impedance can be measured in the lower lobes in the left and right lungs and also in the heart region ,2]. To position the electrodes above the 6th ICS could be difficult because abdominal content and the diaphragm occasionally enter the measurement area.
Electrodes are either single self-adhesive electrodes (e.g., electrocardiogram, ECG) which are placed in a similar spacing between electrodes or are embedded in electrode belts ,22. Self-adhesive stripes are also available for a more user-friendly application [ ,21. Chest tubes, chest wounds (non-conductive) bandages or wire sutures can block or substantially affect EIT measurements. Commercially available EIT devices usually use 16 electrodes, but EIT devices with 8 and 32 electrodes are available (please see Table 2 for details) It is recommended to consult Table 2 for more details. ,2[ 1,2].
Table 2. Available electrical impedance (EIT) devices.
During an EIT test, low AC (e.g. approximately 5 million mA with a frequency of 100 kHz) is applied to different electrodes, and the produced voltages are measured using the remaining electrodes ]. Bioelectrical impedance between the injecting and electrodes that are measuring is determined from the applied current and measured voltages. Most commonly the electrodes adjacent to each other are used to allow AC application in a 16-elektrode setup and 32-elektrode systems typically utilize a skip-pattern (see Table 2) which increases the distance of the electrodes that inject current. The resulting voltages are then measured with other electrodes. Currently, there is an ongoing discussion on different current stimulation patterns , and their unique advantages and disadvantages . In order to obtain an complete EIT data set of bioelectrical measurements both the injecting and electrode pairs measuring are continuously moved around the entire thorax .
1. Current measurements and voltage measurements in the thorax using an EIT system consisting of 16 electrodes. Within milliseconds each of the electrodes for current and an active voltage electrode are repeatedly rotating in the area of the thorax.
The AC utilized during EIT tests are safe for a body surface application and is not detectable 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 recorded in one cycle during AC programs is called a frame . It is comprised of the voltage measurements to generate that initial EIT image. The term frame rate reflects the amount of EIT frames recorded in a second. Frame rates that are at least 10 images/s are necessary for monitoring ventilation and 25 images/s are required to monitor perfusion and cardiac function. Commercially available EIT devices utilize frames with a frame rate between 40 and 50 images/s , depicted in
To produce EIT images from the captured frames, the so-called reconstructing of images is carried out. Reconstruction algorithms attempt to solve the opposite problem of EIT that is the determination of the conductivity distribution in the thorax by analyzing the voltage measurements that have been obtained at the electrodes located on the thorax’s surface. In the beginning, EIT reconstruction assumed that electrodes were placed on an ellipsoid, circular or circular plane, whereas newer algorithms employ information on the anatomical shape of the thorax. Currently, the Sheffield back-projection algorithm , the finite element method (FEM) that is a linearized Newton-Raphson algorithm ], and the Graz consensus reconstruction algorithm for EIT (GREIT) [10are often employed.
The majority of EIT images have a similarity to a computed two-dimensional (CT) image. These images are typically rendered in a way that the operator looks across the entire cranial region when analysing the image. Contrary to an CT image, an EIT image does not display an image “slice” but an “EIT sensitivity region” . The EIT sensitive region is a lens-shaped intra-thoracic volume with impedance-related changes that contribute to the EIT image generation [11(11, 11). The shape and the thickness of the EIT sensitization region is determined by the dimensions, the bioelectric propertiesas well as the structure of the chest, as well depending on the current injection and voltage measurement pattern [1212.
Time-difference imaging can be described as a technique that is employed in EIT reconstruction to show the changes in conductivity rather than total conductivity. It is a technique that uses time to show the change in conductivity. EIT image compares the variation in impedance to the baseline frame. This affords the opportunity to track the time-dependent physiological changes like lung ventilation or perfusion [22. The color coding of EIT images is not unified but typically shows the change in the impedance of the patient to a standard (2). EIT images are typically coded using a rainbow-colored scheme with red representing the highest absolute impedance (e.g., during inspiration) and green representing a medium relative impedance and blue the lowest impedance (e.g. during expiration). For clinical applications the best option is to use color-scales that range from black (no impedance change) or blue (intermediate impedance change) and white (strong impedance change) to code ventilation or from black, to white, and red in order to code mirror perfusion.
2. Different color codings for EIT images in comparison with the CT scan. The rainbow-color scheme utilizes red for the most powerful relative impedance (e.g., during inspiration) and green for a moderate relative impedance, blue for the lowest relative impedance (e.g. when expiration is in progress). The newer color scales employ instead of black (which has no impedance change), blue for an intermediate impedance change, and white for the largest impedance change.
4. Functional Imaging and EIT Waveform Analysis
Analyzing Impedance Analyzers data is based on EIT waveforms that form inside individual image pixels within a series of raw EIT images that are scanned over period of (Figure 3). The term “region of interest” (ROI) can be defined to describe activity in the individual pixels of the image. In any ROI, the waveform displays changes in regional conductivity over the course of time that result from breathing (ventilation-related signal, VRS) or heart activity (cardiac-related signal CRS). Furthermore, electrically conductive contrast agents such as hypertonic Saline can be used to generate an EIT waveshape (indicator-based signal IBS) which may be related to the perfusion of the lung. The CRS can be traced to both the cardiac and lung region and may be partly linked to lung perfusion. Its precise source and composition is not fully understood 13]. Frequency spectrum analysis is frequently used to distinguish between ventilatorand cardiac-related impedance variations. Non-periodic changes in impedance may result from modifications in the settings of the ventilator.
Figure 3. EIT form and function EIT (fEIT) photos are derived from the EIT raw EIT images. EIT waveforms are defined pixel-wise or on a region in interest (ROI). Changes in conductivity are naturally triggered by the process of ventilation (VRS) and cardiac activities (CRS) but they can be produced artificially e.g. or through the injection of bolus (IBS) to measure perfusion. FEIT images present regional physiological parameters including ventilation (V) along with perfusion (Q) as extracted from raw EIT images by applying a mathematical procedure over time.
Functional EIT (fEIT) images are generated by applying a mathematical procedure on the raw images as well as the corresponding pixel EIT spectrums. Because the mathematical process is applied to determine an appropriate physiological parameter for each pixel, physiological regional characteristics such as regional ventilation (V), respiratory system compliance as also respiratory system compliance as well as regional perfusion (Q) are measured to be displayed (Figure 3.). The data collected from EIT waveforms and concurrently recorded pressures of the airways can be used to calculate the lung’s compliance as well as lung opening and closing for each pixel using changes of pressure and impedance (volume). Similar EIT measurements of increments of inflation and deflation in lung volumes allow for the display of pressure-volume curves at an individual level. Depending on the mathematical method used, different types of fEIT photos could address different functional properties within the cardio-pulmonary systems.