Electrical Impedance Tomography for Cardio-Pulmonary Monitoring

Abstract

Electrical Impedance Tomography (EIT) is an instrument for monitoring bedside that noninvasively visualizes local ventilation and , possibly, lung perfusion distribution. In this article, we review and analyzes the clinical and methodological aspects of the thoracic EIT. Initially, researchers addressed the validation of EIT to assess regional ventilation. Present research is focused on clinical applications of EIT to determine the extent of lung collapse, increased tidal flow, 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 looked at EIT as a way to assess regional lung perfusion. The absence of indicators in EIT tests could be enough to continuously monitor cardiac stroke volume. Utilizing a contrast agent, such as saline, might be required in order to determine regional perfusion of the lungs. In the end, EIT-based monitors of regional ventilation and lung perfusion might reveal local perfusion and oxygenation which may be useful in the treatment of patients suffering from chronic respiratory distress syndrome (ARDS).

Keywords: electrical impedance tmography bioimpedance; image reconstruction; thorax; regional ventilation Regional perfusion; monitoring

1. Introduction

Electrical impedance tomography (EIT) is an radiation-free functional imaging method that permits the non-invasive monitoring of bedside regional lung ventilatory and perhaps perfusion. Commercially accessible EIT devices were introduced to allow clinical application of this technique and the thoracic EIT is used in a safe manner in both pediatric and adult patients [ 1., [ 1, 2].

2. Basics of Impedance Spectroscopy

Impedance Spectroscopy can be defined as the biomaterial’s voltage response to an externally applied electronic current (AC). It is usually measured with four electrodes, of which two are employed to inject AC injection, and the remaining two for voltage measurement 3,3. Thoracic EIT measures the regional variation of the intra-thoracic bioimpedance. It is seen as an extension of the four electrode principle to the image plane , which is covered by the electrode belt 1]. Dimensionallyspeaking, electrical impedance (Z) is identical to resistance and the related International System of Units (SI) unit is Ohm (O). It is easily expressed in a complex form, where the real portion is resistance and the imaginary is called reaction, which is the measurement of effects caused by either inductance or capacitance. Capacitance is dependent on biomembranes’ characteristics of a tissues such as ion channel, fatty acids, and gap junctions, whereas resistance is mainly determined by the content and quantity of extracellular fluid 1., 22. When frequencies are below 5 kilohertz (kHz) an electrical current circulates through extracellular fluids and is in a major way dependent on the resistive characteristics of the tissues. At higher frequencies up to 50 kHz, currents are slightly diverted at cells’ membranes which causes an increase in the capacitive properties. In frequencies that exceed 100 kHz electrical current can flow through cell membranes, reducing the capacitive component [ 2]. Therefore, the effects which determine the level of impedance in the tissue depend on the utilized stimulation frequency. Impedance Spectroscopy is usually given as conductivity or resistivity. These normalize resistance or conductance to unit area and length. The SI units for the same is Ohm-meter (O*m) for resistivity and Siemens per meter (S/m) (S/m) for conductivity. The thoracic tissue’s resistance ranges from 150 O*cm in blood, to 700 O*cm for deflated lung tissue, up to 2400 O*cm in air-filled lung tissue ( Table 1). In general, tissue resistance or conductivity is determined by the levels of ion and fluid content. In the case of respiratory lungs it depends on the amount of air present in the alveoli. Although most tissues exhibit isotropic behavior, the heart and muscles of the skeletal are anisotropic. this means that resistivity is heavily dependent on the direction that it’s measured.

Table 1. Electrical resistance of thoracic tissues.

3. EIT Measurements and Image Reconstruction

To perform EIT measurements electrodes are set around the chest in a transverse line that is usually located in the 4th through 5th intercostal areas (ICS) in the parasternal line [5]. The changes in impedance can also be measured in the lower lobes and lobes of the left and right lungs as well as in the heart area ,21. In order to place the electrodes under the 6th ICS might be difficult as the abdominal and diaphragm often enter the measurement plane.

Electrodes can be self-adhesive or single electrodes (e.g. electrocardiogram ECG) which are placed in a similar spacing between electrodes or are embedded in electrode belts [ ,2[ 1,2]. Also, self-adhesive stripes are made available for more user-friendly application [ ,21,2. Chest wounds, chest tubes bandsages that are not conductive or wire sutures may preclude or significantly affect EIT measurements. Commercially available EIT devices typically have 16 electrodes. However, EIT systems with 8 as well as 32 electrodes are available (please check Table 2 for details) The following table shows the electrodes available. ,2].

Table 2. Electrical impedance devices that are commercially accessible. (EIT) equipment.

During an EIT measurements, small AC (e.g. the smallest value of 5 mA at a frequency of 100 kHz) are applied to various electrode pairs, and the output voltages are analyzed using the remaining electrodes ]. Bioelectrical impedance between the injecting and electrodes used for measuring is determined by analyzing the applied current and the measured voltages. Most often nearby electrode pairs are utilized for AC application within a 16-elektrode configuration, while 32-elektrode systems often use a skip pattern (see the table 2) which increases the distance of the electrodes that inject the current. The resulting voltages are measured by using one of the other electrodes. At present, there is a debate ongoing about various kinds of current stimulation, as well as their advantages and disadvantages [7]. To acquire a complete EIT data set of bioelectrical measurements, the injecting and the electrodes used to measure the electrodes are continuously rotated around the entire thorax .

1. The measurements of voltage and current are made within the thorax, using an EIT system consisting of 16 electrodes. Within a few milliseconds, simultaneously, the current electrode and the active voltage electrodes are repeatedly rotated over the entire thorax.

The AC used during the EIT tests are safe for body surface applications 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.

The EIT data set which is recorded over a single cycle that is recorded during one cycle of AC apps is known as a frame and contains the voltage measurements to generate that original EIT image. The term “frame rate” refers to the number of EIT frames recorded in a second. Frame rates that are at least 10 images/s is required for monitoring ventilation and 25 images/s to track heart function or perfusion. Commercially accessible EIT devices use frame rates ranging from 40 to 50 images/s as is shown in

To produce EIT images from recorded frames, so-called image reconstruction method is used. Reconstruction algorithms are designed to address the inverse problem of EIT which is the recovery of the conductivity distribution inside the thorax on the basis of 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 in an ellipsoid or circular plane. Newer methods use information about the anatomical contour of the thorax. At present, there are three main algorithms used for EIT: the Sheffield back-projection algorithm [ , the finite element method (FEM) based linearized Newton-Raphson algorithm [ ] as well as the Graz consensus reconstruction algorithm for EIT (GREIT) [10are commonly used.

As a rule, EIT photographs are similar to a two-dimensional computed (CT) image. These images are typically rendered in a way that the viewer looks from caudal to cranial when looking at the image. In contrast to an CT image one can observe that an EIT image does not display an actual “slice” but an “EIT sensitivity region” [11]. The EIT sensitivity region is a tubular intra-thoracic structure that is the source of impedance variations which contribute to EIT creation of images(11, 11). The shape and size of the EIT sensitive region are determined by the dimensions, the bioelectric propertiesas well as the contour of the thorax as well with the type of voltage measurement and current injection pattern [12The shape and thickness of the EIT sensitivity region is determined by the voltage measurement pattern [.

Time-difference-based imaging is a process that is used for EIT reconstruction to show the changes in conductivity instead of total conductivity. In a time-difference EIT image compares the changes in impedance with the baseline frame. It is an opportunity to study the underlying physiological phenomenon that changes over time such as lung respiration and perfusion [22. Color coding of EIT images is not unified but commonly displays the change in impedance to an appropriate level (2). EIT images are generally encoded using a color scheme that is rainbow-like with red representing the high relative impedance (e.g. during inspiration) with green being a medium relative impedance while blue is the lowest impedance (e.g., during expiration). In clinical settings An interesting approach is to use color scales ranging from black (no change in impedance) up to blue (intermediate impedance changes) and white (strong impedance shift) to code ventilation . between black and white and then red towards mirror perfusion.

2. Different color codes are available for EIT images when compared with the CT scan. The rainbow color scheme uses red for the most powerful value of impedance relative (e.g. during inspiration) while green is used for middle relative impedance and blue for the lowest relative impedance (e.g. during expiration). Newer color scales utilize instead black for no impedance change) and blue for the intermediate impedance change while white is the one with the strongest impedance shift.

4. Functional Imaging and EIT Waveform Analysis

Analysis of Impedance Analyzers data is based on EIT waveforms which are created inside individual image pixels within a series of raw EIT images that are scanned over long periods of (Figure 3.). An area of concern (ROI) is a term used to describe activity in the individual pixels of the image. Within each ROI, the image shows fluctuations in regional conductivity in time as a result of either the process of ventilation (ventilation-related signal, VRS) or cardiac activity (cardiac-related signal CRS). Additionally, electrically conducting contrast agents such as hypertonic saline can be used to obtain the EIT signal (indicator-based signal IBS) and could be connected to the perfusion of the lung. The CRS could be a result of both the lung and the cardiac region and may also be linked to lung perfusion. The exact cause and the composition are not understood fully 1313. Frequency spectrum analysis can be employed to distinguish between ventilationand cardiac-related impedance fluctuations. Impedance changes outside of the periodic cycle could result from modifications in the settings of the ventilator.

Figure 3. EIT waveforms , as well as the functional EIT (fEIT) image originate from the unprocessed EIT images. EIT waves can be defined either pixel-wise or in a region that is of particular interest (ROI). Conductivity fluctuations are the result of ventilation (VRS) or heart activity (CRS) but can be artificially induced, e.g. via injection of bolus (IBS) for the purpose of measuring perfusion. FEIT images depict specific physiological parameters of the region such as perfusion (Q) and ventilation (V) or perfusion (Q) that are extracted from raw EIT images by applying the mathematical process of time over.

Functional EIT (fEIT) images are produced by applying a mathematical calculation on the sequence of raw pictures together with the appropriate pixel EIT waveforms [14]. Because the mathematical process is applied to determine the physiologically relevant parameters for each pixel. The regional physiological features like regional ventilation (V), respiratory system compliance, as well as regional perfusion (Q) are measured as well as displayed (Figure 3). The information derived generated from EIT waveforms , as well as concurrently registered pressures of the airways can be used to determine the lung’s compliance and the opening and closing of the lungs for each pixel using changes of impedance and pressure (volume). Comparable EIT measurements of inflating and deflating lung volumes allow for the display of pressure-volume curves on scales of pixel. Based on the mathematical operation, different kinds of fEIT photographs may address different functional characteristics from the cardio-pulmonary apparatus.