Electrical Impedance Tomography for Cardio-Pulmonary Monitoring


Electrical Impedance Tomography (EIT) is an instrument used to monitor the bed that does not require any surgery to see the local airflow and arguably lung perfusion distribution. This article reviews and discusses both methodological and clinical aspects of thoracic EIT. Initially, researchers addressed the validation of EIT for measuring regional airflow. Present research is focused on clinical applications of EIT to quantify lung collapse, the tidal response, and 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 have evaluated EIT as a way to measure lung perfusion in the region. Indicator-free EIT tests could be enough to continuously measure the heart stroke volume. The use of a contrast agent, such as saline, may be required to measure the regional lung perfusion. In the end, EIT-based monitoring of regional ventilation and lung perfusion may visualize the perfusion match and local ventilation and can prove helpful in the treatment of patients with chronic respiratory distress syndrome (ARDS).

Keywords: Electrical impedance tomography and bioimpedance. Image reconstruction Thorax; regional vent Regional perfusion; monitoring

1. Introduction

The electrical impedance imaging (EIT) is an radiation-free functional imaging modality that permits non-invasive monitoring at bedside of both respiratory ventilation in the region and possibly perfusion. Commercially available EIT devices were introduced for clinical application of this technique, and thoracic EIT has been used safely in both pediatric and adult patients 1, 1.

2. Basics of Impedance Spectroscopy

Impedance Spectroscopy may be described as the electrical response of biological tissue to externally applied alternating electronic current (AC). It is commonly obtained using four electrodes, of which two are used for AC injection and the other two for voltage measurement [ 3,3. Thoracic EIT measures the regional Impedance Spectroscopy of the thoracic region and can be considered as an extension of the four electrode principle into the image-plane spanned by an electrode belt 1]. Dimensionallyspeaking, electrical impedance (Z) is the same as resistance , and the corresponding International System of Units (SI) unit is Ohm (O). It is often expressed as a complex number in which it is the actual portion of resistance and the imaginary is called the reactance, which quantifies effects resulting from either inductance or capacitance. The amount of capacitance is determined by biomembranes’ characteristics of the tissue , including ion channels, fatty acids, and gap junctions. In contrast, resistance is determined by the content and quantity of extracellular fluid [ 1., 22. When frequencies are below 5 kilohertz (kHz) an electrical current is carried by extracellular fluid and is primarily dependent on the resistance characteristics of tissues. For higher frequencies that exceed 50 kHz electrical currents are slightly deflected at cell membranes , leading to an increase in capacitive tissue properties. When frequencies exceed 100kHz the electrical current is able to pass through cell membranes and decrease the capacitive component [ 22. Therefore, the effects which determine the tissue’s impedance depend on the utilized stimulation frequency. Impedance Spectroscopy usually refers to conductivity or resistance, which compares conductance or resistance the area of the unit and the length. The SI units used can be described as Ohm-meter (O*m) for resistivity, and Siemens per meters (S/m) in the case of conductivity. The thoracic tissue’s resistance ranges from 150 O*cm for blood and 700 O*cm in air-filled lung tissue, and up to 2400 O*cm for tissues that have been inflated ( Table 1). In general, the tissue’s resistance or conductivity varies based on volume of the fluid and the amount of ions. Regarding the lungs, it depends on the amount of air inside the alveoli. While most tissues exhibit isotropic response, heart and skeletal muscle behave anisotropic, meaning that resistivity strongly depends on the direction that they are measured.

Table 1. The electrical resistance of the thoracic tissue.

3. EIT Measurements and Image Reconstruction

In order to conduct EIT measurements electrodes are placed around the Thorax in a transverse, usually in the 4th-5th intercostal space (ICS) at the line between parasternal and lateral [55. Subsequently, the changes of impedance can be observed in areas of the lower part of the right and left lungs, as well as within the heart region ,22. To place the electrodes below the 6th ICS might be difficult as abdominal content and the diaphragm periodically enter the measurement plane.

Electrodes can be self-adhesive or single electrodes (e.g. electrocardiogram ECG,) that are placed with equal spacing between the electrodes, or they are integrated into electrode belts ,21 2. Additionally, self-adhesive stripe are designed to be more comfortable for application ,21. Chest tubes, chest wounds Non-conductive bandages and conductive sutures for wires can substantially affect EIT measurements. Commercially available EIT devices typically have 16 electrodes. However, EIT systems with 8 and 32 electrodes are available (please check Table 2 for information) [ ,21 2.

Table 2. Available electrical impedance tomography (EIT) gadgets.

During an EIT measurement sequence, small AC (e.g. approximately 5 million mA with a frequency of 100 kHz) are applied to different electrode pairs, and the resultant voltages are recorded using the other electrodes ]. The bioelectrical impedance between the injecting and the electrode pairs that measure is calculated from the known applied current and measured voltages. Most commonly the electrodes adjacent to each other are used to allow AC application in a 16-elektrode set-up in 32-elektrode devices, whereas 16-elektrode use a skip pattern (see Table 2.) which increases the distance of electrodes used for injecting current. The voltages that result are then measured with an additional electrode. At present, there is an ongoing discussion about different kinds of current stimulation, as well as their unique advantages and disadvantages [77. To get a complete EIT data set that includes bioelectrical tests as well as the injecting and electrodes used to measure the electrodes are continuously rotationally positioned around the entire chest .

1. Current measurements and voltage measurements around the thorax by using an EIT system that includes 16 electrodes. In a matter of milliseconds both the current electrodes and these active electrodes are repeatedly rotated about the chest.

The AC used during EIT tests is safe to use for body surface applications and are not 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.

A EIT data set that is recorded in one cycle within AC programs is known as a frame and contains the voltage measurements needed to produce that raw EIT image. Frame rate refers to the number of EIT frames recorded per second. Frame rates that exceed 10 images/s are essential to monitor ventilation and 25 images/s to monitor perfusion or cardiac function. Commercially available EIT devices employ frame rates of 40 to 50 images/s as described in

In order to create EIT images from recorded frames, the so-called reconstructing of images is carried out. Reconstruction algorithms try to solve the issue that causes EIT which is the determination of the conductivity distribution inside the thorax on the basis of the voltage measurements that have been collected at electrodes on the thorax surface. In the beginning, EIT reconstruction assumed that electrodes were placed on an ellipsoid, circular or circular plane, whereas newer algorithms incorporate information about the anatomical form of the thorax. In the present, there are three main algorithms used for EIT: the Sheffield back-projection algorithm [ as well as the finite-element method (FEM) with a linearized Newton–Raphson algorithm ] and the Graz consensus reconstruction algorithm for EIT (GREIT) [10are commonly used.

In general, EIT images are comparable to a two-dimensional computed tomography (CT) image. These images are normally rendered so that the user looks from caudal to cranial when analyzing the picture. Contrary to a CT image EIT images are not a two-dimensional image. EIT image doesn’t show a “slice” but an “EIT sensitivity region” [1111. The EIT sensitivity region is a lens-shaped intra-thoracic volume with impedance-related changes that contribute to the EIT images [11It is a lens-shaped intra-thoracic volume that contributes to the generation. The shape and size of the EIT sensitization region is determined by the dimensions, the bioelectric propertiesand appearance of the Thorax with the type of current injection and voltage measurement pattern [12The shape and thickness of the EIT sensitivity region is determined by the voltage measurement pattern [.

Time-difference Imaging is a method that is employed in EIT reconstruction to show changes in conductivity and not the pure conductivity amounts. In a time-difference EIT image compares changes in impedance to the baseline frame. This gives the possibility to trace time-varying physiological phenomena such as lung respiration and perfusion [22. Color coded EIT images is not uniform but usually displays the change in the impedance of the patient to a standard (2). EIT images are typically colored using a rainbow color scheme with red indicating the greatest in relative intensity (e.g. when inspiration occurs), green a medium relative impedance and blue the lowest impedance (e.g. for expiration). In clinical settings one option to consider is using color scales which range from black (no impedance changes) through blue (intermediate impedance changes) and white (strong impedance changes) to code ventilation , or from black to white and red up to mirror-perfusion.

2. Different color codings for EIT images as compared to CT scan. The rainbow-color scheme utilizes red for the highest absolute impedance (e.g. when inspiration occurs) Green for a medium relative impedance, and blue, for the lowest ratio of impedance (e.g. at expiration). Newer color scales utilize instead of black (which has no impedance changes) Blue for an intermediate change in impedance, and white for the most powerful impedance change.

4. Functional Imaging and EIT Waveform Analysis

Analysis of Impedance Analyzers data is based on EIT waves that are generated in individual image pixels in an array of raw EIT images over long periods of (Figure 3). The term “region of interest” (ROI) is a term used to represent activity within individual pixels in the image. In each ROI the waveform shows variations in the conductivity of the region over time as a result of breathing (ventilation-related signal, also known as VRS) as well as cardiac activity (cardiac-related signal CRS). Additionally, electrically conducting contrast agents such as hypertonic sodium can be used to produce the EIT pattern (indicator-based signal IBS) which may be related to perfusion in the lung. The CRS could originate from both the heart and lung region and could be attributable to lung perfusion. Its exact origin and composition isn’t fully understood 1313. Frequency spectrum analysis has been used to distinguish between ventilationas well as cardiac-related changes in impedance. Impedance changes outside of the periodic cycle could be caused by changes in ventilator settings.

Figure 3. EIT forms and the functions of EIT (fEIT) photos can be derived from original EIT images. EIT waveforms can be defined by pixel or on a particular region that is of particular interest (ROI). Conductivity variations are caused by breathing (VRS) and cardiac activities (CRS) but they can also be induced artificially, e.g. using the injection of bolus (IBS) to measure perfusion. FEIT images are a visual representation of regional physiological parameters including ventilation (V) and blood flow (Q) as extracted from the raw EIT images by applying an algorithmic operation over time.

Functional EIT (fEIT) images are generated by applying a mathematical operation on the sequence of raw pictures and the corresponding pixel EIT spectrums. Since the mathematical procedure is used to calculate the physiologically relevant parameters for each pixel. The regional physiological parameters like regional respiration (V), respiratory system compliance as in addition to regions perfusion (Q) can be determined as well as displayed (Figure 3.). The data obtained from EIT waveforms along with simultaneously registered airway pressure measurements can be utilized to calculate lung compliance and the rate of lung opening and closing for each pixel through changes in pressure and impedance (volume). The comparable EIT measurements of stepwise inflation and deflation of the lungs allow the displaying of volume-pressure curves at a pixel level. Based on the mathematical operation, different types of fEIT photos could address different functional properties for the cardio-pulmonary system.