Electrical Impedance Tomography for Cardio-Pulmonary Monitoring

Abstract

Electrical Impedance Tomography (EIT) is an instrument for monitoring bedside that visually examines the local environment and perhaps lung perfusion. The article discusses and discusses the methodological and clinical aspects of the thoracic EIT. Initially, researchers focused on the validity of EIT to measure regional ventilation. The current research focuses on clinical applications of EIT to measure lung collapse an increase in tidal volume, and 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 tool to measure regional lung perfusion. Indicator-free EIT tests could be enough for continuous measurement of cardiac stroke volume. The use of a contrast agent, such as saline, might be required to assess the regional perfusion of the lungs. Therefore, EIT-based monitors of regional ventilation and lung perfusion may visualize local ventilation and perfusion matching that could prove useful in the treatment of patients suffering from chronic respiratory distress syndrome (ARDS).

Keywords: electrical impedance tomography bioimpedance; image reconstruction Thorax; regional adrenergic and regional perfusion monitoring.

1. Introduction

Electric impedance tomography (EIT) is one of the non-radiation functional imaging modality that permits non-invasive bedside monitoring of regional lung ventilatory and perhaps perfusion. Commercially available EIT devices were introduced for clinical application of this technique and the thoracic EIT is safe for both pediatric and adult patients 1, 1 2.

2. Basics of Impedance Spectroscopy

Impedance Spectroscopy can be defined as the electrical response of biological tissue to an externally applied alternating electricity (AC). It is typically achieved by using four electrodes, of which two are used to inject AC injection and the other two are used to measure voltage 3,3. 4. Thoracic EIT measures the regional variability of Impedance Spectroscopy in the thoracic area and could be seen by extending the principle of four electrodes to the image plane , which is covered by an electrode belt 11. Dimensionallyspeaking, electrical impedance (Z) is the same as resistance and its equivalent International System of Units (SI) unit is Ohm (O). It can be easily expressed as a complicated number, in which the real component is resistance while the imaginary part is called the reactance, which is the measurement of effects caused by capacitors or the effect of inductance. The capacitance of a cell is determined by the biomembranes’ properties of the tissues, such as ion channels and fatty acids as well as gap junctions. The resistance is mostly determined by the composition and quantity of extracellular fluid [ 1, 22. When frequencies are below 5 kilohertz (kHz) the electrical current circulates through extracellular fluids and is heavily dependent on the properties of the resistive tissues. At higher frequencies up to 50 kHz, electrical impulses are slightly diverted at the cell membranes resulting in an increase in capacitive tissues properties. At frequencies above 100 kHz the electrical current is able to pass through cell membranes, and diminish the capacitive component [ 22. Therefore, the effects which determine the tissue’s impedance depend on the used stimulation frequency. Impedance Spectroscopy is typically described as conductivity or resistance, which will normalize conductance or resistance in relation to units’ area and length. The SI units of equivalent comprise Ohm-meter (O*m) for resistivity and Siemens per meters (S/m) on conductivity. The resistance of thoracic tissue varies from 150 O*cm when blood is present as high as 700 O*cm with lung tissue that is deflated, all the way between 2400 and 2400 O*cm of air-filled lung tissue ( Table 1). In general, the tissue’s resistance or conductivity is a function of quantity of fluid in the tissue and the concentration of ions. When it comes to the lungs, it also depends on the volume of air inside the alveoli. While most tissues exhibit anisotropic behaviour, the heart and skeletal muscle behave anisotropic, this means that resistivity is heavily dependent on the direction that you measure it.

Table 1. The electrical resistivity of the thoracic tissues.

3. EIT Measurements and Image Reconstruction

To carry out EIT measurements electrodes are placed around the chest in a transverse line generally in the 4th to the 5th intercostal space (ICS) near an angle called the parasternal line]. As a result, changes in impedance can be assessed in the lower lobes of both the left and right lungs and also in the heart region [ ,21 2. Placing the electrodes beneath the 6th ICS might be difficult as the diaphragm as well as abdominal content are frequently inserted into the measurement plane.

Electrodes can be self-adhesive or single electrodes (e.g. electrocardiogram, ECG) that are positioned individually with equal spacing between electrodes, or are integrated into electrode belts ,22. Self-adhesive lines are available for a more user-friendly application ,2]. Chest wounds, chest tubes and non-conductive bandages as well as conductive sutures for wires can greatly affect EIT measurements. Commercially available EIT equipment typically uses 16 electrodes. However, EIT systems with 8 or 32 electrodes are also available (please check Table 2 for information) (see Table 2 for details). ,2].

Table 2. Available electrical impedance (EIT) devices.

In an EIT test, low AC (e.g. five mA at a frequency of 100 kHz) are applied to various electrode pairs. The generated voltages are measured with the remaining electrodes 6. Bioelectrical Impedance between the injecting and the electrode pairs that measure is calculated using the applied current and the measured voltages. Most commonly connected electrode pairs are used for AC application within a 16-elektrode configuration for example, while 32-elektrode systems generally utilize a skip-pattern (see Table 2.) to increase the distance between the current injecting electrodes. The resulting voltages can be measured by using one of the other electrodes. Currently, there is a constant debate regarding different types of current stimulation and their unique advantages and disadvantages [7]. To obtain a full EIT data set that includes bioelectrical measurements as well as the injecting and electrode pairs used for measuring are constantly moved around the entire thorax .

1. Measurements of voltage and current around the thorax by using an EIT system with 16 electrodes. Within milliseconds each of the electrodes for current and an active voltage electrode get rotating around the thorax.

The AC that is used in EIT tests are safe to use on the body and remain undetected 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 recorded during one cycle of AC programs is termed a frame . It is comprised of the voltage measurements needed to produce what is known as the initial EIT image. Frame rate is the number of EIT frames recorded in a second. Frame rates of at least 10 images/s are necessary for monitoring ventilation and 25 images/s to monitor heart function or perfusion. Commercially accessible EIT devices use frame rates ranging from 40 to 50 images/s, as depicted in

To produce EIT images from recorded frames, an algorithm known as image reconstruction technique is used. Reconstruction algorithms try to solve the other aspect of EIT that is the determination of the conductivity distribution within the thorax, based on the voltage measurements taken at the electrodes on the thorax surface. In the beginning, EIT reconstruction assumed that electrodes were placed on an ellipsoid, circular or circular plane, but more recent algorithms make use of information regarding the anatomical form of the thorax. The current algorithms include an algorithm called the Sheffield back-projection algorithm and the finite element algorithm (FEM) that is a linearized Newton-Raphson algorithm [ ] and the Graz consensus reconstruction algorithm for EIT (GREIT) [10typically used.

A lot of the time, EIT images are comparable with a two-dimensional computed (CT) image. These images are conventionally rendered so that the viewer is looking from cranial to caudal while analyzing the picture. In contrast to CT images, unlike a CT image, an EIT image doesn’t show an image “slice” but an “EIT sensitivity region” [11]. The EIT sensitive region is a lens-shaped intra-thoracic area where impedance fluctuations contribute to the EIT image generation [1111. Shape and thickness of the EIT sensitivity region are dependent on the dimensions, the bioelectric properties, and the shape of the thorax as according to the particular current injection and voltage measurement pattern [12(13, 14).

Time-difference imaging is a technique that is used for EIT reconstruction to show variations in conductivity, not relative conductivity of the levels. In a time-difference EIT image displays the change in impedance to a base frame. It is an opportunity to study the underlying physiological phenomenon that changes over time like lung ventilation or perfusion [22. Color coding of EIT images isn’t unified but generally displays the change in the impedance of the patient to a standard (2). EIT images are generally coded using a rainbow-color scheme with red representing the greatest relative impedance (e.g., during inspiration) while green is a moderate relative impedance while blue is the lowest impedance (e.g. for expiration). For clinical purposes the best option is to utilize color scales that range from black (no change in impedance) to blue (intermediate impedance change), and white (strong impedance changes) to code ventilation or between black and red, and white towards mirror perfusion.

2. Different available color codings of EIT images in comparison to CT scan. The rainbow-color scheme is based on red for the most powerful percentage of the relative imperceptibility (e.g. during inspiration) and green for a moderate relative impedance, blue is the color that has the lowest relative intensity (e.g. at expiration). A more recent color scale uses instead of black for no impedance change) or blue to indicate the intermediate impedance change and white for the most powerful impedance change.

4. Functional Imaging and EIT Waveform Analysis

Analysis of Impedance Analyzers data is based on EIT waveforms created in the individual pixels of the raw EIT images that are scanned over duration (Figure 3). Region of Interest (ROI) is a term used to show the activity of individual pixels of the image. In any ROI, the image shows changes in the regional conductivity over time resulting from ventilatory activity (ventilation-related signal, also known as VRS) or heart activity (cardiac-related signal CRS). In addition, electrically conductive contrast agents like hypertonic saline could be used to produce an EIT pattern (indicator-based signal IBS) and can be linked to perfusion in the lung. The CRS could come from both the lung as well as the cardiac region, and is possibly linked to lung perfusion. The exact source and composition isn’t fully understood 1313. Frequency spectrum analysis can be utilized to differentiate between ventilationand cardiac-related impedance fluctuations. Impedance fluctuations that are not frequent can result from modifications in the settings of the ventilator.

Figure 3. EIT Waveforms as well as functional EIT (fEIT) image are derived from the unprocessed EIT images. EIT waves can be described by pixel or on a particular region which is of concern (ROI). Conductivity variations are caused by the process of ventilation (VRS) (or cardiac activity (CRS) however they could be produced artificially e.g. or through the injection of bolus (IBS) for the purpose of measuring perfusion. FEIT images are a visual representation of specific physiological parameters of the region, such as ventilation (V) or perfusion (Q), extracted from raw EIT images by using a mathematical procedure over time.

Functional EIT (fEIT) images are generated through the application of a mathematical algorithm on an array of raw images and the corresponding pixel EIT signal waveforms. Because the mathematical process is applied to calculate the physiologically relevant parameters for each pixel, regional physiological features like regional ventilation (V) and respiratory system compliance, as in addition to region-wide perfusion (Q) can be determined in a visual display (Figure 3). The data drawn from EIT waveforms and concurrently recorded pressures of the airways can be used to calculate the lung compliance and lung opening and closing in each pixel using the changes of pressure and impedance (volume). Comparable EIT measurements during the deflation and inflation of the lungs can be used to display of curves representing volume and pressure at an individual pixel. Based on the mathematical operation, different types of fEIT images might address distinct functional characteristics that are associated with the cardiovascular system.