Electrical Impedance Tomography for Cardio-Pulmonary Monitoring

Abstract

Electrical Impedance Tomography (EIT) is a bedside monitor that can be used to visualize the local airflow and , possibly, lung perfusion distribution. The article discusses and discusses the clinical and methodological aspects of the thoracic EIT. Initially, researchers were concerned about the validation of EIT for measuring regional airflow. These studies focus on its clinical applications to measure lung collapse, increased tidal flow, and lung overdistension, in order to determine positive end-expiratory pressure (PEEP) and Tidal volume. In addition, EIT may help to detect pneumothorax. Recent studies evaluated EIT as a way to assess regional lung perfusion. Indicate-free EIT measurements could be adequate to continuously measure the heart stroke volume. A contrast agent such as saline could be necessary to evaluate the regional lung perfusion. Thus, EIT-based monitoring of regional respiration and lung perfusion can reveal local perfusion and oxygenation which may be useful in treating patients suffering from acute respiratory distress syndrome (ARDS).

Keywords: Electrical impedance tomography; bioimpedance; image reconstruction Thorax; regional circulation and regional perfusion monitoring.

1. Introduction

Electronic impedance transmission (EIT) can be described as a radiation-free functional imaging modality that allows non-invasive monitoring of bedside regional lung ventilation as well as arguably perfusion. Commercially available EIT devices were introduced for the clinical use of this technique and thoracic EIT is widely used in both pediatric and adult patients [ 1, ].

2. Basics of Impedance Spectroscopy

Impedance Spectroscopy may be described as the electrical response of biological tissue to externally applied alternating electricity (AC). It is usually measured with four electrodes, of which two are used for AC injection and the other two are for voltage measurement 3,,4. Thoracic EIT measures the regional variation of the intra-thoracic bioimpedance. It can be considered like an extension of four electrode principle to the image plane , which is covered through the electro belt [ 11. Dimensionally, electrical impedance (Z) is the same as resistance and the International System of Units (SI) unit is Ohm (O). It can be conveniently expressed as a complicated number, in which the real part is resistance, while the imaginary portion is called the reactance, which determines the effect of the inductance of capacitance. Capacitance depends on the biomembranes’ features of the tissues, such as ion channels, fatty acids, and gap junctions. Resistance is mainly determined by composition and the amount of extracellular fluid 1., 22. At frequencies below 5 kilohertz (kHz) the electrical current travels through extracellular fluids and is mostly dependent on the resistance characteristics of tissues. For higher frequencies that exceed 50 kHz, electrical impulses are a little deflected by cell membranes which leads to an increase in the tissue’s capacitive properties. When frequencies exceed 100kHz electricity can pass through cell membranes and reduce the capacitive portion 2[ 1, 2]. So, the results which determine the tissue’s impedance depend on the utilized stimulation frequency. Impedance Spectroscopy is usually given as conductivity or resistance, which compares conductance or resistance unit size and length. The SI units that correspond to it can be described as Ohm-meter (O*m) for resistivity and Siemens per meters (S/m) on conductivity. The resistance of the thoracic tissues ranges from 150 O*cm for blood up to 700 O*cm for deflated lung tissue, up to 2400 o*cm for the lung tissue that has been inflated ( Table 1). In general, tissue resistivity or conductivity is dependent on amount of fluid and the ion concentration. For the lung, it also is dependent on the quantity of air that is present in the alveoli. While the majority of tissues exhibit isotropic characteristic, the heart and the muscle in particular exhibit anisotropic properties, in which the degree of resistance depends on the direction that the measurement is made.

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 plan typically in the 4th through 5th intercostal space (ICS) in the parasternal line [55. The changes in the impedance of the lungs can be measured within the lower lobes in the right and left lungs, and also in the heart region ,21. To position the electrodes above the 6th ICS could be difficult because the abdominal and diaphragm occasionally enter the measurement area.

Electrodes are self-adhesive electrodes (e.g. electrocardiogram ECG,) which are placed with equal spacing between electrodes or are incorporated into electrode belts [ ,22. Also, self-adhesive electrodes are available for a more user-friendly application ,21,2. Chest wounds, chest tubes and non-conductive bandages as well as conductive wire sutures could block or significantly impact EIT measurements. Commercially available EIT devices typically use 16 electrodes, but EIT devices with 8 or 32 electrodes is also available (please refer to Table 2 for details) There are also 32 electrodes (please refer to Table ,2[ 1,2.

Table 2. The commercially-available electrical impedance (EIT) equipment.

In an EIT measure sequence, small AC (e.g. 5, microamps at 100 kHz) are applied to several electrode pairs and the results are then measured using the remaining other electrodes [ ]. Bioelectrical impedance that is measured between the injecting and the electrodes that are measuring is determined from the applied current and measured voltages. Most commonly adjacent electrode pairs are utilized for AC application in a 16 electrode system for example, while 32-elektrode systems generally use a skip pattern (see Table 2.) that increases the distance between current injecting electrodes. The resultant voltages are measured by using those remaining electrodes. At present, there is a debate ongoing about various current stimulation patterns and their distinct advantages and disadvantages [7]. To acquire a complete EIT data set that includes bioelectrical measurements, the injecting and the electrodes that measure are constantly rotationally positioned around the entire chest .

1. Current application and voltage measurements around the thorax with an EIT system that includes 16 electrodes. In only a few milliseconds as well as the voltage and current electrodes as well as activated voltage electrodes will be repeatedly moved across the upper thorax.

The AC that is used in EIT measurements are safe for use on body surfaces that is undetectable 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 which is recorded during one cycle that is recorded during one cycle of AC application is known as a frame and contains the voltage measurements required to create that Raw EIT image. Frame rate is the number of EIT frames recorded in a second. Frame rates that exceed 10 frames/s are required in order to monitor ventilation , and 25 images/s in order to monitor perfusion or cardiac function. Commercially available EIT equipment uses frames with a frame rate between 40 and 50 images/s as demonstrated in

To produce EIT images using recorded frames, the technique known as image reconstruction process is employed. Reconstruction algorithms seek to solve the reverse problem of EIT that is the recovery of the conductivity distribution within the thorax based upon the voltage measurements that have been made at the electrodes of the thorax surface. At first, EIT reconstruction assumed that electrodes were placed on an ellipsoid, circular or circular plane, while newer algorithms employ information on the anatomical contour of the thorax. The current algorithms include using the Sheffield back-projection algorithm along with the finite elements method (FEM) built on a linearized Newton and Raffson algorithm [ ] and the Graz consensus reconstruction algorithm for EIT (GREIT) [10are commonly used.

On the whole, EIT photographs are similar to a two-dimensional computed (CT) image: these images are usually rendered so that the operator looks at the cranial and caudal regions when analysing the image. In contrast to the CT image however, an EIT image does not show a “slice” but an “EIT sensitivity region” [11]. The EIT sensitization region is a lens-shaped intra-thoracic volume with impedance-related changes that contribute to EIT image generation [11]. Shape and thickness of the EIT area of sensitivity are dependent upon the dimensions, bioelectrical properties, and also the structure of the chest, as well as on the utilized voltage measurement and current injection pattern [1212.

Time-difference imaging is a technique which is employed for EIT reconstruction to show the changes in conductivity rather than Absolute conductivity values. An time-difference EIT image displays the change in impedance to a baseline frame. This provides the chance to trace time-varying physiological phenomena such as lung respiration and perfusion [22. The color coding of EIT images may not be uniform but usually displays the change in impedance in relation to a reference level (2). EIT images are typically coded using a rainbow-colored scheme with red representing the most significant value of relative imperf (e.g., during inspiration) and green for a middle relative impedance and blue the lowest impedance (e.g. during expiration). For clinical purposes, an interesting option is to use color scales that range from black (no change in impedance) and blue (intermediate impedance change), and white (strong impedance changes) to code ventilation , or from black, to white and red towards mirror perfusion.

2. Different color codes that are available for EIT images in comparison to CT scan. The rainbow color scheme uses red for the most powerful absolute impedance (e.g. during inspiration), green for a intermediate relative impedance, and blue for the lowest relative impedance (e.g. at expiration). Modern color scales make use of instead of black, which has no impedance changes) or blue to indicate an intermediate impedance shift, as well as white for the greatest impedance change.

4. Functional Imaging and EIT Waveform Analysis

Analyzing Impedance Analyzers data is done using EIT waveforms which are created in each image’s pixels an array of raw EIT images over time (Figure 3.). A region of interest (ROI) is a term used to describe activity in the individual pixels in the image. Within each ROI the waveform shows changes in regional conductivity over time as a result of either respiration (ventilation-related signal, also known as VRS) or heart activity (cardiac-related signal, CRS). Additionally, electrically conductive contrast-agents such as hypertonic salinity can be used to get an EIT waveshape (indicator-based signal IBS) and is linked to perfusion in the lung. The CRS may originate from both the lung and the cardiac region and may be partly attributed to lung perfusion. The exact source and composition are incompletely understood [ 13]. Frequency spectrum analysis is commonly employed to distinguish between ventilationand cardiac-related impedance variations. Impedance changes that aren’t periodic may result from changes in settings for the ventilator.

Figure 3. EIT waveforms and functional EIT (fEIT) pictures originate from the original EIT images. EIT waveforms can be identified by pixel or on a particular region or region of interest (ROI). Conductivity changes are a natural result of breathing (VRS) or heart activity (CRS) however, they can also be created artificially, e.g. using IBS (IBS) for measuring perfusion. Images of fEIT show local physiological parameters like perfusion (Q) and ventilation (V) and perfusion (Q) taken from raw EIT images using an algorithmic process over time.

Functional EIT (fEIT) images are created by applying a mathematical operation on the raw images and the corresponding pixel EIT spectrums. Because the mathematical process is used to determine an appropriate physiological parameter for each pixel, regional physiological aspects like regional ventilation (V), respiratory system compliance as also respiratory system compliance as well as regional perfusion (Q) can be assessed to be displayed (Figure 3.). The data from EIT waves and simultaneously registered pressures of the airways can be used to determine the lung compliance and lung closing and opening times for each pixel using changes in pressure and impedance (volume). Comparable EIT measurements taken during gradual inflation and deflation of the lungs allow the displaying of pressure-volume curves on scales of pixel. Based on the mathematical process, different types of fEIT images could be used to analyze different functions within the cardio-pulmonary systems.