Development of Electrical Impedance Technique for Assessment of Lung Function

Abstract

The Pneumothorax, Pulmonary Edema, Asthma, Acute Respiratory Distress Syndrome (ARDS), Chronic Obstructive Pulmonary Disease (COPD), Pneumonia, Influenza, Lung Cancer are the most common lung diseases. Lung diseases are the major public health burden all over the World including Bangladesh. According to the World Health Organization (WHO, 2020) lung diseases are the third leading causes of death in Bangladesh. People with Lung diseases become economic burden because they cannot perform their work properly and diagnostic and treatments cost a huge amount of money. X-rays, CT scan, Magnetic Resonance Imaging (MRI) are the existing techniques for the diagnosis of lung diseases. X-ray and CT scan are very harmful due to radiation. MRI has high resolution but it is very expensive and non-portable diagnostic system. As Lungs contain large volume of airs, so Ultrasound (US) imaging cannot be used for diagnosis of lung diseases. Due to Anatomical and Physiological changes the Electrical Conductivity, Relative Permittivity and Capacitance of cell membrane do change due to lung diseases. In case of Pulmonary Edema (water in the lung) and the Pneumothorax the electrical conductivity and relative permittivity values do change by large amount than that of a healthy lung. As Electrical current at 1mAat 50 KHz to 200 KHz has no known harmful effects on tissues. Therefore, Electrical Impedance Technique (EIT) method can be used for the assessment of various lung functions and lung diseases as a good complementary to other imaging techniques, though its resolution is low. The most used Electrical Impedance Technique (EIT) uses two surface electrodes for injecting a small amount of current 1 mA at 50 kHz and other surface electrodes are used for voltage sensing. There are four existing EIT Protocols for current injection and voltage measurements are namely-(P1) adjacent current drive, (P2) opposite current drive at the vertices of the semi-major axis of the chest cross section, (P3) opposite current drive at the vertices of the semi-minor axis of the chest cross section, (P4) opposite current drive along the right lung. In these existing EIT protocols electrodes are placed on the chest cross section at horizontal plane, the current drive electrodes and voltage sensing electrodes are at the same plane. The Proposed electrical impedance measurement protocol for lung function is Anterior-Posterior Electrical Impedance Technique (APEIT), where two current drive electrodes are placed at the anterior and posterior side of lungs and other eight voltage sensing electrodes at equal spacing in ellipsoid shape at vertical plane at the posterior side following the lung shape and size of lung. vii To see the sensitivity of the above five (5) electrical impedance imaging protocols (P1, P2, P3, P4, and APEIT) of computer simulation is conducted by COMSOL Multiphysics software for small current of 1 mA at 50 kHz for assessing the lung functions at two different conditions like, lung at inspiration and at expiration for healthy lungs. In the computer simulation studies the Relative Electric Potential Change (REPC) values were computed for eight voltage sensing electrodes to see the sensitivity at inspiration and expiration of healthy lungs for all (5) protocols. The average REPC values were 1.41%, 1.84%, 1.34%, 2.29%, and 3.24% for P1, P2, P3, P4, and APEIT EIT protocols respectively. The highest average REPC value for the APEIT protocol clearly indicates that this protocol is the most sensitive among all existing protocols for electrical impedance measurement of lung functions. It was found the sensitivity of eight voltage sensing electrodes are more smoothly distributed than all other EIT protocols. In both counts the proposed APEIT protocol is the best amongst all the protocols for electrical impedance measurement and imaging for lung functions. The average REPC values were also computed for all above five (5) EIT protocols for the following lung conditions:(a) lung at inspiration and pneumothorax, (b)lung at expiration and pneumothorax(c) lung at inspiration and pulmonary edema, and (d) lung at expiration and pulmonary edema. The average REPC values for lung at inspiration and pneumothorax of the mentioned five (5) protocols were: 2.97%, 3.89%, 2.41% ,4.26% and 5.97% respectively. For lung at expiration and pneumothorax the average REPC values were: 1.59 %, 2.10 %, 1.09 %, 2.02% and 2.82 % respectively. The average REPC values for lung at inspiration and pulmonary edema were: 4.50 %, 5.99 %, 3.21 %, 5.80 % and 8.07 % respectively. For lung at expiration and pulmonary edema the average REPC values were: 3.15 %, 4.24 %, 1.90 %, 3.59 % and 4.99 % respectively. Above results clearly indicates that the above average REPC values in all cases the proposed APEIT protocol is the most sensitive than all other existing protocols for electrical impedance measurement and imaging for lung functions and lung diseases. Computer simulation was also conducted for calculating the distinguishing factor for the proposed APEIT protocol for different sizes of tumor like,5%, 10%, and 20% of lung volume at the center of the right lung at inspiration and at expiration. Distinguishing factors indicates whether a disease lung condition like a tumor can be detected. The viii distinguishing factors were 4.18%, 8.57% and 20.79 % for lung at inspiration, and 4.17%, 8.46% and 16.90 % for lung at expiration for 5%, 10%, and 20% of tumor sizes respectively. In both cases even a small size of the tumors less than 5% of lung volume can be detect by using the proposed APEIT protocol. Computer simulation also been conducted for Electrical impedance imaging spectroscopy i.e. imaging at multiple frequencies namely: 20 kHz, 50 kHz, 100 kHz, 150 kHz, 200 kHz for lung at inspiration for fixed electrical conductivity values of : 0.0824 Sm-1 ,0.0927 Sm-1 ,0.103 Sm-1 , 0.1133 Sm-1 , and 0.1236 Sm-1 (i.e. 0.103 Sm-1 ± at steps 10%) and at expiration for fixed electrical conductivity value of : 0.2096 Sm-1 , 0.2358 Sm-1 , 0.2620 Sm-1, 0.2882 Sm-1 and 0.3144 Sm-1 ( i.e. 0.2620 Sm-1 ± steps of 10%) to see the sensitivity to the proposed APEIT protocol to be used as EIT Spectroscopy. The sum of the eight surface electrodes potential for lung at inspiration decreased almost linearly from 591.98 mV to 562.82 mV for electrical conductivity of 0.0824 Sm-1,589.90 mV to 561.20 mV for electrical conductivity of 0.0927 Sm-1, 587.97 mV to 559.63 mV for electrical conductivity of 0.103 Sm-1, 586.17 mV to 558.17 mV for electrical conductivity of 0.1133 Sm-1 and 584.50 mV to 556.75 mV for electrical conductivity of 0.1236 Sm-1 for the range of frequency: 20KHz to 200KHz. In case of lung at expiration the electric potential decreased almost linearly from 573.60 mV to 545.84 mV for electrical conductivity of 0.2096 Sm-1; 571.11 mV to 543.81 mV for electrical conductivity of 0.2358 Sm-1, 568.87 mV to 541.97 mV for electrical conductivity of 0.2620 Sm-1, 566.87 mV to 540.27 mV for electrical conductivity of 0.2882 Sm-1, 565.07 mV to 538.71 mV for electrical conductivity of 0.3144 Sm-1 for the same range of frequency. The simulation results show that the proposed APEIT protocol can also be used in EIT spectroscopy to identify the different lung diseases. The sum of eight surface electrodes potential values of lung at inspiration condition have higher value than at expiration condition, For phantom study the Maltron Bio-Scan 920-11 system is used for electrical impedance measurements and for that a human thorax phantom was designed and made, following the dimensions of multipurpose digital chest phantom (Ref. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4169876). The proposed Anterior- Posterior Electrical Impedance Technique (APEIT) protocol was used for a single channel with four electrodes of Maltron Bio-Scan 920-11 system, where two driving electrodes are used for injecting 1 mA alternating current at 50 kHz and another two electrodes are used for electrical potential measurement. The different conductive material and having ix ellipsoidal shape with porous foam representing lung (2.5cm × 9cm× 22 cm) was inserted at 8cm (fixed) away from the frontal of the chest surface in the phantom. Saline water having concentration of 0.54%, 0.72%, 0.90%, 1.08 %, 1.26% of NaCl, the corresponding electrical conductivities are: 0.952 Sm-1, 1.210Sm-1,1.520Sm-1, 1.858Sm-1, 2.162 Sm-1 were used in phantom study to simulate the different conductive tissues of lungs. Eight voltage sensing electrodes potentials were computed and found that they have distinct electric potentials and the sum of the eight voltage sensing electric potential were 120.40 mV to 54.96 mV at 50 kHz frequency. Electrical conductivity (Sm-1) for different concentrations of NaCl (%) in Phantom solutions have linear relationship and gave a straight line through origin. The sum of eight voltage sensing electrodes electrical potentials (mV) against the above mentioned concentrations of NaCl (%) in Phantom solutions or its corresponding electrical conductivity (Sm-1) is negatively correlated, which is similar to computer simulation studies of the proposed Anterior- Posterior Electrical Impedance Technique (APEIT) protocol. The proposed Anterior- Posterior Electrical Impedance Technique (APEIT) protocol found the most sensitive method for measurements of Electrical Impedances for the assessment of the lungs functions and diseases. However, it needs preclinical and clinical trials before practical use, in clinics and hospitals.