Currently, there are three main fields in medicine where electrical properties of biological tissues are measured and used:
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differentiation of tissues and detection of pathologic processes in tissues on the base of impedance at different frequencies, i.e. impedance spectroscopy
- analysis of organ functioning (e.g. assessment of cardiac output), i.e. impedance cardiography
- visualization of internal structure of the body, i.e. impedance tomography.
1. Impedance Spectroscopy
Electrical impedance spectroscopy is a method of medical diagnostics, which uses values of electrical conductivity or permittivity of a part of body received by way of superficial electrical measurements. Electrical impedance is the term used to signify resistance of an object to current, which has frequency-dependent and frequency-independent properties. Its distinguishing feature is the use of wide-band equipment (up to 1 GHz).
Some cancer cells demonstrate changes of frequency-dependent impedance, which differs significantly from impedance of healthy cells. Therefore, measurement of electrical impedance on different frequencies helps to diagnose tumours.
The chart above shows that mean conductivity of tumour tissue is significantly higher than that of normal tissue on the whole range of frequencies (from 10 Hz to 1MHz).
2. Impedance Cardiography
The method allows to perform non-invasive measurements of blood flow via measurement of electrical conductivity of chest. The main principle is registration of changes in conductivity due to blood output during systole. Change of impedance reflects the change of volume and rate of blood flow in aorta during systole and diastole. Changes of impedance values are used for the calculation of hemodynamic parameters.
Typical impedance cardiographic waves accompanied by electrocardiographic (ECG) signal, which is being registered simultaneously.
Some areas of application of impedance cardiography
Obstetrics
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Monitoring of normal pregnancy
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Monitoring of high-risk pregnancy
Cardiology
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Assessment of cardiac function
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Assessment of hypertension
Critical conditions
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Pulmonary edema
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Heart failure
Nephrology
- Monitoring during hemodialysis
- Assessment of hypertension
Outpatient application
Review of impedance cardiography has revealed its advantages, current and future application and approach to treatment of arterial hypertension (R. Nazário Leão, P. Marques Da Silva Impedance Cardiography in the Evaluation of Patients with Arterial Hypertension. International Journal of Cardiovascular Sciences. 2019;32(1)61-69).
Impedance cardiography is an important non-invasive method allowing to receive comprehensive information about cardiac function (Berntson, Quigley & Lozano, 2007, S. 200).
Impedance cardiography should be the method of choice for functional diagnostics of cardiovascular system of pregnant women (А. Karpov, I. Ilyin, М. Korotkova. Preeclampsia Amplitude-time Characteristics for ΔZ/Δt Curve. World Congress on Medical Physics and Biomedical Engineering, Munich, Germany, 2009).3. Impedance Tomography
The term ‘tomography’ implies reconstruction of internal distribution of multiple external points in such a way so that to ensure layer-by-layer visualization of the object. Electrical impedance tomography is a medical visualization technique, the purpose of which is to create medical image of human body by way of external scanning. Similar to MRI and ultrasonography electrical impedance tomography is non-invasive technology of imaging. And just like magnetic induction tomography, electrical impedance tomography measures electromagnetic phenomena and their changes. Electrical impedance tomography uses the difference in electrical conductivity of different body tissues in order to recreate the image of distribution of electrical conductivity within the borders of a certain body part. Cerebrospinal fluid and blood have minimum electrical impedance. Adipose tissue and bone tissue have the maximum values of electrical impedance. Impedance of other tissues has intermediary values.
Electrical impedance tomographic image of chest. Spinal column with isolator properties, respiratory area and pathways are clearly visualized.
Some areas of application of electrical impedance tomography systems:
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Monitoring of lung ventilation in emergency rooms
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Optimization of ventilation during anesthesia
Assessment of pulmonary edema with newborns and adults with heart failure