There are several approaches to measure bio impedance that should be chosen depending on the desired characteristics of the system built.
Null Techniques: Detection with this technique is very simple and it is based in a simple ampere meter. The most common method used is a Wheatstone bridge. This is a high accurate method with the inconvenient of needing a large number of electronic components and not being time efficient in some applications because of its iterating process.
Deflection Techniques: The impedance estimation is done by measuring the voltage drop or the current through the load as a response to an alternative known current. This method is based on simple electronics using complex operations. Due to measurements of bio impedance and instrumentation, in this application a specific integrated circuit or microcontroller is needed to carry these operations. Its main characteristic is the time efficient, being able to do short time accuracy measurements.
For the impedance estimation we are going to focus only in the deflection techniques that are those that are commonly used in bio impedance measurements. In impedance estimation is it possible to do single frequency and multi-frequency analysis using different techniques, basing the study in the known excitation provided and the measured obtained. So for single frequency or sweep frequency measurements usually Sine Correlation is used and for multi-frequency measurements it is common used the Fourier Transform.
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Symptoms of lung edema usually appear at a very late stage when the amount of fluid in the lungs has already sextupled.
At this stage, the patient typically has dyspnea (short breath) and hypoxia (low partial pressure of oxygen in the blood) because of impaired gas exchange due to the increased interstitial fluid volume. As a consequence, the patient is usually committed to an intensive care unit for medical treatment. Preclinical detection and continuous monitoring of the lung fluid volume during medical treatment would enhance medical care while reducing costs.
Unfortunately, conventional detection methods like radiographic imaging, monitoring pulmonary capillary wedge pressure or double indicator thermodilution are impractical for continuous monitoring. A promising alternative is the Bio Impedance Spectroscopy (BIS). Detection of lung edema using BIS is based on the fact that the amount of fluid in the lungs has significant impact on their electrical impedance. Normally, the lungs have about 5% of fluid and 95% of air, resulting in an electrical impedance in the range of about 10 to 20 Ohm. If the amount of fluid in the lungs increases, electrical impedance decreases because of the much lower electrical impedance of fluid (serum, for example, has a resistance of about 0.6 Ohm).
Compared to other measurement methods, BIS has two significant advantages: the measurement is non-invasive and it can easily be done at the patient’s home. In previous work, BIS was shown to be practical for detection of lung edema and, in a single-frequency version, it is used in some ICDs (implantable cardioverter-defibrillator).
Electrical Bio impedance (EBI) through transthoracic measurements offers a non-invasive option to measure hemodynamic values. In the last decades, advances in the field of electrodes, electronic hardware and the development of textrodes paved the way for smaller, easier-to-use equipment for obtaining EBI measurements.
Impedance Cardiography is the study of the bio impedance of the human body with measurement area limited to the chest cavity. Changes in the bio impedance signal can be monitored to estimate cardiac parameters and assess the cardiac cycle, with monitoring possible by using non-invasive measuring methods using skin electrodes.
Bio Impedance Plethysmography is a method of studying tissue volume changes in a living body by measuring the changes of electrical impedance at the body surface. Impedance Cardiography (ICG) is the Impedance Plethysmography measurement limited to the chest/thorax region.
Electrocardiography (EKG) is the process of recording all the electric excitation phenomena of the heart. The EKG records with a good level of detail the electrical excitation in different parts of the heart muscle: which include the sinus and AV nodes, the atrial and ventricular muscles and nerve fibers in the ventricular muscle - it is crucial in diagnosing several heart diseases. The EKG signal shows a good relation with the ICG signal as some of the waves of the EKG are correlated with waves in the ICG measurement.
As with EKG, ICG measurements can potentially be done non-invasively. Several authors have attempted to correlate invasive methods such as thermo-dilution with ICG with skin electrodes with varying degrees of success. Most of these authors agree that ICG requires more methodological investigation but the measurements are good enough to estimate changes of the Stroke Volume.
The impedance cardiography signal is obtained by injecting a current and then sensing the voltage in the thorax region to estimate the impedance using the Ohms law equation.
The typical current used to measure the impedance is a sinusoidal signal with a constant frequency between 20 and 150 kHz with a constant amplitude of 0.5-5 mA. The impedance usually consists of a Real component, with values between 15Ohm to 30 Ohm, and an often negligible and not used imaginary component. The changes of impedance due to the changes of fluid in the chest area are typically between 100-400 mOhm which results in a change of 0.1-0.4 mV in the signal received when applying 1mA to the patient.
Depending on the type of information to be analyzed, several types of measurements may be taken.
Non-phase BIA measurement: This method measures the total resistance of the body (Z). This does not give determination of the phase angle and, as such, a subdivision of impedance into water and cellular resistance, so that no judgment can be made about the body cell mass or the extra-cellular mass with non-phase measurements .
Phase BIA measurement: The phase sensitive technique of measuring allows the impedance Z to be differentiated into its two components resistance (R), that shows the water resistance, and reactance (Xc), that show the cell resistance. This let us differentiate between the body cell mass and extra-cellular mass.
Phase BIA multi-frequency measurement: BIA is frequency dependent, therefore for a more intensive study we can study the resistance and reactance in a different single low frequencies measurements (1-5 KHz). With this type of measurement it is possible to achieve a subdivision of the total body water into intra-cellular and extra-cellular body water.
Electrical Bio Impedance measurements have been used for many years to study the electrical properties of biologic tissue and to measure physiological events, being applied in several clinical areas, including body composition. The assessment of oedema formation from EBI measurements is based on the dependency of the electrical properties of tissue on its structure and intrinsic constituents, i.e. alterations in the tissue structure produce a modification of its electrical properties.
An immediate physiological consequence of impaired renal function in the kidneys is an excessive accumulation of liquid in the rest of the body, consequently forming an oedema - considered by physicians to be a common indicator of renal failure. Therefore visual inspection by the physician is a regular practice to detect kidney function impairment. Such an inspection is usually performed targeting the limbs, legs and arms, targeting the inspection for peripheral oedema.
There are several methods of assessing extracellular swelling, and the EBI measurement approach is one of the more comfortable ones, due to the fact that the electrical properties of tissue can be measured non-invasively and without tightening the skin. Some studies about the excess of fluid in chronic HD patients, using bio impedance spectroscopy measurements, where performed. The experimental results suggested that HD patients keep their excess fluid volume primarily in the extracellular compartment (interstitial fluid). Bio impedance spectroscopy together with a stable measurement of lean tissue can determine the degree of relative excess hydration. Due to the capacitance effects of the cell membrane, the tissue impedance depends on the measurement frequency. As a consequence, the accuracy of measurements by means of multiple frequency Bio impedance spectroscopy analysis is superior to the accuracy of measurements based on a single frequency for the prediction of extracellular water. Some authors suggest that the best frequency range to assess extra-cellular fluid is up to 10 kHz and the range between 50-100 kHz is a suitable measurement range for a successful assessment of extra- and intra-cellular fluid.
Concluding, the monitoring principle of the EBI measurement system implemented in this thesis work lays as follows: during renal failure the amount of interstitial fluid in the limbs increases, causing extracellular oedema. The consequent interstitial swelling modifies the electrical properties of the tissue, and by means of the combination of non-invasive EBI measurements with skin-surface electrodes and EBI spectroscopy analysis, the ongoing swelling can be detected. Therefore changes in the EBI of the limbs may be used as an efficient indicator for early detection of renal failure.
Electrical impedance is the opposition to the flow of an electrical current, being the ratio between an alternating sinusoidal voltage and an alternating sinusoidal flow. In consequence, impedance is a passive magnitude that does not irradiate energy, therefore energy must be provided in two ways: by exciting the tissue with current or with voltage. In this case, the EBI measurement method used consists of injecting an electrical signal with a known current and measuring the reciprocal complementary voltage magnitude. The value is calculated using the Ohm’s Law, in the frequency domain. (Z = V/I) Where Z is the impedance, V the voltage and I the current.