Feature: Medical
Figure 2: Composition of the human body
Figure 3: Bioelectrical impedance vector - analysis
When studying body composition, we refer to the three-
Techniques measuring bioimpedance differ in the excitation signal’s frequency
supplies the electrodes’ excitation voltage and measures the produced current. Te excitation signal is generated with a digital- to-analogue converter (DAC) connected to a downstream driver. Te DAC is programmed by a microcontroller to enable the setting of the signal’s amplitude and frequency. To measure current, a transimpedance amplifier (TIA) is used,
connected to a high-resolution ADC for precise measurements. Te acquired data is processed by the system’s microcontroller, which extracts the information required for the analysis.
Measurements methods For bioimpedance measurements, the human body is divided into five segments: upper and lower limbs and the torso. Tis distinction is important for understanding the measurement method used, with the most common ones being hand-to-foot, foot-to-foot and hand-to-hand. There are multiple factors to be taken into account during
a bioelectrical impedance analysis (BIA) test, including anthropometric parameters, such as height, weight, skin thickness and build, and factors like gender, age, ethnic group, and, not least, the patient’s state of health, such as malnutrition or dehydration. If these factors are not taken into consideration, the test results can be distorted. Interpretation of the measurements is based on statistical data and equations that take these various factors into account.
compartment model, which includes fat mass, cell mass and extracellular mass. Figure 2 shows these concepts, starting from the well-known terms of lean mass (fat-free mass) and fat mass of the two-compartment model. Te fat mass has two components – essential and storage fat. Te lean mass is divided into body cell mass, composed of protein mass and intracellular water, and extracellular mass, which in turn includes extracellular water and bone mass. A final parameter, fundamental to establishing the degree of hydration, is the total body water given by the sum of the intracellular and extracellular water. From electrical point of view, intracellular and extracellular
electroslytic solutions behave like good conductors, whereas fat and bone tissue are poor conductors.
Bioimpedance measurement techniques Te most common techniques measuring bioimpedance differ in the excitation signal’s frequency. Te simplest instruments are based on measurements at a fixed frequency (single-frequency bioelectrical impedance analysis, or SF-BIA), while some adopt a system with multiple frequencies (multifrequency bioelectrical impedance analysis, or MF-BIA), and the most sophisticated instruments perform spectroscopy over a range of frequencies (bioimpedance spectroscopy, or BIS). Tere are also different techniques for evaluating the results,
of which bioelectrical impedance vector analysis and real-time analysis are the most important. In SF-BIA instruments, the small current injected into the
human body has a frequency of 50kHz. Te operation is based on an inversely-proportional relationship between the measured impedance and the total body water (TBW) – the conductive part of the impedance – consisting of intracellular water (ICW) and extracellular water (ECW). Tis technique provides good results in subjects with normal hydration, whereas it loses validity in subjects with strongly altered hydration, above all due to a limited
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