Test & measurement


current can pass through CM and RI, while low frequency current can only pass through RE. RE, RI, and CM are cell-level parameters that can reflect some health status. RE is related to


the volume of ECF, while RI is related to the volume of ICF. For RE, a smaller RE represents a larger ECF volume, commonly in tissue edema,


ascites, or organ failure. A larger RE is usually found in dehydration. For RI, a smaller RI represents a larger ICF volume. Since muscle cells have a higher water content than fat cells, a


smaller RI is commonly seen in people with developed muscles. On the contrary, people with


high body fat usually have a larger RI. CM reflects the integrity of the cell membrane. A


larger CM often represents good cell function, while a smaller CM indicates poor cell function. In Figure 1, we can see that human body


bioimpedance Z is a complex impedance, which can be expressed as


And its polar form is


Here, the real part of Z is the resistance R, and the imaginary part is the reactance X. R mainly represents the overall water content of the human body, while the absolute value of X mainly represents the overall tissue density. The modulus of Z is |Z|, and the phase angle


Instrumentation Monthly May 2024


If we expand Equation 3 in the form of complex impedance, and combine it with equations 1 and 2, we can further express R, X, |Z|, and φ as functions of ω, namely


Figure 2. Bioimpedance parameter vs. angular frequency plot.


Since RE, RI, and CM are microcosmic parameters in the cell level, they cannot be


measured directly. However, R, X, |Z|, and φ can be measured by the methods introduced in


the following sections. Therefore, these equations and Figure 2 can not only quantitatively determine the relation between


parameters (namely R, X, |Z|, and φ) and the angular frequency, but also help us calculate RE, RI, and CM indirectly. This associates the macrocosmic human body with the microcosmic cells. To conclude, these equations can facilitate the user in assessing their health status comprehensively and completely by bioimpedance measurements. The physiological


Continued on page 30... 29


is φ. A smaller |Z| often indicates higher water content, tumors, or inflammation, while


a larger |Z| indicates a dehydration state, tissue necrosis, or damage. The absolute value


of φ can reflect the nutritional status of the human body, and a lower value usually indicates malnutrition.


According to the equivalent circuit model in


Figure 1, RI and CM are connected in series and they are connected in parallel with RE. So interested readers can get


Here ω is the angular frequency of the stimulus signal. The plot of the previous


parameters changing with the angular frequency is shown in Figure 2.


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