Laws of physics dictate the calculation of phase angle. In a capacitor AC (impedance) circuit, reactance of the current through cells causes the current to lag behind the voltage; consequently, current (blue) and voltage (red) do not meet at the same time. The time difference in the period between this voltage peak and current peak is called the phase shift.
Current flows by the movement of ions; movement impeded by the viscosity of the medium is known as resistance and measured in Ohms. It should be noted that the human body does not adhere to Ohm’s Law (Current = Voltage / Resistance) due to the presence of dielectrics (or insulators) in the body. Groups of cells perform specialised functions and form part of a complex communicative network, sending signals throughout the body via an ion concentration or gradient. These electromagnetic gradients or dielectrics absorb some of the current causing the cells to become electronically charged, an essential function for cellular survival. It is these cells that store electricity that act as capacitors (measured in Farad F).
The impedance obtained by the BIA device is a combination of capacitive and resistive type elements, the calculation of which is shown opposite. Consequently, BIA measures resistance (R) and capacitance in combination. Reactance (Xc) is subsequently calculated from capacitance and representative of the opposition to the electrical current or voltage. Furthermore, the phase shift is always relative to the resistance line as the resistance line is always in phase with the voltage. When the device receives this signal it is ‘decoded’ using a series of electronics called synchronous demodulators. This is why the quality of the electronics within the device is fundamental in obtaining accurate and consistent measurements.
Once the phase shift data is received, a series of complex number equations are then applied to derive the resistance and the reactance and thus the Impedance measurements (refer to figure). From this the phase angle (φ) is calculated.
Conventional BIA systems CALCULATE Phase Angle . This relies on some assumptions and unknown values and is therefore no more than an estimate of the real value.
Bodystat systems directly MEASURES the Phase Angle and is calibrated against a known value.
Independent certification is supplied for each device as to the accuracy of Phase Angle measurement
Phase Angle is commonly accepted as a prognostic indicator of morbidity and mortality. As health status changes, so does the relationship between Intra/ extracellular water. With bioelectrical impedance analysis measurements, volume of TBW can be ascertained from the resistance, in turn reactance reflects the ability of the cell membrane to act as an imperfect capacitor. Therefore the phase angle is an indicator of the intra and extra cellular area. This relationship is represented by changes in the phase angle. A low phase angle is associated with a reduced survival time, conversely a higher phase angle is associated with improved cellular health.
This has been demonstrated in various population groups including oncology, HIV, liver cirrhosis, COPD, heart failure, haemodialysis and sepsis.
Additionally, muscle strength and phase angle correlate which is suggestive of a lower phase angle being associated with decreasing functioning status.
Many research papers have examined the relationship between phase angle and malnutrition and have found a correlation between low phase angle and higher nutritional risk. Population groups used in both research and clinical practice include nephrology, HIV, oncology and surgical patients.
Bioelectrical Impedance Analysis is becoming a preferred method to establish and monitor malnutrition. Alternative methods, such as blood tests, arm circumferences and skin-fold tests are time consuming, require training and maybe affected by other nutritional changes. Traditional methods may also miss subtle changes in body cell mass (intracellular water and metabolic tissue).
Malnutrition is characterised by changes in the integrity of the cellular membrane, marked by fluid shifts. Study of phase angle, as a reflection of water distribution between ICW/ECW water, is an easy, quick, non-invasive way to ascertain nutritional status.