Phase angle as an index of physiological status: validating bioelectrical assessments of hydration and cell mass in health and disease.

Bioelectrical impedance (BI) is a practical method to assess body composition in health and disease. This method relies on the passive conduction of an applied, safe, low-level alternating current through water and electrolytes in the body. Using a phase-sensitive device, BI yields measurements of impedance (Z) and its components, resistance (R) and reactance (Xc), that are related geometrically as phase angle (PhA). In vitro studies provide empirical evidence relating BI measurements to physiological variables. Cooking raw food samples results in greater decreases in PhA, predominantly Xc, with smaller reductions R indicating destruction of cell membrane integrity with simultaneous movement of fluid from intracellular to extracellular space. Infusion of saline into a cell-free model shows a proportional decrease in R with increases in volume. Saline infusion in a composite model of cells disproportionately decreases Xc and PhA, compared to R, demonstrating greater relative expansion of extracellular water (ECW) with a lesser relative increase in total fluid volume. Surgical patients treated with fluid infusion and diuresis demonstrate changes in Xc predominantly indicating relative changes in ECW with lesser variations in R indicating fluctuations in total fluid volume. Proteomics studies disclose strong independent associations of PhA with protein markers of fluid overload and protein proliferation. Interpretations of PhA measurements for body cell mass should be examined in the context of hydration status.