What is bioimpedance spectroscopy and how is it used?
Bioelectrical impedance analysis refers to several related techniques for determining body composition based on measuring electrical impedance or opposition to flow of a small alternating current applied to the body. Electrical impedance is the alternating current corollary to resistance in direct current circuits and is composed of resistance as well as reactance, which is made up itself of inductance (current induced by magnetic fields) and capacitance (the ability of circuit components to store charge). Body composition is determined by modeling the human body as an alternating current circuit using estimation equations derived from physical properties of the human body. Fat-free mass has lower impedance given its higher electrolyte-rich water content, whereas fat mass is relatively anhydrous and has higher impedance. In general, the greater the fluid content, the lower the impedance.
Whole-body BIS is the most validated of these methods. In this method, electrodes are placed on a hand and foot and alternating currents over a broad band of frequencies are applied to the body to estimate impedance. BIS has been validated against deuterium, bromide, and radioactive potassium radioisotope dilution techniques for determination of total body water (TBW), ECF, and intracellular fluid (ICF), respectively. BIS appears to have value in detecting occult fluid overload patients with end-stage kidney disease (ESKD) on HD. In retrospective cohort analysis, fluid overload as measured by BIS has been associated with increased all-cause mortality. In a large prospective cohort, fluid overload as measured by BIS was strongly and independently associated with all-cause mortality. Duration of exposure to fluid overload was associated with all-cause mortality in a dose-dependent fashion. These effects were durable across blood pressure tertiles. In two small clinical trials, estimation of dry weight enhanced by BIS showed improvement in all-cause mortality versus clinical evaluation alone. Similarly, in heart failure literature, one study validated transthoracic impedance to specifically measure ECF in the lung and found improvements in cardiovascular outcomes, all-cause mortality, and readmission. However, a multi-center randomized-controlled trial including 50 patients with fluid overload of 15% or more as demonstrated by BIS comparing three methods of dry weight reduction showed a high (31%) rate of dialysis-related complications across all groups demonstrating the difficulty of fluid removal in the dialysis population.