Bioimpedance

Bioimpedance analysis passes a small, safe alternating current (typically 50kHz, <1mA) through the body between electrodes and measures the resulting voltage to calculate tissue impedance. Different tissues exhibit distinct impedance characteristics: muscle tissue (high water content, ~73%) conducts well with low impedance, adipose tissue (low water content, ~10%) conducts poorly with high impedance, and bone is intermediate. The impedance has two components—resistance (R) from tissue fluids and reactance (Xc) from cell membranes acting as capacitors. Multi-frequency BIA (1kHz to 1MHz) can distinguish between intracellular and extracellular fluid compartments. Clinical-grade systems use tetrapolar electrode configurations to eliminate contact impedance errors. Wearables apply BIA for body composition estimation, hydration status monitoring, respiration rate detection via thoracic impedance changes, and advanced cardiac output monitoring.