Noninvasive Estimation of Plasma Sodium Concentration During Hemodialysis via Capacitively Coupled Electrical Impedance Spectroscopy

This paper presents a compact, low-cost, and noninvasive system for real-time estimation of plasma sodium concentration ([Na]Pl) during a hemodialysis (HD) session with state-of-the-art accuracy. It is based on the electrical impedance spectroscopy (EIS) performed with a capacitively coupled impedance sensing cell and a high-frequency measurement device, both custom-built. The EIS data are processed to infer the resistance of the liquid inside the cell, which is used together with an optical hemoglobin sensor to estimate the [Na]Pl. Validation of the EIS was performed by estimating the conductivity of the blood-mimicking fluid (BMF). The complete method was validated using the whole bovine blood, comparing the results to those obtained with standard instruments. The system was able to estimate the [Na]Pl with sufficient accuracy (root-mean-square error (RMSE) of 3.0 mol/m
³
with respect to reference data] to provide clinically useful information. The proof-of-concept hardware can be converted to a cheap and compact circuit board for integration into an HD machine.