Contactless and continuous sodium concentration monitoring during continuous renal replacement therapy

Abstract

A frequent complication of critically ill patients in intensive care units suffering from acute kidney injury is a severe dysnatremia requiring a continuous renal replacement therapy. For such patients, an individual dialysis treatment with an individual composition of the dialysate to limit the rate of serum sodium normalization has a positive effect on patients’ recovery. However, a precise control of the serum sodium concentration change requires continuous sodium monitoring. Drawing blood samples and analyzing such samples with a blood gas analyzer gives the serum sodium concentration with an absolute accuracy of about 2 mmol/L and a precision of 0.6 mmol/L. Nevertheless, blood gas analyzers do not allow for continuous serum sodium monitoring. Therefore, we present a new sensing concept based on a differential transformer consisting of printed circuit board coils for contactless and noninvasive monitoring of the serum sodium concentration during continuous renal replacement therapy to enable an individual sodium management. This sensing concept covers a linear operating range from 100 mmol/L to 170 mmol/L, comparable to commercially available blood gas analyzers. In a preclinical investigation using reconfigured human packed red blood cells as test medium, we could continuously measure the serum sodium concentration with an absolute accuracy of 4 mmol/L compared to the reference and a precision of 0.3 mmol/L. Moreover, we have established an analytical expression to optimize the differential transformer coils, resulting in a high sensitivity of 192 mV/mol/L.