On-line Monitoring of the Intravascular Volume during Haemodialysis by Continuous Refractometry

The control of intravascular volume (IVV) by continuous on-line measurement of protein concentration would optimise the patients’ specific rate of ultrafiltration. To prove the accuracy of a refractometric device, plasma was continuously drawn by haemofiltration during 10 haemodialysis treatments of male patients. Refractometry reflects highly significant changes in the concentrations of filtrate proteins (r = 0.862, p < 0.001) and blood proteins (rtotal = 0.593, ptotal < 0.001). In vitro, the refractometric device detected a change of protein concentration of 0.041 g/L through a refraction increase of 0.1 mV. The power of discrimination was 0.067 % of IVV. However, in vivo, the accuracy of IVV refractometric monitoring is reduced by interference factors such as sodium (0.141 mV/mmol/L), glucose (0.034 mV/mg/dl) and triglycerides (–0.040 mV/mg/dl). Adjustment of the refraction data using sodium and glucose electrodes and plasma filters with a cutoff below the size of chylomicrons is recommended.