NONINVASIVE CONDUCTIVITY TECHNIQUE TO DETECT CHANGES IN HEMATOCRIT - IN-VITRO VALIDATION

An on-line haematocrit measurement in extracorporeal circuits might be useful under some clinical circumstances (e.g. haemodialysis or cardi ac surgery). As no such measurement exists, a device has been develope d that makes it possible to detect haematocrit (Ht) continuously witho ut a loss of blood. It is a multi-frequency system for the detection o f electrical conductivities. The aim of this study was to investigate whether this device can measure Ht alterations properly. Ht alteration s were induced by adding pure mannitol and 20% mannitol to fresh human blood. Furthermore, the effect of both mannitol substances on the int racellular ion content, intracellular conductivity and Ht were investi gated. Alternations in Ht were established by the addition of 1000, 80 0, 600, 400, 200 and 0 mg of pure mannitol to 10 ml of fresh human blo od, and 3.0, 2.5, 2.0, 2.0, 1.5, 1.0, 0.5 and 0 ml of 20% mannitol to fresh human blood until a total volume of 10 ml was achieved. Although their effects were significantly different, pure mannitol and 20% man nitol both caused a reduction in mean cellular volume, and thus in Ht. A highly significant correlation was found between Ht and intracellul ar conductivity (r=0.90, p<0.001). In addition to these effects, addit ion of pure mannitol and 20% mannitol had different effects on the int racellular ion content. Pure mannitol caused an increase in intracellu lar ion content due to a transcellular ion shift, whereas 20% mannitol induced a decrease. From this study, it can be concluded that the mul ti-frequency conductivity method observed changes in Ht (and intracell ular fluid volume) in an accurate manner. Changes in intracellular ion content of erythrocytes depend on the sort of mannitol substance that is added. The intracellular ion concentration can be calculated from measured intracellular conductivity and Ht. The total number of intrac ellular ions can be derived from intracellular conductivity and the nu mber of erythrocytes.