Non-invasive conductivity technique to detect changes in haematocrit: in vitro validation.

An on-line haematocrit measurement in extracorporeal circuits might be useful under some clinical circumstances (e.g. haemodialysis or cardiac surgery). As no such measurement exists, a device has been developed that makes it possible to detect haematocrit (Ht) continuously without a loss of blood. It is a multi-frequency system for the detection of electrical conductivities. The aim of this study was to investigate whether this device can measure Ht alterations properly. Ht alterations were induced by adding pure mannitol and 20% mannitol to fresh human blood. Furthermore, the effect of both mannitol substances on the intracellular ion content, intracellular conductivity and Ht were investigated. Alternations in Ht were established by the addition of 1000, 800, 600, 400, 200 and 0 mg of pure mannitol to 10 ml of fresh human blood, 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% mannitol both caused a reduction in mean cellular volume, and thus in Ht. A highly significant correlation was found between Ht and intracellular conductivity (r = 0.90, p < 0.001). In addition to these effects, addition of pure mannitol and 20% mannitol had different effects on the intracellular ion content. Pure mannitol caused an increase in intracellular ion content due to a transcellular ion shift, whereas 20% mannitol induced a decrease. From this study, it can be concluded that the multi-frequency conductivity method observes changes in Ht (and intracellular fluid volume) in an accurate manner.(ABSTRACT TRUNCATED AT 250 WORDS)