Effect of hemolysis and free hemoglobin on optical hematocrit measurements in the extracorporeal circulation

Clinically significant hemolysis is a rare but serious problem in dialysis. Because hemolysis affects red blood cell count and optical density of plasma it has been speculated whether techniques used for online blood volume monitoring would be useful to detect hemolysis. In this study the influence of free hemoglobin on hematocrit and relative blood volume changes measured by optical means (CritLine, HemaMetrics, Kaysville, UT) were examined using an in vitro model with bovine blood. Free hemoglobin solutions were added in steps to circulating whole blood at baseline hematocrits covering a range from 30\% to 60\% and at blood flows of approximate to 200 and 400 ml/min. The free hemoglobin concentration reached was in the range of 2 to 3 g/di. The presence of free hemoglobin led to a relative increase in hematocrit in the range of 0.3\% per 0.1 g of free hemoglobin per dl (+3\% dl/g). As an increase in hematocrit is interpreted as a decrease in blood volume, this change referred to an apparent decrease in relative blood volume in the same order of magnitude (-3\% dl/g). Effects were more pronounced at low baseline hematocrit. Thus, although optical hematocrit readings are affected by the presence of free hemoglobin the changes at levels associated with clinical symptoms appear to be too small to be accurately detected in the in vivo situation where the hematocrit and the resulting optical signal is affected by various physiological processes and therefore much noisier.