Continuous monitoring of circulating blood hematocrit

The relationship between the hematocrit value and the absolute value of impedance of whole blood was analyzed. The results indicated that the hematocrit value was linearly proportional to the logarithm of the impedance of whole blood over the range of hematocrit values of 10 to 60% with a correlation coefficient of 0.995. The circuit used for detection of the impedance of whole blood was described. It required no adjustments during measurements and had a wide linear range. The sources of error during continuous measurements were the rate of blood flow through the sensor cell, the temperature of the blood, and the concentrations of plasma proteins and ions. Accordingly, the flow and temperature of blood were kept constant during measurements. Changes in the concentrations of plasma proteins and ions within physiological ranges caused errors of +/- 2.0%. Typical results of continuous measurements were presented, showing that the hematocrit values obtained by this method agreed well with those obtained by the conventional methods from the packed cell volume. An advantage of this method over the conventional methods was that it could follow rapid and transient changes in the hematocrit.