Left ventricular volume measurement by the conductance catheter and variations in the hematocrit in small animals.

Cardiac performance is quantitatively and continuously assessed from pressure-volume signals by using the conductance catheter technique even in small animals. Conductivity of blood, however, is dependent on hematocrit (Hct). Interdependence between hematocrit and volume measurement by the conductance catheter has been evaluated. In 12 male Wistar rats weighing 400-475 g, anesthetized and artificially ventilated, Hct was gradually lowered by isovolumic hemodilution ranging from 50% to 7%. Heparinized blood samples were drawn at decreasing Hct levels for centrifugation, for automated Hct measurement by a blood gas analyzer, and for conductance catheter volume measurements (CCV) in calibrated cuvettes. Substitution of about 2 ml colloid solution lowered the Hct initially from 47 +/- 2% to 36 +/- 3%; at the same time, CCV output rose by 36 +/- 14% for definite blood volume. There is a strong inverse linear relationship (absolute value of r > 0.96; P < 0.0001) between relative volume units (RVU) displayed by the volume acquisition device and the hematocrit for any calibrated blood cuvette. Slopes of the regression lines increase proportionally to the calibration volumes (28.3 microl: -0.25; 63.6 microl: -0.57; 113.1 microl: -0.92). These data document the direct interdependence between Hct and CCV. Consequently, careful Hct correction of the RVU recordings is necessary especially in small animals where even small amounts of substituted solutions result in a marked decrease in Hct and, thus, in pronounced blood volume misreadings.