Arterial microsphere concentrations in cats are not affected by changes in hematocrit.

BACKGROUND AND PURPOSE Acute anemia may lead to erroneously low arterial reference sample concentrations of radioactive microspheres, depending on the sampling rate and the size of the artery from which the reference samples are withdrawn. Because this error would lead to falsely high cerebral blood flow values in studies involving hemodilution caused by hemorrhage and fluid resuscitation, we studied the effects of hematocrit, withdrawal rate, and vessel location and size on arterial microsphere concentrations in anesthetized adult cats. METHODS Cats were anesthetized with ketamine, isoflurane, and nitrous oxide; both brachial arteries were cannulated with polyethylene tubing, as was the abdominal aorta through the femoral artery. Sequential left atrial microsphere injections were made using several doses of each of five isotopes. The rate of reference sample withdrawal from the three sampling catheters was randomized to 1.03 mL.min-1 or 2.06 mL.min-1. We analyzed the ratio of the number of microspheres in paired reference samples using the factors hematocrit, rate of withdrawal, and site. A ratio less than 1 indicates an underestimation of arterial microsphere concentration, which would lead to erroneously high cerebral blood flow values. The procedure was repeated after isovolemic hemodilution with 10% hetastarch to hemoglobin levels approximating 85%, 70%, 55%, and 40% of baseline. RESULTS No significant effects of hematocrit on ratios of microsphere concentrations existed at any withdrawal rate or site. Ratios of microsphere concentrations in reference samples withdrawn slowly (1.03 mL.min-1) from the aorta and ratios of microsphere concentrations withdrawn either rapidly (2.06 mL.min-1) or slowly from the brachial arteries were significantly (P < .001) less than 1. CONCLUSIONS Hemodilution did not affect microsphere concentrations in arterial reference samples at any withdrawal site or rate and therefore does not affect the accuracy of microsphere blood flow determinations. However, slow withdrawal from a large vessel may underestimate actual microsphere concentrations.