ARTERIAL MICROSPHERE CONCENTRATIONS IN CATS ARE NOT AFFECTED BY CHANGES IN HEMATOCRIT

Background and Purpose Acute anemia may lead to erroneously low arteri al reference sample concentrations of radioactive microspheres, depend ing on the sampling rate and the size of the artery from which the ref erence samples are withdrawn. Because this error would lead to falsely high cerebral blood flow values in studies involving hemodilution cau sed by hemorrhage and fluid resuscitation, we studied the effects of h ematocrit, withdrawal rate, and vessel location and size on arterial m icrosphere concentrations in anesthetized adult cats. Methods Cats wer e anesthetized with ketamine, isoflurane, and nitrous oxide; both brac hial arteries were cannulated with polyethylene tubing, as was the abd ominal aorta through the femoral artery. Sequential left atrial micros phere injections were made using several doses of each of five isotope s. The rate of reference sample withdrawal from the three sampling cat heters was randomized to 1.03 mL . min(-1) or 2.06 mL . min(-1). We an alyzed the ratio of the number of microspheres in paired reference sam ples using the factors hematocrit, rate of withdrawal, and site. A rat io less than 1 indicates an underestimation of arterial microsphere co ncentration, 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 4 0% 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 Ratios of microsphere concentrations in reference samples withdrawn s lowly (1.03 mL . min(-1)) from the aorta and ratios of microsphere con centrations withdrawn either rapidly (2.06 mL . min(-1)) or slowly fro m the brachial arteries were significantly (P<.001) less than 1. Concl usions Hemodilution did not affect microsphere concentrations in arter ial reference samples at any withdrawal site or rate and therefore doe s not affect the accuracy of microsphere blood flow determinations. Ho wever, slow withdrawal from a large vessel may underestimate actual mi crosphere concentrations.