Effect of sampling site on early kinetics of blood radioiodide and pertechnetate

Radioiodide (I) and radiopertechnetate (TcO4) were sampled simultaneously from left ventricle, right atrium, and femoral vein early after intravenous injection to pentobarbital sodium-anesthetized cats. Concentrations of both tracers in femoral vein blood was lower than in left ventricular (arterial) blood, with rapid narrowing of the differences over 20 min and gradual approach to steady state. Right atrial blood concentrations were intermediate. When these data were fitted to three-compartment mamillary models, representing blood and tissue pools, compartment size and kinetics differed according to which sampling site was taken as representative of each cat's blood. We have developed a model incorporating subcompartmentalization of the blood with an exchange compartment connecting to the pulmonary circulation as well as compartments exchanging with peripheral sites. With the use of this model, all data sets fitted well. The pulmonary exchange compartment and a large rapid exchange compartment in the femoral vein drainage areas are essential to reconciling the differences among sampling sites.