Validation studies for the use of the microsphere method in cats and young minipigs.

Radioactive microspheres are suitable for measuring cardiac output, its distribution and organ blood flow if certain criteria are met. Cardiac output may be determined with the reference flow method if microspheres do not recirculate. In case 10 micron microspheres were extracted completely by the lungs. The use of microspheres for the determination of cardiac output is therefore not limited by recirculation of microspheres down to this size. Under baseline conditions we found no preferential streaming with 50 micron spheres in young minipigs and with 25 and 15 micron spheres in cats At high cardiac output values induced by the administration of 0.4 mg/kg of Dihydralazine in cats, 25 micron but not 15 micron microspheres showed a small, non significant tendency to stream preferentially into the upper body. Thus, under rapid flow conditions 15 micron microspheres may be preferable to larger ones from the rheological point of view. Under baseline conditions the distribution of cardiac output to a wide range of organs was very similar when comparing 25 and 15 micron microspheres in cats and 50 and 15 micron microspheres in minipigs, whereas 10 micron microspheres were poorly extracted by most organ in cats. 15 micron microspheres were found to be suitable for the determination of cardiac output and its distribution in both species provided that bronchial and arteriovenous shunt flows can be neglected.