Trapping and intramyocardial distribution of microspheres with different diameters in cat and rabbit hearts in vitro.

Microsphere trapping was determined in isolated perfused Langendorff hearts of cats and rabbits with 7.9-, 8.6-and 14.6-micrometers microspheres. Only the largest spheres were completely trapped whereas significant and relevant fractions of the smaller spheres were found in the perfusate leaving the heart. These fractions were 8.3 and 3.0% of the total activity for the cat and 6.7 and 2.1% for the rabbit hearts. The density of spheres trapped in the outer layer was similar with all three sizes of microspheres, whereas the density in the subendocardial layer increased with increasing microsphere size and paralleled overall trapping in both species. The complete trapping of 14.4-micrometer microspheres proves that there were no leaks and no arterio-venous shunts. The results obtained with the two smaller sphere sizes suggest regional differences in trapping and therefore regional differences in capillary size. Only selected batches of commercially available 7-10 micrometer spheres should be used. The mean diameter should be 10 micrometer with a standard deviation no larger than 1 micrometer. 15-micrometer spheres may not be ideal but, nevertheless, give reasonable values, for intramyocardial distribution of blood flow in cats and rabbits.