The radioactive microsphere method for the assessment of regional myocardial blood flow after coronary artery occlusion

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Abstract
In this study, we have tried to determine the magnitude of the inaccuracy of the radioactive microsphere method — due to variations in the diameter distribution of the spheres — for measuring regional myocardial blood flow after coronary artery occlusion. In 5 mongrel dogs, three types of 15 μm microsperes, labelled with125I,141Ce or85Sr, were injected simultaneously after the descending branch of the left coronary artery had been ligated. Myocardial samples were taken from the left ventricle and divided into four groups according to the number of spheres per sample. The radioactivity of the various isotopes per gram tissue was expressed as percentage of their activity per milliliter of the reference sample. The diameter distribution of microspheres, labelled with each of the isotopes, was determined light-microscopically in suspensions belonging to three different batches. The relative error, as determined from the difference in relative radioactivity of the various types of microspheres in the tissue samples, was higher than the theoretical error for each of the number of spheres per sample. It is very likely that this discrepancy is caused by the differences in diameter distribution of the various types of microspheres, resulting in non-random error. The smaller spheres tended to go to low flow areas and the larger ones to high flow areas. Because of the non-randomness, the error due to diameter variations in the spheres can be diminished by randomizing the order of injection of the various isotopes. The present study indicates that the relatively high degree of accuracy of the microsphere method for the determination of blood flow to large parts of the myocardium with an unimpeded coronary circulation, as was described in literature, cannot be extrapolated to the determination of regional myocardial blood flow after coronary artery occlusion, when the combination of small tissue samples, variations in the diameter distribution of the spheres and an unevenly distributed myocardial blood flow unfavourably affect the accuracy of the method.