Radioactive microspheres effectively measure regional myocardial blood flow, revealing differential effects of propranolol and isoprenaline on endocardial versus epicardial perfusion.
Regional myocardial blood flow was studied in the anaesthetized dog by determining the distribution of 15 and 50μ radioactive microspheres after left atrial injection. Doses of the 15μ spheres below 4 × 106 did not significantly alter coronary or systemic haemodynamics. Larger doses produced a significant increase in mean aortic pressure. Less than 1% of the 15μ spheres delivered to the heart passed into the coronary sinus. The myocardial distribution of 15 and 50μ spheres was not significantly different except for the ratio of left ventricular endocardium to epicardium (1·12 for 15μ and 1·45 for 50μ, P<0.001). Using 15μ spheres, it was found that the ratio of endocardial to epicardial radioactivity in the left ventricular wall was increased by propranolol, decreased by isoprenaline, and unchanged by nitroglycerin.
Fortuin et al. (1971) studied this question.