Potassium-43 and Rubidium-81 Myocardial Imaging at Rest, at Exercise, and During Angina Pectoris
The myocardial accumulation of radioactive intracellular cations, such as potassium and rubidium, was first demonstrated by Love in 1954.1 Eight to ten years then elapsed before Carr and his co-workers applied this observation to the study of static imaging of the myocardium.2,3 In these prototype studies, using the model of experimental canine infarction, significant imageable radioactivity was detected within a normal left ventricular myocardium following intravenous administration of radionuclide. Regions of myocardial infarction appeared as “cold spots,” or zones of relatively decreased or absent radioactivity. Following these initial studies, a number of groups have attempted to use various radionuclides to image the atherosclerotic myocardium. Most work has focused on radioactive tracers, such as potassium and ions that are capable of exchange with the myocardial intracellular cation pools. Potassium-43 was introduced as a clinical imaging radionuclide in 1971.4 In addition to potassium, the physiologically similar monovalent cations rubidium,5 cesium,6 thallium,7,8 and ammonium ion9,10 have been employed. Other classes of compounds investigated include labeled fatty acids11, norepinephrine,12 amino acids,13 toluidine blue,14 and bretylium analogues.15 In all instances the region of interest, be it infarction or is chemic zone, is visualized as an area of decreased activity surrounded by areas of normal uptake in adjacent and/or overlying portions of the left ventricle. This undoubtedly limits resolution in comparison to that which can be obtained when the abnormal area is visualized as a “hot spot,” or region of increased radioactivity, as has been demonstrated in acute infarction with the bone-imaging agents. A recent novel approach for visualizing regions of coronary stenosis or occlusion as hot spots, while employing traditional cold-spot agents, such as potassium-43, has been suggested by Cibulski et a1. 16 This approach involves retrograde injection of tracers into the coronary sinus during transient, proximal coronary-sinus closure. The radionuclide is then maximally distributed to lowpressure—low-arterial-flow zones, and defects are seen as hot spots. Although such an approach is of interest, it obviously lacks broad clinical applicability.
KeywordsExercise Stress Myocardial Image Acute Infarction Splanchnic Blood Flow Regional Myocardial Perfusion
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