Isotope clearance and myocardial blood flow

Abstract

R adioisotope techniques for the study of the circulation have evolved at a progressively accelerating rate during the 3.5 years since the earliest semiquantitative estimates of the linear velocity of blood flow were made with radium C and a Wilson cloud chamber.’ A wide variety of techniques have followed which are also based on the use of externally placed detectors to record the flow of an intravascular bolus of radioisotope through the body. Methods of quantitation have been developed that are based on adaptation and expansion of the indicator-dilution concepts of Stewart, as developed by Hamilton and others. The use of the Fick calculation and clearance concepts has provided another major avenue of approach. The speed of clearance of injected tracer from a site of injection is closely related to the volume rate of blood flow to the area.2 However, the results are difficult to quantitate in terms of blood flow, even “effective” blood flow, and this technique is not easily applied to the viscera of man. The entrance of isotopes into tissue cells from the circulating blood has also been made the basis for measurements of regional blood flo~.~ This general approach is readily applied to the hear!, because there is a rapid exchange of POtassium ions across the myocardial capillaries and cellular membranes and a large reservoir of potassium within the muscle cells. More than one half of any KJ2 or Rbss in coronary arterial blood enters the cardiac cells, from which it returns only slowly to the blood stream. Since the radioactivity of the human heart can be assessed by external monitors, the possibility of evaluating coronary blood flow by innocuous methods has been of widespread interest. Unfortunately, the dynamics of Rbs6 and K42 exchange are complex, and their speed of uptake by the myocardium is only partly determined by the volume rate of coronary blood flow. Therefore, there is a real possibility that the results of isotope uptake studies may be seriously misinterpreted. When a plateau concentration of isotope is maintained in arterial blood by infusing Rbs6 at a progressively decreasing rate, the behavior of the myocardium toward the tracer can be analyzed as if it were eschanging with an infinite pool of tagged blood. During the first minute of isotope infusion, an amount of potassium equal to more than three times that present in the interstitial fluid of the heart is delivered