Myocardial kinetics of ¹²³I-labeled-16-hexadecanoic acid

To determine if the myocardial clearance of omega ¹²³I-16-hexadecanoic acid (¹²³I-HDA) is affected by decreased coronary blood flow, six anesthetized dogs had partial occlusion of the left anterior descending coronary artery. One hour later, ¹¹³Sn-microspheres were injected into the left atrium, followed immediately by the IV administration of ¹²³I-HDA. Following injection, regional myocardial ¹²³I activities were monitored continuously with miniature cadmium telluride radiation detectors placed against the endocardium in both ischemic and nonischemic zones. After 3 h continuous monitoring, 46 Sc microspheres were injected into the left atrium and the dogs were killed. Ischemic and nonischemic areas of myocardium were sectioned and counted in a well counter. Nonischemic myocardial ¹²³I activity reached 99%±1% (mean±1 SD) of peak activity within 1 min after injection, peaked in 1.8±0.2 min, then decreased biexponentially over the next 3 h (decay constant λ₁=0.0406 ± 0.0156 min^-1, λ₂=0.0033 ± 0.0009 min^-1). Ischemic myocardial ¹²³I activity reached 99%±2% of peak activity within 1 min after injection, peaked in 1.2±0.4 min, then decreased bi-exponentially over the next 3 h λ₁=0.0525 ± 0.0199 min^-1, λ₂=0.0027 ± 0.0009 min^-1; P=N.S. compared with nonischemic zone. There were linear correlations between ¹²³I-activity initially and the initial microsphere-determined regional myocardial blood flow for each dog (r=0.88-0.96). Ischemic and nonischemic zone myocardial blood flows and hemodynamics did not change significantly during the experiment. Cardiac images of excellent quality were obtained after ¹²³I-HDA administration in two additional dogs with left anterior descending occlusions and two additional dogs with no occlusion using a conventional gamma scintillation camera and a low-energy collimator. These data suggest that the initial distribution of ¹²³I-HDA is flow-related and that ¹²³I-HDA clearance rates are not altered by a decrease in myocardial perfusion. As a result, the extrapolation of ¹²³I-HDA clearance rates to myocardial metabolism must be done with caution. However, ¹²³I-HDA is a promising cardiac perfusion agent due to (1) the linear relationship of activity with regional myocardial blood flow immediately after administration, (2) the favorable 159 keV photon energy, and (3) the short biological and physical half-lives, thus reducing the radiation burden per study and allowing multiple studies over relatively short intervals.