Fatty acid analogue accumulation: A marker of myocyte viability in ischemic-reperfused myocardium

A 3-methyl substituted radioiodinated long chain fatty acid analogue was evaluated as an agent for the nonivasive detection of altered fatty acid uptake in reperfused, postischemic myocardium. This iodinated fatty acid analogue, 15-(para-iodophenyl)-3-methyl pentadecanoic acid, was given intravenously at 3 hours of reperfusion following 15 minutes (Group 1, n = 5 dogs) or 60 minutes (Group 2, n = 5 dogs) of left anterior descending coronary artery occlusion. Myocardial blood flow (MBF) was measured during occlusion and reperfusion with radiolabeled microspheres administered via the left atrium. Paired ultrasonic subendocardial crystals were placed in the ischemic perfusion bed to assess regional left ventricular systolic function at baseline, during ischemia and reperfusion. Electron microscopic analysis and staining with triphenyltetrazolium chloride (TTC) was performed. Groups 1 and 2 dogs had similar (p = NS) myocardial blood flows during occlusion. TTC positive 1 g endocardial segments from Group 1 (n = 98) and Group 2 (n = 71) had 37% greater fatty acid analogue activity (0.26 ± 0.04 vs. 0.19 ± 0.09 percent injected dose per gram; p < 0.05) compared with TTC negative segments from Group 2 dogs (n = 37). When fatty acid analogue activity was related to near simultaneous reperfusion blood flow, this ratio was 27% greater (p < 0.05) in TTC positive segments (0.38 ± 0.1) compared with TTC negative (0.30 ± 0.16) segments, and 9% greater than normal (0.35 ± 0.09; p < 0.05). While ischemic regions from both Groups 1 and 2 dogs became similarly dyskinetic during occlusion (systolic shortening, -11 ± 6 vs. -11 ± 2%; p = NS), TTC negative segments remained akinetic (=1 ± 7%) at 3 hours of reperfusion while TTC positive zones had recovered partial systolic function (8 ± 22%). Electron microscopy confirmed the presence of reversible ultrastructural changes in TTC positive regions. A 60-minute occlusion, 3-hour reperfusion model adapted for in vivo single photon emission computed tomography showed a similar excess of ¹²³I fatty acid activity over flow when compared to perfusion (as measured with ²⁰¹Tl) in the ischemic border zone of 4/4 canine myocardial infarcts. We conclude that the accumulation of this non-β-oxidized fatty acid analogue noninvasively identifies zones of discordance between fatty acid and flow distribution that are characteristic of ischemically 'stunned' but viable myocardium.