Localization of viable, ischemic myocardium by positron-emission tomography with 11C-palmitate.

Abstract

Regional metabolism of exogenous fatty acid extracted by myocardium depends primarily on oxidation. Positron-emission tomography (PET) delineates necrotic myocardium by virtue of its failure to extract labeled fatty acid. In ischemic myocardium, fatty acid is extracted, but metabolized slowly. Whether viable, but ischemic, tissue could be detected and localized in vivo based on external detection of impaired fatty acid metabolism was studied. Regional clearance of 11C-palmitate was assessed by sequential PET in 15 anesthetized dogs. Clearance was consistently monoexponential from 5-15 min after administration of the tracer. In the absence of coronary stenosis (n = 7), clearance was homogeneous throughout the heart, with an average rate constant (.hivin.k) of -0.060 .+-. 0.005 min-1 and a coefficient of variation (CV) of 11.1 .+-. 2.1% in each heart. Homogeneity persisted when the heart rate was increased from 84.4 .+-. 6.0 to 202.7 .+-. 11.5 beats/min with atropine (CV 13.2 .+-. 3.5%). With left circumflex coronary stenosis (.ltoreq. 70% reduction in vessel diameter), homogeneity of 11C-clearance under control conditions and with tachycardia did not differ from clearance in hearts without coronary stenosis. With stenosis > 70% sufficient to induce ischemia without gross infarction, regional clearance of 11C became markedly heterogeneous under control conditions (CV 28.1 .+-. 5.5%, P < 0.01 compared with normal hearts) and with tachycardia (CV 34.8 .+-. 5.4%, P < 0.01). The heterogeneity resulted from reduced clearance of 11C in regions supplied by the stenotic vessel (.hivin.k = -0.044 .+-. 0.011 min-1) compared with clearance in well perfused regions (.hivin.k = -0.064 .+-. 0.011 min-1, P < 0.025), a difference accentuated by tachycardia. Sequential PET after i.v. injection of 11C-palmitate delineates zones of viable, ischemic myocardium that characteristically exhibit impaired oxidation of extracted fatty acid.