Background Stunned myocardium reflects postreperfusion dysfunction in myocardium that is destined to ultimately fully recover.Most investigators attribute postreperfusion stunning to a primary defect in excitation-contraction coupling or to an altered sensitivity of the myofilaments to calcium. The aim of the present study was to evaluate the interrelation between myocardial perfusion, oxidative metabolism, and function in an effort to better characterize the phenomenon of myocardial stunning, to define the regional efficiency of stunned myocardium, and to characterize its reserve capacity.Methods and Results Regional myocardial perfusion (measured with radiolabeled microspheres), myocardial oxygen consumption (MVo2) (quantified with positron emission tomography using 1-Carbon-11-acetate), and myocardial function (assessed with two-dimensional echocardiography) were evaluated in 12 anesthetized, closed-chest dogs subjected to 15 minutes of left anterior descending coronary artery occlusion followed by reperfusion. To evaluate flow, oxidative, and functional reserve after measurements were obtained 1 hour after reperfusion, dogs were subjected to paired pacing (an inotropic stimulus that does not alter systemic hemodynamics), and measurements were repeated. One hour after reperfusion, stunned myocardium was characterized by near-normal levels of myocardial perfusion (0.57+-0.13 mL/g per minute, 81+-13% of that in remote, normal regions) but severe dyskinesis (echo score, 2.6+-0.7; percent wall thickening, 14+-20%). Despite the low level of contractile function, MVo2averaged 1.72+-0.7 micromole/g per minute, 71+-27% of that observed in remote myocardium. Regional myocardial efficiency (systolic wall thickening divided by MVo2) was markedly diminished. With paired pacing, myocardial perfusion increased proportional to that in remote myocardium, systolic function improved (echo score, 1.4+-0.7; percent wall thickening, 30+-15%), and regional MVo2nearly doubled (to 3.41+-1.82 micromole/g per minute, P<.05 for each paired measurement). Importantly, with paired pacing, regional myocardial efficiency nearly normalized in reperfused myocardium.Conclusions Stunned myocardium is characterized by near-normal levels of perfusion and oxygen consumption despite marked dyskinesis. Myocardial efficiency is poor. With inotropic stimulation (in the present study, paired pacing), reperfused myocardium demonstrated considerable perfusion, oxidative, and functional reserve and a dramatic improvement in myocardial efficiency. These results may have implications for the treatment of postreperfusion pump failure. (Circulation. 1994;89:2290-2296.)