Objectives: Recent clinical studies suggest that contractile reserve may occur in a minority of viable, chronically dysfunctional segments with reduced resting flow (hibernating myocardium). We hypothesized that epicardial artery patency might predict which segments have critically reduced subendocardial flow reserve and limited contractile reserve.
Methods: Pigs were chronically instrumented with a fixed stenosis on the left anterior descending coronary artery (LAD) to produce hibernating myocardium. At least 3 months later, flow at rest and during adenosine vasodilation (microspheres), ventricular function and contractile reserve (contrast ventriculography), and (18)F-2-deoxyglucose (FDG) deposition (ex vivo tissue counting) were quantified.
Results: Hibernating myocardium (regional dysfunction with reduced resting perfusion) was present in animals with an occluded (n=40) or patent (n=19) LAD. Viability was confirmed by histology and FDG deposition. In collateral-dependent hibernating myocardium, subendocardial flow did not increase above baseline levels during epinephrine or adenosine stimulation, consistent with exhausted subendocardial flow reserve at rest. This was associated with limited contractile reserve and regionally increased FDG deposition. In contrast, subendocardial flow reserve was present in hibernating myocardium distal to a patent artery. Contractile reserve during epinephrine infusion in this group was significantly greater than in animals with an occluded artery.
Conclusions: The physiology and metabolism of hibernating myocardium was dependent upon stenosis severity and its effects on subendocardial flow reserve. In collateral-dependent hibernating myocardium, contractile reserve was limited in the setting of exhausted subendocardial flow reserve, thus supporting the hypothesis that metabolic imaging may be preferable for determining viability distal to a complete occlusion.