Myocardial Adaptation During and After Sustained, Demand-Induced Ischemia

BACKGROUND: We tested the hypotheses that prolonged, demand-induced myocardial ischemia plateaus and that on relief of stress, myocardial function remains depressed, with proportionate reductions in blood flow and oxygen consumption indicative of hibernation. METHODS AND RESULTS: Closed-chest swine (n = 20) were prepared with an 80% coronary stenosis. Hemodynamics, myocardial blood flow, oxygen, and lactate metabolism were measured in group 1 (n = 9) (1) at baseline, (2) at 10 and 30 minutes of atrial pacing plus intravenous norepinephrine infusion, and (3) in 5 of 9 (group 1a) at approximately 50 minutes after stress. Group 1a had ischemia assessed with 99mTc-labeled BMS 181321. In group 2 (n = 11), myocardial function was determined with radionuclide ventriculography (n = 8), and myocardial necrosis was looked for with trichlorotetrazolium chloride staining (n = 7), histology (n = 10), and myocardial creatine kinase concentration (n = 4). Baseline stenotic-zone endocardial blood flow was reduced versus the normal zone (0.94 +/- 0.33 versus 1.38 +/- 0.27 mL.min-1.g-1, mean +/- SD; P < .05), whereas epicardial flows were comparable (1.15 +/- 0.36 versus 1.16 +/- 0.26 mL.min-1.g-1). Stenotic-zone endocardial flow was unchanged versus baseline at 10 and 30 minutes of stress, whereas epicardial flow increased (1.62 +/- 0.53 mL.min-1.g-1 at 10 minutes and 1.44 +/- 0.51 mL.min-1.g-1 at 30 minutes, both P < .05). Myocardial oxygen consumption increased versus baseline (10.8 +/- 2.9 mL.min-1.100 g-1) at 10 and 30 minutes of stress (14.9 +/- 5.2 and 13.9 +/- 4.5 mL.min-1.100 g-1, both P < .05). After stress, stenotic-zone blood flow and oxygen consumption were reduced approximately 30% (P < .01) versus baseline. In group 2, stenotic-zone contraction with stress declined versus baseline and remained depressed throughout recovery. Histological and biochemical evidence of myocardial necrosis was absent in group 2. CONCLUSIONS: Myocardial ischemia induced by a sustained increase in oxygen demand may not progress to necrosis but may instead plateau. After relief of stress, myocardial function remains depressed, with a proportionate reduction in blood flow and oxygen consumption consistent with myocardial hibernation.