Regional ischemic 'preconditioning' protects remote virgin myocardium from subsequent sustained coronary occlusion.
One or more brief episodes of coronary artery occlusion protect or "precondition" the myocardium perfused by that artery from a subsequent episode of sustained ischemia. We sought to determine whether ischemic preconditioning protects only those myocytes subjected to brief coronary occlusion or whether brief occlusions in one vascular bed also limit infarct size and/or attenuate contractile dysfunction in remote virgin myocardium subjected to subsequent sustained coronary occlusion.
Methods and Results
In the preliminary limb of the study, six anesthetized dogs underwent four episodes of 5-minute circumflex branch occlusion plus 5-minute reperfusion, followed by 1 hour of sustained left anterior descending coronary artery occlusion and 4.5 hours of reflow. Subendocardial blood flow during left anterior descending coronary artery occlusion (measured by injection of radiolabeled microspheres) was 0.07 +/- 0.03 mL.min-1 x g tissue-1, similar to the value of 0.07 +/- 0.02 mL.min-1 x g-1 observed in a group of eight concurrent control dogs. However, infarct size (assessed by triphenyltetrazolium staining) in the circumflex preconditioned group averaged 4 +/- 1% of the myocardium at risk, significantly less (p < 0.05) than the value of 13 +/- 4% observed in the concurrent controls. An additional 18 dogs were then randomized to undergo either four episodes of circumflex branch occlusion (n = 8) or no intervention (n = 10) before 1 hour of left anterior descending coronary artery occlusion and 4.5 hours of reflow. Subendocardial blood flow averaged 0.08 +/- 0.02 versus 0.08 +/- 0.03 mL.min-1 x g-1 in the control versus circumflex preconditioned groups, yet infarct size was significantly smaller in circumflex preconditioned dogs than in the controls (6 +/- 2% versus 16 +/- 5% of the risk region; p < 0.05). At 4.5 hours following reperfusion, segment shortening in the left anterior descending coronary artery bed (assessed by sonomicrometry) averaged -21 +/- 19% of baseline in control animals versus 13 +/- 12% of baseline in the preconditioned group (p = NS). Circumflex preconditioning did not, however, have an independent beneficial effect on contractile function: Regression analysis revealed that the trend toward improved function in circumflex preconditioned dogs reflected the smaller infarct sizes in this group.
Brief episodes of ischemia in one vascular bed protect remote, virgin myocardium from subsequent sustained coronary artery occlusion in this canine model. These data imply that preconditioning may be mediated by factor(s) activated, produced, or transported throughout the heart during brief ischemia/reperfusion.