Background: Despite numerous reports that one or more episodes of brief coronary artery occlusion preconditions the myocardium and dramatically reduces myocardial infarct size produced by a subsequent prolonged ischemia, we recently demonstrated that preconditioning does not attenuate contractile dysfunction in the peri-infarct tissue. However, the specific effects of preconditioning on myocardium in which wall motion has not been compromised by the preconditioning regimen per se and is further submitted to a short ischemic insult (that is, not confounded by necrosis) remain unknown.
Methods and results: We addressed these issues in the canine model of myocardial stunning. Eighteen anesthetized dogs underwent 15 minutes of coronary occlusion followed by 3 hours of reperfusion. Before the 15-minute coronary occlusion, each dog received one of three treatments: no intervention (control group, n = 6), one episode of 5-minute coronary occlusion/5-minute reperfusion (PC5 group, n = 6), or one episode of 2.5-minute coronary occlusion/5-minute reperfusion (PC2.5 group, n = 6). Segment shortening (SS) in the ischemic/reperfused midmyocardium was monitored by sonomicrometry, and myocardial blood flow was assessed by injection of radiolabeled microspheres. All three groups were equally ischemic during the 15-minute coronary occlusion: Midmyocardial blood flow averaged 0.05 +/- 0.02, 0.07 +/- 0.04, and 0.08 +/- 0.03 ml/min/g in control, PC2.5, and PC5 groups, respectively. Before the 15-minute coronary occlusion, PC5 dogs exhibited significant stunning (SS = 55% baseline; p less than 0.01 versus control), whereas PC2.5 dogs did not (SS = 91% baseline; p = NS versus control). However, segment shortening during the subsequent 15-minute coronary occlusion was equally depressed at -25% to -42% of baseline values among the three groups. Furthermore, all three groups demonstrated a similar degree of stunning after reperfusion: SS at 3 hours after reflow averaged 24 +/- 12%, 34 +/- 16%, and 48 +/- 12% of baseline in control, PC2.5, and PC5 groups, respectively (p = NS). The degree of recovery of function after reperfusion correlated with the amount of midmyocardial blood flow during coronary artery occlusion. However, this relation was not different among the three groups: Specifically, for any given collateral flow during ischemia, preconditioning did not reduce the degree of stunning.
Conclusions: Preconditioning neither preserves contractile function during a reversible ischemic insult nor prevents myocardial stunning during the initial hours of reflow.