Ischemic preconditioning. Is always a beneficial phenomenon?

Introduction and objectives. Hearts exposed to reversible ischemia stand a subsequent prolonged episode of coronary artery occlusion (ischemic precond itioning) better. The reduction of infarct size by means of preconditioning has been amply demonstrated, but the relationship between preconditioning and contractile function remains less well defined. In this study we assess the effect of a later ischemia on the regional contractility in a stunned- preconditioned myocardium. Methods. We analyze the shortening fraction in the ischemic (dependent on t he left anterior descending coronary artery), periischemic and control zone (dependent on the left circumflex coronary artery), using chronic implants of ultrasonic crystals in 17 adult mongrel dogs. In the control series, we quantified the effects of partial (30-60% reduction of coronary flow from the basal) and transitory (15 minutes) ischemic episode in the regional myo cardial function in a ((virgin,, myocardium. In two other series, the myoca rdium was previously stunned-preconditioned through brief and repeated isch emias. Afterwards, at 5th day (series B) and at 15th day (series C), the do gs were subjected to ischemic episode similar to control ones. Results. After comparing the results with the control series, we observed t hat the shortening fraction of the ischemic zone was decreased by 107% (p < 0.01) during partial ischemic episode when it was induced on the 5th day o f the stunning-preconditioning (series B). Conclusions. In dogs, the brief and repeated episodes of ischemia could condition the contractile function so that a later partial and transitory reduction of coronary flow could ind uce a severe affectation of contractility expressed as a diskinetic area.