Clinically useful cardioprotection: ischemic preconditioning then and now.

Ischemic preconditioning (IP) is the most effective, reproducible form of protection against myocardial cell death yet described. The mechanism of classic IP has not been identified, but recent investigations have focused on the mitochondrial permeability transition pore (mPTP). Similarly, the mechanism of the "second window of protection" (SWOP) is not known but thought to involve increased expression of important gene products. Currently, IP in the clinical arena is limited to cardiac surgery, planned angioplasty, and organ preservation protocols. To move preconditioning into a broader clinical arena will require resolution of important fundamental yet stubborn problems involving both basic and clinical science. Important unresolved issues include the mechanisms involved in the transition from reversible to irreversible injury, the amount of potential salvageable myocardium present at the onset of reperfusion, the identity and signaling of the mPTP, the optimization of protocols, the identity of end effectors (SWOP), and the identification of the best experimental model systems. From a clinical standpoint, important issues include the influence of comorbidities on cardioprotection, identification of appropriate animal models, the lack of a biologic marker of the cardioprotective state, the influence of coexistent therapeutic drugs, potential toxicity of pharmacologic mimics, and the window of opportunity for significant protection. Ischemic preconditioning has yielded promising results in other organs including the brain as well as tissue preservation for certain surgical procedures that will require definition of the underlying mechanism(s) to be fully exploited clinically. Over the past 25 years, the scientific community has learned much regarding the biology and potential mechanisms of IP and the concept has been expanded to many other organ systems in many other clinically relevant scenarios. To realize the full clinical potential will require continued investigation into the mechanism.