Preconditioning and arrhythmias.

Abstract

A brief episode of myocardial ischemia confers endogenous cardioprotection during a subsequent prolonged period of ischemia. This phenomenon, known as preconditioning,1 offers one of the most powerful mechanisms for reducing the speed and extent of myocardial cell damage resulting from an acute or sustained ischemic insult. This protection has been shown to occur in a wide variety of animal species, including man, and exists in two forms, namely an early or classic preconditioning1 and delayed or second window of protection.2 The “gold standard” used in these animal studies for assessing a preconditioning effect has been measurement of the extent of limitation of infarct size.3 Studies in humans therefore necessarily rely on a less direct approach.4 For example, clinical observation has shown that the occurrence of episodes of angina shortly before a myocardial infarction reduces mortality and morbidity.4,5⇓ In the setting of percutaneous transluminal coronary angiography, it has been shown that measures of ischemia, such as angina and ST elevation, in the ECG occurring during the period when the coronary artery is occluded by the inflated balloon are less during a second balloon inflation compared with the first.6–8⇓⇓ These studies are not always easy to interpret, however, because of confounding variables such as uncertainties about the recruitment of collaterals in response to the preconditioning ischemia, the adequacy of the very short See p 3091 duration of ischemia sometimes used during the preconditioning phase, and the use of the amount of ST segment shift in the ECG as a quantitative measure of ischemia.4 One study largely overcomes the uncertainties with regard to collateral flow in patients undergoing percutaneous transluminal coronary angiography and demonstrates an antiarrhythmic effect of preconditioning.9 Possibly the most direct evidence for the occurrence of preconditioning in humans …