BACKGROUND:Ischemic preconditioning occur refers to the occurrence of many unstable angina pectoris before acute myocardial ischemia(AMI).It is generally recognized that ischemic preconditioning occur has protective effect on heart and can reduce infarction scope and improve contraction function of heart.Recently some authors thinks ischemic preconditioning occur will cause the exacerbation of patients condition during the time of hospitalization and increase the incidence rate of angina and pectoris and death.