Abstract： Objective To evaluate clinical significance of the grade of ischemia by QRS complex on the admission electrocardiogram(ECG)to predict severe arrithmia in patients with acute ST-segment elevation myocardial infarction(STEMI).Methods Patients with acute ST-segment elevation myocardial infarction(STEMI)admitted to emergency department from July 2003 to April 2008 were enrolled.A total of 223 patients met the criteria(ischemic chest pain ≥ 30 min,2 or more adjacent leads of ST segment elevation and onset time within 12 h).Exclusion criteria were bundle branch block and left ventricular hypertrophy.All enrolled patients were divided into two groups based on the enrollment electrocardiogram:grade 2 ischemia(ST elevation without terminal QRS distortion; n =134)and grade 3 ischemia(ST elevation with terminal QRS distortion; n =89).Patients of the two groups had comparable genderproportion,average age and coronary heart disease risk factors etc.All patients received thrombolytic therapy.The incidence rate of ST segment resolution(STR)and severe arrithmia in hospital stay were observed.Numerical variables were expressed mean ± standard deviation and compared by unpaired Student't test,Categorical variables were expressed percentage and compared by chi square test.Multiple logistic regression analysis was used to determine independent predictors of severe arrithmia.Results Patients with grade 3 ischemia had greater Σ ST on admission and 2 h after thrombolysis ECGs(P ＜ 0.01),the incidence rate of STR in patients with grade 3 ischemia was lower than that in patients with grade 2 ischemia(P ＜0.01).The peak creatine kinase MB fraction was higher in patients with grade 3 ischemia than that in patients with grade 2 ischemia(P ＜ 0.01).There was no significant difference of the incidence of severe arrithmia,such as ventricular premature beat,ventricular tachycardia or fibrillation,second-degree or third-degree atrioventricular block,and sinus arrest between the two groups(P ＞ 0.05),but there was a trend of higher incidence of severe arrithmia in patients with grade 3 ischemia compared with that in patients with grade 2 ischemia.Multiple logistic regression analysis demonstrated that the independent predictors of severe arrithmia were duration from symptom to thrombolysis and initial.Σ ST,whereas grade 3 ischemia remained a strong predictor of severe arrithmia.Conclusions Grade 3 ischemia on admission is associated with lower incidence of STR in patients with ST-segment elevation myocardial infarction(STEMI)after thrombolysis and a strong predictor of severe arrithmia.