- Background Failure of ST-segment resolution (STR) after primary percutaneous coronary intervention (PPCI) for ST-elevation myocardial infarction is associated with adverse outcome but currently cannot be predicted on admission. Our aim was to determine whether failure of STR can be predicted from clinical and electrocardiographic data available on admission and whether the adverse outcome associated with grade 3 ischemia (distortion of the terminal portion of the QRS complex) is mediated through impaired tissue reperfusion. Methods We prospectively studied 100 consecutive patients who underwent PPCI for a first ST-elevation myocardial infarction. Multiple variables available on admission were analyzed as predictors of STR. Electrocardiograms and angiograms were analyzed by blinded investigators. Results Grade 2 ischemia was found in 71 patients (71%) and 29 (29%) had grade 3 ischemia. Complete STR was observed in 42 (59%) of 71 patients with grade 2 ischemia as compared to 8 (28%) of 29 patients with grade 3 ischemia (P = .004). In a multivariate model, grade 3 ischemia was the sole predictor of failure of STR (odds ratio [OR] 0.26, 95% CI 0.1-0.72) and the strongest predictor of failure to achieve TIMI grade 3 flow (OR 0.07, CI 0.02-0.3) and TIMI myocardial perfusion grade 3 (OR 0.09, CI 0.02-0.4) after the procedure. Conclusions Grade 3 ischemia is a strong independent predictor available on admission of failure to achieve myocardial reperfusion after PPCI, as assessed both electrocardiographically and angiographically. This association may underlie the larger infarcts associated with grade 3 ischemia and may allow the identification upon admission of patients who require more aggressive management to improve reperfusion.