Background: Acute coronary syndrome (ACS) patients undergoing primary percutaneous coronary intervention (PCI) may experience coronary slow flow (CSF) or the no-reflow (NR) phenomenon, where myocardial perfusion remains insufficient despite restored epicardial artery patency. This study aimed to study the role of QRS complex configurations and ST segment shifts on ECG as predictors for coronary slow flow (CSF) and no-reflow (NR) phenomena in patients undergoing primary percutaneous coronary intervention (PCI) for acute coronary syndrome (ACS). Method: This observational cohort study included 66 ACS patients were divided into two groups: 33 with ST-segment elevation (STEMI) and 33 without (NSTE-ACS). Coronary flow was assessed using TIMI flow grade and myocardial blush grade (MBG) pre- and post-PCI. Results: Among STEMI patients, 92% of those with fragmented or distorted QRS (QRS+) had TIMI flow <3, indicating no-reflow or slow-flow, compared to 50% of patients with normal QRS (P=0.020). A statistically significant correlation was observed between QRS+ and suboptimal coronary flow post-PCI (P=0.002). In NSTE-ACS patients, an ST segment shift >3 mm was significantly associated with no-reflow (P=0.05). Conclusion: QRS configuration and ST segment shifts on ECG are valuable predictors of coronary slow flow and no-reflow phenomena in ACS patients after primary PCI, aiding in risk stratification and clinical decision-making.