Background: Coronary artery disease is one of the most common causes of morbidity and mortality worldwide. Aim: This study aimed to evaluate the effect of implementing arrhythmia guidelines on improving myocardial infarction outcomes among patients undergoing primary percutaneous coronary intervention. Design: A quasi-experimental research design. Setting: Study was carried out in the coronary care unit at Assiut University Heart Hospital. Patients: A purposive sampling of 150 patients. Tools: Three tools were utilized to collect data. Tool l: Patient assessment tool; Tool II: Prediction of life-threatening arrhythmia post primary percutaneous coronary intervention assessment tool and Tool III: Patients' outcomes tool. Result: Among study and control groups (12% and 20%, respectively) were high-risk for developing life-threatening arrhythmia; the area under the curve (AUC) of the predictive risk score of life-threatening arrhythmia and prediction model was 0.858 and 0.841, respectively, which demonstrate good accuracy. There were statistically significant differences between the study and control groups regarding discharge criteria and length of stay at CCU (P values of 0.005 and 0.000, respectively). Conclusion: The study confirmed that patients' outcomes can be improved through early prediction of complications; scores provide the nurses with a quicker and more reliable predictor of life threatening arrhythmia shortly since the arrival and effective in improving patients' outcomes. Recommendation: Implementation of predictive life threatening arrhythmia scores in CCU. An educational program should be made for nurses about the implementation of the predictive life-threatening arrhythmia scores and nursing guidelines in the coronary care unit.