Background: QT duration represents the time of the whole summated electric cardiac ventricular activity involving stepwise depolarization followed by repolarization. There has been a long history of using the surface electrocardiogram (ECG) to identify ventricular repolarization problems. The 1960s were a turning point for precise mathematical methodologies. It has been customary in clinical practice to use only the QT interval and the polarity and shape of the T wave when evaluating cardiac repolarization using an electrocardiogram (ECG). This terminology, such as "non-specific ST segment and T wave variations are widely used. An earlier theory on interlead disparities in QTI length was resurrected in a 1990 report by the group led by Professor John Campbell. The "QT dispersion" range of durations was proposed as a measure of ventricular recovery time spatial dispersion. Objective: Determine the relevance of QTd in prediction of myocardial and its severity. Conclusion: For cardiac patients, QTd is an easy-to-use, rapid, affordable, and helpful tool for helping with study interpretation, clinical management, and therapeutic orientation.