AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram Part IV: The ST Segment, T and U Waves, and the QT Interval

The present article is the fourth in a series of 6 documentsfocused on providing current guidelines for the standard-ization and interpretation of the electrocardiogram (ECG). The project was initiated by the Council on Clinical Cardiology of the American Heart Association. The rationale for this project and the process for its implementation were described earlier (1). Abnormalities in the ST segment, T wave, and duration of the QT interval reflect abnormalities in ventricular repolar-ization. These abnormalities are common and often difficult to interpret. The U wave most likely represents an electric-mechanical phenomenon that occurs after repolarization is completed. However, it is frequently included in discussions of repolarization and is discussed in this section. The ST segment corresponds to the plateau phase of the ventricular transmembrane action potential. Under normal con-ditions, the transmembrane voltage changes slowly during thi