Where is repolarization located on an electrocardiogram (ECG)?

From the Guidelines

Repolarization on an ECG is primarily represented by the T wave, which corresponds to the phase of rapid ventricular repolarization (phase 3) of the ventricular action potential. This process is crucial for the heart's electrical activity and overall cardiac health. The T wave appears after the QRS complex and reflects the recovery phase of the ventricles as they return to their resting electrical state following contraction. According to the study by Rautaharju et al. 1, the ST segment corresponds to the plateau phase of the ventricular transmembrane action potential, and while it is part of the repolarization process, the T wave is the primary representation of repolarization. Key points to consider about repolarization on an ECG include:

• The T wave should be upright in most leads, particularly leads I, II, and V3-V6, with a smooth, asymmetric shape.
• Abnormalities in T wave morphology or ST segment changes can indicate various cardiac conditions such as ischemia, electrolyte disturbances, or drug effects.
• The direction and timing of repolarization waves provide valuable diagnostic information about the heart's electrical activity and overall cardiac health. As noted in the study 1, abnormalities in the ST segment, T wave, and duration of the QT interval reflect abnormalities in ventricular repolarization, highlighting the importance of careful interpretation of these elements on an ECG.

From the Research

Location of Repolarization on ECG

• Repolarization on an electrocardiogram (ECG) can be identified by several components, including J waves, ST-segments, and T- and U-waves 2.
• The J wave is associated with the early repolarization syndrome and Brugada syndrome, and is caused by a transmural voltage gradient during initial ventricular repolarization 2.
• The ST-segment elevation, as seen in Brugada syndrome and acute myocardial ischemia, may be largely secondary to a loss of the action potential dome in the epicardium, but not endocardium 2.
• The T-wave is a symbol of transmural dispersion of repolarization, and its peak (Tp) concurs with the time at which 25% of ventricular sites are repolarized 3.
• The TpTe_total, which represents the time from the first Tpeak in any lead to the last Tend in any lead, correlates with the dispersion of repolarization time in the entire heart 3.

Relation to Ventricular Repolarization

• Ventricular repolarization is nonlinearly distributed in time, and its sequence can be correlated with simultaneously recorded T waves 3.
• Repolarization differences along all anatomic axes, including left ventricle-right ventricle, apico-basal, anterior-posterior, and transmural, contribute to the T wave 3.
• The R-on-T phenomenon, which is an extrasystole originating on the T-wave of a preceding ventricular beat, is probably due to transmural propagation of phase 2 re-entry or phase 2 early after depolarization 2.

Clinical Significance

• The ECG provides a low sensitivity but high specificity for the diagnosis of myocardial infarction using the QRS complex, but one cannot determine the age of an MI from the QRS complex 4.
• Primary ST-segment abnormalities strongly suggest the diagnosis of acute MI or severe ischemia, and the possible need for emergency revascularization 4.
• Modern ECG systems with vector-based electrocardiography can facilitate and optimize the detection of ischemic ECG alterations, including those related to repolarization 5.