What part of the electrocardiogram (ECG) represents the repolarization phase of the cardiac cycle?

ECG Repolarization Phases

Ventricular repolarization on the ECG is represented by the ST segment (plateau phase), the T wave (rapid repolarization phase), and extends through the end of the T wave, after which the TP segment represents the electrical diastole between repolarization completion and the next depolarization. 1

Understanding the Repolarization Components

ST Segment: Early Repolarization (Plateau Phase)

• The ST segment corresponds to the plateau phase (phase 2) of the ventricular action potential, during which transmembrane voltage changes slowly and remains at approximately the same level in all ventricular myocardial cells 1
• During this phase, only small voltage gradients are present, which explains why the ST segment is normally nearly flat and isoelectric 1
• The ST segment extends from the end of the QRS complex (J point) to the beginning of the T wave 1

T Wave: Rapid Repolarization Phase

• The T wave corresponds to phase 3 of the ventricular action potential—the phase of rapid ventricular repolarization 1, 2
• During this phase, the transmembrane action potential repolarizes from its plateau voltage of approximately +10 mV to its resting level of approximately -85 mV 1
• The interventricular and intraventricular voltage gradients created as cells undergo rapid sequential repolarization generate the T wave on the body surface ECG 1
• Repolarization proceeds from epicardium to endocardium, opposite to the direction of ventricular depolarization 1, 2

TP Segment: Electrical Diastole (Post-Repolarization)

• The TP segment represents electrical diastole—the period from the end of repolarization to the onset of the next depolarization 1
• During this phase, ventricular myocardial cells are at their resting transmembrane potential of approximately -85 mV 1
• The absence of significant voltage gradients during this phase explains why the TP segment is normally flat and isoelectric, similar to the ST segment 1

Clarifying Your Question About Specific Intervals

Based on your question about "end of P wave to beginning of QRS" and other intervals:

• End of P wave to beginning of QRS (PR segment): This represents atrial repolarization and AV nodal conduction delay, not ventricular repolarization 1
• End of QRS (S wave) to beginning of T wave (ST segment): This represents the plateau phase of ventricular repolarization 1
• End of T wave to beginning of P wave (TP segment): This represents the completion of ventricular repolarization and electrical diastole before the next cardiac cycle 1

The U Wave: Post-Repolarization Phenomenon

• The U wave is primarily a mechanoelectric phenomenon that occurs after repolarization is completed, appearing as a low-amplitude deflection after the T wave 3, 4
• The U wave is most evident in leads V2 and V3, with normal amplitude approximately 0.33 mV or 11% of the T wave 3
• While the U wave appears after repolarization is complete, it may represent delayed repolarization of the Purkinje system or electrical-mechanical interactions 3, 4

Clinical Pitfall to Avoid

A common misconception is that the PR interval represents ventricular repolarization—it does not. The PR interval (end of P wave to beginning of QRS) represents atrial depolarization completion and AV nodal conduction. Ventricular repolarization specifically begins with the ST segment and continues through the T wave, ending at the start of the TP segment 1.