Ventricular Repolarization and the ST Segment
No, ventricular repolarization does not occur during the ST segment—the ST segment represents the plateau phase of the ventricular action potential when all ventricular myocytes are fully depolarized and maintaining a stable membrane voltage before repolarization begins. 1
Electrophysiological Basis of the ST Segment
The ST segment corresponds to Phase 2 (the plateau phase) of the cardiac action potential, not the repolarization phase:
During the plateau phase, the transmembrane voltage remains at approximately +10 to -10 mV, which is markedly positive compared to the resting membrane potential of approximately -85 mV 1
This plateau voltage is maintained by slow inward calcium currents that balance outward potassium currents, keeping the membrane depolarized for an extended period 1
The voltage changes slowly during this phase and remains at approximately the same level across all ventricular myocardial cells, which explains why the ST segment appears isoelectric (flat) on the ECG 1
When Repolarization Actually Occurs
Ventricular repolarization occurs after the ST segment, during the T wave:
The membrane potential repolarizes from the plateau voltage of approximately +10 to -10 mV down to the resting level of approximately -85 mV during Phase 3 (rapid repolarization) 1
This rapid repolarization phase corresponds to the T wave on the surface ECG and is mediated primarily by time- and voltage-dependent potassium currents 1
The T-wave represents transmural dispersion of repolarization across different layers of the ventricular wall 2
Clinical Implications
Understanding this distinction is critical for ECG interpretation:
ST segment abnormalities reflect alterations in the plateau phase voltage gradients between different regions of myocardium, not repolarization abnormalities per se 3, 1
ST segment elevation in conditions like acute myocardial ischemia results from loss of the action potential dome in the epicardium but not endocardium during the plateau phase 2
Abnormalities in the ST segment and T wave are caused by abnormal voltage gradients during the plateau and rapid repolarization phases of the action potential 3
Drug effects and electrolyte disturbances can alter the duration and voltage of the plateau phase, affecting both contractility and arrhythmia susceptibility 1