Ionic Mechanism of Repolarization
The process is triggered by a change in charge polarity across the cell membrane. Once a positive charge accumulates inside the cell following depolarization, the following sequence of events occurs:
- Channel Closure — Sodium ($Na^+$) and calcium ($Ca^{2+}$) channels close.
- Opening of Potassium Pores — Voltage-gated potassium channels open.
- Potassium Efflux — A massive outward current of $K^+$ ions flows out of the cell.
- Charge Restoration — The outer surface of the membrane becomes positive (+) again, while the inside returns to negative values (resting potential).
- Pump Activity — To restore ionic balance, the $Na^+,K^+$-ATPase pump activates and actively redistributes the ions. In sodium channels, the m-gates close and h-gates open, while the voltage-gated potassium channels close (at rest, potassium leaves the cell exclusively through constitutively open leak channels).
Cardiac Repolarization Characteristics
In the myocardium, repolarization shapes the heart's electrical field:
- In the Atria: Membrane recovery occurs while excitation is already propagating through the ventricles.
- In the Ventricles: The disappearance of excitation proceeds in a direction from the heart apex toward its base. Excitation resolves last in the papillary muscles and trabeculae of the atrioventricular valves.
- Field Generation: Since some myocardial regions have already recovered (outer surface +) while others remain excited (outer surface -), a voltage gradient arises between them. This generates local currents and an electromagnetic field, creating potential differences on the body surface.
How Repolarization Appears on the ECG
On the electrocardiogram, ventricular repolarization forms the T wave. The late phase of this process may manifest as the U wave.
- Normal and Pathological Evaluation: During systematic ECG analysis, each lead is evaluated for the polarity (positive or negative), shape, and amplitude of the T wave. Pathological changes in these parameters often indicate myocardial ischemia.
- During Arrhythmias: Ventricular premature complexes (extrasystoles) and tachycardias are characterized by discordance: the ST segment and T wave point in the direction opposite to the main deflection of the widened and deformed QRS complex.
Pharmacological Regulation
In electrophysiology, final repolarization corresponds to phase 3 of the action potential.
- Drug Target: The blockade of potassium channels by antiarrhythmic agents leads to the prolongation (lengthening) of this phase.
- Clinical Effect: Prolonged repolarization increases the effective refractory period (ERP). This is a universal strategy for treating tachyarrhythmias and extrasystoles of any origin, as it reduces the risk of the ventricles responding to a premature (ectopic) electrical stimulus.