Repolarization

Ionic mechanism of repolarization

The process is triggered by a change in the charge sign on the cell membrane. When 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 channel gates open.
• Potassium efflux — a massive outward current of $K^+$ ions from the cell to the outside occurs.
• Charge restoration — the outer surface of the membrane becomes positive (+) again, and the inner surface returns to negative values (resting potential).
• Pump operation — to restore ionic balance, the $Na^+,K^+$-ATPase is activated, which actively redistributes ions. In sodium channels, m-gates close and h-gates open, while the voltage-gated gates of potassium channels close (at rest, potassium leaves only through constitutively open leak channels).

Features of the process in the heart

In the myocardium, repolarization participates in the formation of the heart's electrical field:

• In the atria: Membrane restoration occurs at the moment when excitation is already spreading through the ventricles.
• In the ventricles: The process of extinction of excitation proceeds in the direction from the apex of the heart to its base. Excitation disappears last in the papillary muscles and trabeculae of the atrioventricular valves.
• Field formation: Since some myocardial regions have already recovered (outside +), while others remain excited (outside -), a charge difference arises between them. This generates local currents and an electromagnetic field that creates a potential difference on the body surface.

How does repolarization reflect on the ECG?

On the electrocardiogram, ventricular repolarization forms the T wave. The late phase of this process may manifest as the U wave.

• Assessment in health and disease: During sequential ECG analysis in each lead, the polarity (positive or negative), shape, and amplitude of the T wave are evaluated. Pathological changes in these parameters often indicate myocardial ischemia.
• During rhythm disturbances: Ventricular extrasystoles and tachycardias are characterized by discordance: the ST segment and T wave are directed opposite to the main vector of the widened and deformed QRS complex.

Pharmacological regulation

In electrophysiology, final repolarization corresponds to phase 3 of the action potential.

• Drug target: Blockade of potassium channels by antiarrhythmic agents leads to the prolongation (lengthening) of this phase.
• Clinical effect: Prolongation of repolarization increases the effective refractory period (ERP). This is a universal strategy for treating tachyarrhythmias and extrasystoles of any localization, as it reduces the risk of ventricular response to an untimely (premature) electrical stimulus.

Class and exam questions

What else to study on this topic

• Cardiomyocyte action potential
• Membrane depolarization
• Electrocardiography (ECG)
• Ventricular rhythm disorders

Other topics in "Physiology concepts"

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