Triggered Activity

Triggered Activity is a mechanism of cardiac arrhythmogenesis characterized by abnormal depolarizations that occur during or immediately after the repolarization phase of the cardiac action potential. These abnormal depolarizations can reach threshold and generate premature or repetitive action potentials, which may lead to arrhythmias. Triggered activity differs fundamentally from automaticity, which arises from spontaneous phase 4 depolarization, and from reentry, which involves a self-sustaining circuit of conduction.

Types of Triggered Activity

Triggered activity is primarily classified into two types based on the timing of the abnormal depolarizations relative to the action potential phases: Early Afterdepolarizations (EADs) and Delayed Afterdepolarizations (DADs).

Early Afterdepolarizations (EADs)

EADs occur during the late phase 2 or phase 3 of the cardiac action potential, which corresponds to the plateau or repolarization phases. These afterdepolarizations arise when repolarization is prolonged or unstable, allowing reactivation of depolarizing currents. EADs can lead to premature action potentials if the membrane potential reaches threshold. Prolonged action potential duration, as seen in conditions like long QT syndrome or during exposure to certain drugs, favors the development of EADs.

The main ionic mechanisms underlying EADs include reactivation of L-type calcium channels and, in some cases, sodium-calcium exchanger currents. EADs are more likely to occur at slower heart rates, where action potentials are longer, facilitating the reactivation of inward currents during repolarization.

Delayed Afterdepolarizations (DADs)

DADs occur after full repolarization has been completed, during phase 4 of the action potential cycle, before the next normal depolarization. They are linked to intracellular calcium overload, which causes spontaneous release of calcium from the sarcoplasmic reticulum. This calcium release activates the sodium-calcium exchanger, generating an inward depolarizing current that can produce these afterdepolarizations.

DADs are commonly observed under conditions of increased sympathetic stimulation, digitalis toxicity, or myocardial ischemia. They tend to occur at higher heart rates due to calcium overload from rapid pacing or adrenergic stimulation.

Ionic and Cellular Mechanisms

The genesis of triggered activity depends on abnormal ionic currents that disrupt normal membrane potential dynamics:

• Calcium handling abnormalities: Spontaneous calcium release from the sarcoplasmic reticulum contributes to DADs by increasing intracellular calcium and activating inward currents.
• Reactivation of voltage-gated channels: In EADs, incomplete inactivation or reactivation of L-type calcium channels during repolarization provides inward current that delays or reverses repolarization.
• Sodium-calcium exchanger (NCX): NCX operates by extruding one Ca²⁺ in exchange for three Na⁺ ions, creating a net inward positive current. This current is crucial in both EADs and DADs, especially in DADs where spontaneous calcium release triggers NCX activity.
• Other currents: Late sodium current and T-type calcium currents may also contribute to triggered depolarizations.

These ionic disturbances disrupt the balance of inward and outward currents that normally stabilize the membrane potential, enabling afterdepolarizations to reach threshold and initiate premature beats.

Clinical Significance

Triggered activity plays a significant role in the initiation and maintenance of various cardiac arrhythmias, particularly in settings where ionic homeostasis is disturbed:

• Long QT syndrome: EAD-induced triggered activity may cause torsades de pointes, a potentially life-threatening polymorphic ventricular tachycardia.
• Catecholaminergic polymorphic ventricular tachycardia (CPVT): DAD-mediated triggered activity under adrenergic stimulation leads to ventricular arrhythmias.
• Digitalis toxicity: Excess intracellular calcium promotes DADs and subsequent arrhythmias.
• Ischemia and reperfusion: Calcium overload in ischemic myocardium predisposes to DADs and triggered beats.

Recognition of triggered activity as an arrhythmia mechanism has implications for therapy, including the use of antiarrhythmic drugs that modulate calcium handling, sympathetic blockade, and interventions that shorten action potential duration.

Experimental and Diagnostic Considerations

Triggered activity can be elicited and studied in isolated cardiac tissue and myocytes by manipulating ionic currents or pacing rates. In clinical electrophysiology:

• Programmed electrical stimulation may provoke triggered beats.
• Electrocardiographic features such as premature ventricular complexes with specific coupling intervals can suggest triggered activity.
• Pharmacologic testing with agents that prolong repolarization or increase intracellular calcium can reveal susceptibility.

Understanding the cellular basis of triggered activity aids in the design of targeted antiarrhythmic therapies and risk stratification in patients prone to arrhythmias.

Summary of Key Concepts

Triggered activity represents a critical arrhythmogenic mechanism grounded in cellular electrophysiology and calcium dynamics. Its understanding bridges basic science and clinical cardiology, guiding diagnosis and therapy of complex cardiac arrhythmias.