Reentry

Reentry refers to a pathophysiological mechanism in cardiac electrophysiology wherein an electrical impulse persistently circulates within a loop of excitable tissue, repeatedly activating the myocardium. This phenomenon underlies many types of cardiac arrhythmias, particularly tachyarrhythmias, by generating rapid, self-sustaining excitation that disrupts the normal rhythm of the heart.

Requirements for Reentrant Excitation

Reentry requires three fundamental conditions to be present simultaneously within cardiac tissue:

1. A Closed Conduction Pathway

There must be a loop or circuit through which an electrical impulse can continuously travel. This pathway can be anatomical, formed by fixed structures, or functional, emerging from transient electrophysiological properties of the tissue.

2. Unidirectional Block

Conduction must be blocked in one direction within the loop, preventing the impulse from traveling back along the same path immediately. This unidirectional block allows the impulse to circulate around the blocked area rather than extinguishing.

3. Slow Conduction and Recovery Time

The impulse must travel slowly enough around the loop so that by the time it returns to its point of origin, the previously excited tissue has recovered excitability (refractoriness has ended). This timing ensures the circulating wavefront can re-excite the tissue, perpetuating the cycle.

These conditions together establish a continuous circuit of excitation, producing sustained arrhythmias.

Anatomical Reentry

Anatomical reentry occurs when the reentrant circuit is defined by fixed structural obstacles within the heart, such as:

• Fibrotic tissue
• Valvular annuli
• Scar tissue from prior myocardial infarction or surgery
• Anatomical features like the atrioventricular node or the bundle of His

In anatomical reentry, the impulse travels around these physical barriers. Since the circuit depends on tangible anatomical boundaries, the pathway remains constant and reproducible. Classic examples include atrial flutter where the reentry circuit revolves around the tricuspid valve annulus, and ventricular tachycardia associated with post-infarction scar tissue.

Functional Reentry

Functional reentry arises in the absence of fixed anatomical obstacles. Instead, it depends on transient electrophysiological heterogeneities and dynamic properties within the myocardium. This type of reentry is characterized by:

• Circuits formed by areas of differing refractoriness or conduction velocity
• Wavefronts that revolve around zones of transient conduction block or slow conduction
• Spiral or figure-of-eight wave patterns that can shift location over time

Functional reentry is often observed in atrial fibrillation and polymorphic ventricular tachycardia, where reentrant circuits are unstable and nonfixed. These circuits can meander through the myocardium, making them more complex to map and terminate.

Mechanisms of Reentry Maintenance and Termination

The perpetuation of reentry depends critically on the balance between conduction velocity and refractory period. If conduction is too fast or refractoriness too long, the wavefront extinguishes; if conduction is too slow or refractoriness too short, wavefronts may fragment or degenerate into fibrillation.

Termination of reentry can be achieved by:

• Interrupting the circuit with ablation therapy targeting critical isthmuses or pathways
• Modifying refractoriness or conduction velocity pharmacologically to prevent re-excitation
• Altering the substrate by reversing ischemia or fibrosis to eliminate the circuit

Understanding these dynamics is essential for diagnosis and treatment of reentrant arrhythmias.

Electrophysiological Characteristics of Reentry

Conduction Velocity

The speed at which the electrical impulse travels through the myocardium influences whether the impulse can successfully re-excite tissue. Slow conduction favors reentry by allowing time for recovery.

Refractory Period

The refractory period is the time post-excitation during which cardiac cells cannot be re-excited. Shortening or heterogeneity in refractory periods can facilitate reentry by creating excitable gaps.

Excitable Gap

This is the portion of the reentrant circuit where tissue has recovered excitability but has not yet been re-excited by the circulating impulse. The presence of an excitable gap enables the wavefront to continue propagating.

Clinical Significance

Reentry is the fundamental mechanism responsible for many clinically important arrhythmias, including:

• Atrial flutter and fibrillation
• Paroxysmal supraventricular tachycardia (PSVT)
• Ventricular tachycardia and fibrillation

Recognition of reentry mechanisms guides therapeutic strategies such as catheter ablation, antiarrhythmic drug use, and device implantation to restore normal cardiac rhythm and prevent sudden cardiac death.

Summary Diagram of Reentry Components

This illustration depicts a circular reentrant pathway around an area of unidirectional block, demonstrating the continuous, circular propagation of the excitation wavefront essential for reentry.