Reentry is a term used across several fields, but in medicine it refers to one of the most common causes of abnormal heart rhythms. A reentrant arrhythmia occurs when an electrical signal in the heart gets trapped in a loop, circling back on itself instead of traveling its normal one-way path. This looping signal forces the heart to beat too fast, sometimes dangerously so. Reentry is responsible for the majority of clinically significant fast heart rhythms, from relatively benign episodes of rapid heartbeat to life-threatening ventricular tachycardia.
How a Reentry Circuit Forms
Your heart normally conducts electrical signals in a single direction, like traffic on a one-way street. Each region of heart muscle fires, then enters a brief resting period (called a refractory period) during which it can’t fire again. This built-in cooldown prevents signals from doubling back. Reentry happens when that system breaks down in a specific way.
Three conditions must exist simultaneously for a reentry circuit to form. First, there needs to be a potential loop, meaning two separate pathways that connect at both ends. Second, one of those pathways must block the signal in one direction (unidirectional block) while still allowing it to pass the other way. Third, conduction through the circuit must be slow enough that by the time the signal completes the loop, the tissue at the starting point has recovered and is ready to fire again. If the signal arrives too early, it hits tissue that’s still resting and the circuit dies out. If it arrives at just the right moment, the loop sustains itself, potentially firing hundreds of times per minute.
What Creates the Conditions for Reentry
Scar tissue is one of the most important physical substrates for reentry. After a heart attack, the damaged area heals with fibrous scar tissue, but surviving strands of heart muscle often remain woven through the scar in a disorganized pattern. Electrical signals must follow a zigzag course through these branching and merging muscle bundles, which dramatically slows conduction. The fibrous tissue between the surviving strands creates natural barriers, forming the walls of potential circuits. Researchers who have painstakingly traced surviving heart muscle cells through scar tissue under a microscope have been able to reconstruct hypothetical reentry circuits just by following the zigzag routes of muscle fibers separated by fibrosis.
Not all reentry requires scar tissue, though. Some people are born with an extra electrical pathway or have naturally occurring differences in how their heart’s conduction system is wired. The AV node (the electrical gateway between the upper and lower chambers) can contain two pathways with different speeds and recovery times, creating a built-in setup for a reentry loop without any structural damage at all.
Common Arrhythmias Caused by Reentry
The most common reentrant arrhythmia involving rapid heartbeat from above the ventricles is AV nodal reentrant tachycardia (AVNRT). It affects roughly one to two people per 1,000 in the general population. AVNRT uses the AV node itself as the circuit: a premature heartbeat travels down a slow pathway because the fast pathway hasn’t recovered yet. By the time the signal reaches the bottom of the slow pathway, the fast pathway has recovered, so the signal shoots back up the fast pathway. When this back-and-forth motion sustains itself, the heart races, often at 150 to 250 beats per minute. People typically experience this as a sudden onset of pounding in the chest that stops just as abruptly.
AV reciprocating tachycardia (AVRT) works on a similar principle but uses an accessory pathway, an extra electrical connection between the upper and lower chambers that exists from birth. The circuit travels down the normal conduction system and back up through the accessory pathway (or vice versa), creating a larger loop.
Atrial flutter involves a larger reentry circuit that typically loops around the right upper chamber of the heart. The atria beat at around 300 times per minute, though the ventricles usually respond at a slower rate because the AV node acts as a gatekeeper. Atrial fibrillation, the most common sustained arrhythmia overall, involves multiple simultaneous reentry circuits in the left upper chamber and the pulmonary veins, creating the chaotic, irregular rhythm it’s known for.
Ventricular tachycardia caused by reentry is the most dangerous form. The circuit typically runs through surviving muscle strands within scar tissue from a prior heart attack. Because the ventricles are pumping out of sync with normal rhythm and often too fast, this arrhythmia can cause dangerously low blood pressure, loss of consciousness, or cardiac arrest.
How Reentrant Arrhythmias Are Diagnosed
An EKG can identify the type of tachycardia based on the shape, width, and regularity of the electrical tracings, but distinguishing reentry from other mechanisms (like a rogue group of cells firing on their own) often requires an electrophysiology study. During this procedure, thin wires are threaded into the heart through a vein, and small electrical impulses are delivered to map the heart’s conduction and provoke the arrhythmia under controlled conditions. One hallmark of reentrant rhythms is that they can be reliably started and stopped with precisely timed electrical impulses, something that isn’t true of arrhythmias caused by abnormal automaticity.
Treatment Options
Medications used for reentrant arrhythmias work by altering the electrical properties of the circuit. The goal is to either lengthen the resting period of heart tissue so the circling signal encounters unresponsive cells and stops, or to slow conduction enough that the circuit can no longer sustain itself. Some drugs do both. However, medications can occasionally make things worse: by slowing conduction without sufficiently lengthening the resting period, they can theoretically stabilize a circuit that would otherwise have fizzled out, or even speed up a tachycardia.
Catheter ablation is a more definitive treatment. A catheter is guided into the heart and uses heat or freezing energy to destroy a small, critical section of the reentry circuit. For AVNRT, this means targeting the slow pathway in the AV node. For atrial flutter, it means creating a line of scar across the circuit’s path. For ventricular tachycardia related to scar tissue, the procedure maps the circuit and targets the narrow channels of surviving muscle that the signal depends on. Success rates vary by the type and complexity of the arrhythmia. In patients with congenital heart disease and complex atrial surgery, initial ablation successfully eliminates all reentry circuits in about 69% of cases, but recurrence rates can be as high as 48% over several years, with older age and prior complex surgery predicting worse outcomes. Simpler circuits like typical atrial flutter and AVNRT have substantially higher long-term success rates.
Reentry Outside of Cardiology
The term “reentry” appears in two other common contexts. In aerospace, atmospheric reentry refers to the return of a spacecraft through a planet’s atmosphere. As a vehicle descends at extreme speed, it compresses the air ahead of it, generating shock waves and temperatures reaching thousands of degrees Celsius. The Perseverance rover’s heat shield hit about 1,300°C (2,370°F) roughly 80 seconds after entering the Martian atmosphere. Thermal protection systems are designed to absorb and deflect this heat to keep the spacecraft and its occupants or instruments intact.
In criminal justice and social services, reentry refers to the process of individuals returning to the community after incarceration. This transition carries serious health risks: people with incarceration histories are 12 to 13 times more likely to die within the first two weeks after release compared to the general population, driven largely by overdose, untreated medical conditions, and disrupted access to care.