Atrioventricular Nodal Reentrant Tachycardia (AVNRT) is a frequent type of supraventricular tachycardia (SVT) that causes the heart to beat unusually fast. This condition is characterized by episodes of a rapid, regular heart rhythm originating within the atrioventricular (AV) node. Although AVNRT is not considered life-threatening and usually occurs in people with otherwise healthy hearts, the sudden onset of a racing heart can be distressing and significantly impact quality of life.
Understanding the Heart’s Electrical Short Circuit
The heart’s normal electrical system relies on the AV node to act as a gatekeeper, delaying the electrical signal traveling from the atria to the ventricles. In AVNRT, the AV node contains two distinct pathways for electrical conduction, known as dual AV nodal physiology. These are the fast pathway, which conducts signals quickly but recovers slowly, and the slow pathway, which conducts signals sluggishly but recovers rapidly.
This dual pathway structure forms the anatomical substrate for a re-entry circuit, often described as an electrical short circuit. An AVNRT episode is triggered when a premature beat arrives at the AV node, finding the fast pathway blocked because it is still recovering. The signal is then forced to travel down the slow pathway.
Once the signal conducts down the slow pathway, the fast pathway recovers. The electrical impulse then travels backward (retrogradely) up the fast pathway and re-enters the slow pathway, creating a continuous, circular loop of electrical activity. This sustained re-entry circuit causes the ventricles to be stimulated rapidly, resulting in the sudden, high heart rate characteristic of AVNRT.
Identifying the Signs and Symptoms
An episode of AVNRT is marked by the abrupt start and equally abrupt cessation of a rapid heartbeat, which can occur without a specific trigger. During an event, the heart rate typically jumps to a range between 140 and 280 beats per minute. This fast rhythm is felt as regular palpitations or a fluttering sensation in the chest.
A unique symptom is a pounding or throbbing sensation in the neck, sometimes called “frog sign.” This occurs because the atria and ventricles contract nearly simultaneously due to the circuit, pushing blood back against the closed valves into the neck veins.
The rapid heart rate can also lead to lightheadedness, shortness of breath, or anxiety. Some people may experience mild chest discomfort or pressure due to the heart’s increased workload.
How Doctors Confirm the Diagnosis
The primary method for confirming AVNRT is capturing an episode on an Electrocardiogram (ECG) during the tachycardia. The ECG will show a regular, fast rhythm with narrow QRS complexes. A specific finding in typical AVNRT is that the electrical signal returning to the atria is so close to the ventricular signal that the P wave is often hidden within the QRS complex.
The retrograde P wave may sometimes appear as a subtle distortion on the ECG, such as a small notch or “pseudo-R prime” wave in lead V1. Since episodes are intermittent, doctors may prescribe ambulatory monitoring devices, such as a Holter or event monitor, to record the heart rhythm over an extended period.
For a definitive diagnosis, especially when planning treatment, an Electrophysiology (EP) study may be performed. During an EP study, thin catheters are guided into the heart to map the electrical pathways and intentionally trigger the AVNRT circuit. This study allows the cardiologist to precisely locate the dual pathways and confirm the re-entry mechanism, providing information for planning the most effective long-term treatment.
Managing and Eliminating AVNRT
The initial management of an acute AVNRT episode involves simple techniques known as vagal maneuvers, which stimulate the vagus nerve to slow conduction through the AV node. A common maneuver is the Valsalva technique, which involves bearing down for about 10 to 15 seconds. The modified Valsalva maneuver, which adds leg elevation immediately after the strain, is often more effective in terminating the rhythm.
If vagal maneuvers are unsuccessful, medical intervention is required, typically with intravenous adenosine in an emergency setting. Adenosine temporarily blocks conduction through the AV node, interrupting the re-entry circuit and restoring a normal rhythm. For those with infrequent or mild symptoms, long-term management involves oral medications like beta-blockers or calcium channel blockers to reduce the frequency and severity of episodes.
For patients experiencing frequent or debilitating episodes, the definitive treatment is catheter ablation. This procedure is guided by the mapping performed during the EP study and aims to permanently disrupt the slow pathway tissue. Using specialized catheters, the physician delivers energy—most commonly radiofrequency (heat) or sometimes cryoablation (cold)—to a targeted area near the slow pathway.
Catheter ablation of the slow pathway has a very high success rate, often exceeding 95%, making it a curative option for most people with AVNRT. The procedure carries a low complication risk, including a small chance of damaging the normal conduction system. By eliminating the slow pathway, the electrical short circuit is broken, preventing future episodes.