Atrioventricular Nodal Reentrant Tachycardia (AVNRT) is a common form of supraventricular tachycardia (SVT), a rapid, regular heartbeat originating above the lower chambers of the heart. This condition involves episodes where the heart rate suddenly speeds up, typically ranging from 120 to 200 beats per minute. While generally not life-threatening, these sudden episodes can be highly disruptive. This article explains the electrical cause of AVNRT, the symptoms it produces, and the modern methods used for diagnosis and resolution.
Understanding the Electrical Short Circuit
The heart’s electrical system relies on the atrioventricular (AV) node to regulate the flow of impulses from the upper chambers (atria) to the lower chambers (ventricles). Normally, signals pass through a single path. In people with AVNRT, the AV node tissue contains two distinct electrical pathways: a “fast” pathway and a “slow” pathway.
These two tracks have different conductive properties. The fast pathway has a longer refractory period, meaning it takes longer to reset after conducting a signal. Conversely, the slow pathway conducts signals more sluggishly but resets more quickly. This difference creates the potential for a reentrant circuit—a short circuit where an electrical impulse gets caught in a loop.
When a premature beat occurs, it may find the fast pathway still recovering (refractory) and be forced down the slow pathway. By the time the impulse emerges from the slow pathway, the fast pathway has recovered and conducts the signal backward. This movement down the slow path and immediately back up the fast path creates a continuous, circular electrical current within the AV node. This rapidly spinning circuit drives the ventricles at an abnormally fast rate, causing the abrupt onset of tachycardia.
Recognizing Symptoms and Common Triggers
The most recognizable symptom of AVNRT is the sudden onset of rapid, regular palpitations, often described as a pounding or fluttering sensation in the chest. During an episode, the heart rate can jump from a resting rate to over 150 beats per minute almost instantly. This rapid rate may lead to anxiety, dizziness, or lightheadedness because the heart does not have enough time to properly fill with blood between beats.
Some individuals report an uncomfortable pulsing sensation in their neck, sometimes called “frog sign.” This symptom occurs because the upper and lower heart chambers contract almost simultaneously during tachycardia. Since the atria cannot effectively empty into the ventricles, blood backs up into the jugular veins in the neck.
Episodes can be initiated by non-cardiac factors that increase the heart’s electrical excitability. Common triggers include stimulants like caffeine or alcohol, and periods of intense physical exertion. Other triggers involve emotional stress, anxiety, lack of sleep, or certain over-the-counter cold and allergy medications.
Confirming the Diagnosis and Stopping Acute Episodes
A definitive diagnosis of AVNRT relies on capturing an episode on an Electrocardiogram (ECG). During an AVNRT episode, the ECG typically shows a rapid, narrow-complex rhythm. The P wave (atrial activation) is often hidden within or appears immediately after the QRS complex (ventricular activation). This specific electrical pattern helps distinguish AVNRT from other supraventricular tachycardias.
For uncertain diagnoses, an Electrophysiology (EP) Study offers the most precise confirmation. This invasive procedure involves threading thin catheters through the veins into the heart to map its electrical system. The arrhythmia is intentionally induced under controlled conditions, allowing the EP study to pinpoint the dual pathways and the exact location of the reentrant circuit.
Patients can be taught management techniques to interrupt an acute episode at home, known as vagal maneuvers. These physical actions stimulate the vagus nerve, which has a direct inhibitory effect on the AV node and can slow the heart rate. Common maneuvers include the Valsalva maneuver (bearing down) or immersing the face in ice-cold water. These are typically the first-line intervention for a stable, acute episode.
Long-Term Solutions and Curative Procedures
For patients experiencing frequent or highly symptomatic episodes, the preferred long-term solution is Catheter Ablation. This procedure is considered curative for AVNRT and has a high success rate, often exceeding 95%. During the ablation, an electrophysiologist uses mapping information from the EP study to navigate a catheter to the slow pathway within the AV node.
Once positioned, the catheter delivers energy, usually radiofrequency (heat) or cryotherapy (cold), to create a tiny, precise scar. This controlled injury permanently neutralizes the slow pathway, eliminating one of the two necessary components of the short circuit. Interrupting the reentrant loop prevents the rapid heart rhythm from starting again.
For individuals with infrequent or mild symptoms, or those who prefer not to undergo ablation, medications can manage the condition. Oral medications, such as beta-blockers or calcium channel blockers (like verapamil and diltiazem), slow conduction through the AV node. These drugs reduce the frequency or severity of episodes, but they do not offer a permanent cure.