Atrioventricular nodal reentrant tachycardia (AVNRT) produces a distinctive pattern on the electrocardiogram: a regular, narrow-complex tachycardia where the P wave is either buried inside the QRS complex or appears as a tiny distortion just after it. The hallmark clues are a small upward deflection at the end of the QRS in lead V1 (called a pseudo r’ wave) and a small downward notch at the tail end of the QRS in the inferior leads (called a pseudo S wave). These features are absent on the patient’s baseline ECG when the heart is in normal rhythm, which is what makes them so useful. But recognizing AVNRT on a rhythm strip is trickier than textbooks suggest, and the ECG can look quite different depending on which subtype of AVNRT is running.
The Signature ECG Findings
The two most widely taught ECG markers for typical AVNRT are the pseudo r’ wave in lead V1 and the pseudo S wave in leads II, III, and aVF. Neither of these deflections represents actual ventricular activity. Instead, they are caused by the retrograde P wave (atrial depolarization traveling backward from the AV node toward the atria) occurring almost simultaneously with the QRS complex. Because atrial and ventricular activation happen at nearly the same instant in typical AVNRT, the P wave gets superimposed on the QRS. It peeks out as a tiny bump or notch that wasn’t there on the patient’s sinus rhythm tracing.
These classic criteria remain the most commonly used approach to diagnosing AVNRT on the surface ECG, even though newer criteria involving leads aVL and aVR have been proposed over the past decade.1PubMed Central. Electrocardiographic differentiation of narrow QRS complex tachycardia: validation of the new aVL and aVR criteria and a unifying approach to ECG analysis Additional proposed markers include a notch in lead aVL, a pseudo r’ wave in lead aVR, and any deflection appearing more than 100 milliseconds after the QRS complex during tachycardia.2PubMed. Diagnostic Accuracy of Several Electrocardiographic Criteria for the Prediction of Atrioventricular Nodal Reentrant Tachycardia In practice, though, most clinicians still look first for the pseudo r’ in V1 and pseudo S in the inferior leads, partly because they are intuitive and partly because the newer criteria have not consistently outperformed them.
A third scenario is even simpler: sometimes no P waves are visible at all during the tachycardia. When atrial and ventricular activation are perfectly simultaneous, the retrograde P wave is completely hidden within the QRS. The absence of any visible P wave during a regular narrow-complex tachycardia is itself a clue that you may be looking at AVNRT.
Why the P Wave Hides in the QRS
AVNRT exists because the AV node region contains two functionally distinct pathways with different conduction speeds and recovery times. One pathway conducts quickly but takes longer to recover (the fast pathway), while the other conducts more slowly but recovers sooner (the slow pathway). In sinus rhythm, the electrical impulse travels down both, but the fast pathway wins the race and determines the PR interval. The slow pathway’s impulse arrives late and gets canceled out. You never see it on the ECG.
When a premature atrial beat arrives at just the right moment, the fast pathway may still be recovering and can’t conduct. The impulse then travels exclusively down the slow pathway, reaches the ventricle late, and by the time it gets there the fast pathway has recovered. The impulse can now travel backward (retrograde) up the fast pathway, re-excite the atria, and send another impulse down the slow pathway, setting up a self-sustaining loop.3PubMed. Determinants of fast- and slow-pathway conduction in patients with dual atrioventricular nodal pathways In the common “slow-fast” form, the impulse goes down the slow pathway and back up the fast pathway. Because retrograde conduction over the fast pathway is rapid, the atria get activated almost at the same time the ventricles do. That near-simultaneous activation is why the P wave sits right on top of the QRS.
Short RP Versus Long RP and What It Means
One of the most practical things the ECG tells you about a supraventricular tachycardia is the relationship between the P wave and the QRS complexes around it. In typical (slow-fast) AVNRT, the retrograde P wave falls very close to the preceding QRS, creating what is called a short RP tachycardia. The RP interval in typical AVNRT is usually extremely short, often 90 milliseconds or less when a retrograde P wave is visible at all.4PubMed. Electrocardiographic differentiation of common type atrioventricular nodal reentrant tachycardia from atrioventricular reciprocating tachycardia via a concealed accessory pathway
Atypical AVNRT flips this relationship. In the fast-slow form, the impulse goes down the fast pathway and returns up the slow pathway. Because retrograde conduction is now slow, the P wave appears well after the QRS, closer to the next QRS than to the previous one. This produces a long RP tachycardia.5Heart Rhythm Case Reports. Typical atrioventricular nodal reentrant tachycardia presenting as short RP tachycardia: A case report The distinction matters because the differential diagnosis changes entirely. Short RP tachycardia raises the question of AVNRT versus orthodromic AVRT (a tachycardia using a hidden accessory pathway). Long RP tachycardia brings atypical AVNRT, atrial tachycardia, and a rare form of AVRT called permanent junctional reciprocating tachycardia (PJRT) into the picture.
Telling AVNRT Apart from Other Narrow-Complex Tachycardias
Distinguishing AVNRT from AVRT using a concealed accessory pathway is one of the classic challenges in ECG interpretation. Both can present as regular narrow-complex tachycardias with rates in the 150 to 250 range. Both can have visible retrograde P waves. Several ECG features help tilt the diagnosis one way or the other.
The pseudo r’ wave in V1 strongly favors AVNRT. One study found it had a specificity of about 97% for AVNRT over AVRT, meaning it very rarely appears in accessory pathway tachycardias. Its sensitivity is modest, though, around 39%, so its absence doesn’t rule AVNRT out. The pseudo S wave in inferior leads performed similarly: highly specific (about 94%) but not very sensitive (roughly 28%).6PubMed. Electrocardiographic differentiation of common type atrioventricular nodal reentrant tachycardia from atrioventricular reciprocating tachycardia via a concealed accessory pathway In other words, when you see these features, you can be fairly confident it’s AVNRT, but many AVNRT episodes won’t show them clearly.
QRS alternans (a beat-to-beat variation in QRS height) tends to favor AVRT. One analysis reported a sensitivity of 50% and specificity of about 89% for AVRT when alternans was present. ST-segment changes during tachycardia also showed up more frequently in AVRT patients.7PubMed. Differentiating atrioventricular nodal reentrant tachycardia from tachycardia via concealed accessory pathway
For long RP tachycardias, the differential is different. Here you’re trying to separate atypical AVNRT from atrial tachycardia. One approach uses the ratio of the RP interval to the PR interval. A study found that atrial tachycardia tended to have a higher RP/PR ratio (averaging about 2.0) compared to non-atrial-tachycardia long RP tachycardias (averaging about 1.6), with an optimal cutoff of 1.65.8Journal of Arrhythmia. Differentiation of atrial tachycardia from other long RP tachycardias by electrocardiographic characteristics The test isn’t definitive on its own, but it adds a piece to the puzzle.
Atypical Forms and Their ECG Signatures
Typical slow-fast AVNRT accounts for the vast majority of cases, but the atypical variants have their own ECG personalities that are worth recognizing. In the fast-slow form, the circuit runs in reverse: antegrade conduction goes through the fast pathway and retrograde return travels up a slow pathway. The ECG shows a long RP interval with retrograde P waves that may be inverted in the inferior leads, and the overall appearance can closely mimic atrial tachycardia.
Within the atypical group, there’s further variation depending on which slow pathway the circuit uses. A “superior” slow pathway variant produces retrograde atrial activation above the His bundle region and a long RP interval.9PubMed Central. Atrioventricular Ring Tachycardias: Atypical Fast-Slow Atrioventricular Nodal Reentrant Tachycardia and Atrial Tachycardia Share a Common Arrhythmogenic Substrate—A Unifying Proposal Another variant uses a slow pathway that extends toward the lower right atrium along the tricuspid annulus, also showing a long RP interval but with the earliest retrograde atrial activation in a different location.10Circulation Reports. Atypical Fast-Slow Atrioventricular Nodal Reentrant Tachycardia Utilizing a Slow Pathway Extending to the Inferolateral Right Atrium These distinctions are hard to make from the surface ECG alone and usually require an invasive electrophysiology study to sort out, but the key surface clue is the long RP interval with negative P waves in the inferior leads.
The slow-slow form is a third variant where both the antegrade and retrograde limbs of the circuit use slow pathways. On the surface ECG, this can look somewhat like the typical form (the retrograde P wave may appear just after the QRS), but the overall rate tends to be slower because conduction in both directions takes longer. During an electrophysiology study, this form is identified by a prolonged interval in both the forward and backward limbs of the circuit.11Arrhythmia & Electrophysiology Review. Classification, Electrophysiological Features and Therapy of Atrioventricular Nodal Reentrant Tachycardia
ST-Segment Depression During AVNRT
ST depression during an AVNRT episode can look alarming, especially in an older patient or someone with cardiac risk factors, because it can mimic the ECG pattern of cardiac ischemia. Clinicians sometimes order cardiac catheterizations or troponin workups based on these ST changes. But research consistently shows that this ST depression is a rate-related artifact, not a sign of inadequate blood flow to the heart.
The mechanism is straightforward. At fast heart rates, each QRS complex starts overlapping with the T wave of the beat before it. This shifts the electrical baseline of the ECG, making the ST segment appear depressed relative to the new baseline. Studies have confirmed that the degree of ST depression during AVNRT correlates with the tachycardia cycle length: the faster the rate, the more pronounced the apparent depression.12PubMed Central. ST-segment depression in atrioventricular nodal reentrant tachycardia: Important finding or just an artifact? The same group concluded that ischemia could be excluded as the cause of these ST changes in their patient cohort.13Advances in Clinical and Experimental Medicine. ST-segment depression in atrioventricular nodal reentrant tachycardia: Preliminary results
An additional factor specific to AVNRT involves the very short interval between ventricular and atrial activation. Patients with a shorter VA interval (the gap between when the ventricle fires and when the atrium fires) were more likely to have ST depression at a given heart rate, at least for AVNRT, though this relationship did not hold for AVRT.14EP Europace. Reason of corrected ST segment depression during a narrow QRS complex tachycardia (AVNRT and AVRT) The practical takeaway: ST depression during an SVT episode should be interpreted cautiously and not automatically attributed to coronary artery disease.
When AVNRT Looks Wide
AVNRT is considered a narrow-complex tachycardia, but it doesn’t always stay narrow. Intermittent aberrancy, where one of the bundle branches temporarily fails to conduct because it hasn’t recovered from the prior beat, can widen the QRS during some or all beats of the tachycardia. This produces an ECG that can closely resemble ventricular tachycardia, a much more dangerous rhythm.
One clue that you’re dealing with AVNRT with aberrancy rather than true ventricular tachycardia is that the cycle length (the interval between beats) stays the same when the QRS transitions from narrow to wide. If the tachycardia mechanism is the same and only the QRS shape is changing, the timing of the circuit doesn’t shift.15JACC: Case Reports. AVNRT With Intermittent Aberrancy-Mimicking Ventricular Tachycardia: One Tachycardia, Two QRS Morphologies This is a subtle finding that requires careful measurement, but it can prevent unnecessary escalation of treatment in a hemodynamically stable patient.
How the ECG Responds to Vagal Maneuvers and Adenosine
Because AVNRT depends on the AV node for its circuit, anything that transiently slows AV nodal conduction can break the loop and terminate the tachycardia. This gives both vagal maneuvers and adenosine a dual role: they are treatments, and the ECG’s response to them provides diagnostic information.
Vagal maneuvers such as the Valsalva maneuver work by increasing parasympathetic tone to the AV node. In AVNRT, the tachycardia typically ends abruptly: the rhythm snaps back to sinus without gradually slowing down first. A study of 48 patients with AVNRT found that when vagal maneuvers successfully terminated the arrhythmia, they did so by blocking conduction in either the slow pathway (about 14% of cases) or the fast pathway (about 19%).16PubMed. Electrophysiological mechanisms and determinants of vagal maneuvers for termination of paroxysmal supraventricular tachycardia The modified Valsalva maneuver, where the patient strains and then is quickly repositioned supine with legs elevated, appears to augment the vagal effect by adding a boost in venous return that further activates the baroreceptors.17PubMed Central. Pursuit of Optimal Vagal Maneuvers in Stable Supraventricular Tachycardia: A Network Meta-Analysis
Adenosine works by briefly shutting down AV nodal conduction. In one study, adenosine terminated AVNRT in 86% of episodes and AVRT in 85%, so the raw termination rate doesn’t help you distinguish between them. But the pattern of termination can. In typical AVNRT, the VA interval (the time from ventricular to atrial activation) did not change before the rhythm stopped, while in AVRT, the VA interval lengthened in about 13% of cases before termination.18PubMed. Electrophysiologic effects of adenosine in patients with supraventricular tachycardia If adenosine causes the tachycardia to continue but with AV block (the ventricles stop responding while the atrial rhythm persists), that points strongly toward atrial tachycardia rather than AVNRT, since AVNRT can’t persist without the AV node conducting.19Journal of the American College of Cardiology. Diagnostic and therapeutic use of adenosine in patients with supraventricular tachyarrhythmias
What the Electrophysiology Study Reveals That the Surface ECG Cannot
The surface ECG gives you the broad strokes, but pinning down the exact AVNRT subtype and confirming the diagnosis often requires an invasive electrophysiology study (EPS). During an EPS, catheters placed inside the heart record electrical signals directly from the AV node region, the His bundle, and the coronary sinus. This allows precise measurement of intervals that the surface ECG can only hint at.
In typical slow-fast AVNRT, the HA interval (the time from the His bundle signal to the earliest atrial activation in that region) is usually less than 70 milliseconds, and the VA interval measured from the surface QRS onset to the atrial electrogram is less than 60 milliseconds. In atypical fast-slow AVNRT, the HA interval stretches beyond 70 milliseconds, and the VA interval exceeds 60 milliseconds. The slow-slow variant has a prolonged HA interval like the fast-slow form, but the forward limb is also slow, with the AH interval exceeding 200 milliseconds.20Arrhythmia & Electrophysiology Review. Classification, Electrophysiological Features and Therapy of Atrioventricular Nodal Reentrant Tachycardia These measurements are what definitively categorize a patient’s AVNRT into one of the subtypes and guide ablation strategy.
One characteristic finding during EPS for the fast-slow variant is that the retrograde conduction time through the slow pathway actually shortens after the initial induction of the tachycardia. Researchers measured this by comparing the HA interval in the first beat after induction to the HA interval once the tachycardia stabilized, and found a consistent decrease.21PubMed. Unique electrophysiological properties of fast-slow atrioventricular nodal reentrant tachycardia characterized by a shortening of retrograde conduction time via a slow pathway manifested during atrial induction This kind of detail is invisible on a surface ECG but helps the electrophysiologist confirm which pathway is doing what.
ECG Changes After Catheter Ablation
Catheter ablation of the slow pathway is the definitive treatment for AVNRT, with high success rates and a low risk of complications. But the procedure does leave a footprint on the ECG. After ablation, studies have found that the heart rate at rest tends to increase slightly, the QRS duration shows a small but measurable prolongation, and the PR interval lengthens by a modest amount as well. Importantly, the corrected QT interval does not change significantly.22Eskisehir Medical Journal. The Effects of AVNRT Ablation on the Conduction System These changes are typically minor and don’t cause symptoms, but they’re worth knowing about because a slightly longer PR interval after ablation could raise unnecessary concern about heart block if the baseline ECG isn’t compared.
The main complication to watch for is inadvertent damage to the fast pathway, which can produce AV block requiring a pacemaker. The risk is small but not zero. During ablation, electrophysiologists track several safety markers to stay away from the fast pathway. One key metric is the distance between the atrial signal recorded at the His bundle catheter and the atrial signal at the ablation catheter tip: when this interval is too short, the ablation catheter is dangerously close to the compact AV node and fast pathway.23PubMed. Prediction of atrioventricular block during radiofrequency ablation of the slow pathway of the atrioventricular node Cases where the slow pathway is the dominant conducting pathway present a particular challenge, since ablating it could unmask inadequate fast pathway function. Even in these unusual scenarios, successful ablation is achievable with careful monitoring of fast pathway conduction throughout the procedure.24PubMed Central. Ablation of AVNRT in a Setting of Slow Pathway-Dominant Conduction
AVNRT in Children and Adolescents
AVNRT is the most common supraventricular tachycardia in adults, but it also shows up in pediatric patients, particularly in adolescents. In one series of 190 children who underwent catheter ablation for various tachyarrhythmias, about a quarter were diagnosed with AVNRT, and the average age was around 13 years.25Clinical and Experimental Pediatrics. Radiofrequency catheter ablation of atrioventricular nodal reentry tachycardia in children and adolescents: a single center experience The ECG features are the same as in adults: pseudo r’ in V1, pseudo S in inferior leads, and a regular narrow-complex tachycardia at a rate that often surprises parents and pediatricians alike.
The challenge in children is partly practical. Younger patients may not be able to describe palpitations clearly, and brief episodes of SVT can be dismissed as anxiety or exertion-related. The recurrence rate after ablation in this series was roughly 12% (confirmed by ECG during follow-up), which is somewhat higher than the recurrence rates typically reported in adults. Whether this reflects ongoing maturation of the AV node region or the technical challenge of ablating smaller structures in younger patients remains an active area of investigation. Clinicians tend to be more conservative about recommending ablation in very young children, reserving it for cases that are frequent, symptomatic, or medication-resistant, partly because of the proximity of the ablation target to the AV node and the stakes of accidentally creating heart block in a patient with decades of life ahead.