Wide-Complex Tachycardia and SVT: How to Tell Atrial Tachycardia, AVNRT and AVRT Apart in Clinical Practice
- Faraz Afzal
- 6 days ago
- 12 min read
SVT is a term we use frequently in clinical practice. But SVT is not one single diagnosis. It is an umbrella term for several different supraventricular tachycardias, each with its own mechanism, ECG appearance and treatment.
This article is the first part of a series on SVT and ECG interpretation in clinical practice.
We start with a clinical case where the diagnosis is not immediately obvious: a fast, regular tachycardia with a wide QRS complex, known right bundle branch block, no effect from beta-blocker therapy, adenosine causing AV block without terminating the arrhythmia — and later a Wenckebach pattern after amiodarone.
This is exactly what ECG interpretation often looks like in real life. You do not always have the final answer immediately. You have to interpret the rhythm step by step.
A Patient With Palpitations, Fever and Diarrhoea
A man in his 60s is admitted with palpitations, fever and diarrhoea. The ECG shows a fast, regular tachycardia with a heart rate around 182/min.
The QRS complexes are wide, but the patient is known to have a right bundle branch block from previous ECGs. The QRS morphology during the tachycardia is unchanged compared with old ECGs.
This initially raises suspicion of a supraventricular tachycardia with pre-existing bundle branch block.
The most important differential diagnoses are:
AVNRT
AVRT
atrial tachycardia
atrial flutter with fixed conduction
ventricular tachycardia
The patient receives a beta-blocker without effect. Adenosine is then administered. On telemetry, a short pause with AV block is seen, but the tachycardia continues afterwards with the same rate and the same QRS morphology.
This is where the diagnostic reasoning becomes interesting.
Briefly Explained: What Is Actually Happening?
At first glance, this looks like a typical SVT. The rhythm is fast and regular, and the wide QRS can be explained by the known right bundle branch block.
But the response to adenosine does not fit well with classic AVNRT or typical orthodromic AVRT.*
In AVNRT and orthodromic AVRT, the AV node is a necessary part of the re-entry circuit. When adenosine blocks the AV node, the circuit is usually interrupted immediately. This is why these tachycardias often terminate abruptly after adenosine.
In this case, something else happens:
Adenosine causes transient AV block.
The ventricles pause.
But the atrial rhythm continues.
When the effect of adenosine wears off, the tachycardia returns with the same rate and the same QRS morphology.
This means that the AV node is not the actual driver of the arrhythmia. It is only the conduction pathway between the atria and the ventricles.
This points towards atrial tachycardia.
Failure of tachycardia termination despite clear adenosine-induced AV block strongly argues against classic AVNRT or orthodromic AVRT and supports an atrial tachycardia mechanism.
* Orthodromic AVRT is a re-entry tachycardia where the impulse travels down to the ventricles through the AV node and returns to the atria through an accessory pathway. The QRS complex is therefore usually narrow unless bundle branch block is present.
What Is Atrial Tachycardia?
Atrial tachycardia is a supraventricular tachycardia in which the impulses arise in the atria, without the AV node being necessary to maintain the arrhythmia.
This is what separates atrial tachycardia from AVNRT and AVRT.
In atrial tachycardia, the focus is located in the atrium. The atria fire on their own, and the impulses must then pass through the AV node to reach the ventricles.
In AVNRT, the AV node is part of the re-entry circuit itself.
In AVRT, the re-entry circuit usually travels through the AV node in one direction and through an accessory pathway in the other direction.
Why This Difference Matters
In atrial tachycardia, the driver is located in the atrium. In AVNRT and orthodromic AVRT, the AV node is part of the arrhythmia circuit itself.
This is why atrial tachycardia can continue even when the AV node is slowed or blocked. The AV node only determines how many impulses are allowed to pass through to the ventricles.
Atrial Tachycardia Can Look Like Any Other SVT
Atrial tachycardia can be difficult to recognize, especially at high rates. The P waves may be hidden in the T wave, the ST segment or very close to the QRS complex.
For this reason, the rhythm can be mistaken for AVNRT, AVRT, atrial flutter with fixed conduction, sinus tachycardia during acute illness — and, if the QRS is wide, ventricular tachycardia.
This is why the response to adenosine and the pattern of AV conduction become important.
Adenosine in SVT: Treatment and Diagnostic Test
Adenosine is commonly used in fast, regular SVT. But in clinical practice, the key question is not only whether the tachycardia stops.
The most important thing is to observe what happens to AV conduction and atrial activity while adenosine is active.
Adenosine causes a transient block of the AV node. This can either terminate an AV node-dependent arrhythmia or reveal atrial activity that is otherwise hidden.
If the tachycardia stops abruptly, this supports AVNRT or orthodromic AVRT, because the AV node is then a necessary part of the re-entry circuit.
If adenosine causes AV block but atrial activity continues, this instead supports atrial tachycardia or atrial flutter. In that situation, the AV node is not the driver of the arrhythmia, but only the gate between the atria and the ventricles.
In this case, we see exactly that: the AV node is briefly blocked, but the arrhythmia continues. This is a central diagnostic finding.
Table 1: Mechanism and Adenosine Response in AVNRT, AVRT and Atrial Tachycardia
Rhythm | Where does the arrhythmia arise? | Is the AV node part of the circuit? | What does adenosine do? |
AVNRT | In or near the AV node | Yes | Often terminates the tachycardia |
AVRT | Between the AV node and an accessory pathway | Yes | Often terminates the tachycardia |
Atrial tachycardia | In the atrium | No | May cause AV block, but the arrhythmia continues |
Why Does the AV Node Produce Wenckebach?
After amiodarone, so-called group beatings are seen: the rhythm appears in groups, and some beats are dropped.
This fits with the Wenckebach phenomenon, also called second-degree AV block, type 1 or Mobitz I.
To understand why this happens, we need to understand an important property of the AV node: decremental conduction.
Decremental Conduction: The Special Property of the AV Node
Decremental conduction means that the AV node conducts more slowly when impulses arrive rapidly and closely together.
The shorter the time between impulses, the more refractory the AV node becomes. Eventually, an impulse arrives while the AV node is still not ready to conduct. That impulse is then blocked.
This is a particularly important property of the AV node.
In electrophysiology, decremental conduction is often used as a sign that the impulses are actually passing through the AV node, and not through a typical accessory pathway. Ordinary conduction pathways conduct more directly. The AV node behaves differently: it slows, delays and filters.
A simple way to say it:
The AV node does not merely conduct; it also functions as a biological filter.
This is why the AV node can produce a Wenckebach pattern when impulses arrive too rapidly.
A Practical Way to Understand AV-Conduction
It can be useful to think of AV conduction as active work.
The AV node does not simply let signals pass through passively. It must continuously recover between impulses. When the atria fire rapidly, the workload on the AV node increases.
In atrial tachycardia around 180/min, impulses arrive very close together. Each new impulse meets an AV node that may not yet have fully recovered. In atrial tachycardia, the impulses arrive so rapidly that the AV node gradually conducts more slowly until one impulse is blocked. The pause after the dropped beat allows the AV node to recover again. This results in the Wenckebach phenomenon, which represents the decremental property of the AV node.
The AV Node Is Influenced by Autonomic Tone and Medication
The number of impulses that the AV node allows through can vary. It is determined not only by the atrial rate, but also by autonomic tone, refractory period, medications and acute illness.
Table 2: Factors That Influence AV Nodal Conduction
Factor | Effect on AV conduction | Clinical significance |
High atrial rate | The AV node has less time to recover | Can cause 2:1 conduction, variable conduction or Wenckebach |
Increased sympathetic tone | Makes the AV node more willing to conduct | More impulses may pass through to the ventricles |
Increased vagal tone | Makes the AV node slower and more refractory | Fewer impulses pass through |
Beta-blockers, calcium channel blockers, digoxin and amiodarone | Slow the AV node | Can reduce ventricular rate or cause AV block/Wenckebach |
Electrolyte disturbances and acute illness | Can change refractoriness and conduction properties | The same atrial tachycardia can produce different ventricular responses |
This is why the same atrial tachycardia can present with 1:1 conduction, 2:1 conduction, variable conduction or a Wenckebach pattern. It does not necessarily mean that the atrial tachycardia itself has changed — it may be the AV node that has changed how much it allows through.
Practical ECG Interpretation in Fast Regular Tachycardia
When standing at the bedside, you rarely have the final answer immediately. A systematic approach is useful: start with the patient, assess QRS width, look for regularity and P waves — and carefully interpret what happens during adenosine.
Table 3: Stepwise ECG Interpretation in Fast Regular Tachycardia
Step | Question | What are you looking for? | Practical interpretation |
1 | Is the patient stable? | Hypotension, chest pain, syncope, reduced consciousness, heart failure, pulmonary oedema or shock. | Always start with the patient, not the ECG. In an unstable patient, the treatment is synchronized cardioversion. |
2 | Is the QRS narrow or wide? | A narrow QRS often supports supraventricular tachycardia. A wide QRS can be caused by ventricular tachycardia, SVT with known bundle branch block, rate-dependent aberrancy, pre-excitation, electrolyte disturbance or drug effect. | In wide QRS tachycardia, always compare with old ECGs. Known bundle branch block may explain the wide QRS, but ventricular tachycardia must always be considered. |
3 | Is the rhythm regular? | A regular rhythm fits with AVNRT, AVRT, atrial tachycardia, atrial flutter with fixed conduction or monomorphic ventricular tachycardia. | An irregular rhythm suggests other differential diagnoses, particularly atrial fibrillation, atrial flutter with variable conduction and polymorphic ventricular tachycardia. |
4 | Can you see P waves or atrial activity? | P waves may be hidden in the T wave, the ST segment, immediately after the QRS or just before the QRS. | Adenosine or other AV nodal slowing can make P waves, flutter waves or other atrial activity visible by briefly blocking the ventricular response. |
5 | What happens with adenosine? | The tachycardia terminates, AV block occurs, atrial activity continues, flutter waves become visible — or very little happens. | The most important thing is to observe the rhythm while adenosine is active. Keep ECG/telemetry connected, because the response itself often gives the diagnosis. |
Table 4: How to Interpret the Response to Adenosine
Adenosine response | Practical interpretation |
The tachycardia terminates abruptly | Supports AVNRT or orthodromic AVRT |
AV block, but atrial activity continues | Supports atrial tachycardia or atrial flutter |
Flutter waves become visible | Supports atrial flutter |
No clear AV block | Consider dose, IV access, administration technique or another mechanism |
Wide-complex rhythm continues without AV response | Ventricular tachycardia must be strongly considered |
Why Does Wenckebach Support the Diagnosis of Atrial Tachycardia?
Wenckebach during ongoing tachycardia is diagnostically useful because it shows that the atria continue to fire, while the AV node allows only some of the impulses through.
This fits well with atrial tachycardia.
In atrial tachycardia, the rhythm focus is located in the atrium. The atria continue to send impulses independently of the AV node. The AV node only determines how many impulses reach the ventricles.
In classic AVNRT or orthodromic AVRT, this is different. There, the AV node is part of the re-entry circuit itself. If the AV node is blocked, the circuit is usually interrupted and the tachycardia terminates.
Therefore, the combination of:
persistent regular atrial activity
AV block or Wenckebach
tachycardia that does not terminate
is a strong argument that the rhythm is driven from the atria.
Atrial tachycardia continues even if the AV node slows or blocks conduction, because the arrhythmia is driven from the atrium. AVNRT and orthodromic AVRT often stop when the AV node is blocked, because the AV node is part of the re-entry circuit itself.
Wide QRS: SVT With Bundle Branch Block or Ventricular Tachycardia?
This patient had a wide QRS during tachycardia. That can be concerning, because wide-complex tachycardia must always be considered possible ventricular tachycardia.
In this case, several findings still support a supraventricular origin:
The patient has known right bundle branch block.
The QRS morphology is identical to previous ECGs.
The rhythm is regular.
Adenosine causes AV block.
The tachycardia continues through AV block, supporting an atrial mechanism.
But precision is important here:
A wide QRS does not always mean ventricular tachycardia, but ventricular tachycardia must always be considered in wide-complex tachycardia.
If the patient is unstable, one should not spend too much time on detailed ECG diagnosis. In that situation, synchronized cardioversion is the correct treatment. The ESC guidelines for SVT recommend urgent synchronized cardioversion in hemodynamically unstable supraventricular tachycardia.
Cardioversion in Persistent Atrial Tachycardia
Because the arrhythmia persists, the patient is electrically cardioverted to sinus rhythm.
This is reasonable when the tachycardia cannot be controlled with medication, when the patient is symptomatic or when there is hemodynamic compromise.
In instability, synchronized cardioversion is the first-line treatment.
Digression: Tachycardia Is Not Only an Electrical Problem
The telemetry also records photoplethysmography, often through the pulse oximeter. The PPG waveform reflects pulsatile changes in blood volume in peripheral tissue.
During the tachycardia, a clearly reduced pulse amplitude is seen. When the heart rate is around 180/min, diastole becomes short. The ventricles have less time to fill. Stroke volume falls, and the peripheral pulse wave becomes weaker. This is why the pleth waveform becomes lower.
This is an important clinical reminder:
An arrhythmia is not only an ECG phenomenon. It affects the circulation beat by beat. In a patient with fever, diarrhoea, possible dehydration and reduced physiological reserve, fast atrial tachycardia can cause significant hemodynamic compromise.
Clinical Learning Points
SVT is not one single diagnosis, but an umbrella term for arrhythmias conducted through the AV node.
Adenosine is both a treatment and a diagnostic test.
ECG should be connected while adenosine is administered.
AV block without termination supports an atrial mechanism; that is, the rhythm is driven by the atrium.
In AVNRT and orthodromic AVRT, the AV node is part of the re-entry circuit.
Decremental conduction is a characteristic property of the AV node and can be used electrophysiologically to understand where impulses are conducted.
Wenckebach during tachycardia supports the idea that the AV node is filtering a fast atrial rhythm.
Known bundle branch block can make SVT a wide-complex tachycardia.
In wide-complex tachycardia, ventricular tachycardia must always be considered.
PPG can show the hemodynamic consequence of tachycardia.
Conclusion
This case shows how a fast, regular tachycardia with a wide QRS complex can be misinterpreted if one only looks at the rate and QRS width.
The key finding is that adenosine causes AV block without terminating the arrhythmia. Together with the later Wenckebach pattern, this points towards atrial tachycardia, where the rhythm is driven from the atria and the AV node merely filters impulses onwards to the ventricles.
In SVT, it is not enough to ask whether adenosine “worked”. Look at what happens to AV conduction and atrial activity while adenosine is active. And always keep continuous ECG monitoring running while adenosine is administered.
Frequently Asked Questions
What does SVT mean?
SVT stands for supraventricular tachycardia. It is not one single diagnosis, but an umbrella term for several fast heart rhythms that originate above the ventricles or are conducted through the AV node. Examples include AVNRT, AVRT, atrial tachycardia and atrial flutter.
Can SVT have a wide QRS complex?
Yes. SVT can appear as a wide-complex tachycardia if the patient has a pre-existing bundle branch block, rate-dependent aberrancy, pre-excitation, electrolyte abnormalities or drug-related conduction changes. However, ventricular tachycardia must always be considered when the QRS complex is wide.
How can you tell the difference between SVT and ventricular tachycardia?
The first step is to assess the patient clinically. If the patient is unstable, synchronized cardioversion is the appropriate treatment. In stable patients, ECG interpretation includes QRS morphology, comparison with previous ECGs, rhythm regularity, AV dissociation, capture or fusion beats, and the response to adenosine. A known unchanged bundle branch block may support SVT, but ventricular tachycardia must never be dismissed too early.
Why is adenosine useful in SVT?
Adenosine temporarily blocks conduction through the AV node. This can terminate AV node-dependent tachycardias such as AVNRT and orthodromic AVRT. It can also reveal hidden atrial activity, such as atrial tachycardia or atrial flutter, by briefly blocking the ventricular response.
What does it mean if adenosine causes AV block but the tachycardia continues?
If adenosine causes clear AV block but the atrial rhythm continues, the AV node is unlikely to be the driver of the arrhythmia. This supports an atrial mechanism, such as atrial tachycardia or atrial flutter, rather than classic AVNRT or orthodromic AVRT.
How does atrial tachycardia differ from AVNRT and AVRT?
In atrial tachycardia, the rhythm originates from a focus in the atrium. The AV node is not required to maintain the arrhythmia; it only conducts some of the atrial impulses to the ventricles. In AVNRT and orthodromic AVRT, the AV node is usually part of the re-entry circuit itself.
Why does AVNRT often stop with adenosine?
AVNRT depends on the AV node as part of the re-entry circuit. When adenosine temporarily blocks the AV node, the circuit is interrupted and the tachycardia often terminates abruptly.
Why can atrial tachycardia continue after adenosine?
Atrial tachycardia can continue after adenosine because the arrhythmia is driven from the atrium, not from the AV node. Adenosine may block conduction to the ventricles temporarily, but the atrial rhythm itself can keep going.
What is Wenckebach during tachycardia?
Wenckebach, also called second-degree AV block type I or Mobitz I, occurs when AV nodal conduction gradually slows until one atrial impulse fails to conduct to the ventricles. During atrial tachycardia, this can happen because rapid atrial impulses repeatedly reach an AV node that has not fully recovered.
Why does Wenckebach support atrial tachycardia?
Wenckebach during an ongoing tachycardia suggests that the atria continue to fire while the AV node filters the impulses. This fits well with atrial tachycardia, where the arrhythmia continues independently of AV nodal conduction.
What is decremental conduction?
Decremental conduction means that the AV node conducts more slowly when impulses arrive rapidly and close together. This is a characteristic property of the AV node. It allows the AV node to act as a biological filter between the atria and the ventricles.
When should synchronized cardioversion be used in SVT?
Synchronized cardioversion should be used when the patient is hemodynamically unstable, for example with hypotension, syncope, reduced consciousness, chest pain, heart failure, pulmonary oedema or shock. In these situations, treatment should not be delayed by prolonged ECG analysis.