ECG criteria
- Narrow QRS — usually less than 120 ms; duration is measured from the widest complex in available leads. If the QRS reaches 120 ms or more in even a single lead, the tachycardia is classified as wide complex
- Heart rate above 100 bpm; the range of 150–250 bpm is typical primarily for paroxysmal reentrant SVTs (AVNRT, AVRT), whereas sinus tachycardia, AF, and multifocal atrial tachycardia frequently present with a heart rate of 101–149 bpm
- Assess RR regularity: in case of an irregular rhythm, consider atrial fibrillation, atrial flutter with variable conduction, or multifocal atrial tachycardia; a regular rhythm makes typical AF unlikely, but does not completely rule it out, for instance, in the presence of complete AV block with a regular escape rhythm amidst ongoing atrial fibrillation
- Identify the P wave: presence, morphology, position relative to the QRS (before, inside, after), and its polarity in II and V1
- In atrial flutter, search for sawtooth F waves in II, III, aVF at a rate of approximately 300 bpm (with 2:1 conduction — a ventricular rate of about 150 bpm)
- Assess response to vagal maneuvers or adenosine: termination of tachycardia, transient AV block unmasking P waves, or no effect
- In AV nodal tachycardia (AVNRT), look for pseudo-R' in V1 and pseudo-S in II, III, aVF due to simultaneous atrial and ventricular activation
What it looks like on ECG
The baseline for analysis is a QRS duration of less than 120 ms, which immediately narrows down the differential diagnosis to supraventricular mechanisms (barring rare exceptions such as fascicular or septal VT, discussed in the topic "VT vs. SVT with aberrancy"). Next, evaluate rhythm regularity and the P wave.
| Rhythm | P wave | Typical picture |
|---|---|---|
| Sinus tachycardia | before each QRS, normal morphology | gradual onset and termination, heart rate often 100–150 bpm |
| Atrial tachycardia | before QRS, morphology differs from sinus | PR interval may be prolonged |
| AVNRT (typical) | hidden in QRS or immediately after it | pseudo-R' in V1, pseudo-S in II, III, aVF, heart rate usually 150–250 bpm |
| Atrioventricular reentrant tachycardia (orthodromic in WPW) | after QRS, RP shorter than PR | RP usually greater than 70 ms, heart rate typically 150–250 bpm |
| Atrial flutter | F waves instead of P, sawtooth pattern in II, III, aVF | rate around 300 bpm, often 2:1 conduction |
| Atrial fibrillation | absent, baseline is undulating | rhythm absolutely irregular, heart rate variable |
| Multifocal atrial tachycardia | at least 3 different P wave morphologies | irregular PP and RR intervals, variable PR intervals, isoelectric line between P waves, heart rate above 100 bpm |
The RP interval is important for dividing narrow tachycardias into short-RP tachycardias (RP shorter than half the RR interval, typically less than 70 ms for typical AVNRT) and long-RP tachycardias (RP longer than half the RR interval). Long-RP tachycardia is seen in focal atrial tachycardia, atypical AVNRT, permanent junctional reciprocating tachycardia (PJRT — a chronic form of AV reciprocating tachycardia utilizing a slowly conducting concealed accessory pathway), and certain variants of orthodromic AVRT with slow retrograde conduction; RP alone does not definitively establish the mechanism.
Mechanism
A narrow QRS is formed when the ventricles are depolarized via the normal His-Purkinje system, regardless of where the impulse originated — in the sinus node, atria, AV node, or via a reentrant mechanism involving an accessory pathway. Most paroxysmal forms are based on a reentrant mechanism: a macro-reentrant circuit within the AV node (AVNRT) or utilizing an accessory pathway between the atria and ventricles (atrioventricular reciprocating tachycardia). Focal atrial tachycardia may be automatic, triggered, or due to micro-reentry; accelerated junctional rhythm is the result of enhanced automaticity, whereas AVNRT and most paroxysmal AV reciprocating tachycardias are due to reentry. In atrial flutter and fibrillation, the mechanism is macro-reentry and multiple chaotic wavelets of activation in the atria, respectively, while a narrow QRS is preserved because the AV node and His-Purkinje system remain intact.
Causes
- Sinus tachycardia: fever, hypovolemia, pain, anxiety, thyrotoxicosis, anemia, physical exertion
- AVNRT: the most common form of paroxysmal SVT, functional longitudinal dissociation of the AV node into two conduction pathways
- AV reciprocating tachycardia: presence of an accessory pathway (WPW syndrome, concealed pathways)
- Atrial tachycardia: focal ectopic activity, often in the setting of structural heart disease, COPD, digoxin toxicity
- Multifocal atrial tachycardia: severe COPD, hypoxia, electrolyte abnormalities
- Atrial flutter and fibrillation: structural heart disease, arterial hypertension, thyrotoxicosis, post-cardiac surgery
- Junctional tachycardia: digoxin toxicity, post-cardiac surgery, inferior wall ischemia
Differential diagnosis
- Sinus tachycardia vs. atrial tachycardia — P wave morphology: in sinus tachycardia, the P wave is identical to the baseline sinus P, whereas in atrial tachycardia, the morphology differs
- AVNRT vs. AV reciprocating tachycardia — P wave position relative to QRS: in AVNRT, the P wave is hidden within the QRS or immediately follows it (short RP), whereas in orthodromic AVRT via an accessory pathway, the RP interval is typically markedly greater than 70 ms
- Atrial flutter 2:1 vs. sinus tachycardia with a heart rate around 150 — meticulous search for F waves in II, III, aVF, performance of vagal maneuvers or administration of adenosine to unmask them
- Narrow QRS tachycardia vs. wide QRS tachycardia with aberrant conduction — a QRS duration of less than 120 ms generally renders the Brugada and Vereckei algorithms used for wide QRS unnecessary, but does not preclude clinical evaluation when fascicular VT is suspected
- Multifocal atrial tachycardia vs. atrial fibrillation — in MAT, distinct P waves of varying morphologies are visible with an isoelectric segment between them, whereas in AF, there are no organized P waves
Clinical significance and management
Danger depends not on QRS width, but on the underlying etiology and hemodynamic impact. In hemodynamic instability (hypotension, pulmonary edema, ischemia, altered mental status) caused directly by the tachyarrhythmia, emergent synchronized electrical cardioversion is indicated. Conversely, if a narrow QRS tachycardia is compensatory — for example, sinus tachycardia secondary to shock, blood loss, or sepsis — cardioversion will not resolve the cause of instability and may delay necessary treatment; the primary etiology must be sought and treated. Multifocal atrial tachycardia is also generally not treated with cardioversion; management focuses on the underlying condition (hypoxia, COPD, electrolyte disturbances). In a stable patient with a regular tachycardia of unclear etiology, vagal maneuvers and intravenous adenosine serve as both diagnostic and therapeutic tools: termination of the tachycardia supports an AV node-dependent reentrant mechanism (AVNRT or AVRT), but does not absolutely prove it, as adenosine can also suppress certain focal atrial tachycardias by affecting automaticity or triggered activity; transient AV block without tachycardia termination unmasks atrial activity in atrial flutter or atrial tachycardia. Atrial fibrillation and flutter are managed according to distinct rate and rhythm control algorithms with assessment of thromboembolic risk. Sinus tachycardia is not treated with antiarrhythmic drugs — the underlying cause is identified and corrected.
Common interpretation errors
- Mistaking a P wave superimposed on the preceding T wave for T wave abnormalities
- Measuring QRS width in only a single lead and missing a wider complex in another lead
- Diagnosing sinus tachycardia without comparing P wave morphology to the patient's baseline ECG
- Confusing 2:1 atrial flutter with sinus or atrial tachycardia without searching for F waves
- In an irregular tachycardia with predominantly narrow complexes, mistaking an isolated wide complex due to the Ashman phenomenon for a premature ventricular contraction
1. Which QRS duration criterion is used to classify a tachycardia as "narrow complex"?
The 120 ms threshold is the standard criterion separating narrow QRS from wide QRS in the differential diagnosis of tachycardias.
2. Which ECG finding is most characteristic of typical atrioventricular nodal reentrant tachycardia?
In AVNRT, the P wave is buried within the terminal portion of the QRS, creating a pseudo-R' in V1 and a pseudo-S in II, III, aVF.
3. What is the purpose of administering adenosine in stable regular narrow-complex tachycardia?
Adenosine transiently blocks AV conduction, which either terminates AV node-dependent tachycardia or unmasks atrial activity (flutter, atrial tachycardia).
Frequently asked questions
How to distinguish narrow QRS tachycardia from wide QRS tachycardia?
The QRS width is measured using the widest complex among the available leads: less than 120 ms indicates a narrow complex, which usually suggests a supraventricular origin with normal conduction through the His-Purkinje system. For a width of 120 ms or more, specific differential diagnosis algorithms for VT and SVT with aberrancy are applied.
Is narrow QRS tachycardia dangerous?
A narrow QRS by itself does not indicate danger. Its clinical significance is determined by the cause (ranging from benign sinus tachycardia to atrial fibrillation with stroke risk) and the patient's hemodynamic status at the time of the episode.
Why do vagal maneuvers and adenosine aid in diagnosis?
They temporarily block conduction through the AV node. Termination of tachycardia is most characteristic of AV node-dependent re-entry (AVNRT, AVRT), but it is not absolutely specific—adenosine can also terminate or slow certain focal atrial rhythms. If the rhythm does not terminate, but AV block appears, flutter waves or atrial tachycardia are unmasked.
What is the difference between short and long RP tachycardia?
With a short RP interval (less than half the RR interval), the P wave is hidden within or immediately after the QRS complex—typical for AVNRT. With a long RP interval, the P wave is visible well before the next QRS complex—characteristic of atrial tachycardia and atypical forms of re-entrant tachycardias, including permanent junctional reciprocating tachycardia (PJRT).
Can ventricular tachycardia be immediately ruled out based on a narrow QRS?
Practically yes: VT almost always presents with a wide QRS because the impulse bypasses the normal conduction system. A rare exception is fascicular or septal VT, which can produce a relatively narrow complex; therefore, in cases of clinical doubt, one should not rely solely on QRS width.
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Related terms
More in the section "Criteria, Formulas, Algorithms"
Sources
- Goldberger's Clinical Electrocardiography
- Hampton. The ECG in Practice
- Surawicz, Knilans. Chou's Electrocardiography in Clinical Practice
- Orlov V.N. Guide to Electrocardiography
Educational material for medical students and physicians. Does not replace clinical guidelines and physical examination; the ECG must always be interpreted in conjunction with the clinical picture.