[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Clinical skills](https://durgical.com/en/clinical-skills/) › [The ECG](https://durgical.com/en/clinical-skills/areas/ecg/) › Narrow-complex tachycardia (SVT)

# Narrow-complex tachycardia (SVT)

- ECG tracing

A fast heartbeat that starts above the ventricles; the narrow QRS and the position of the P waves on the ECG show which kind it is.

Updated Nov 13, 2026

## Key points

- Narrow-complex tachycardia: rate over 100/min and QRS under 120 ms; first decide regular or irregular.
- AVNRT: regular at 150–250/min, P waves hidden in the QRS or seen as a pseudo-r' in V1 and pseudo-S in II, III and aVF.
- AVRT (orthodromic): retrograde P waves just after the QRS, in the ST segment; look for a delta wave on the sinus ECG.
- Stable patient: vagal manoeuvres, then adenosine with a rhythm strip running; unstable: synchronised cardioversion.
- Never assume a broad-complex tachycardia is SVT with aberrancy: treat it as VT unless clearly proven otherwise.

On this page

- The basics Key points
- What it is
- How to recognise it
- Why it happens
- Step by step
- Regular narrow-complex tachycardias: where the P wave is and how adenosine helps.
- Next step
- Memory aid
- Elsewhere in DurgiPedia

- Clinical Differential and pitfalls

- Study Test yourself

- Connections Connections

## What it is

A narrow-complex tachycardia has a rate over 100/min and a QRS under 120 ms, which means the ventricles are activated through the normal His–Purkinje system, so the rhythm starts at or above the AV node. In practice «SVT» usually means paroxysmal supraventricular tachycardia: a regular rhythm, typically 150–250/min, that starts and stops abruptly. Its two main forms are reentrant circuits that use the AV node: AV nodal reentrant tachycardia (AVNRT), the commonest, with a circuit inside the node, and AV reentrant tachycardia (AVRT), with a circuit through an accessory pathway, as in Wolff–Parkinson–White syndrome. Focal atrial tachycardia is the third. Other narrow tachycardias must be told apart: sinus tachycardia, atrial flutter (often 2:1 at about 150/min), atrial fibrillation and multifocal atrial tachycardia. The reading rests on three questions: is it regular, where are the P waves (before, inside or just after the QRS), and what happens when the AV node is blocked for a moment with vagal manoeuvres or adenosine. Because AVNRT and AVRT need the AV node, blocking it usually ends them; flutter and atrial tachycardia continue but become visible.

## How to recognise it

- Width: QRS under 120 ms (3 small squares) in all leads means supraventricular origin with normal conduction.
- Regular or irregular: irregular points to AF, flutter with variable block or multifocal atrial tachycardia; regular points to sinus tachycardia, AVNRT, AVRT, atrial tachycardia or flutter with fixed block.
- Onset: abrupt start and stop favours reentry (AVNRT, AVRT); gradual rise and fall favours sinus tachycardia.
- RP interval: short RP (P within or just after the QRS) suggests AVNRT or AVRT; long RP (P closer to the next QRS) suggests atrial tachycardia, sinus tachycardia or atypical AVNRT.
- AVNRT: the retrograde P is buried in the QRS or appears at its end, under about 70 ms after QRS onset.
- AVRT: the retrograde P sits in the ST segment, usually over 70 ms after QRS onset; QRS alternans and rate-related ST depression can occur.
- Atrial tachycardia: abnormal P waves before each QRS, an atrial rate usually 100–250/min, and it can continue with AV block.
- A regular rate close to 150/min should always prompt a search for flutter waves in II, III, aVF and V1.

## Why it happens

- AVNRT uses two pathways inside the AV node region, a slow and a fast one; an ectopic beat that blocks in one starts a loop down one and up the other.
- In typical (slow–fast) AVNRT the atria and ventricles are activated almost at the same time, so the P wave hides inside the QRS.
- Orthodromic AVRT goes down the AV node and back up an accessory pathway; the ventricles fire first and the atria a little later, so the P falls after the QRS.
- Antidromic AVRT goes down the accessory pathway, so the QRS is broad and looks like VT.
- Focal atrial tachycardia comes from one atrial site firing fast; it does not need the AV node, which is why it can continue during AV block.
- Triggers include caffeine, alcohol, stimulant drugs, fatigue, thyroid excess and electrolyte disturbance, though many episodes have no trigger.

## Step by step

- Check the patient before the paper — shock, syncope, ischaemic chest pain or heart failure make it unstable and change the plan.
- Confirm the QRS is narrow in all 12 leads — any broad complex tachycardia is treated as VT until proven otherwise.
- Decide regular or irregular — irregular narrow tachycardia is usually AF.
- Look for P waves and measure the RP interval — position of the P separates AVNRT, AVRT and atrial tachycardia.
- Look at V1 and the inferior leads for a pseudo-r' or pseudo-S — they are the hidden P waves of AVNRT.
- Search for flutter waves at about 300/min — 2:1 flutter is the classic look-alike at 150/min.
- Record a continuous rhythm strip during vagal manoeuvres or adenosine — the response is diagnostic as well as therapeutic.
- Record a 12-lead ECG after reversion — a short PR and delta wave reveal a pre-excitation pathway.

## Regular narrow-complex tachycardias: where the P wave is and how adenosine helps.

| Rhythm | Usual rate | P wave | Response to adenosine |
| --- | --- | --- | --- |
| Sinus tachycardia | 100 to about 180/min, gradual | Normal P before each QRS | Brief slowing, then returns |
| [AVNRT](https://durgical.com/en/conditions/supraventricular-tachycardia/) | 150–250/min, abrupt | Hidden in QRS; pseudo-r' in V1 | Usually terminates |
| [Orthodromic AVRT](https://durgical.com/en/conditions/wolff-parkinson-white-syndrome/) | 150–250/min, abrupt | Retrograde, in the ST segment | Usually terminates |
| Atrial tachycardia | 100–250/min | Abnormal P before QRS (long RP) | AV block shows the P waves; may persist |
| [Atrial flutter 2:1](https://durgical.com/en/conditions/atrial-flutter/) | About 150/min | Sawtooth waves at about 300/min | Unmasks flutter waves; does not terminate |

## Differential and pitfalls

- Sinus tachycardia is the most common narrow tachycardia: it starts and ends gradually and has a cause (fever, pain, bleeding, sepsis, hypoxia); treat the cause, not the rhythm.
- 2:1 atrial flutter at about 150/min is often read as sinus tachycardia or AVNRT; adenosine slows the ventricles and unmasks the flutter waves.
- SVT with a bundle branch block or rate-related aberrancy gives a broad QRS and can look like VT; if in doubt, manage as VT.
- Pre-excited AF is irregular and broad; adenosine and other AV-node blockers can push it into ventricular fibrillation.
- ST depression during SVT is common and often rate-related; it does not mean infarction on its own, but recheck after reversion.
- Adenosine causes brief flushing, chest tightness and a few seconds of asystole; warn the patient beforehand and use it with caution in severe asthma.

## Next step

- Unstable: call a senior and arrange synchronised cardioversion at once.
- Stable and regular: vagal manoeuvres (such as the modified Valsalva), then adenosine through a large proximal vein with a rhythm strip recording.
- If adenosine fails or is contraindicated: a senior chooses verapamil, diltiazem or a beta-blocker; refer early if the patient has known pre-excitation.
- After reversion: 12-lead ECG, electrolytes and thyroid function; look for a delta wave and for structural heart disease.
- Refer to cardiology for frequent, prolonged or poorly tolerated episodes, and for any pre-excitation: catheter ablation often cures the circuit.
- Teach the patient vagal manoeuvres and when to come back: chest pain, fainting or episodes that do not stop.

## Memory aid

Narrow and regular? Find the P: inside the QRS = AVNRT, just after = AVRT, before = atrial or sinus.

## Test yourself

Think of the answer, then tap the question to see it.

## A 25-year-old has sudden palpitations: regular narrow QRS at 190/min, no visible P waves, small r' in V1 that is absent in her sinus ECG. Most likely rhythm?

AV nodal reentrant tachycardia: the pseudo-r' in V1 is the retrograde P wave at the end of the QRS. If stable, start with vagal manoeuvres, then adenosine with a rhythm strip.

## In a regular narrow tachycardia, a retrograde P wave sits in the ST segment and the sinus ECG shows a short PR with a delta wave. What is it?

Orthodromic AV reentrant tachycardia using an accessory pathway (Wolff–Parkinson–White). Refer for electrophysiology study and ablation.

## Adenosine is given for a regular tachycardia at 150/min and the strip shows regular sawtooth waves at 300/min with no QRS. What does this show?

Atrial flutter with 2:1 block: adenosine blocked the AV node and unmasked the flutter waves. It does not stop the flutter; plan rate control and anticoagulation.

## Why is a broad-complex regular tachycardia not treated as SVT with aberrancy by default?

Most broad-complex tachycardias are ventricular, especially with heart disease; drugs given for SVT, such as verapamil, can cause collapse in VT. Manage it as VT unless clearly proven otherwise.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/clinical-skills/narrow-complex-tachycardia/)

## Elsewhere in DurgiPedia

- [ECG basics: waves, intervals and leads](https://durgical.com/en/physiology/ecg-basics-waves-intervals-and-leads/)
- [How arrhythmias arise](https://durgical.com/en/physiology/how-arrhythmias-arise/)
- [The cardiac conduction system](https://durgical.com/en/physiology/the-cardiac-conduction-system/)
- [Arterial pulse: rate, rhythm and amplitude](https://durgical.com/en/clinical-exam/arterial-pulse-rate-rhythm-and-amplitude/)
- [Cardiac monitoring at the bedside: leads, alarms and rhythm changes to report](https://durgical.com/en/nursing/cardiac-monitoring-at-the-bedside-leads-alarms-and-rhythm-changes-to-report/)

## Connections

## Related 9

- [Reading an ECG step by step](https://durgical.com/en/clinical-skills/reading-an-ecg-step-by-step/)
- [Atrial fibrillation and atrial flutter](https://durgical.com/en/clinical-skills/atrial-fibrillation-and-atrial-flutter/)
- [Sinus rhythm, sinus tachycardia, bradycardia and arrhythmia](https://durgical.com/en/clinical-skills/sinus-rhythm-sinus-tachycardia-bradycardia-and-arrhythmia/)
- [Broad-complex tachycardia, VF and the arrest rhythms](https://durgical.com/en/clinical-skills/broad-complex-tachycardia-vf-and-the-arrest-rhythms/)
- [Pre-excitation: Wolff–Parkinson–White](https://durgical.com/en/clinical-skills/pre-excitation-wolff-parkinson-white/)
- [Vagal manoeuvres and carotid sinus massage](https://durgical.com/en/clinical-skills/vagal-manoeuvres-and-carotid-sinus-massage/)
- [Defibrillation and synchronised cardioversion](https://durgical.com/en/clinical-skills/defibrillation-and-synchronised-cardioversion/)
- [Thyroid function tests](https://durgical.com/en/clinical-skills/thyroid-function-tests/)
- [Holter and ambulatory blood-pressure monitoring](https://durgical.com/en/clinical-skills/holter-and-ambulatory-blood-pressure-monitoring/)

## Related conditions 5

- [Supraventricular tachycardia (AVNRT, AVRT)](https://durgical.com/en/conditions/supraventricular-tachycardia/)
- [Wolff-Parkinson-White syndrome](https://durgical.com/en/conditions/wolff-parkinson-white-syndrome/)
- [Atrial flutter](https://durgical.com/en/conditions/atrial-flutter/)
- [Atrial fibrillation](https://durgical.com/en/conditions/atrial-fibrillation/)
- [Graves disease (hyperthyroidism)](https://durgical.com/en/conditions/graves-disease/)

## In Pharmacology 6

- [Adenosine](https://durgical.com/en/pharmacology/adenosine/)
- [Verapamil](https://durgical.com/en/pharmacology/verapamil/)
- [Diltiazem](https://durgical.com/en/pharmacology/diltiazem/)
- [Beta blockers](https://durgical.com/en/pharmacology/classes/beta-blockers/)
- [Metoprolol](https://durgical.com/en/pharmacology/metoprolol/)
- [Flecainide](https://durgical.com/en/pharmacology/flecainide/)

[← All clinical skills](https://durgical.com/en/clinical-skills/)

Educational content; it does not replace supervised hands-on training, your institution's protocols or a prescriber's order. A procedure is learnt with a teacher and done only once you are trained to do it.

Sources: [Supraventricular Tachycardia](https://www.ncbi.nlm.nih.gov/books/NBK441972/) (StatPearls (NCBI Bookshelf), 2025); [Paroxysmal Supraventricular Tachycardia](https://www.ncbi.nlm.nih.gov/books/NBK507699/) (StatPearls (NCBI Bookshelf), 2024).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/clinical-skills/narrow-complex-tachycardia/