Supraventricular Tachycardia (SVT)

The AV node is the only normal route from the atria to the ventricles. Narrow-complex tachycardia needs one of two things: a re-entrant circuit or an atrial focus. The ventricles still fire through the normal His–Purkinje system, which is why the QRS stays narrow. Re-entry is a loop with two limbs, one slow and one fast. Add a timing condition that lets the impulse keep circling, and the loop runs.

Three mechanisms explain most regular narrow-complex tachycardias on this page:

• AV nodal re-entry (AVNRT): the circuit sits inside the AV node, usually between a slow and a fast pathway. Atria and ventricles both activate through the same node. That is why the retrograde P wave often hides in the QRS.
• AV re-entry (AVRT): the circuit runs through the AV node and an accessory pathway. That pathway is a strand of conducting tissue outside the node, and it can conduct in one direction or both.
• Atrial tachycardia: a focus in the atria fires fast and needs no AV circuit. The Focal Atrial Tachycardia section below covers it. Flutter and atrial fibrillation run on their own mechanisms, and Atrial Rhythms teaches those.

AVNRT and AVRT are paroxysmal. They start and stop all at once.

Every narrow or wide tachycardia gets the same six questions. The table on this page sorts each rhythm with its first-line action. Stability sets the first move. No table replaces the bedside check.

1. Find the rate: Count the R waves over 6 seconds and multiply by 10. The usual range for these re-entrant tachycardias in adults and older children is 150–250 bpm; infants run faster (see Pediatric & Special Populations).
2. Check regularity: Very regular points to AVNRT, AVRT, focal atrial tachycardia or flutter with fixed block. Irregularly irregular means atrial fibrillation, multifocal atrial tachycardia or pre-excited AF.
3. Look for P waves: Look in lead II and V1. Where a Lewis lead is available, it shows retrograde P waves more often than standard leads.
4. Measure the RP interval: Time it from the R wave to the next P wave. A short RP is the usual pattern for typical AVNRT. A long RP, with the P wave well after the QRS, is not typical AVNRT.
5. Measure the QRS: Narrow is supraventricular unless aberrancy is proven. Wide is VT, antidromic AVRT or pre-excited AF until proven otherwise.
6. Assess stability first: Hypotension, altered mentation, ischemic chest pain or acute heart failure makes the patient unstable. That patient needs cardioversion now.

Normal Conduction Values

AVNRT is a paroxysmal re-entrant tachycardia that stays inside the AV node. Along with orthodromic AVRT, it is one of the most frequent kinds of paroxysmal SVT. Expect it in adolescents and adults. It is less common in young children.

ECG Criteria

• QRS: Narrow, under 120 ms, unless there is a bundle branch block or rate-related aberrancy.
• Rhythm: Fast and very regular.
• P waves: The retrograde P often hides inside the QRS. A pseudo-r′ in V1 or a pseudo-S in the inferior leads supports AVNRT when you see it. Don't rule AVNRT out because they are missing. In one study they were present in only 37% and 20% of AVNRT patients.
• Onset and offset: Abrupt. The rhythm starts and stops suddenly.

Causes

• Substrate: Dual AV nodal physiology is the setup. A slow limb and a fast limb sit inside the node, and together they allow the re-entrant loop.
• Age: Less common in young children; more frequent after age 10.

Clinical Significance

• Symptoms: Expect palpitations, lightheadedness and chest discomfort. A rate this fast can drop the blood pressure and bring on angina.
• Treatment: Vagal maneuvers and adenosine can end it. AV nodal blockers can too, because the node is part of the circuit. For recurrent episodes, catheter ablation is the treatment. The 2025 pediatric review lists radiofrequency or cryoablation as first-line.

Look-Alikes

• Orthodromic AVRT: the RP interval separates the two (see the RP section below).
• Sinus tachycardia: see Differential Diagnosis & Mimics.

AVRT is a re-entrant circuit built from the AV node and an accessory pathway, a strand of conducting tissue outside the node. Orthodromic AVRT has a narrow QRS, and it is one of the most common paroxysmal SVTs. Antidromic AVRT has a wide QRS and can pass for VT.

ECG Criteria

• Orthodromic (narrow): Anterograde conduction runs down the AV node and retrograde conduction climbs the accessory pathway. The QRS is narrow. The retrograde P follows the QRS, and the RP interval is usually longer than in typical AVNRT.
• Antidromic (wide): Conduction runs down the accessory pathway and back up the AV node. The QRS is wide, so it mimics VT.
• Rate: Usually 150–250 bpm.
• Sinus rhythm clue: In sinus rhythm, a manifest pathway shows PR ≤120 ms, a delta wave and a QRS over 120 ms. The full WPW pattern is taught on PR Interval.

Causes

• Accessory pathway: The pathway is the substrate. A concealed pathway conducts only backward, so it shows no pre-excitation in sinus rhythm.
• Wolff-Parkinson-White (WPW): In fetal life and the first year of life, about 80% of SVTs are linked to WPW. In children older than 10, that falls to about 65%.

Clinical Significance

• The dangerous forms: Antidromic AVRT gets mistaken for VT. An AV nodal blocker given in pre-excited AF can drive the patient into VF.
• Definitive treatment: Ablation of the accessory pathway is the definitive treatment. The 2023 AF guideline recommends it for pre-excited AF. The 2025 pediatric review lists radiofrequency or cryoablation as first-line for AVRT.

Focal atrial tachycardia is a fast-firing focus in the atria that uses no AV circuit. It is usually regular. When the P waves are visible, it is a common look-alike for AVNRT. The 2015 ACC/AHA/HRS SVT guideline (References) covers the adult approach.

ECG Criteria

• P-wave morphology: The P wave looks different from a sinus P wave. Its shape and axis reflect where the focus sits. Compare it with a sinus P wave on an earlier strip.
• RP interval: Often long, with the P wave falling well after the QRS. That is the main reason to suspect atrial tachycardia over typical AVNRT.
• Onset: The rate often ramps up gradually and winds down at the end. AVNRT and AVRT start abruptly.
• AV block: 2:1 or other AV block can appear while the atrial rate keeps going.

Adenosine Response

• Adenosine can cause a transient AV block that unmasks the atrial rate. The atrial tachycardia usually keeps going, so an unchanged rhythm after adenosine does not rule out a focus.
• Most focal atrial tachycardias continue, with transient AV block. A minority are adenosine-sensitive and terminate. Termination alone does not fully rule out an atrial focus.

Regular and Irregular Atrial Rhythms

• Regular: focal atrial tachycardia, AVNRT, AVRT and atrial flutter.
• Irregular: multifocal atrial tachycardia (MAT), with at least three P-wave morphologies and a rate usually of 100 bpm or more; wandering atrial pacemaker (WAP), slower than 100 bpm with varying P-wave morphology.

Treat the cause. In stable, regular narrow-complex tachycardia, IV diltiazem or verapamil (COR 2a, LOE B-R) can be effective. IV beta-blockers have also been shown to terminate it. If the patient is unstable, use synchronized cardioversion (2025 AHA Part 9). See Bedside Management & Escalation.

Most regular narrow-complex tachycardias sort into three groups: typical AVNRT, AVRT and an atrial source. At the bedside, the RP interval and the response to adenosine help separate them. Pinning down the final mechanism may need an electrophysiology study.

RP Interval

• Typical AVNRT: Its RP interval is generally shorter than in AVRT.
• AVRT or focal atrial tachycardia (see its section): A P wave well after the QRS, a long RP, is not typical AVNRT.
• Limit: P waves are hard to see inside a narrow QRS. In one study, a Lewis lead, a bipolar chest lead, showed retrograde P waves in 70% (21 of 30) of AVNRT patients.

Adenosine as a Diagnostic Tool

• Mechanism: Adenosine slows conduction through the AV node. It can interrupt AV-node re-entry, so it treats the circuit and tests it at the same time.
• Termination: An abrupt stop points to an AV-node-dependent circuit. Adenosine does not convert atrial flutter or atrial fibrillation.
• Slowing: In flutter or fibrillation, adenosine can slow the ventricular response a little, and only for a moment. That slowing can unmask flutter waves or AF. Use it to reveal the rhythm, not to treat it.

Compare three things: the rhythm before the tachycardia, the QRS width and the response to a vagal maneuver or adenosine.

Artifact & Lead Misplacement

• Motion and shivering can mimic a fast regular rhythm or flutter waves on the monitor. Confirm it on a 12-lead.
• Limb lead reversal can invert the P wave in lead I and shift the apparent P axis. A sinus rhythm can then look atrial.
• Transport vibration adds noise that can look like a regular tachycardia. Pause or steady the patient before you judge the rhythm.

Wide and regular is VT until proven otherwise. For the full picture, see Ventricular Rhythms.

Stability comes first. An unstable patient gets synchronized cardioversion first, and drugs do not delay the shock. The 2025 AHA tachyarrhythmia algorithm (Box 6) also lists adenosine for a regular narrow-complex rhythm. A stable patient with a narrow, regular tachycardia gets a vagal maneuver, then adenosine. Where 2025 AHA states a dose, the doses below follow it. Verapamil, diltiazem and metoprolol doses come from the E3 medication pages.

Stability Approach

Therapy Table

Pediatrics

• Most common SVT: In infants and young children, AVRT is the most common SVT. In fetal life and the first year, about 80% of SVTs are linked to WPW.
• Heart rate: Infant SVT usually runs at or above 220 bpm. Sinus tachycardia usually stays below 220. Children's SVT is usually 180 bpm or more (2025 AHA PALS tachyarrhythmia algorithm). Check the age-based values on pediatric vital signs.
• Stable: Start with vagal stimulation (2025 AHA Part 8, COR 1, LOE C-LD). In infants, face immersion in cold water (the diving reflex) is one option. AHA notes the success rate of each maneuver is unknown. If vagal maneuvers fail and you have IV or IO access, adenosine is the next step (COR 1, LOE C-LD).
• Unstable: Synchronized cardioversion at 0.5–1 J/kg. If it fails, go to 2 J/kg (COR 2a, LOE C-LD).
• Drugs: Avoid verapamil in infants. Severe, sometimes fatal, hemodynamic reactions have followed IV doses in neonates and infants. Older children get it only with pediatric cardiology guidance. Diltiazem and metoprolol are not established in children. Dose by weight, using the weight-based dosing reference, and cap every drug at its maximum.

Other Populations

• Older adults: Give verapamil over at least 3 minutes. Expect lower doses in older patients, and expect more hypotension and bradycardia.
• Heart transplant: A denervated heart can react strongly to adenosine. 1 mg IV may be enough to terminate the rhythm (2025 AHA Part 9). Have the defibrillator ready.
• Pacemaker or ICD: Check the device before you cardiovert. The device therapy may already have ended the rhythm.
• Pregnancy: Vagal maneuvers and synchronized cardioversion are first-line, and neither depends on drugs. The adenosine label says studies have not been conducted in pregnant women. Use it when the benefit outweighs the risk, with obstetric and cardiology input. Avoid amiodarone and other drugs without specialist review.
• Known WPW: Treat any irregular wide rhythm as possible pre-excited AF. Adenosine can precipitate AF. If that AF turns wide and irregular, treat it as pre-excited AF. The WPW pattern in sinus rhythm is taught on PR Interval.

1. Check the patient first: Note blood pressure, mental status, chest pain and heart failure signs. Unstable means cardiovert now.
2. Measure the QRS in the widest lead: Narrow or wide sets the drug pathway.
3. Check regularity: Very regular points to AVNRT, AVRT or flutter. Irregular points to AF or pre-excited AF.
4. Find the P waves and the RP interval: Use lead II and V1, and a Lewis lead if available.
5. Choose the pathway: Stable narrow regular goes to a modified Valsalva, then adenosine. Wide is VT until proven otherwise. Irregular wide gets no AV nodal blocker and is cardioverted if unstable.
6. Reassess after each step: Check rate, blood pressure and rhythm after every drug or shock.
7. Document: Record the time of each drug and shock, and the rhythm after conversion.

Common Pitfalls

• Calling an irregular wide tachycardia SVT before you check for pre-excitation.
• Giving diltiazem or verapamil before you confirm the rhythm is narrow and regular.
• Ruling out AVNRT because pseudo-r′ or pseudo-S waves are missing. They are often missing.
• Giving another adenosine dose after high-level AV block on the first dose. The label says not to give more doses in that case.
• Giving adenosine to a patient with asthma or active bronchospasm. It is contraindicated (2025 AHA Part 9). Use synchronized cardioversion if unstable, or IV diltiazem or verapamil (Class 2a) if stable and regular.
• Treating sinus tachycardia as SVT. Find a normal P wave before each QRS.