Narrow Complex Tachycardia

NCT = Narrow QRS (<120 ms) fast rate (>100 bpm) regardless of origin

SVT = Any tachyarrhythmia origin above or within the His bundle (i.e., supraventricular), often used synonymously with re-entrant tachycardias involving the AV node or atria

“SVT” in practice is often shorthand for paroxysmal, re-entrant, regular NCTs like AVNRT and AVRT.

But technically, any atrial or AV junction rhythm is supraventricular — including sinus tachycardia and atrial fibrillation

🔚 In Short:

SVT (clinical use) = AVNRT, AVRT, atrial tachycardia
NCT = ECG pattern
Some rhythms like sinus tachycardia, AF, and MAT are NCTs but are not typically labelled as SVT in clinical parlance.

also note if aberrant conduction (e.g., bundle branch block) or pre-excitation (e.g., WPW) is present, the QRS can appear wide — even though it’s still an SVT.

Rhythm	Is it a Narrow Complex Tachycardia (NCT)?	Is it an SVT? (anatomically)	Is it called SVT in practice?	Explanation
Sinus tachycardia	✅ Yes	✅ Yes (from SA node)	❌ Often excluded	Though supraventricular in origin, not usually grouped with “SVT” in electrophysiology
Inappropriate sinus tachycardia	✅ Yes	✅ Yes	❌ Often excluded	Autonomic dysfunction, not a pathological circuit
Atrial tachycardia (focal)	✅ Yes	✅ Yes	✅ Yes	True SVT, arises from ectopic atrial focus
Atrial flutter	✅ Yes (unless aberrant)	✅ Yes	✅ Sometimes	Re-entrant, atrial macro-circuit → considered SVT in many guidelines
Atrial fibrillation	✅ Yes (unless aberrant)	✅ Yes	❌ Usually excluded	Irregular chaotic atrial activation, not grouped under “paroxysmal SVT”
Multifocal atrial tachycardia (MAT)	✅ Yes	✅ Yes	❌ Rarely	Multiple atrial foci; irregular rhythm → not typically called SVT
AVNRT	✅ Yes	✅ Yes	✅ Yes	Prototypical SVT
AVRT (e.g. WPW, orthodromic)	✅ Yes	✅ Yes	✅ Yes	SVT involving accessory pathway and AV node
Junctional ectopic tachycardia	✅ Yes	✅ Yes	✅ Sometimes	Often seen post-op in peds; not a re-entrant SVT but included due to location

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🔷 A. CLASSIFICATION BY MECHANISM

Category	Mechanism	Examples
1. Automatic	Increased automaticity	Sinus tachycardia, focal atrial tachycardia
2. Triggered	Afterdepolarisations	Some atrial tachycardias
3. Re-entrant	Re-entrant circuits	AVNRT, AVRT, atrial flutter, atrial tachycardia

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🔷 B. CLASSIFICATION BY AV NODE DEPENDENCE

AV Node–Dependent NCTs	AV Node–Independent NCTs
▪ AVNRT (most common PSVT)	▪ Sinus tachycardia
▪ AVRT (e.g., WPW orthodromic)	▪ Atrial fibrillation
▪ Junctional ectopic tachycardia	▪ Atrial flutter (esp. typical)
	▪ Atrial tachycardia (unifocal/multifocal)
	▪ Multifocal atrial tachycardia (MAT)

🔑 Therapeutic clue: Adenosine or vagal manoeuvres terminate AV node–dependent arrhythmias but only slow ventricular response in AV node–independent rhythms.

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🔷 C. CLASSIFICATION BY ECG REGULARITY

Regular NCTs	Irregular NCTs
▪ Sinus tachycardia	▪ Atrial fibrillation
▪ AVNRT	▪ Multifocal atrial tachycardia (MAT)
▪ AVRT	▪ Atrial flutter with variable AV block
▪ Atrial tachycardia (focal, organized)

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🔷 D. CLASSIFICATION BY SITE OF ORIGIN

Location	Examples
1. SA Node	▪ Sinus tachycardia ▪ Inappropriate sinus tachycardia
2. Atria	▪ Atrial tachycardia ▪ Atrial flutter ▪ Atrial fibrillation
3. AV Node	▪ AVNRT ▪ Junctional ectopic tachycardia
4. Accessory Pathway	▪ AVRT (e.g. WPW with orthodromic conduction)

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🔷 E. CLASSIFICATION BY CLINICAL CONTEXT

Type	Associated Conditions
Sinus tachycardia	Fever, hypovolaemia, pain, thyrotoxicosis
AVNRT	Paroxysmal palpitations, often young women
AVRT (e.g., WPW)	Palpitations, syncopal episodes
Atrial flutter/fibrillation	Structural heart disease, post-op state
MAT	COPD, metabolic derangement

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CLASSIFICATION BY AV NODE DEPENDENCE

1. AV Node–Independent Tachycardias

Definition: The arrhythmia originates and persists without relying on the AV node for its generation or maintenance.

The ventricular rate in these cases still passes through the AV node, but blocking the AV node doesn’t terminate the underlying rhythm.

Implication: AV nodal blockers (e.g., adenosine) will slow the ventricular response or cause transient AV block, but the atrial arrhythmia continues.

In some cases (like pre-excited AF, e.g., WPW), AV nodal blockade can be dangerous by promoting conduction down the accessory pathway → very rapid ventricular response → ventricular fibrillation.

• Sinus tachycardia
• Atrial tachycardia (unifocal or multifocal)
• Atrial flutter
• Atrial fibrillation
• Multifocal atrial tachycardia (MAT)

Tachycardia Type	Mechanism	Key ECG Clues	Adenosine Effect
Sinus Tachycardia	Enhanced automaticity	Upright P waves before each QRS, gradual onset	Transient AV block only
Atrial Tachycardia	Ectopic atrial focus	Abnormal P morphology, fixed RP interval	Does not terminate
Atrial Flutter	Macro-reentry (usually RA)	Sawtooth flutter waves, often 2:1 block	AV block → unmask flutter waves
Atrial Fibrillation	Multiple wavelets in atria	Irregularly irregular, no distinct P waves	Slows response, rhythm persists
Multifocal Atrial Tachycardia (MAT)	Multiple atrial foci	≥3 different P wave morphologies, irregular	Does not terminate

2. AV Node–Dependent Tachycardias

Definition: The generation and maintenance of the arrhythmia requires conduction through the AV node.

Implication: If AV node conduction is blocked (e.g. with adenosine, vagal manoeuvres, β-blockers, or non-dihydropyridine calcium channel blockers), the arrhythmia stops.

• AVNRT (AV nodal re-entrant tachycardia)
• AVRT (AV re-entrant tachycardia via accessory pathway – e.g., WPW)
• Junctional tachycardia (rare, usually in children or post-op cardiac surgery)

Tachycardia Type	Mechanism	Key ECG Clues	Adenosine Effect

AVNRT

Re-entry within AV node

Regular, no visible P waves or retrograde P

Terminates

AVRT (orthodromic WPW)

Re-entry via AV node + accessory pathway

Retrograde P after QRS (RP interval short)

Terminates

Junctional Tachycardia

Automatic or re-entry in AV node

Regular, narrow QRS, P may be inverted/absent

May slow or terminate

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🔹 Initial Management of Undifferentiated Narrow Complex Tachycardia

• Vagal maneuvers (modified Valsalva preferred)
• Adenosine 6–12 mg IV push
  • Halve dose if: heart transplant, central line, or on dipyridamole
  • AVNRT / AVRT: likely terminates
  • Atrial flutter / atrial tachycardia / AF: transient AV block; rhythm persists atrial rhythm persists despite AV node blockade.

❗ Transient ventricular asystole (standstill) after adenosine is normal. Treat only if prolonged (>10 sec) or hemodynamically unstable.

🔚 In Short:

AV node dependent = tachycardia stops if AV node conduction is blocked.

AV node independent = atrial rhythm persists despite AV node blockade.

Always consider the underlying rhythm, not just the ventricular rate (i.e., don’t confuse tachycardia with the arrhythmia generating it).

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🔹 AV Node–Independent Tachycardias: Management

🟢 Sinus Tachycardia

• Treat underlying cause (e.g., fever, dehydration, pain)

🟢 Atrial Tachycardia

• Correct electrolytes (K⁺, Mg²⁺)
• Stop pro-arrhythmic agents (digoxin, theophylline)
• Medications:
  • β-blockers (e.g., metoprolol, sotalol for unifocal)
  • Amiodarone
  • Verapamil or diltiazem
• Cardioversion if unstable

🟢 Atrial Flutter

• Type I: typical (rate ~300 bpm) – may respond to overdrive pacing
• Rate control: β-blockers, diltiazem, digoxin
• Rhythm control: amiodarone, flecainide
• DC cardioversion if unstable or persistent
• Catheter ablation is definitive

🟢 Atrial Fibrillation

• Rate control: β-blockers, NDHP CCBs, amiodarone, digoxin
• Rhythm control: flecainide, amiodarone, or synchronized cardioversion
• Anticoagulation: based on CHA₂DS₂-VASc score
• Avoid AV nodal blockers in WPW!

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🔹 AV Node–Dependent Tachycardias: Management

🟢 AVNRT / Orthodromic AVRT

• Vagal maneuvers
• Adenosine
• β-blockers, verapamil, diltiazem
• Flecainide or amiodarone if recurrent
• Synchronized cardioversion if unstable
• Catheter ablation for definitive cure

⚠️ Avoid verapamil/diltiazem in WPW with pre-excited AF

🟢 Junctional Tachycardia

• Often automatic, not re-entrant
• Address underlying causes (e.g. post-op, myocarditis)
• Medications: amiodarone, flecainide