How do you classify supraventricular tachycardia (SVT)?

Classification of Supraventricular Tachycardia

Supraventricular tachycardia is classified into three primary anatomic categories based on the tissue involved in the tachycardia circuit: atrial tachycardias (originating from atrial tissue), atrioventricular junctional tachycardias (involving the AV node), and atrioventricular tachycardias (requiring an accessory pathway). 1

Primary Classification Framework

The most clinically useful classification system divides SVT by anatomic origin and mechanism 1:

1. Atrial Tachycardias

These originate from atrial tissue above the AV node and include 1:

• Sinus tachycardias:

  • Physiological sinus tachycardia (appropriate response to exercise, stress, or other stimuli) 1
  • Inappropriate sinus tachycardia (sinus rate >100 bpm at rest with mean 24-hour rate >90 bpm without physiological cause) 1, 2
  • Sinus node reentrant tachycardia (microreentry within sinus node complex with P-wave morphology indistinguishable from sinus rhythm) 1

• Focal atrial tachycardia: Arises from a localized atrial site with regular, organized atrial activity and discrete P waves with isoelectric segments between them 1, 2

• Multifocal atrial tachycardia (MAT): Irregular SVT with ≥3 distinct P-wave morphologies at different rates 1

• Macroreentrant atrial tachycardias (atrial flutter):

  • Cavotricuspid isthmus-dependent (typical atrial flutter, counter-clockwise or clockwise) 1
  • Non-cavotricuspid isthmus-dependent (atypical flutter including mitral isthmus-dependent and other left or right atrial circuits) 1

2. Atrioventricular Junctional Tachycardias

These involve the AV node as the critical component 1:

• Atrioventricular nodal reentrant tachycardia (AVNRT): The most common form of paroxysmal SVT, caused by reentry within the AV node using dual pathways (fast and slow conduction pathways) 2, 3

  • Typical (slow-fast) AVNRT: Atrial activation nearly simultaneous with QRS, producing pseudo S waves in inferior leads and pseudo R' in V1 1
  • Atypical (fast-slow or slow-slow) AVNRT: P wave closer to subsequent QRS, producing long RP interval 1

• Non-paroxysmal junctional tachycardia: Automatic focus in AV junction 1

• Focal junctional tachycardia: Ectopic focus within junctional tissue 1

3. Atrioventricular Tachycardias (Accessory Pathway-Mediated)

These require an accessory pathway connecting atrium and ventricle 1:

• Wolff-Parkinson-White (WPW) syndrome: Documented SVT or symptoms consistent with SVT in a patient with ventricular pre-excitation during sinus rhythm (incidence 0.1-0.3% in general population) 2

• Atrioventricular reentrant tachycardia (AVRT):

  • Orthodromic AVRT: Anterograde conduction down AV node, retrograde up accessory pathway (narrow QRS with P wave in early ST segment, >70 ms after QRS) 1
  • Antidromic AVRT: Anterograde conduction down accessory pathway, retrograde up AV node (wide QRS) 1
  • Permanent form of junctional reciprocating tachycardia (PJRT): Orthodromic AVRT via slowly conducting accessory pathway with decremental properties, producing long RP interval 1

• Concealed accessory pathways: Capable only of retrograde conduction 1

Practical ECG-Based Classification Algorithm

When evaluating SVT on ECG, use this systematic approach 1:

Step 1: Assess QRS Width

• Narrow QRS (<120 ms): Almost always SVT 4
• Wide QRS (>120 ms): May be SVT with bundle branch block, SVT with accessory pathway conduction, or ventricular tachycardia—when uncertain, treat as VT 1, 4

Step 2: Assess Rhythm Regularity

• Irregular ventricular rate: Suggests atrial fibrillation, MAT, or atrial flutter with variable AV conduction 1
• Regular ventricular rate: Proceed to Step 3 1

Step 3: Compare Atrial and Ventricular Rates

• Atrial rate exceeds ventricular rate: Atrial flutter or atrial tachycardia (with 2:1 or variable block) 1
• 1:1 AV relationship: Proceed to Step 4 1

Step 4: Identify P Wave Location (RP Interval)

• No visible P waves (hidden in QRS): Most likely AVNRT 4
• Pseudo R' in V1 or pseudo S in inferior leads: Pathognomonic for typical AVNRT 1, 4
• Short RP (P wave closer to prior QRS, RP <70 ms): Typical AVNRT or orthodromic AVRT 1
• Long RP (P wave closer to subsequent QRS): Atypical AVNRT, PJRT, or atrial tachycardia 1
• P wave in ST segment separated from QRS by >70 ms: Orthodromic AVRT most likely 1, 4

Critical Distinguishing Features for Wide QRS Tachycardia

When QRS is >120 ms, differentiate VT from SVT using 1:

• AV dissociation with ventricular rate faster than atrial rate: Proves VT (visible in only 30% of cases) 1, 4
• Fusion complexes: Pathognomonic for VT 1, 4
• Capture beats: Diagnostic of VT 4
• Concordance of precordial QRS complexes (all positive or all negative): Suggests VT or pre-excitation 1
• QRS identical to sinus rhythm: Consistent with SVT with pre-existing bundle branch block 1

Common Pitfalls to Avoid

• Never assume hemodynamic stability indicates SVT—VT can be well-tolerated, especially in younger patients without structural heart disease 4
• Atrial fibrillation with rapid ventricular response may appear regular at very fast rates and be misdiagnosed as regular SVT 1, 4
• Pre-excitation on baseline ECG mandates electrophysiology referral due to sudden death risk, particularly if atrial fibrillation develops 4
• Automatic ECG interpretations are unreliable—always interpret manually 4
• In patients with prior myocardial infarction, wide-complex tachycardia is VT until proven otherwise 4