What is the pathophysiology, ECG characteristics, and management of Supraventricular Tachycardia (SVT), Atrioventricular Nodal Reentrant Tachycardia (AVNRT), and Atrioventricular Reentrant Tachycardia (AVRT), including orthodromic and antidromic types?

Supraventricular Tachycardias: Pathophysiology, ECG Characteristics, and Management

Catheter ablation is the recommended first-line treatment for recurrent, symptomatic SVTs including AVNRT and AVRT due to its high success rate (>95%) and low complication rate. 1, 2

Pathophysiology of SVT Types

AVNRT (Atrioventricular Nodal Reentrant Tachycardia)

• Mechanism: Reentry within the AV node using dual AV nodal physiology
• Anatomic Substrate: Functional/anatomic duality of pathways in the AV node region 3
• Types:
  • Typical (Common) AVNRT (90% of cases):
    • Anterograde conduction through slow pathway
    • Retrograde conduction through fast pathway
  • Atypical (Uncommon) AVNRT:
    • Anterograde conduction through fast pathway
    • Retrograde conduction through slow pathway

AVRT (Atrioventricular Reentrant Tachycardia)

• Mechanism: Reentry using an accessory pathway and the normal conduction system
• Types:
  • Orthodromic AVRT:
    • Anterograde conduction through AV node
    • Retrograde conduction through accessory pathway
  • Antidromic AVRT:
    • Anterograde conduction through accessory pathway
    • Retrograde conduction through AV node or second accessory pathway 1

ECG Characteristics

AVNRT

• Typical AVNRT:

  • Narrow QRS (unless aberrancy)
  • Heart rate 180-200 bpm (range 110-250 bpm) 1
  • P waves hidden within or immediately after QRS complex (RP < 90 ms) 4
  • Pseudo-R' wave in V1 and/or pseudo-S waves in inferior leads 1
  • Short RP interval tachycardia

• Atypical AVNRT:

  • Retrograde P waves with long RP interval
  • Negative P waves in inferior leads 1

AVRT

• Orthodromic AVRT:

  • Narrow QRS (unless aberrancy)
  • P wave visible in ST segment, separated from QRS by ≥70 ms 1
  • RP interval longer than in typical AVNRT (RP >90 ms) 4

• Antidromic AVRT:

  • Wide QRS complex with LBBB morphology
  • Pre-excitation pattern during tachycardia 1

Differential Diagnosis of SVT

Narrow QRS Tachycardia (<120 ms)

• Almost always supraventricular in origin
• If regular with no visible P waves: Likely AVNRT
• If P waves in ST segment (RP < PR): Likely orthodromic AVRT
• If RP > PR: Consider atypical AVNRT, PJRT, or atrial tachycardia 1

Wide QRS Tachycardia (>120 ms)

• Must differentiate between SVT with aberrancy and ventricular tachycardia
• SVT with wide QRS can be due to:
  1. Pre-existing bundle branch block
  2. Rate-related aberrancy
  3. Antidromic conduction over accessory pathway 1

Acute Management

Hemodynamically Unstable Patients

• Synchronized cardioversion is first-line treatment 1, 2

Hemodynamically Stable Patients

1. Vagal maneuvers (Class I recommendation) 1

   • Valsalva maneuver: Patient bears down against closed glottis for 10-30 seconds
   • Carotid sinus massage: Apply steady pressure over carotid sinus for 5-10 seconds after confirming absence of bruit
   • Success rates vary by mechanism:
     • AVRT: 53% (33% terminate in antegrade limb, 20% in retrograde limb)
     • AVNRT: 33% (14% terminate in antegrade slow pathway, 19% in retrograde fast pathway) 5

2. Adenosine (Class I recommendation) if vagal maneuvers fail 1

   • Caution: Can cause transient AV block, bronchospasm, chest discomfort
   • Contraindicated in patients with severe asthma or high-grade AV block 6

3. IV beta-blockers, diltiazem, or verapamil (Class IIa recommendation) 1

   • Effective for rate control and potential conversion
   • Avoid in patients with severe heart failure or severe pulmonary disease

4. IV amiodarone (Class IIb recommendation) when other therapies fail 1

Long-term Management

Pharmacological Options

• Oral verapamil or diltiazem (Class I recommendation) for AVNRT 1
• Oral beta-blockers (Class I recommendation) for AVNRT 1
• Flecainide or propafenone (Class IIa recommendation) for patients without structural heart disease 1

Definitive Treatment

• Catheter ablation (Class I recommendation) 1, 7, 2
  • Success rate >95% for AVNRT and AVRT
  • Low complication rate (<2% risk of AV block)
  • For AVNRT: Slow pathway ablation preferred over fast pathway ablation (lower risk of AV block) 3, 7
  • For AVRT: Ablation of the accessory pathway

Special Considerations

• Patients with pre-excitation syndromes (e.g., WPW) should avoid AV nodal blocking agents (verapamil, diltiazem, digoxin) during AF as they can accelerate conduction through the accessory pathway
• Persistent SVT can lead to tachycardia-mediated cardiomyopathy 4
• Continuous cardiac monitoring is essential to detect intermittent arrhythmias 4
• Avoid verapamil or diltiazem in wide-complex tachycardias of unknown origin as they can cause hemodynamic collapse if the rhythm is VT 1

By understanding the pathophysiology, ECG characteristics, and management options for SVTs, clinicians can provide effective treatment for these common arrhythmias while minimizing complications.