What are the primary types of Supraventricular Tachycardia (SVT)?

Primary Types of Supraventricular Tachycardia (SVT)

Supraventricular tachycardia (SVT) encompasses several distinct types, with the most common being atrioventricular nodal reentrant tachycardia (AVNRT), atrioventricular reentrant tachycardia (AVRT), and atrial tachycardia (AT). 1

Major Categories of SVT

1. Atrioventricular Nodal Reentrant Tachycardia (AVNRT)

• Most common type of SVT in adults, especially in middle-aged patients
• More prevalent in females than males
• Mechanism: Reentry involving two functionally distinct pathways ("fast" and "slow") within the AV node
• ECG characteristics:
  • Short RP interval (RP < PR)
  • RP < 90 ms
  • P waves often buried in QRS complex or appearing as a pseudo S wave in inferior leads
  • Pseudo R' in lead V1
• Clinical presentation often includes "shirt flapping" or "neck pounding" sensations due to cannon a-waves 1

2. Atrioventricular Reentrant Tachycardia (AVRT)

• More common in younger patients
• Requires an accessory pathway between atrium and ventricle
• Two main subtypes:
  • Orthodromic AVRT: Conducts anterograde through AV node and retrograde through accessory pathway
    • Narrow QRS unless pre-existing bundle branch block
    • P wave visible in early ST-T segment
    • Short RP interval
  • Antidromic AVRT: Conducts anterograde through accessory pathway and retrograde through AV node
    • Wide QRS (maximally pre-excited)
    • Less common than orthodromic AVRT 1
• Wolff-Parkinson-White (WPW) syndrome: Characterized by pre-excitation on ECG plus documented SVT or symptoms consistent with SVT 1
• Permanent form of junctional reciprocating tachycardia (PJRT): Rare form of nearly incessant orthodromic AVRT with a slowly conducting concealed accessory pathway (usually posteroseptal) 1

3. Atrial Tachycardia (AT)

• Several subtypes:
  • Focal AT: Arises from a localized atrial site with discrete P waves and typically an isoelectric segment between P waves
  • Sinus node reentry tachycardia: Microreentry within sinus node complex with P wave morphology identical to sinus rhythm
  • Multifocal atrial tachycardia (MAT): Irregular rhythm with ≥3 distinct P-wave morphologies at different rates 1
• ECG characteristics:
  • Long RP interval (RP > PR)
  • P wave morphology different from sinus rhythm
  • P waves typically seen near end of or shortly after T wave 1

4. Atrial Flutter

• Macroreentrant atrial tachycardia
• Two main types:
  • Cavotricuspid isthmus-dependent (typical) atrial flutter:
    • Counterclockwise reentry around tricuspid annulus
    • Negative "sawtooth" flutter waves in leads II, III, aVF
    • Atrial rate typically 300 bpm
  • Reverse typical (clockwise) atrial flutter:
    • Positive flutter waves in inferior leads
    • Negative in V1
  • Atypical or non-cavotricuspid isthmus-dependent atrial flutter:
    • Various reentrant circuits not involving cavotricuspid isthmus 1

5. Other Types

• Junctional tachycardia: Nonreentrant SVT arising from AV junction (including His bundle)
• Inappropriate sinus tachycardia: Sinus heart rate >100 bpm at rest with mean 24-hour heart rate >90 bpm without appropriate physiological cause 1

Distinguishing Features on ECG

The key to differentiating SVT types is analyzing the relationship between P waves and QRS complexes:

1. Regular vs. Irregular:

   • Regular: AVNRT, AVRT, focal AT, typical atrial flutter
   • Irregular: Atrial fibrillation, MAT, atrial flutter with variable conduction 1

2. RP Interval:

   • Short RP (<90 ms): Typical AVNRT
   • Short RP (>90 ms but RP < PR): Orthodromic AVRT
   • Long RP (RP > PR): Atypical AVNRT, AT, PJRT 1

3. P Wave Morphology:

   • P waves buried in QRS or appearing immediately after: AVNRT
   • P waves in ST segment: Orthodromic AVRT
   • P waves with morphology different from sinus rhythm: AT 1

Clinical Pearls

• AVNRT is more common in middle-aged adults, especially women, while AVRT is more common in younger patients 1
• Patients with AVNRT more frequently describe "shirt flapping" or "neck pounding" sensations compared to those with AVRT 1
• Pre-excitation on ECG during sinus rhythm (short PR interval with delta wave) suggests potential for AVRT and warrants referral to an electrophysiologist 1
• When distinguishing from ventricular tachycardia, remember that SVT typically has a narrow QRS (<120 ms) unless there is pre-existing bundle branch block or aberrant conduction 1
• Polyuria is particularly common with AVNRT due to higher right atrial pressures and elevated atrial natriuretic protein levels 1

Understanding these distinct types of SVT is crucial for appropriate diagnosis and management, as treatment approaches may vary based on the specific mechanism involved.