What is Supraventricular Tachycardia (SVT)?

Supraventricular Tachycardia (SVT)

Supraventricular tachycardia is a rapid, regular heart rhythm originating above the ventricles, typically characterized by heart rates between 150-250 beats per minute with sudden onset and termination. SVT is a group of arrhythmias that originate from or require participation of atrial or atrioventricular nodal tissue, with the most common types being atrioventricular nodal reentrant tachycardia (AVNRT) and atrioventricular reciprocating tachycardia (AVRT). 1

Clinical Presentation

SVT presents with various symptoms depending on heart rate, duration, and individual factors:

• Common symptoms:

  • Palpitations (most common)
  • Chest discomfort/pressure
  • Lightheadedness
  • Dyspnea
  • Fatigue
  • Presyncope
  • Syncope (in approximately 15% of patients) 1

• Characteristic features:

  • Sudden onset and termination (paroxysmal)
  • Regular rhythm
  • Heart rates typically 150-250 bpm (usually 260-300 bpm in infants) 1
  • Polyuria may occur during prolonged episodes due to atrial natriuretic peptide release 1

ECG Characteristics

The diagnosis of SVT is primarily based on 12-lead ECG findings:

• Narrow QRS complex (<120 ms) in most cases (>90% of cases) 1
• Regular R-R intervals after the first 10-20 beats 1
• P waves may be:
  • Visible in 60% of cases but different from sinus P waves 1
  • Hidden within the QRS complex (in typical AVNRT)
  • Visible after the QRS (in AVRT)
  • Visible before each QRS (in atrial tachycardia)

Types of SVT

1. Atrioventricular Nodal Reentrant Tachycardia (AVNRT):

   • Most common type (50-60% of SVTs)
   • Involves dual pathways within the AV node
   • Typical AVNRT: anterograde conduction over slow pathway, retrograde over fast pathway
   • Atypical AVNRT: anterograde conduction over fast pathway, retrograde over slow pathway 1

2. Atrioventricular Reciprocating Tachycardia (AVRT):

   • Involves an accessory pathway between atria and ventricles
   • Orthodromic AVRT: anterograde conduction over AV node, retrograde over accessory pathway
   • Antidromic AVRT: anterograde conduction over accessory pathway, retrograde over AV node 1

3. Atrial Tachycardia (AT):

   • Originates from a focal area in the atria
   • Usually regular with 1:1 AV conduction 1

Differential Diagnosis

Important to distinguish SVT from:

• Sinus tachycardia: Gradual onset/offset, rates rarely exceed 180 bpm
• Atrial flutter: Regular atrial rate 250-350 bpm with variable AV conduction
• Atrial fibrillation: Irregular rhythm with variable ventricular response
• Ventricular tachycardia: Wide QRS complex (>120 ms), different QRS morphology from sinus rhythm 1

Acute Management

1. First-line: Vagal maneuvers for hemodynamically stable patients:

   • Valsalva maneuver (most effective): bearing down against closed glottis for 10-30 seconds, equivalent to 30-40 mmHg pressure 1
   • Modified Valsalva: standard Valsalva followed by lying flat with passive leg raise (improved success rates from 17% to 43%) 2
   • Carotid sinus massage (after confirming absence of carotid bruit)
   • Facial immersion in cold water 1

2. Second-line: Pharmacological therapy if vagal maneuvers fail:

   • Adenosine: 6 mg rapid IV push, followed by 12 mg if needed
   • Calcium channel blockers: Verapamil or diltiazem
   • Beta-blockers 1, 3

3. For hemodynamically unstable patients:

   • Immediate synchronized cardioversion 3

Long-term Management

1. Catheter ablation:

   • First-line therapy for recurrent, symptomatic SVT
   • High success rate (>95% for AVNRT and AVRT)
   • Low complication rate 3

2. Pharmacological therapy:

   • Beta-blockers or calcium channel blockers for symptom control
   • Class IC or III antiarrhythmic drugs for refractory cases 1

Important Pitfalls and Caveats

• Never administer verapamil or diltiazem for wide-complex tachycardias of unknown origin as they may precipitate hemodynamic collapse if the rhythm is actually ventricular tachycardia 1

• Patients with pre-excitation (Wolff-Parkinson-White syndrome) and atrial fibrillation require immediate electrophysiological evaluation due to risk of sudden death 1

• SVT with aberrancy can mimic ventricular tachycardia; when in doubt, treat as VT 1

• Prolonged SVT with rapid ventricular rates can lead to tachycardia-induced cardiomyopathy 1

• All patients with documented SVT should be referred to a cardiac electrophysiologist for definitive management 3