How do you differentiate between Supraventricular Tachycardia (SVT) and Sinus Tachycardia (ST)?

Differentiating Supraventricular Tachycardia (SVT) from Sinus Tachycardia (ST)

The key distinction between SVT and sinus tachycardia is that SVT is paroxysmal with abrupt onset and termination, while sinus tachycardia gradually accelerates and terminates with an identifiable underlying cause. 1

Clinical Features

Sinus Tachycardia

• Heart rate typically 100-180 bpm
• Gradual onset and termination
• Usually has an identifiable cause:
  • Physiologic: exercise, anxiety, pain, fever
  • Pathologic: hyperthyroidism, anemia, hypovolemia, infection
• P waves have normal sinus morphology (upright in leads I, II, aVF)
• Fixed or gradually changing PR interval
• Regular rhythm

Supraventricular Tachycardia

• Heart rate typically 150-250 bpm
• Abrupt/paroxysmal onset and termination
• Often occurs without clear precipitating factors
• P waves may be:
  • Hidden within QRS complexes (AVNRT)
  • Visible after QRS (AVRT)
  • Abnormal morphology (atrial tachycardia)
• May respond to vagal maneuvers

ECG Diagnostic Criteria

Key ECG Findings in Sinus Tachycardia:

• Normal P wave morphology identical to patient's baseline sinus rhythm
• 1:1 P:QRS relationship
• PR interval within normal limits (120-200 ms)
• Gradual rate changes visible when monitoring is continuous

Key ECG Findings in SVT:

1. AVNRT (most common type):

   • P waves often hidden in QRS or appearing as pseudo-R' in V1
   • Pseudo-S waves in inferior leads (II, III, aVF) 1
   • RP interval shorter than PR interval ("short RP tachycardia")

2. AVRT:

   • P waves visible in ST segment, typically 70-150 ms after QRS
   • RP interval shorter than PR interval (except in PJRT)
   • May see pre-excitation (delta wave) during sinus rhythm if WPW present

3. Atrial Tachycardia:

   • Abnormal P wave morphology
   • Long RP interval (P wave closer to subsequent QRS)
   • May have variable AV block

Diagnostic Algorithm

1. Assess onset and termination pattern:

   • Abrupt onset/offset suggests SVT
   • Gradual onset/offset suggests sinus tachycardia 1

2. Evaluate P wave morphology:

   • Normal P waves identical to sinus rhythm → ST
   • Abnormal or hidden P waves → SVT

3. Analyze response to vagal maneuvers:

   • Gradual slowing with return to original rate → ST
   • Abrupt termination → SVT (especially AVNRT or AVRT)
   • Transient AV block revealing atrial activity → Atrial tachycardia

4. Consider associated symptoms:

   • "Neck pounding" or "shirt flapping" sensation → AVNRT 1
   • Polyuria during episodes → AVNRT 1
   • Symptoms associated with specific triggers (exercise, stress) → ST

Special Considerations

• Caution: When evaluating a wide-complex tachycardia, always consider ventricular tachycardia first, as misdiagnosis can be life-threatening 1

• Pitfall: Inappropriate sinus tachycardia can be confused with SVT but is characterized by resting heart rate >100 bpm, mean 24-hour heart rate >90 bpm, without appropriate physiological cause 1

• Caveat: In patients with WPW syndrome, the presence of pre-excitation on a resting ECG with history of paroxysmal palpitations is sufficient for presumptive diagnosis of AVRT 1

• Warning: If the diagnosis is uncertain in a patient with wide-complex tachycardia, treat as ventricular tachycardia until proven otherwise 1

Diagnostic Confirmation

If the diagnosis remains uncertain after initial evaluation, consider:

1. 12-lead ECG during tachycardia (gold standard)
2. Extended cardiac monitoring (Holter, event recorder)
3. Adenosine administration under controlled conditions with ECG monitoring
4. Referral for electrophysiologic study for definitive diagnosis

Remember that patients with recurrent, symptomatic SVT should be referred to a cardiac electrophysiologist for consideration of catheter ablation, which is curative in the majority of cases 2.