How to diagnose Supraventricular Tachycardia (SVT)?

Diagnosis of Supraventricular Tachycardia (SVT)

The diagnosis of SVT requires a 12-lead ECG during tachycardia, which should be obtained whenever possible but should not delay treatment in hemodynamically unstable patients. 1

Initial ECG Classification

Narrow QRS Complex Tachycardia (QRS <120 ms)

When the QRS complex is narrow (<120 ms), the tachycardia is almost always supraventricular. The differential diagnosis relates to its mechanism:

1. Regular RR intervals with no visible P waves:

   • Most likely AVNRT (Atrioventricular Nodal Reentrant Tachycardia)
   • Look for pseudo-R wave in lead V1 and/or pseudo-S wave in inferior leads (pathognomonic for AVNRT) 1

2. P wave visible in ST segment (separated from QRS by 70 ms):

   • Most likely AVRT (Atrioventricular Reciprocating Tachycardia) 1

3. RP interval longer than PR interval:

   • Consider atypical AVNRT, PJRT (Permanent form of Junctional Reciprocating Tachycardia), or AT (Atrial Tachycardia) 1

4. Atrial rate exceeds ventricular rate:

   • Consider atrial flutter or atrial tachycardia 1

5. Irregular ventricular rate:

   • Consider atrial fibrillation, multifocal atrial tachycardia, or atrial flutter with variable AV conduction 1

Wide QRS Complex Tachycardia (QRS ≥120 ms)

When the QRS complex is wide (≥120 ms), it is critical to differentiate between SVT with aberrancy and ventricular tachycardia (VT):

1. Key diagnostic features for VT:

   • AV dissociation (ventricular rate faster than atrial rate)
   • Fusion complexes (pathognomonic for VT)
   • QRS width >140 ms with RBBB pattern or >160 ms with LBBB pattern
   • RS interval >100 ms in any precordial lead
   • QRS pattern with negative concordance in precordial leads
   • QR complexes (indicate myocardial scar) 1

2. SVT with wide QRS may be due to:

   • Pre-existing bundle branch block
   • Rate-related aberrancy
   • Conduction over an accessory pathway 1

Diagnostic Tools and Maneuvers

1. Vagal Maneuvers:

   • Valsalva maneuver, carotid sinus massage, facial immersion in cold water
   • Record 12-lead ECG during maneuvers
   • Response can aid in differential diagnosis 1, 2

2. Adenosine Administration:

   • Useful diagnostic tool for narrow QRS tachycardias
   • Record 12-lead ECG during administration
   • Different responses help identify the mechanism 1, 2

3. Esophageal Pill Electrodes:

   • Helpful when P waves are not visible on standard ECG 1

Specific ECG Patterns for Common SVTs

1. AVNRT (most common type):

   • Regular tachycardia with heart rates 150-250 bpm
   • P waves often hidden within QRS complex
   • Pseudo-R' in V1 and pseudo-S waves in inferior leads (II, III, aVF) 1, 2

2. AVRT:

   • Regular tachycardia with heart rates 150-250 bpm
   • P waves visible in early ST segment (RP <70 ms)
   • In orthodromic AVRT, QRS is usually narrow unless aberrancy is present 1, 2

3. Atrial Tachycardia:

   • Regular rhythm with 1:1 AV conduction
   • P wave morphology differs from sinus P waves
   • RP interval typically longer than PR interval 2

Important Pitfalls to Avoid

1. Misdiagnosing VT as SVT:

   • When in doubt, treat wide QRS tachycardia as VT
   • Administering calcium channel blockers (verapamil/diltiazem) to patients with VT can cause hemodynamic collapse 1, 3

2. Overlooking pre-excitation:

   • Check for delta waves on resting ECG during sinus rhythm
   • Patients with pre-excitation (WPW syndrome) require prompt referral to a cardiac electrophysiologist 1, 2

3. Relying solely on stable vital signs:

   • Stable vital signs do not help distinguish SVT from VT 1

4. Missing atrial fibrillation with rapid ventricular response:

   • Can be misdiagnosed as regular SVT when very rapid 1

Diagnostic Algorithm for SVT

1. Assess hemodynamic stability:

   • If unstable, obtain at least a monitor strip before cardioversion 1

2. Determine QRS width:

   • <120 ms: Likely SVT
   • ≥120 ms: Could be SVT with aberrancy or VT

3. For narrow QRS tachycardia:

   • Assess regularity of rhythm
   • Look for P waves and their relationship to QRS
   • Apply vagal maneuvers or adenosine while recording ECG

4. For wide QRS tachycardia:

   • Look for AV dissociation and fusion beats
   • Assess QRS morphology and width
   • When in doubt, treat as VT 1

By following this systematic approach to ECG interpretation, the correct diagnosis of SVT can be made, allowing for appropriate treatment and management.