What is the initial approach and differential diagnosis for a patient presenting with suspected Atrioventricular Nodal Reentrant Tachycardia (AVNRT)?

Differential Diagnosis for AVNRT

When evaluating a patient with suspected AVNRT, the primary differential diagnoses include other forms of supraventricular tachycardia—specifically orthodromic AVRT, atypical AVNRT, atrial tachycardia, and in cases with wide QRS complexes, ventricular tachycardia must be excluded. 1

Narrow QRS Complex Tachycardia Differential

Typical AVNRT (Most Common)

• P waves are hidden within or immediately after the QRS complex, creating a pseudo R' wave in lead V1 and pseudo S waves in inferior leads (II, III, aVF) 1
• The reentrant circuit uses anterograde conduction over the slow AV nodal pathway and retrograde conduction over the fast pathway, producing a short RP tachycardia 1
• Accounts for approximately 90% of AVNRT cases 2
• Heart rate typically ranges from 180-200 bpm but can vary from 110 to >250 bpm 1, 3

Atypical AVNRT (Fast-Slow Form)

• P waves appear with a long RP interval, showing retrograde P waves with negative polarity in inferior leads 1
• Uses anterograde conduction over the fast pathway and retrograde conduction over the slow pathway 1
• Represents only 5-10% of AVNRT cases and may be more resistant to pharmacological therapy 4
• This ECG pattern does not exclude PJRT or low septal atrial tachycardia, which can appear very similar 1

Orthodromic AVRT

• P waves are visible in the ST segment, separated from the QRS by >70 ms 1
• Conducts anterograde through the AV node and retrograde through an accessory pathway 3
• Produces narrow QRS complexes unless aberrant conduction is present 3

Atrial Tachycardia

• P wave morphology differs from sinus rhythm and is typically seen near the end of or shortly after the T wave 1
• The rhythm is driven by the atrium with normal conduction to the ventricles 1
• Long RP interval is characteristic 1

Permanent Form of Junctional Reciprocating Tachycardia (PJRT)

• An uncommon form of AVRT with a decremental (slowly conducting) accessory pathway producing delayed atrial activation 1
• Creates a long RP interval that can mimic atypical AVNRT or atrial tachycardia 1

Wide QRS Complex Tachycardia Differential

Critical Distinction: VT vs SVT with Aberrancy

If the QRS is >120 ms, ventricular tachycardia must be excluded first, as misdiagnosis can be fatal 1

Ventricular Tachycardia (VT)

• Presence of AV dissociation is diagnostic for VT 1
• Negative concordance (all precordial leads showing QS complexes) is diagnostic for VT 1
• Ventricular fusion beats indicate ventricular origin 1
• QR complexes indicate myocardial scar and are present in approximately 40% of VT cases after myocardial infarction 1
• History of previous myocardial infarction strongly suggests VT 1

SVT with Bundle Branch Block or Aberrancy

• Pre-existing bundle branch block pattern on baseline ECG supports SVT with aberrancy 1
• QRS morphology during tachycardia matches the baseline BBB pattern 1

Antidromic AVRT (Pre-excited Tachycardia)

• Conducts anterograde through the accessory pathway and retrograde through the AV node 3
• Produces wide QRS complexes with left bundle branch block morphology 3
• The QRS is generally wider (more pre-excited) compared with sinus rhythm 1

Diagnostic Approach Using Adenosine Response

Adenosine can aid in differential diagnosis but must be used with extreme caution 1

AVNRT and Orthodromic AVRT

• Adenosine terminates the tachycardia in 90-95% of cases by blocking AV nodal conduction 3, 5, 6

Atrial Tachycardia

• Adenosine may transiently slow or block AV conduction, revealing underlying atrial activity without terminating the tachycardia 1

Critical Pitfall

Never use adenosine in wide-complex tachycardia of uncertain etiology if pre-excited atrial fibrillation is possible, as it may produce ventricular fibrillation in patients with accessory pathways and rapid ventricular rates 1, 3

Key Distinguishing Features on 12-Lead ECG

When P Waves Are Not Visible

• AVNRT is most likely if the RR interval is regular and no P waves are apparent 1
• Use esophageal pill electrodes if P waves cannot be identified on surface ECG 1

When P Waves Are Visible

• RP interval <70 ms: typical AVNRT 1
• RP interval >70 ms but <PR interval: orthodromic AVRT 1
• RP interval >PR interval (long RP tachycardia): atypical AVNRT, PJRT, or atrial tachycardia 1

Comparison with Sinus Rhythm

Always compare the tachycardia ECG with sinus rhythm to identify the pseudo R' in V1 and accentuated S waves in inferior leads that are pathognomonic for AVNRT 1

Management Implications Based on Diagnosis

If AVNRT or Orthodromic AVRT Confirmed

• Vagal maneuvers and adenosine are highly effective 1, 5
• AV nodal blocking agents (calcium channel blockers, beta-blockers) are appropriate 1, 5

If Wide-Complex Tachycardia Cannot Be Definitively Diagnosed

Treat as ventricular tachycardia 1

• Never administer verapamil or diltiazem, as these may cause hemodynamic collapse in VT 1, 3

If Pre-excited Atrial Fibrillation Suspected

Never use AV nodal blocking agents, as they may accelerate ventricular rate and precipitate ventricular fibrillation 3