What is the difference between Atrioventricular Nodal Reentrant Tachycardia (AVNRT) and Supraventricular Tachycardia (SVT) with retrograde conduction?

Understanding the Difference Between AVNRT and SVT with Retrograde Conduction

AVNRT is itself a specific type of SVT with retrograde conduction, but the key distinction lies in the anatomic circuit: AVNRT uses dual pathways confined within the AV node itself, while other SVTs with retrograde conduction (particularly AVRT) require an accessory pathway connecting atrium to ventricle outside the normal conduction system. 1

Anatomic and Circuit Differences

AVNRT Circuit Characteristics

• The reentrant circuit is confined to the AV node and perinodal atrial tissue, involving two functionally distinct pathways within the AV node termed "fast" and "slow" pathways 1
• In typical AVNRT (90% of cases), anterograde conduction occurs down the slow pathway and retrograde conduction travels up the fast pathway 1, 2
• No accessory pathway exists outside the normal conduction system 1

AVRT Circuit Characteristics (The Main "Other" SVT with Retrograde Conduction)

• The electrical pathway requires an accessory pathway, the atrium, AV node, and ventricle, with the accessory pathway being an extranodal connection between atrial and ventricular myocardium across the AV groove 1
• In orthodromic AVRT, the reentrant impulse uses the accessory pathway in the retrograde direction from ventricle to atrium, and the AV node in the anterograde direction 3
• This accessory pathway is anatomically separate from the AV node, distinguishing it fundamentally from AVNRT 3

ECG Differentiation: The Critical Clinical Tool

P-Wave Timing and Location

• In AVNRT, atrial activation occurs nearly simultaneously with ventricular activation, causing P waves to be buried within or at the end of the QRS complex, creating a very short RP interval (RP < 90 ms) 1, 4
• In AVRT, retrograde P waves are usually clearly visible in the early part of the ST-T segment (90-95% of episodes), separate from the QRS complex with a longer RP interval than AVNRT but still creating a "short RP" tachycardia (RP < PR) 1, 4

Specific ECG Patterns

• AVNRT characteristically shows pseudo-R' waves in lead V1 and/or pseudo-S waves in inferior leads (II, III, aVF) due to simultaneous atrial and ventricular activation 3
• In AVRT, the P wave morphology reflects the location of the accessory pathway, with deeply inverted P waves in inferior leads when the pathway is posteroseptal 4

Important Caveat on P-Wave Visibility

• Age and gender affect P-wave visibility in AVNRT: elderly patients show visible P waves in 29% versus 12% in younger patients, mainly due to delayed nodal retrograde conduction 5
• When visible P waves are present, AVRT becomes more likely in men (70%) while AVNRT remains more common in women (64%) 5

Clinical Presentation Patterns

Age and Gender Distribution

• AVNRT is more common in middle-aged or older patients with female predominance, with mean symptom onset at 32±18 years 3, 1
• AVRT is more prevalent in adolescents and younger adults, with mean symptom onset at 23±14 years 3, 1

Symptom Characteristics

• Patients with AVNRT more frequently describe "shirt flapping" or "neck pounding" sensations, relating to pulsatile reversed flow when the right atrium contracts against a closed tricuspid valve 1
• These symptoms are less common in AVRT due to the sequential rather than simultaneous atrial-ventricular activation 1

Atypical Variants Creating Diagnostic Confusion

Long RP Tachycardias

• Atypical (fast-slow) AVNRT can produce long RP intervals where the P wave appears closer to the subsequent QRS complex, mimicking atrial tachycardia or PJRT 4
• PJRT (Permanent Junctional Reciprocating Tachycardia) is a rare form of AVRT involving a slowly conducting, concealed, usually posteroseptal accessory pathway with decremental properties 3, 6
• In PJRT, the RP interval exceeds the PR interval (RP > PR) with deeply inverted P waves in inferior leads 6

Anterior Type AVNRT

• A rare anterior type AVNRT exists where an anteriorly located slow pathway is used retrogradely and a posteriorly located fast pathway anterogradely, representing an anatomical reversal of typical AVNRT 7
• This variant shows shallow negative P waves in inferior leads rather than deeply negative ones 7

Critical Treatment Implications

Acute Management Considerations

• Both AVNRT and orthodromic AVRT respond to vagal maneuvers and adenosine by blocking AV nodal conduction 3, 2
• Vagal maneuvers terminate AVRT in 53% of cases (33% in antegrade limb, 20% in retrograde limb) and AVNRT in 33% of cases 8

Dangerous Pitfall with Pre-Excited Tachycardias

• In AVRT with pre-excitation (antidromic AVRT or pre-excited AF), verapamil or diltiazem administration may cause hemodynamic collapse or ventricular fibrillation 1
• This risk does not exist with AVNRT, as there is no accessory pathway capable of rapid anterograde conduction to the ventricles 1

Definitive Treatment Differences

• AVNRT is treated with catheter ablation of either the fast or slow pathway (slow pathway ablation preferred due to lower risk of AV block) 2
• AVRT is treated with catheter ablation of the accessory pathway itself 3

Practical Diagnostic Algorithm

When evaluating narrow QRS tachycardia with retrograde P waves:

1. Assess P-wave timing relative to QRS:

   • P wave buried in or immediately after QRS (RP < 90 ms) → likely AVNRT 4
   • P wave clearly visible in ST segment (RP 90-200 ms) → likely AVRT 4
   • P wave closer to next QRS (RP > PR) → consider atypical AVNRT, PJRT, or atrial tachycardia 6

2. Look for pseudo-R' in V1 or pseudo-S in inferior leads → pathognomonic for AVNRT 3

3. Consider patient demographics:

   • Middle-aged/older woman → AVNRT more likely (91% in women) 5
   • Adolescent/young adult → AVRT more likely 3

4. Check for pre-excitation on baseline ECG → if present, AVRT is the mechanism (though concealed pathways show no pre-excitation) 3