What is the difference between Atrioventricular Reentrant Tachycardia (AVRT) and Atrioventricular Nodal Reentrant Tachycardia (AVNRT)?

Key Differences Between AVRT and AVNRT

AVNRT and AVRT differ fundamentally in their reentrant circuits: AVNRT uses dual pathways within the AV node itself, while AVRT requires an accessory pathway connecting atrium to ventricle outside the normal conduction system. 1

Anatomic Circuit Differences

AVNRT Circuit

• The reentrant circuit is confined to the AV node and small amount of perinodal atrial tissue 1, 2
• Involves two functionally distinct pathways within the AV node, termed "fast" and "slow" pathways 1
• The fast pathway is typically located near the apex of Koch's triangle, while the slow pathway is inferoposterior to the compact AV node 1
• In typical AVNRT (most common), anterograde conduction occurs down the slow pathway and retrograde conduction up the fast pathway 1

AVRT Circuit

• The electrical pathway requires an accessory pathway (bypass tract), the atrium, AV node, and ventricle 1
• The accessory pathway is an extranodal connection between atrial and ventricular myocardium across the AV groove 1
• In orthodromic AVRT (most common form), anterograde conduction occurs down the AV node and retrograde conduction up the accessory pathway 1
• The QRS is typically narrow in orthodromic AVRT unless bundle branch block or aberrancy is present 1

ECG Characteristics: P Wave Location

AVNRT P Wave Features

• Atrial activation occurs nearly simultaneously with ventricular activation, causing P waves to be buried within or at the end of the QRS complex 1, 3
• P waves appear as a narrow negative deflection (pseudo S wave) in inferior leads or slightly positive deflection (pseudo R′) at the end of QRS in lead V1 1, 3
• The RP interval is very short (RP < 90 ms), creating a "short RP" tachycardia 3
• P waves may be completely invisible in 27% of AVNRT cases 4
• The characteristic R′ in V1 and/or S wave in inferior leads is seen in 57% of AVNRT 4

AVRT P Wave Features

• Retrograde P waves are usually clearly visible in the early part of the ST-T segment, separate from the QRS complex 1, 3
• P waves are visible after the QRS in 100% of AVRT cases 4
• The RP interval is longer than in AVNRT but still creates a "short RP" tachycardia (RP < PR) 3
• Retrograde conduction occurs over the accessory pathway rather than through the AV node 3

Electrophysiologic Distinctions

VA Interval Variability at Tachycardia Onset

• AVRT demonstrates minimal VA interval variability at tachycardia induction (median ΔVA = 0 ms), with the VA interval stabilizing within 1-3 beats 5
• Atypical AVNRT shows significant VA interval variability (median ΔVA = 40 ms), requiring 4-7 beats for stabilization 5
• A ΔVA < 10 ms distinguishes AVRT from atypical AVNRT with 100% sensitivity and specificity 5

Entrainment Response

• Differential entrainment from RV apex versus base can distinguish these arrhythmias 6
• The [SA-VA]apex - [SA-VA]base difference is negative (-9.4 ± 6.6 ms) for all AVNRT cases 6
• This difference is positive (10 ± 11.3 ms) for all AVRT with septal accessory pathways 6

Clinical Presentation Differences

Demographics

• AVNRT is more common in middle-aged or older patients and has female predominance (91% in women vs 72% in men) 1, 4
• AVRT is more prevalent in adolescents and younger adults, with mean symptom onset at 23 years versus 32 years for AVNRT 1
• The relative frequency of AVRT decreases with age 1

Symptom Characteristics

• Patients with AVNRT more frequently describe "shirt flapping" or "neck pounding" sensations 1
• These symptoms relate to pulsatile reversed flow when the right atrium contracts against a closed tricuspid valve (cannon a-waves) 1
• This occurs because atrial and ventricular contraction are nearly simultaneous in AVNRT 1

Important Clinical Caveats

ECG Interpretation Pitfalls

• Visible P waves can occur in 16% of AVNRT cases, particularly in elderly patients (29% vs 12% in younger patients) 4
• In elderly women with visible P waves, AVNRT is still the mechanism in 71% of cases 4
• Bundle branch block makes retrograde P waves more difficult to discern in both arrhythmias 3
• Atypical AVNRT variants (fast-slow or slow-slow) can produce long RP intervals that mimic atrial tachycardia 1, 3

Treatment Implications

• Both arrhythmias respond to AV nodal blocking agents (adenosine, calcium channel blockers, beta-blockers) for acute termination 2, 7
• Critical distinction: In AVRT with pre-excitation (WPW syndrome), verapamil or diltiazem administration during pre-excited atrial fibrillation may cause hemodynamic collapse or ventricular fibrillation 1, 7
• Radiofrequency catheter ablation targets different anatomic sites: slow pathway modification for AVNRT versus accessory pathway ablation for AVRT 2, 7