AVNRT and AVRT - Supraventricular Tachycardias

Overview

AVNRT - AV Nodal Reentrant Tachycardia

Mechanism

• Slow pathway - slower conduction, shorter refractory period (posterior)
• Fast pathway - faster conduction, longer refractory period (anterior)

Types

Typical AVNRT ("slow-fast") - Most common

• Antegrade conduction: down the slow pathway
• Retrograde conduction: up the fast pathway
• Atrial and ventricular activation occur nearly simultaneously
• Result: Short RP tachycardia

• P wave buried in or at the terminal end of the QRS
• Pseudo-r' in V1 (most characteristic sign - ~45% of cases)
• Pseudo-S wave in inferior leads (II, III, aVF)
• RP interval < 70 ms (often < 1/2 of the RR interval)
• Rate: typically 150-250 bpm, regular

Atypical AVNRT ("fast-slow") - Less common

• Antegrade: down the fast pathway; Retrograde: up the slow pathway
• Long RP tachycardia
• Retrograde P wave inscribed well after the QRS in the second half of the RR interval

AVRT - AV Reentrant Tachycardia

Mechanism

• The atrium
• The AV node and His-Purkinje system
• The ventricle
• An accessory pathway (AP) / bypass tract connecting atria to ventricles outside the AV node

Types

Orthodromic AVRT (~95% of AVRT)

• Antegrade: down the AV node → narrow QRS
• Retrograde: up the accessory pathway
• Short RP tachycardia (RP > 70 ms, P visible after QRS, usually separated from it)
• Most common SVT in patients with WPW syndrome
• Can occur with a concealed AP (no preexcitation on resting ECG - retrograde conduction only)

Antidromic AVRT (<5% of AVRT)

• Antegrade: down the accessory pathway
• Retrograde: up the AV node (or second AP)
• Produces a wide, fully preexcited QRS - can mimic VT
• Seen only in patients with WPW syndrome (manifest AP)

Wolff-Parkinson-White Syndrome

• Defined by preexcitation + tachyarrhythmia
• Resting ECG: short PR interval + delta wave (slurred QRS upstroke)
• Risk: AF with rapid conduction through AP → ventricular fibrillation (if AP has short refractory period)

ECG Comparison: Key Differentiating Features

Key differentiator from AT: Both AVNRT and AVRT have a fixed, non-varying RP relationship. Variability in the RP interval points to AT instead.

Acute Management (both conditions)

1. Vagal maneuvers (Valsalva, carotid sinus massage) - first-line
2. IV Adenosine 6 mg rapid bolus (can repeat 12 mg x2) - terminates most cases by blocking AV node
3. IV Beta-blockers or non-dihydropyridine CCBs (diltiazem, verapamil) - if adenosine fails
4. Synchronized DC cardioversion - if hemodynamically unstable

Caution: Avoid AV nodal blockers (adenosine, verapamil, digoxin) in antidromic AVRT or AF with WPW - they can accelerate conduction through the AP and precipitate VF. Use procainamide or ibutilide instead.

Long-Term / Definitive Management