What are the features and management of Orthodromic Atrioventricular Reentrant Tachycardia (OAVRT)?

Orthodromic Atrioventricular Reentrant Tachycardia (OAVRT): Features and Management

Orthodromic AVRT is a reentrant tachycardia in which the circuit uses the AV node for anterograde conduction and an accessory pathway for retrograde conduction from the ventricle to the atrium, typically presenting with a narrow QRS complex unless there is pre-existing bundle branch block or aberrant conduction. 1

Pathophysiology and Mechanism

Orthodromic AVRT is the most common form of atrioventricular reentrant tachycardia, accounting for approximately 90-95% of AVRT episodes in patients with accessory pathways 1. The reentrant circuit involves:

• Anterograde conduction through the AV node and His-Purkinje system
• Ventricular activation via normal conduction
• Retrograde conduction through the accessory pathway back to the atrium
• Completion of the circuit through atrial tissue back to the AV node

This creates a continuous electrical loop that sustains the tachycardia.

Clinical Presentation

Patients with orthodromic AVRT typically present with:

• Paroxysmal palpitations
• Light-headedness (common)
• Chest discomfort
• Fatigue during episodes
• True syncope (infrequent but possible) 1

The rate of AVRT tends to be faster when induced during exercise, and women report more symptoms than men. Notably, 57% of patients with SVT experience episodes while driving, with 24% considering it an obstacle to driving 1.

ECG Features

Key ECG characteristics of orthodromic AVRT include:

• Regular narrow QRS complex tachycardia (unless pre-existing bundle branch block is present)
• Ventricular rate typically between 150-250 bpm
• P waves visible in the early part of the ST-T segment (retrograde P waves)
• RP interval shorter than PR interval ("short RP tachycardia") 1
• RP interval typically <90 ms on surface ECG 1

This ECG pattern helps differentiate orthodromic AVRT from other SVTs, particularly AVNRT (where P waves are often buried in the QRS or appear as a pseudo-S wave in inferior leads).

Differential Diagnosis

Orthodromic AVRT must be distinguished from:

• AVNRT (most common SVT)
• Atrial tachycardia
• Atypical AVNRT
• Permanent form of junctional reciprocating tachycardia (PJRT) - a rare form of nearly incessant orthodromic AVRT involving a slowly conducting, concealed posteroseptal accessory pathway 1

Acute Management

For acute termination of orthodromic AVRT:

1. First-line: Vagal maneuvers (Class I recommendation) 1
2. Second-line: IV adenosine (highly effective with extremely short half-life) 2
3. Alternative options:
   • IV calcium channel blockers (verapamil, diltiazem)
   • IV beta-blockers
   • Synchronized cardioversion for hemodynamically unstable patients

Long-term Management

Pharmacological Therapy

For prevention of recurrent episodes:

• No single drug class stands out as therapy of choice 2
• Options include:
  • Beta-blockers
  • Calcium channel blockers
  • Digitalis
  • Class IC antiarrhythmic drugs (flecainide, propafenone)
  • Class III antiarrhythmic drugs (sotalol, amiodarone)

Definitive Treatment

Catheter ablation is the definitive treatment of choice for symptomatic patients with AVRT. 1, 2

Catheter ablation:

• High success rate (>95%)
• Low complication rates
• Special considerations for pathways in particular locations (e.g., septal region)
• Cryoablation may be preferred for pathways close to normal conduction system 3

Special Considerations

Permanent Form of Junctional Reciprocating Tachycardia (PJRT)

• A rare form of nearly incessant orthodromic AVRT
• Involves a concealed accessory pathway with decremental conduction properties
• Usually located in the posteroseptal region
• ECG shows deeply inverted retrograde P waves in leads II, III, and aVF
• Long RP interval due to the decremental conduction properties
• May result in tachycardia-induced cardiomyopathy if left untreated 1

Wolff-Parkinson-White Syndrome

While orthodromic AVRT is the most common arrhythmia in WPW syndrome, these patients are at risk for:

• Antidromic AVRT (rare, <5% of patients with WPW)
• Pre-excited atrial fibrillation with risk of rapid ventricular response and sudden cardiac death 1, 3

Pitfalls and Caveats

1. Misdiagnosis: Orthodromic AVRT can be misdiagnosed as AVNRT due to similar presentation with narrow complex tachycardia.

2. Concealed pathways: Some accessory pathways conduct only in the retrograde direction (concealed) and do not show pre-excitation on the resting ECG, making diagnosis challenging 1.

3. Bundle branch block: The presence of bundle branch block during orthodromic AVRT can make the QRS complex wide, potentially leading to misdiagnosis as ventricular tachycardia.

4. Drug therapy caution: AV nodal blocking agents should be used with caution in patients with suspected pre-excitation, as they may facilitate rapid conduction through the accessory pathway during atrial fibrillation, potentially leading to ventricular fibrillation 1.

5. Risk assessment: In patients with WPW syndrome and orthodromic AVRT, evaluation for risk of sudden cardiac death is important, particularly in those with shortest pre-excited R-R interval <250 ms during AF 1.