What are the signs on an electrocardiogram (ECG) of an extranodal pathway?

ECG Signs of Extranodal Pathways

The classic electrocardiographic signs of an extranodal pathway (accessory pathway) include a short PR interval (<120 ms), delta wave (slurring of the initial QRS upstroke), and widened QRS complex (>120 ms) due to ventricular pre-excitation. 1

Mechanism and Pathophysiology

Extranodal pathways (also known as accessory pathways) are abnormal electrical connections between the atria and ventricles that bypass the normal AV nodal conduction system. During normal sinus rhythm, electrical impulses travel simultaneously through:

• The normal AV nodal-His Purkinje system (with physiologic delay)
• The accessory pathway (without physiologic delay)

This creates fusion of ventricular activation from both pathways, resulting in the characteristic pre-excitation pattern on ECG 1.

Specific ECG Findings

Manifest Pre-excitation (Wolff-Parkinson-White Pattern)

• Short PR interval: <120 ms 2, 1
• Delta wave: Slurred upstroke at the beginning of the QRS complex 1
• Widened QRS complex: ≥120 ms 1
• The degree of pre-excitation depends on the relative contribution from normal AV nodal conduction versus accessory pathway conduction 1

Concealed Accessory Pathways

• No visible pre-excitation during sinus rhythm
• Only conduct retrogradely (ventricle to atrium)
• May only be detected during tachycardia episodes 2

Intermittent Pre-excitation

• Pre-excitation pattern appears and disappears on serial ECGs
• Indicates a relatively long refractory period of the accessory pathway
• Associated with lower risk of rapid ventricular rates during atrial fibrillation 2, 3

Localization of Accessory Pathways

The location of the accessory pathway can often be determined by analyzing the delta wave and QRS morphology:

• Left-sided pathways: Typically show positive delta waves in leads I and aVL, negative in V1 4
• Right-sided pathways: Often show negative delta waves in leads I and aVL, positive in V1 4
• Posteroseptal pathways: Typically show negative delta waves in leads II, III, and aVF 4
• Anteroseptal pathways: Often show positive delta waves in leads II, III, and aVF 4

Modern algorithms like EASY-WPW have improved the accuracy of pre-procedural localization to 93% compared to older methods 5.

Types of Accessory Pathway-Related Tachycardias

When an accessory pathway participates in a tachycardia, it can manifest as:

1. Orthodromic AVRT (most common, 95%):

   • Antegrade conduction through AV node
   • Retrograde conduction through accessory pathway
   • Presents as narrow QRS tachycardia (unless aberrancy) 2, 6

2. Antidromic AVRT (less common):

   • Antegrade conduction through accessory pathway
   • Retrograde conduction through AV node
   • Presents as wide QRS tachycardia with maximal pre-excitation 2

3. Pre-excited atrial fibrillation:

   • Potentially life-threatening
   • Rapid conduction through accessory pathway can lead to ventricular fibrillation
   • Irregular wide-complex tachycardia with variable QRS morphology 1

Variants of Pre-excitation

Several variants of pre-excitation exist with distinct ECG patterns:

• Permanent junctional reciprocating tachycardia (PJRT): Involves a slowly conducting, concealed posteroseptal accessory pathway with long RP interval 2

• Fasciculoventricular pathway: Connection between the His-Purkinje system and ventricular myocardium

  • Fixed degree of pre-excitation that doesn't change with autonomic maneuvers
  • PR interval remains normal 7

• Mahaim fibers: Nodoventricular or fasciculoventricular connections with decremental properties

  • May show minimal or no pre-excitation during sinus rhythm
  • Can produce left bundle branch block pattern during tachycardia 2

Clinical Implications

The presence of an accessory pathway on ECG has important clinical implications:

• Risk stratification: Shortest pre-excited R-R interval <250 ms during atrial fibrillation indicates high risk for sudden cardiac death 1

• Intermittent pre-excitation: Generally indicates a lower-risk pathway with longer refractory period 2

• Abrupt loss of pre-excitation during exercise testing: Suggests a low-risk accessory pathway 2

Common Pitfalls in Diagnosis

1. Misdiagnosis as bundle branch block: The delta wave of pre-excitation can be mistaken for a conduction delay

2. Intermittent pre-excitation: May be missed if not present on all ECGs 3

3. Concealed pathways: No pre-excitation on baseline ECG, only detectable during tachycardia 2

4. Short PR interval without delta wave: May represent Lown-Ganong-Levine syndrome or other cardiac conditions like HCM or Fabry's disease 2

By carefully analyzing the ECG for these characteristic findings, clinicians can identify extranodal pathways and determine appropriate management strategies based on the patient's risk profile.