Pre-excitation syndromes

This page covers the pathophysiology and ECG features of pre-excitation syndromes in sinus rhythm. The two main forms of tachyarrhythmias that occur due to accessory pathways are discussed separately — see atrioventricular re-entry tachycardia (AVRT) and atrial fibrillation/flutter in pre-excitation.

Overview of Wolff-Parkinson-White (WPW) Syndrome

• WPW Syndrome refers to the presence of a congenital accessory pathway (AP) and episodes of tachyarrhythmias. The term is often used interchangeablely with pre-excitation syndrome
• First described in 1930 by Louis Wolff, John Parkinson and Paul Dudley White
• Incidence is 0.1 – 3.0 per 1000
• Associated with a small risk of sudden cardiac death

ECG features of WPW in sinus rhythm

Pathophysiology of pre-excitation and APs

Pre-excitation refers to early activation of the ventricles due to impulses bypassing the AV node via an AP. Also known as bypass tracts, APs are abnormal conduction pathways formed during cardiac development and can exist in a variety of anatomical locations and in some patients there may be multiple pathways. In WPW, the AP is sometimes referred to as the Bundle of Kent, or atrioventricular bypass tract.

An AP can conduct impulses in three ways:

• In both directions (majority)
• Retrograde only, away from the ventricle (15%)
• Anterograde only, towards the ventricle (rare)

The direction of conduction affects the appearance of the ECG in sinus rhythm and during tachyarrhythmias.

APs can be left-sided or right-sided, and ECG features will vary depending on this:

• Left-sided AP: produces a positive delta wave in all precordial leads, with R/S > 1 in V₁. Sometimes referred to as a type A WPW pattern
• Right-sided AP: produces a negative delta wave in leads V₁ and V₂. Sometimes referred to as a type B WPW pattern

Note that the features of pre-excitation may be subtle, or present only intermittently. Pre-excitation may be more pronounced with increased vagal tone e.g. during Valsalva manoeuvres, or with AV blockade e.g. drug therapy.

Concealed pathway

In patients with retrograde-only accessory conduction, all anterograde conduction occurs via the AV node. No pre-excitation occurs and therefore no features of WPW are seen on the ECG in sinus rhythm. This is termed a “concealed pathway”. These patients can still experience tachyarrhythmias, as the pathway can still form part of a re-entry circuit.

Tachyarrhythmias in WPW

There are only two main forms of tachyarrhythmias that occur in patients with WPW — these are discussed separately:

• Atrial fibrillation or flutter. Due to direct conduction from atria to ventricles via an AP, bypassing the AV node
• Atrioventricular re-entry tachycardia (AVRT). Due to formation of a re-entry circuit involving the AP

Other Pre-Excitation Syndromes / Accessory Pathways

Lown-Ganong-Levine (LGL) Syndrome

Proposed pre-excitation syndrome. AP composed of James fibres.  

ECG features:

The term should not be used in the absence of paroxysmal tachycardia. Existence is disputed.

Mahaim-Type Pre-excitation

Right sided APs connecting either AV node to ventricles, fascicles to ventricles, or atria to fascicles.

ECG features:

ECG Examples

Sinus Rhythm – Type A Pattern

Example 1

• Sinus rhythm with a very short PR interval (< 120 ms)
• Broad QRS complexes with a slurred upstroke to the QRS complex — the delta wave
• Dominant R wave in V1 suggests a left-sided AP, and is sometimes referred to as “Type A” WPW
• Tall R waves and inverted T waves in V1-3 mimicking right ventricular hypertrophy (RVH) — these changes are due to WPW and do not indicate underlying RVH
• Negative delta wave in aVL simulating the Q waves of lateral infarction — this is referred to as the “pseudo-infarction” pattern

Example 2

Another example of a left-sided AP (type A WPW), with dominant R wave in V1 and right precordial T-wave inversions simulating RVH.

Sinus rhythm – Type B Pattern

Example 3

• Sinus rhythm with very short PR interval (< 120 ms)
• Broad QRS complexes with a slurred upstroke to the QRS complexes — the delta wave
• Dominant S wave in V1 indicates a right-sided AP — sometimes referred to as “Type B” WPW
• Tall R waves and inverted T waves in the inferior leads and V4-6 mimic the appearance of left ventricular hypertrophy (LVH) — again, this is due to WPW and does not indicate underlying LVH

Example 4

Another example of a right-sided AP (Type B WPW):

• Negative delta waves in leads III and aVF simulate the Q waves of prior inferior MI (= pseudo-infarction pattern)

Sinus Rhythm – Paediatric ECGs

Example 5

An example of WPW in a 5-year old boy — the ECG changes are more subtle than in adults:

• The PR interval is short even allowing for the patient’s age
• The QRS complexes do not appear particularly broad — however, there is definite slurring of the upstroke of each R wave, most obvious in leads II, III, aVF and V4 (= delta waves)
• The RSR’ pattern with T wave inversion in V1-2 is a normal variant in children of this age; this is still a Type B pattern due to absence of a dominant R wave in V1
• There are pseudo-infarction Q waves in lead aVL simulating lateral infarction

For more information on interpretation of the paediatric ECG, check out our Guide to Paediatric ECG Interpretation.

Example 6

Another example of paediatric WPW, this time in a 7-year old child:

• Slight QRS widening and delta waves are more evident in the older child
• Again, there are pseudo-infarction Q waves in aVL
• It is difficult to determine if this is a left or right-sided AP, as a dominant R wave in V1 is normal for the child’s age

Lown-Ganong-Levine Syndrome

Example 7

This is a possible example of LGL syndrome:

• Very short PR interval
• Narrow QRS complexes
• No evidence of delta waves

Clinical Cases

• Top 100 ECG – Case 002
• Top 100 ECG – Case 038
• Top 100 ECG – Case 073
• Top 100 ECG – Case 092
• Top 100 ECG – Case 136

Related Topics

• Pre-excitation syndromes
• Pre-excitation with atrial fibrillation/flutter
• Atrioventricular Re-entry Tachycardia (AVRT)
• Atrioventricular Nodal Reentrant Tachycardia (AVNRT)
• Supraventricular tachycardia
• VT versus SVT with aberrancy
• ECG Medical Training – WPW Syndrome Part 1
• ECG Medical Training – WPW Syndrome Part 2

Advanced Reading

Online

• Wiesbauer F, Kühn P. ECG Yellow Belt online course: Become an ECG expert. Medmastery
• Wiesbauer F, Kühn P. ECG Blue Belt online course: Learn to diagnose any rhythm problem. Medmastery
• Rawshani A. Clinical ECG Interpretation ECG Waves
• Smith SW. Dr Smith’s ECG blog.

Textbooks

• Mattu A, Tabas JA, Brady WJ. Electrocardiography in Emergency, Acute, and Critical Care. 2e, 2019
• Brady WJ, Lipinski MJ et al. Electrocardiogram in Clinical Medicine. 1e, 2020
• Straus DG, Schocken DD. Marriott’s Practical Electrocardiography 13e, 2021
• Hampton J. The ECG Made Practical 7e, 2019
• Grauer K. ECG Pocket Brain (Expanded) 6e, 2014
• Brady WJ, Truwit JD. Critical Decisions in Emergency and Acute Care Electrocardiography 1e, 2009
• Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric 6e, 2008
• Mattu A, Brady W. ECG’s for the Emergency Physician Part I 1e, 2003 and Part II
• Chan TC. ECG in Emergency Medicine and Acute Care 1e, 2004
• Smith SW. The ECG in Acute MI. 2002 [PDF]

LITFL Further Reading

• ECG Library Basics – Waves, Intervals, Segments and Clinical Interpretation
• ECG A to Z by diagnosis – ECG interpretation in clinical context
• ECG Exigency and Cardiovascular Curveball – ECG Clinical Cases
• 100 ECG Quiz – Self-assessment tool for examination practice
• ECG Reference SITES and BOOKS – the best of the rest