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
  • PR interval < 120ms
  • Delta wave: slurring slow rise of initial portion of the QRS
  • QRS prolongation > 110ms
  • Discordant ST-segment and T-wave changes (i.e. in the opposite direction to the major component of the QRS complex)
  • Pseudo-infarction pattern in up to 70% of patients — due to negatively deflected delta waves in inferior/anterior leads (“pseudo-Q waves”), or prominent R waves in V1-3 (mimicking posterior infarction)
ECG features in normal conduction (left) and WPW (right)

Understanding ECG changes makes them easier to recognise
  • Aberrant conduction via an AP leads to early ventricular depolarisation (pre-excitation), which manifests as a short PR interval and slurred upstroke of the QRS complex
  • Conduction still occurs via the AV node, and is actually a brake on AP conduction, ceasing its propagation path in the ventricle
  • QRS duration is thus prolonged, but not to the same extent as in bundle branch block
  • Because there is an initial abnormal direction of depolarisation (e.g. originating from left ventricle in left-sided AP), there will be an abnormal direction of repolarisation. This produces appropriately discordant ST segment and T wave changes (see above diagram)

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.

WPW Delta wave pre=excitation
An example of a right-sided AP, with anterograde conduction occurring in sinus rhythm. Note that in most patients, retrograde conduction can also occur and forms the basis of re-entry tachycardias

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:

Wolff-Parkinson-White Syndrome WPW reentry circuit
Re-entry circuit during AVRT (retrograde conduction via Bundle of Kent)

Other Pre-Excitation Syndromes / Accessory Pathways
Lown-Ganong-Levine (LGL) Syndrome

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

ECG features:

  • PR interval < 120ms
  • Normal QRS morphology

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:

  • Sinus rhythm ECG may be normal
  • May result in variation in ventricular morphology
  • Re-entry tachycardia typically has LBBB morphology

ECG Examples
Sinus Rhythm – Type A Pattern
Example 1
Type A WPW ECG 3
  • 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
Type A WPW ECG 1

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
WPW Type B ECG 2

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
WPW-in-5yo-boy 2

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
WPW-in-a-7-year-old 2

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
Lown–Ganong–Levine syndrome (LGL)

This is a possible example of LGL syndrome:

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

Clinical Cases

Advanced Reading



LITFL Further Reading



MBBS (UWA) CCPU (RCE, Biliary, DVT, E-FAST, AAA) Emergency Medicine Advanced Trainee in Melbourne, Australia. Special interests in diagnostic and procedural ultrasound, medical education, and ECG interpretation. Editor-in-chief of the LITFL ECG Library. Twitter: @rob_buttner

Emergency Physician in Prehospital and Retrieval Medicine in Sydney, Australia. He has a passion for ECG interpretation and medical education | ECG Library |

One comment

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.