Pre-excitation Syndromes

Wolff-Parkinson-White Syndrome

Wolff-Parkinson-White (WPW) Syndrome is a combination of the presence of a congenital accessory pathway and episodes of tachyarrhythmia

• First described in 1930 by Louis Wolff, John Parkinson and Paul Dudley White
• Associated with a small risk of sudden cardiac death.
• Incidence 0.1 – 3.0 per 1000

Pre-Excitation and Accessory Pathways

Pre-excitation refers to early activation of the ventricles due to impulses bypassing the AV node via an accessory pathway.

Accessory pathways, also known as bypass tracts, 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

An accessory pathway can conduct impulses either anterograde, towards the ventricle, retrograde, away from the ventricle, or in both directions.

In WPW the accessory pathway is often referred to as the Bundle of Kent, or atrioventricular bypass tract.

The majority of pathways allow conduction in both directions, with retrograde only conduction occurring in 15% of cases, and anterograde only conduction rarely seen.

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

Tachyarrythmia can be facilitated by the formation of a reentry circuit involving the accessory pathway, termed atrioventricular reentry tachycardias (AVRT).

Tachyarrythmia may also be facilitated by direct conduction from the atria to the ventricles via the accessory pathway, bypassing the AV node, seen with atrial fibrillation or atrial flutter in conjunction with WPW

ECG in Sinus Rhythm

The presence of a pre-excitation pathway results in a number of changes to the ECG in sinus rhythm.

ECG features of WPW in sinus rhythm are:

• PR interval <120ms
• Delta wave – slurring slow rise of initial portion of the QRS
• QRS prolongation >110ms
• ST Segment and T wave discordant changes – i.e. in the opposite direction to the major component of the QRS complex
• Pseudo-infarction pattern can be seen in up to 70% of patients – due to negatively deflected delta waves in the inferior / anterior leads (“pseudo-Q waves”), or as a prominent R wave in V1-3 (mimicking posterior infarction).

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.

WPW may be described as type A or B.

• Type A: positive delta wave in all precordial leads with R/S > 1 in V₁
• Type B: negative delta wave in leads V₁ and V₂

In patients with retrograde-only accessory conduction all antegrade conduction occurs via the AV node, thus no features of WPW are seen on the ECG in sinus rhythm (as no pre-excitation occurs). This is termed a “concealed pathway”.

Patients with a concealed pathway can experience tachyarrythmias as the pathway can still form part of a re-entry circuit

Atrioventricular Reentry Tachycardias (AVRT)

AVRT is a form of paroxysmal supraventricular tachycardia. A reentry circuit is formed by the normal conduction system and the accessory pathway resulting in circus movement.

• During tachyarrythmias the features of pre-excitation are lost as the accessory pathway forms part of the reentry circuit.
• AVRT often triggered by premature atrial or premature ventricular beats.
• AVRT are further divided in to orthodromic or antidromic conduction based on direction of reentry conduction and ECG morphology.

AVRT with Orthodromic Conduction

In orthodromic AVRT anterograde conduction occurs via the AV node with retrograde conduction occurring via the accessory pathway. This can occur in patients with a concealed pathway.

ECG features of AVRT with orthodromic conduction are:

• Rate usually 200 – 300 bpm
• P waves may be buried in QRS complex or retrograde
• QRS Complex usually <120 ms unless pre-existing bundle branch block, or rate-related aberrant conduction
• QRS Alternans – phasic variation in QRS amplitude associated with AVNRT and AVRT, distinguished from electrical alternans by a normal QRS amplitude
• T wave inversion common
• ST segment depression

Treatment of orthodromic AVRT

• Treatment of AVRT is based on the presence of haemodynamic instability e.g. hypotension, altered mental state, or pulmonary oedema.
• In patients who are haemodynamically stable vagal manoeuvres may be successful, followed by adenosine or calcium-channel blockers, and DC cardioversion may be considered if non-repsonsive to medical therapy.
• In a haemodynamically unstable patient urgent synchronised DC cardioversion is required.

AVRT with Antidromic Conduction

In antidromic AVRT anterograde conduction occurs via the accessory pathway with retrograde conduction via the AV node. Much less common than orthodromic AVRT occurring in ~5% of patients with WPW.

ECG features of AVRT with antidromic conduction are:

• Rate usually 200 – 300 bpm.
• Wide QRS complexes due to abnormal ventricular depolarisation via accessory pathway.

Treatment of antidromic AVRT

• AVRT with antidromic conduction results in a wide complex tachycardia which may be mistaken for Ventricular Tachycardia.
• For discussion on differentiating wide complex tachycardias see here, here, and here.
• Stable patients may respond to drug therapy including amiodarone, procainamide or ibutilide, but may require DC cardioversion
• In a haemodynamically unstable patient urgent synchronised DC cardioversion is required.
• If in doubt treat as VT

Atrial Fibrillation & Atrial Flutter in WPW

• Atrial fibrillation can occur in up to 20% of patients with WPW.
• Atrial flutter can occur in up to 7% of patients with WPW.
• The accessory pathway allows for rapid conduction directly to the ventricles bypassing the AV node.
• Rapid ventricular rates may result in degeneration to VT or VF.

ECG features of Atrial Fibrillation in WPW are:

• Rate > 200 bpm
• Irregular rhythm
• Wide QRS complexes due to abnormal ventricular depolarisation via accessory pathway
• QRS Complexes change in shape and morphology
• Axis remains stable unlike Polymorphic VT

Atrial Flutter results in the same features as AF in WPW except the rhythm is regular and may be mistaken for VT.

Treatment of AF with WPW

• Treatment with AV nodal blocking drugs e.g. adenosine, calcium-channel blockers, beta-blockers may increase conduction via the accessory pathway with a resultant increase in ventricular rate and possible degeneration into VT or VF
• In a haemodynamically unstable patient urgent synchronised DC cardioversion is required.
• Medical treatment options in a stable patient include procainamide or ibutilide, although DC cardioversion may be preferred.

Other Pre-Excitation Syndromes / Accessory Pathways

Lown-Ganong-Levine (LGL) Syndrome

Proposed pre-excitation syndrome. Accessory pathway 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 and may not exist

Mahaim-Type Pre-excitation

Right sided accessory pathways 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
• Reentry tachycardia typically has LBBB morphology

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 — this pattern is known as “Type A” WPW and is associated with a left-sided accessory pathway.
• Tall R waves and inverted T waves in V1-3 mimicking right ventricular hypertrophy — 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 the Type A WPW pattern 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 — this pattern is known as “Type B” WPW and indicates a right-sided accessory pathway.
• Tall R waves and inverted T waves in the inferior leads and V4-6 mimic the appearance of left ventricular hypertrophy — again, this is due to WPW and does not indicate underlying LVH.

Example 4

Another example of WPW Type B:

• 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 categorise this ECG as type A or B given that a dominant R wave in V1 is normal for the child’s age.

Dysrhythmias

Example 7

Orthodromic atrioventricular re-entry tachycardia (AVRT)

• Regular, narrow complex tachycardia at 225 bpm.
• No discernible P-waves.
• The QRS complexes are narrow because impulses are being transmitted in an orthodromic direction (A -> V) via the AV node.
• This rhythm is indistinguishable from AV-nodal re-entry tachycardia (AVNRT).

NB. Widespread ST depression (“rate-related change”) is present; this in common in SVT and does not necessarily indicate myocardial ischaemia

Example 7B

Example 8

• Narrow complex tachycardia at 180 bpm with no discernible P waves – this is another example of orthodromic AVRT.

Example 8B

Antidromic atrioventricular re-entry tachycardia (AVRT)

Example 9

Antidromic AVRT in a 5-year old boy with WPW:

• There is a regular, broad complex tachycardia at ~280 bpm; this would be very difficult to distinguish from VT.
• However, given the child’s age, VT is very unlikely: > 95% of broad complex tachycardias in children are actually some form of SVT with aberrancy.
• This was the presenting ECG of the 5-year old boy from Example 5 (see above for his baseline ECG); the antidromic AVRT resolved with vagal manoeuvres.

Read more about differentiating VT from SVT with aberrancy.

Example 10

• Another example of broad complex tachycardia due to antidromic AVRT in a 15-year old boy with WPW.
• The AVRT resolved with vagal manoeuvres.

Atrial Fibrillation with WPW

Example 11

Atrial fibrillation in a patient with WPW:

• Rapid, irregular, broad complex tachycardia (overall rate ~ 200 bpm) with a LBBB morphology (dominant S wave in V1).
• This could easily be mistaken for AF with LBBB.
• However, the morphology is not typical of LBBB, the rate is too rapid (up to 300 bpm in places, i.e. too rapid to be conducted via the AV node) and there is a subtle beat-to-beat variation in the QRS width which is more typical of WPW (LBBB usually has fixed width QRS complexes).

Example 12

• Another example of AF with WPW resulting in a very rapid (up to 300 bpm in places), irregular broad-complex tachycardia with varying QRS width.
• There are two narrow complexes (in V1-3), where the atrial impulses are presumably conducted via the AV node instead of via the AP.
• This rhythm is extremely difficult to differentiate from polymorphic VT; however it does not demonstrate the twisting morphology characteristic of torsades de pointes.

NB. Regardless of the aetiology, the most appropriate treatment for this rhythm (if sustained) would be immediate electrical cardioversion.

Intermittent WPW

Example 13

AF with WPW showing intermittent pre-excitation:

• Some of the impulses are transmitted via the AP (= pre-excited beats), producing characteristic delta waves.
• Other impulses are transmitted via the AV node, producing narrow QRS complexes.

Example 14

• Another example of AF with WPW with intermittent pre-excitation — characteristic delta waves are best seen in V2.

Lown-Ganong-Levine Syndrome

Example 15

This is a possible example of LGL syndrome:

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

Related Topics

• Supraventricular tachycardia
• VT versus SVT with aberrancy
• ECG Medical Training – WPW Syndrome Part 1
• ECG Medical Training – WPW Syndrome Part 2

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

Advanced Reading

• Brady WJ, Truwit JD. Critical Decisions in Emergency and Acute Care Electrocardiography
• Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric
• Wagner GS. Marriott’s Practical Electrocardiography 12e
• Chan TC. ECG in Emergency Medicine and Acute Care
• Rawshani A. Clinical ECG Interpretation
• Mattu A. ECG’s for the Emergency Physician
• Hampton JR. The ECG In Practice, 6e