Pre-excitation syndrome

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Overview[edit | edit source]

Pre-excitation syndrome is a condition where the the ventricles of the heart become depolarized too early, which leads to their partially premature contraction. Normally, the atria (chambers taking venous blood) and the ventriculi (chambers pro-pulsing blood towards organs) are electrically isolated, and only electrical passage exists at "atrioventricular node". In all pre-excitation syndromes, there is at least one more conductive pathway is present. Physiologically, the electrical depolarization wave 'waits' in atrioventricular node to allow atria contract before ventriculi. However, there is no such property exists in abnormal pathway, so electrical stimulus passes to ventricle by this tracts far before normal atrioventricular-his system, and ventricles are depolarized (excited) before (pre-) normal conduction system. The term pre-excitation derives from this condition.

It is usually caused by a secondary conduction pathway (other than the bundle of His)

Epidemiology and Demographics[edit | edit source]

• First described by Louis Wolff, John Parkinson and Paul Dudley White in 1930
• They found the association of these conditions with a small risk of sudden cardiac death
• Incidence 0.1 – 3.0 per 1000

Risk Factors[edit | edit source]

• High risk population for sudden cardiac death in Wolff-Parkinson-White syndrome include:
  • Policemen
  • Sports player/Athletes
  • Firemen
  • Air pilots Risk factors for the development of atrial fibrillation in WPW syndrome include:
  • Gender- Male
  • Age (Peak age 30-50 yrs)
  • Past history of syncope

Pathophysiology [edit | edit source]

WPW Syndrome

Pathophysiology of Pre-Excitation syndromes

• Pre-excitation refers to the early activation of the ventricles as a result of impulses bypassing the AV node via an accessory pathway. The latter are abnormal conduction pathways formed during cardiac development. These can conduct impulses either
  • towards ventricles (Anterograde conduction, rarely seen) ,
  • Away from the ventricles (Retrograde conduction, in approx 15%),
  • in both the directions ( Majority of cases).

• In WPW syndrome which is a type of pre-excitation syndrome the abnormal conduction pathways are called Bundle of Kent or AV bypass tract.
• The accessory pathways facilitates formation of Tachyarrhythmias by mainly forming reentry circuit , termed as AVRT. Even the direct conduction through the accessory pathways from A to V ( Bypassing AV node) can result in the formation of Tachyarrhythmias, seen most frequently in A. Fib with RVR.

Classification[edit | edit source]

Type	Conduction pathway	PR interval	QRS interval	Delta wave?
Wolff-Parkinson-White syndrome	Bundle of Kent (atria to ventricles)	short	long	yes
Lown-Ganong-Levine syndrome	"James bundle" (atria to bundle of His)	short	normal	no
Mahaim-type	Mahaim fibers	normal	long

WPW Syndrome

WPW is a combination of presence of congenital accessory pathways along with episodic tachyarrhythmias. Here the accessory pathways are reffered to as Bundle of Kent or AV bypass tracts.

The features of pre excitation are subtle, intermittent and are aggravated by increase in vagal tone ( Valsalva maneuver, AV blockage by drugs).

• ECG Features of WPW
  • Shortened PR interval (Less than 120ms)
  • Delta wave – slow/slurring in the rise of initial portion of the QRS
  • Widening of QRS complex
  • ST Segment and T wave discordant changes – i.e. in the opposite direction to the major component of the QRS complex
  • WPW is mainly categorized 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

Lown-Ganong-Levine (LGL) Syndrome

Here the Accessory pathway are composed of James fibres.  

ECG features:

• PR interval <120ms
• Normal QRS morphology

The important point to be noted is that this tern is not relevant or shouldn't be used in the absence of paroxysmal tachycardia. Its existence is disputed and it 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

Clinical Features[edit | edit source]

• People with Pre- Excitation syndromes are usually asymptomatic , however the individual may experience following symptoms
  • Palpitations
  • Dizziness or lightheadedness.
  • Shortness of breath.
  • Chest pain
  • Fatigue.
  • Anxiety.
  • Fainting
  • Difficulty breathing

Diagnosis and Treatment [edit | edit source]

Atrioventricular Reentry Tachycardia's (AVRT)

AVRT is a form of PSVT. Reentry circuit results from the combination of signal transduction from normal conduction system and accessory pathway.

• During tachyarrythmias, the accessory pathway forms part of the reentry circuit that results in the disappearance of features of tachyarrythmias..
• AVRT are further divided into
  • Orthodromic or Antidromic conduction based on ECG morphology and direction of formation of re-entry circuit.

AVRT ( Orthodromic and Antidromic)

1) AVRT with Orthodromic Conduction

In this the anterograde conduction occurs via the AV node and retrograde conduction occurs via accessory pathway.

ECG features of AVRT with orthodromic conduction

• 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 alterns by a normal QRS amplitude
• T wave inversion common
• ST segment depression

Treatment Of Orthodromic AVRT:- Hemodynamically Unstable patients( Low BP, Altered mental state, pulmonary edema)- Synchronized DC Cardioversion. In patients who are hemodynamically stable- Vagal maneuvers, Adenosine, CCB and DC cardioversion as a last resort only if patient not responding to medical therapy.

2) AVRT with Antidromic Conduction

In this the anterograde conduction occurs via the accessory pathway and retrograde conduction via the AV node. Occurring only in app. 5% of patients with WPW.

ECG features are:

• Rate usually 200 – 300 bpm.
• Wide QRS complexes due to abnormal accessory pathway ventricular depolarisation.
• Due to wide complex, Commonly mistaken for Ventricular Tachycardia.

Treatment of antidromic AVRT: Hemodynamically unstable patients:- Urgent synchronized DC cardio version. Hemodynamically stable patients:- Amiodarone, procainamide or ibutilide. 

3) Atrial Fib/Atrial Flutter in WPW

• In 20% of the patients WPW Atrial fibrillation can occur and in approx 7% of patients with WPW atrial flutter can occur. Accessory pathways plays major role by allowing the rapid conduction of impulses directly to the ventricles without involving AV node, in extreme cases may lead to VT or VF.

ECG features

• 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 presents with same features as atrial fibrillation in WPW except rhythm is regular and commonly mistaken for VT

Treatment of AF with WPW:- Hemodynamically unstable patients: Urgent synchronized DC cardioversion. Hemodynamically stable patients:- Procainamide or ibutilide. Caution: Adenosine, CCB, Beta blockers results in increase in conduction via accessory pathway which results in worsening of condition with possible degeneration into VT or VF

Differentiating from other Diseases[edit | edit source]

• Pre-Excitation syndrome must be differentiated from other diseases. The conditions that needs to be ruled out while making the diagnosis are:
  • Atrial Flutter
  • Atrial Fibrillation
  • Atrioventricular Nodal Reentry Tachycardia
  • Atrial Tachycardia
  • Ventricular Tachycardia
  • Paroxysmal Supraventricular Tachycardia
  • Ebstein Anomaly
  • Genetics of Glycogen-Storage Disease Type I
  • Genetics of Glycogen-Storage Disease Type II (Pompe Disease)
  • Lown-Ganong-Levine Syndrome
  • Syncope
  • Danon Disease

Prevention[edit | edit source]

• Electrical conduction system of the heart