Pre-excitation syndrome

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1], Associate Editor-In-Chief: Shivam Singla, M.D.[2]

Overview

Pre-excitation syndrome is a condition where 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).

Historical Perspective

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

Classification

pre-excitation syndrome may be classified into sub-types^[1]

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 the presence of congenital accessory pathways along with episodic tachyarrhythmias. Here the accessory pathways are referred to as Bundle of Kent or AV bypass tracts.

The features of pre-excitation are subtle, intermittent, and are aggravated by an 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 an 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 fibers.

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

Pathophysiology

Pathophysiology of Pre-Excitation syndrome
- 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 facilitate the formation of Tachyarrhythmias by mainly forming a reentry circuit, termed as AVRT (80%). Even in cases of direct conduction through the accessory pathways from A to V ( Bypassing AV node), there can be the resultant formation of Tachyarrhythmias, seen most frequently in the condition of A. Fib with RVR.

Clinical Features

People with Pre- Excitation syndromes may be asymptomatic, however, the individual may experience following symptoms

Palpitations
Dizziness or lightheadedness.
Shortness of breath.
Chest pain
Fatigue.
Anxiety.
Fainting
Difficulty breathing

Differentiating Pre-excitation Syndrome from other Diseases


Arrhythmia	Rhythm	Rate	P wave	PR Interval	QRS Complex	Response to Maneuvers	Epidemiology	Co-existing Conditions
'Atrial Fibrillation (AFib)^[2]^[3]'	Irregularly irregular	On a 10-second 12-lead EKG strip, multiply number of QRS complexes by 6	Absent Fibrillatory waves	Absent	Less than 0.12 seconds, consistent, and normal in morphology in the absence of aberrant conduction	Does not break with adenosine or vagal maneuvers	2.7–6.1 million people in the United States have AFib 2% of people younger than age 65 have AFib, while about 9% of people aged 65 years or older have AFib	Elderly Following bypass surgery Mitral valve disease Hyperthyroidism Diabetes Heart failure Ischemic heart disease Chronic kidney disease Heavy alcohol use Left chamber enlargement
Atrial Flutter^[4]	Regular or Irregular	75 (4:1 block), 100 (3:1 block) and 150 (2:1 block) beats per minute (bpm), but 150 is more common	Sawtooth pattern of P waves at 250 to 350 bpm Biphasic deflection in V1	Varies depending upon the magnitude of the block, but is short	Less than 0.12 seconds, consistent, and normal in morphology	Conduction may vary in response to drugs and maneuvers dropping the rate from 150 to 100 or to 75 bpm	Incidence: 88 per 100,000 individuals	Elderly Alcohol
Atrioventricular nodal reentry tachycardia (AVNRT)^[5]^[6]^[7]^[8]	Regular	140-280 bpm	slow-fast AVNRT: Pseudo-S wave in leads II, III, and AVF Pseudo-R' in lead V1. Fast-Slow AVNRT P waves between the QRS and T waves (QRS-P-T complexes) Slow-Slow AVNRT Late P waves after a QRS Often appears as atrial tachycardia. Inverted, superimposed on or buried within the QRS complex (pseudo R prime in V1/pseudo S wave in inferior leads)	Absent (P wave can appear after the QRS complex and before the T wave, and in atypical AVNRT, the P wave can appear just before the QRS complex)	Less than 0.12 seconds, consistent, and normal in morphology in the absence of aberrant conduction QRS alternans may be present	May break with adenosine or vagal maneuvers	60%-70% of all supraventricular tachycardias	Structural heart disease Atrial tachyarrhythmias
'Multifocal Atrial Tachycardia^[9]^[10]	Irregular	Atrial rate is > 100 beats per minute	Varying morphology from at least three different foci Absence of one dominant atrial pacemaker, can be mistaken for atrial fibrillation if the P waves are of low amplitude	Variable PR intervals, RR intervals, and PP intervals	Less than 0.12 seconds, consistent, and normal in morphology	Does not terminate with adenosine or vagal maneuvers	0.05% to 0.32% of electrocardiograms in general hospital admissions	Elderly Chronic obstructive pulmonary disease (COPD)
Paroxysmal Supraventricular Tachycardia	Regular	150 and 240 bpm	Absent Hidden in QRS	Absent	Narrow complexes (< 0.12 s)	Breaks with vagal maneuvers, adenosine, diving reflex, oculocardiac reflex	Prevalence: 0.023 per 100,000	Alcohol Caffeine Nicotine Psychological stress Wolff-Parkinson-White syndrome
Premature Atrial Contractrions (PAC)^[11]^[12]	Regular except when disturbed by premature beat(s)	80-120 bpm	Upright	> 0.12 second Maybe shorter than that in normal sinus rhythm (NSR) if the origin of PAC is located closer to the AV node Ashman’s Phenomenon: PAC displaying a right bundle branch block pattern	Usually narrow (< 0.12 s)	Breaks with vagal maneuvers, adenosine, diving reflex, oculocardiac reflex		Infants Cardiomyopathy Myocarditis Elderly Coronary artery disease Stroke Increased atrial natriuretic peptide (ANP) Hypercholesterolemia
'Wolff-Parkinson-White Syndrome^[13]^[14]		Regular	Atrial rate is nearly 300 bpm and the ventricular rate is at 150 bpm	With orthodromic conduction due to a bypass tract, the P wave generally follows the QRS complex, whereas in AVNRT, the P wave is generally buried in the QRS complex.	Less than 0.12 seconds	A delta wave and evidence of ventricular pre-excitation if there is conduction to the ventricle via ante-grade conduction down an accessory pathway A delta wave and pre-excitation may not be present because bypass tracts do not conduct ante-grade.	May break in response to procainamide, adenosine, vagal maneuvers	Worldwide prevalence of WPW syndrome is 100 - 300 per 100,000	Ebstein's anomaly Mitral valve prolapse: This cardiac disorder, if present, is associated with left-sided accessory pathways. Hypertrophic cardiomyopathy: This disorder is associated with familial/inherited form of WPW syndrome. Hypokalemic periodic paralysis Pompe disease Tuberous sclerosis
Ventricular Fibrillation (VF)^[15]^[16]^[17]	Irregular	150 to 500 bpm	Absent	Absent	Absent (R on T phenomenon in the setting of ischemia)	Does not break in response to procainamide, adenosine, vagal maneuvers	3-12% cases of acute myocardial infarction (AMI) Out of 356,500 out of hospital cardiac arrests, 23% have VF as initial rhythm	Myocardial ischemia / infarction Cardiomyopathy Channelopathies e.g. Long QT (acquired / congenital) Electrolyte abnormalities (hypokalemia/hyperkalemia, hypomagnesemia) Aortic stenosis Aortic dissection Myocarditis Cardiac tamponade Blunt trauma (Commotio Cordis) Sepsis Hypothermia Pneumothorax Seizures Stroke
Ventricular Tachycardia^[18]^[19]	Regular	> 100 bpm (150-200 bpm common)	Absent	Absent Initial R wave in V1, initial r > 40 ms in V1/V2, notched S in V1, initial R in aVR, lead II R wave peak time ≥50 ms, no RS in V1-V6, and atrioventricular dissociation	Wide complex, QRS duration > 120 milliseconds	Does not break in response to procainamide, adenosine, vagal maneuvers	5-10% of patients presenting with AMI	Coronary artery disease Aortic stenosis Cardiomyopathy Electrolyte imbalances (e.g., hypokalemia, hypomagnesemia) Inherited channelopathies (e.g., long-QT syndrome) Catecholaminergic polymorphic ventricular tachycardia Arrhythmogenic right ventricular dysplasia Myocardial infarction Torsades de pointes is a form of polymorphic VT that is often associated with a prolonged QT interval

Epidemiology and Demographics

Incidence 0.1 – 3.0 per 1000
LGL syndrome is a rare Man > woman.
prognosis is good with SCD is noted in only 0.1% (rare)

Risk Factors

High risk population for sudden cardiac death in Wolff-Parkinson-White syndrome include:

Policemen
Athletes
Firemen
Pilots
Steelworkers

Risk factors for the development of atrial fibrillation in WPW syndrome include:

Male gender
Age (peak ages for the development of atrial fibrillation include 30 years and 50 years)
Past history of syncope

Natural History, Complications and Prognosis[edit | edit source]

The majority of patients with [disease name] remain asymptomatic for [duration/years].
Early clinical features include [manifestation 1], [manifestation 2], and [manifestation 3].
If left untreated, [#%] of patients with [disease name] may progress to develop [manifestation 1], [manifestation 2], and [manifestation 3].
Common complications of [disease name] include [complication 1], [complication 2], and [complication 3].
Prognosis is generally [excellent/good/poor], and the [1/5/10year mortality/survival rate] of patients with [disease name] is approximately [#%].

Diagnosis

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.( See: Diagram)^[20]

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

1) AVRT with Orthodromic Conduction

In this, the anterograde conduction occurs via the AV node and retrograde conduction occurs via an 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 altrens by a normal QRS amplitude
T wave inversion common
ST segment depression

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.

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.