• Accessory Pathways and Conduction System
• Terminology and Characteristics of Accessory Pathways
• Typical Accessory Pathways
• Atypical Accessory Pathways
• Mahaim Pathways and Tachycardia
• Pre-excitation and Typical Accessory Pathways
• Pre-excitation and Atypical Accessory Pathways
• Adenosine and Accessory Pathways
• Sinus Rhythm and Atypical Pathways
• Atrio-Fascicular (AF) Pathway
• Long Atrio-Ventricular (AV) Pathway
• Short Atrio-Ventricular (AV) Pathway
• Nodo-Ventricular (NV) Pathway
• Nodo-Fascicular (NF) Pathway
• Atrio-Hisian (AH) Pathway
• Fasciculo-Ventricular (FV) Pathway

• Physiologically, the atria and ventricles are electrically connected only through the AV node.
• The accessory pathway is an additional non-physiological electrical connection,
• principally, it is an electrical AV nodal bypass.
• Pathways are congenital and may not be electrically active from birth.
• Their electrical conductivity may manifest, for example, after the age of 40.
• Approximately 0.3% of the population has an accessory pathway.
• Rarely, a patient may have multiple accessory pathways.
• About 10-30% of patients with Ebstein's anomaly (EA) have an accessory pathway.
• EA is a congenital heart defect occurring in 1/20,000 people.
• The main feature of EA is apical displacement of the tricuspid valve.
• Accessory pathways are classified into typical and atypical:
• Typical (95%)
• Atypical (5%)
• The majority of atypical pathways (80%) are:
• Atrio-fascicular
• Long AV decremental
• Other atypical pathways are very rare.
• 10% of patients with an atypical pathway have another pathway.
• 6% of patients with SVT with LBBB morphology have an atypical pathway.
• Atypical pathways are mostly right-sided,
• thus, in antidromic AVRT, they create an SVT pattern with an LBBB morphology.

• Characteristic terms and classifications of accessory pathways (typical, atypical) include the following:
• Short pathway
• Length: 0.5-1cm, width 1-7mm
• Atrial and ventricular insertion is located just above and below the annulus.
• Long pathway
• Length: 3-5cm, width 1-7mm
• Atrial insertion is just above the annulus.
• Ventricular insertion of the pathway is further below the annulus.
• Ventricular insertion often branches (arborizes),
• occupying an area with a diameter of 0.5-2cm.
• Fast pathway
• Conduction time <30ms (time for the pathway to conduct an impulse)
• Slow pathway
• Conduction time >30ms
• Decremental pathway
• Conduction time of the pathway prolongs (>30ms) with increasing frequency.
• Non-decremental pathway
• Conduction time of the pathway does not prolong with increasing frequency.
• or extends by max. 10-15ms (<30ms).
• Conduction capacity of the pathway
• Max. frequency at which the pathway conducts impulses with a 1:1 conduction.
• For example, the pathway conducts impulses at a rate of 160/min. (375ms) with a 1:1 conduction,
• but impulses at a rate of 159/min. are conducted with Wenckebach (4:3).
• So, the conduction capacity of the pathway will be: 160/min. (375ms).
• SPERRI (Shortest Pre-Exited RR Interval)
• If the pathway conducts impulses and activates the myocardium around the ventricular insertion
• there is pre-excitation present (delta wave on ECG).
• If the pathway blocks, the present delta wave disappears.
• SPERRI is the shortest RR interval still having a delta wave.
• meaning that this frequency (RR) is still conducted by the pathway.
• Malignant pathway
• It's a pathway with SPERRI ≤250ms (240/min.)
• An RR interval of 250 ms corresponds to an RR frequency of 240/min.
• Which is approximately 6 small squares on ECG paper.
• If the patient has atrial fibrillation, there is a risk of ventricular fibrillation,
• because the pathway still conducts at a frequency of 240/min.
• Therefore, pathways with SPERRI ≤250ms are considered malignant,
• there is a risk of ventricular fibrillation and sudden cardiac death.
• Anterograde pathway
• Conducts impulse from atrium to ventricle, depending on whether it causes ventricular pre-excitation (delta wave), anterograde pathway is divided into:
• Manifest pathway
• Anterograde conduction elicits pre-excitation
• We see a delta wave on ECG
• Latent pathway
• Anterograde conduction does not elicit pre-excitation.
• For example, the pathway is located laterally on the mitral annulus.
• So, when the impulse arrives at the lateral pathway, the ventricles are already activated.
• We do not see a delta wave on ECG.
• Intermittent pathway
• Anterograde conduction elicits pre-excitation but intermittently,
• because the pathway's speed changes according to the autonomic nervous system.
• Sympathetic - accelerates conduction, parasympathetic slows conduction.
• Therefore, the delta wave appears and disappears during the day.
• Retrograde pathway
• Conducts impulse from ventricle to atrium.
• We never see it during sinus rhythm on ECG as a delta wave,
• so it is labeled as concealed.
• It creates the terrain for orthodromic AVRT.
• Bidirectional pathway
• Conducts impulse in both directions, anterograde and retrograde.

Typical Accessory Pathways

• Constitute 95% of all accessory pathways.
• Mostly:
• Short
• Fast
• Non-decremental
• Located in the area of the left or right annulus.
• Almost always seen on ECG as a delta wave during sinus rhythm
• Create terrain for WPW syndrome
• If the pathway causes AVRT (which the patient feels as palpitations, syncope)
• then we say the patient has WPW syndrome
• If the patient has a delta wave on ECG, they may not have WPW syndrome
• because they have a pathway that does not cause AVRT.

Atypical Accessory Pathways

• Constitute 5% of all accessory pathways.
• Mostly:
• Long
• Slow
• Decremental
• Exceptions include: Short AV pathway and Atrio-His pathway.
• Mostly anterograde and right-sided (on the lateral tricuspid annulus),
• creating terrain for antidromic AVRT, which presents as SVT with LBBB morphology.
• May not be crucial for the course of SVT,
• they can act as a bystander in AVNRT, flutter, and atrial fibrillation.
• For example, in a patient with AVNRT, the ventricles are partially activated through the pathway.
• Patients with an accessory pathway may have
• AV nodal duality without AVNRT (30%)
• AVNRT (10%)
• Another pathway (10%)
• Atrial fibrillation (2%)
• Atypical pathways according to the location of atrial and ventricular insertion are divided into:
• Atrio-Fascicular (AF)
• Long Atrio-Ventricular (AV)
• Short Atrio-Ventricular (AV)
• Nodo-Ventricular (NV)
• Nodo-Fascicular (NF)
• Atrio-His (AH)
• Fasciculo-Ventricular (FV)

Mahaim Pathways and Tachycardia

• Mahaim and Benatt discovered the Fasciculo-Ventricular (FV) pathway in 1937.
• Often, atypical pathways (there is no consensus) are referred to as
• Mahaim pathways
• which is incorrect.
• Often, antidromic AVRT with Atrio-Fascicular pathway is termed as
• Mahaim tachycardia.
• which is also incorrect
• When describing an accessory pathway, it is crucial to precisely define:
• the anatomical atrial and ventricular insertion of the pathway, and
• electrical properties of the pathway: antegrade, retrograde, slow, etc.

• The heart weighs approximately 300g, which corresponds to 300cm³ (ml)
• Left ventricle (150cm³)
• Right ventricle (80cm³)
• Left atrium (30cm³)
• Right atrium (30cm³)
• Minimal volume of depolarized myocardium
• that causes a change in the EKG waveform is approximately 1cm³.
• Depolarization of 1cm³ volume takes about 30ms.
• Ventricles depolarize diffusely from the Purkinje fibers
• thus, the entire ventricular depolarization lasts <120ms
• Pre-excitation means that the ventricles are excited (depolarized) earlier.
• We see pre-excitation on the EKG as a change in the QRS complex or as a delta wave.
• A change in the QRS complex (not a typical delta wave) occurs on the EKG
• if a portion of the myocardium (>1cm³) depolarizes through the pathway earlier
• than depolarization through the AVN-HPS (AV node-His Purkinje system).
• To produce a typical delta wave on the EKG,
• more than 10cm³ of ventricular myocardium must be depolarized.
• Typical pathways are fast, short, and nondecremental,
• thus, they usually manage to depolarize a larger volume of myocardium >10cm³,
• resulting in a typical delta wave.

Pre-excitation and Atypical Accessory Pathways

• Atypical pathways are almost always slow, long, decremental.
• An exception is: Short AV pathway and Atrio-His pathway.
• Because atypical pathways are slow, during sinus rhythm on EKG
• they do not manage to depolarize (pre-excitate) >10cm³ of ventricular myocardium.
• Thus, no typical delta wave appears on the EKG as with the typical pathway.
• Atypical pathways during sinus rhythm evoke minimal pre-excitation.
• They pre-excite approximately 1-5cm³ of myocardium,
• thus, no typical delta wave appears on the EKG.
• Minimal pre-excitation through slow atypical pathway
• can sometimes be seen during sinus rhythm,
• as minimal change in the QRS complex. For example:
• Atrio-Fascicular (AF) pathway
• does not create septal lateral q waves (I, aVL, V5-6)

• Adenosine is used in diagnosing accessory pathways.
• It blocks the decremental part of the heart's electrical system:
• AV node
• Decremental pathways (Atypical pathways)
• An exception is made for atypical short AV pathways which do not respond to adenosine.
• Adenosine does not block non-decremental pathways (Typical pathways)
• AV node is more sensitive to adenosine than Atypical pathways,
• Adenosine first prolongs conduction until it blocks the AV node and then the atypical pathway.
• Adenosine rarely (<1%) causes atrial fibrillation
• through hyperpolarization of atrial myocardium.
• If a patient has anterograde typical pathway (manifest or concealed),
• after adenosine administration, a situation may occur (risk is <1%):
• Adenosine blocks the AV node and triggers atrial fibrillation
• atrial fibrillation starts to pass to the ventricles through the pathway without slowing down
• and ventricular fibrillation is threatened.
• A defibrillator must always be available when administering adenosine.
• Adenosine highlights pre-excitation, thus revealing delta wave on EKG in:
• Typical pathways
• Atypical pathways
• except for the Fasciculo-Ventricular pathway (delta wave remains unchanged after adenosine).

Sinus Rhythm and Atypical Pathways

• Atypical pathways are long, slow, and decremental. Exceptions include:
• Short AV pathway (Short, slow, and decremental)
• Atrio-His (AH) pathway (Short, fast, non-decremental)
• It has characteristics like a typical pathway, but a typical pathway does not connect to AVN-HPS (AV node - His Purkinje System).
• All atypical pathways (except AH) are slow (conduction time >30ms)
• Atypical pathways usually do not create pre-excitation during sinus rhythm,
• because the ventricles are activated through AVN-HPS.
• Atypical pathways sometimes induce minimal pre-excitation during sinus rhythm.
• An atypical pathway activates part of the myocardium before activation from HPS.
• Because only a small portion of ventricular myocardium 1-5cm³ is activated,
• no typical delta wave is generated as with a typical pathway.
• A typical delta wave requires pre-excited myocardium of about 10cm³
• such mass is activated quickly through typical pathways.
• Atypical pathways create minimal changes to QRS during sinus rhythm.
• The principle is that minimally pre-excited depolarized myocardial mass
• creates an electrical vector at a specific time during QRS formation.
• The pre-excited small vector alters the typical QRS complex.
• For example, the Atrio-Fascicular (AF) pathway has insertion
• in the distal part of the right bundle branch.
• So, the right bundle branch activates the ventricular septum rapidly through the AF pathway.
• The septum is simultaneously activated through the right and left bundle branches.
• It is not activated only from the left bundle branch left to right,
• which creates a septal q wave in lateral leads (I, aVL, V5-6)
• If a patient has an Atrio-Fascicular pathway,
• there is no septal q wave in lateral leads during sinus rhythm.

• It is the most common atypical pathway.
• AF and Long AV pathway form 80% of all atypical pathways.
• Most commonly found lateral to the tricuspid annulus.
• It is anterograde.
• It resembles the properties of the second AV node, because the AF pathway is
• Decremental (decrementality is in the atrial part of the pathway)
• Anterograde
• Creates Weckebach periods (during atrial pacing)
• Responds to adenosine
• The AF pathway is essentially the second AVN-HPS conduction.
• It runs along the free wall of the right ventricle through the moderator band.
• It connects to the distal part of the right Tawara bundle.
• It never enters the apex of the right ventricle,
• which is essential for characteristic changes in the QRS complex.
  
  
• ECG features:
• No pre-excitation (mostly)
• rS (III)
• Absence of septal q (I, aVL, V5-V6)
• Normal PR interval
• QRS ≤140ms
• Narrow initial r (V1-V2)
• LBBB morphology
• Notch, slurring in terminal part of QRS (I, V5-V6)
• Differential diagnosis:
• Long AV decremental pathway
• Nodo-Fascicular (NF) pathway
• Partial LBBB
• Left anterior hemiblock
• Variant of normal (normal ECG)

• AF and Long AV pathway form 80% of all atypical pathways.
• Most commonly found lateral to the tricuspid annulus.
• It is anterograde.
• It resembles the properties of the second AV node, because the long AV pathway is
• Decremental (decrementality is in the atrial part of the pathway)
• Anterograde
• Creates Weckebach periods (during atrial pacing)
• Responds to adenosine
• The long AV pathway is essentially the second AVN-HPS conduction.
• It runs along the free wall of the right ventricle through the moderator band (MB).
• It terminates at MB and does not connect to the distal part of the right Tawara bundle
• It never enters the apex of the right ventricle,
• which is essential for characteristic changes in the QRS complex.
• It creates a wider QRS than AF pathway, because it connects to the right bundle.


• ECG features
• No pre-excitation (mostly)
• rS (III)
• Absence of septal q (I, aVL, V5-V6)
• Normal PR interval
• QRS ≤140ms
• With increasing degree of pre-excitation
• Narrow initial r (V1-V2)
• LBBB morphology
• Differential diagnosis:
• Atrio-Fascicular (AF) pathway
• Partial LBBB
• Left anterior hemiblock
• Variant of normal (normal EKG)
• If the Long AV pathway is significantly pre-excited, it resembles:
• LBBB
• Typical right lateral pathway

• Atrial and ventricular insertion is located just above and below the annulus.
• Approximately 10mm in length
• Most commonly found lateral to the tricuspid annulus.
• Rarely septal or lateral to the mitral annulus
• The Short AV pathway is
• Decremental
• Anterograde
• Creates Weckebach periods (during atrial pacing)
• Always unresponsive to adenosine
• May occur after incomplete radiofrequency ablation of the typical pathway
• The typical pathway changes to atypical, as it acquires decremental conduction.
  
  
• ECG features
• Pre-excitation in the form of a delta wave (mostly)
• Not as prominent as in the typical pathway
• Shortened or Normal PR interval
• QRS ≤130ms
• Unresponsive to adenosine (mostly)
• Differential diagnosis:
• Typical accessory pathway

• Begins in the AV node and terminates in the para-Hisian region of the ventricular myocardium.
• Decrementality and slow conduction originate from the AV node to which it is attached.
• Bypasses the His bundle
• Degree of pre-excitation depends on the site of insertion on the AV node.
• The more proximal it bypasses the decremental part of the AV node,
• the greater the pre-excitation on ECG.
• The NV pathway can be
• Anterograde
• Retrograde
• Bidirectional
  
  
• ECG features
• No pre-excitation (mostly)
• Shortened or Normal PR interval
• LBBB morphology (with pre-excitation)
• QRS ≤130ms
• Differential diagnosis:
• LBBB
• Antero-septal typical pathway
• Mid-septal typical pathway
• Fasciculo-Ventricular pathway

• Begins in the AV node and terminates in the distal part of the right bundle branch.
• Similar termination as AF pathway.
• Decrementality and slow conduction originate from the AV node to which it is attached.
• Bypasses the His bundle
• Degree of pre-excitation depends on the site of insertion on the AV node.
• The more proximal it bypasses the decremental part of the AV node,
• the greater the pre-excitation on ECG.
• The NF pathway creates lesser pre-excitation than NV pathway
• Because the ventricular insertion of the NF pathway is directly in the right bundle branch
• The NF pathway can be:
• Anterograde
• Retrograde
• Bidirectional
  
  
• ECG features
• No pre-excitation (mostly)
• rS (III)
• Normal PR interval
• LBBB morphology (with pre-excitation)
• QRS ≤130ms
• Differential diagnosis:
• LBBB
• Typical pathway
• Atrio-Fascicular (AF) pathway

• Atrial insertion is in the inferior part of the right atrium,
• Ventricular insertion in the His bundle.
• Bypasses the decremental part of the AV node.
• The AH pathway is fast, short, non-decremental (just like the Typical pathway)
• Ventricular insertion is in the conduction system (in the His bundle).
• The typical pathway always has ventricular insertion in the ventricular myocardium.
• It is anterograde.
• Often manifests as:
• Rapid atrial fibrillation
• Unblocked atrial flutter
• Almost never creates reentry tachycardia (AVNRT, AVRT)
• On ECG, there is a shortened PQ interval and normal QRS complex.
• Exactly the same ECG is seen in enhanced AVN conduction (EAVNC)
• EAVNC is a "fast" AV node
• It has decremental conduction
• It responds to adenosine
• AH pathway
• It has non-decremental conduction
• Does not respond to adenosine
  
  
• ECG features
• Short PQ (≤120ms)
• No pre-excitation
• QRS ≤120ms
• Differential diagnosis
• Enhanced AVN conduction

• The FV pathway was described by Mahaim in 1937.
• It is very rare, accounting for about 1-5% of all atypical pathways.
• Begins in the His bundle or just below the His bundle and ends antero-septally in the ventricular myocardium.
• It is benign
• never causing reentry arrhythmia (AVRT)
• Begins distal to the decremental part of the AV node,
• therefore, unblocked flutter or rapid atrial fibrillation does not occur
• The decremental property is due to the AV node, not the FV pathway itself.
• Must be distinguished from a typical antero-septal (AS) pathway
• which has the same ventricular insertion but begins in the atrial myocardium.
• It causes AVRT, rapid atrial fibrillation, and unblocked atrial flutter.
• The FV pathway and typical AS pathway can be distinguished by adenosine:
• FV pathway:
• Adenosine prolongs the PQ interval, the Delta wave does not change
• AS pathway
• Adenosine prolongs the PQ interval, the Delta wave increases
• The most accurate method is an electrophysiological study.
• Occurs in PRKAG2 gene mutation,
• which causes PRKAG2 hypertrophic cardiomyopathy (HKMP)
• The prevalence of HKMP is 1/500, of which 1% is PRKAG2 HKMP,
• which manifests as:
• Conduction system disorders (AV blocks)
• Syncope, arrhythmias (ventricular, atrial)
  
  
• ECG features:
• Pre-excitation (mostly)
• Negative delta wave (V1)
• Positive delta wave (aVF)
• Normal or short PQ
• QRS ≤130ms
• QRS >130ms if PRKG2 HKMP is present
• S (V1) <20ms
• Notch in the descending part of S (V1)
• Normal QRS axis
• Adenosine does not change the Delta wave
• only prolongs the PQ interval
• Differential diagnosis:
• Antero-septal typical pathway
• Mid-septal typical pathway
• Nodo-Ventricular (NV) pathway