AVRT and AVNRT Flashcards
What are accessory pathways?
“By definition, accessory atrioventricular pathways are aberrant muscle bundles that connect the atrium to a ventricle outside of the regular atrioventricular conduction system”.
In what ways can accessory pathways conduct impulses?
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 beleft-sidedorright-sided, and ECG features will vary depending on this.
Bundle of Kent
Abnormal extra connection between atrial and ventricular myocardium, which is present in 0.1-0.3% of the general population.
Can exist in the AV groove either left, right or septally.
The accessory pathway is called the Bundle of Kent and gives rise to Wolff Parkinson White syndrome if the AP can conduct anterogradely i.e. from atria to ventricles.
Wolff-Parkinson-White Syndrome
Presence of congenital accessory pathway connecting the atria to the ventricles, which bypasses the AVN, in addition to episodes of tachyarrhythmias.
First described in 1930.
The term ‘WPW syndrome’ is used interchangeably with ‘pre-excitation syndrome’.
Re-entrant circuit.
<0.1% of people die of VF.
ECG Features of WPW
Normal P wave
Short PR interval <120 ms/0.12 seconds.
Delta wave – slurring of initial portion of QRS (depolarisation through AP, distorts early part of QRS).
Slow depolarisation is rapidly overtaken by depolarisations propagated via conduction system (rest of QRS appears normal).
QRS prolongation > 120 ms/0.12 seconds.
Discordant ST-segment and T-wave changes (i.e. in the opposite direction to the major component of the QRS complex).
Classification of Wolff- Parkinson-White syndrome
Left-sided AP: dominant R wave in V1. Sometimes referred to as atype A WPW pattern.
Right-sided AP: produces a negative delta wave in leads V1and V2. Sometimes referred to as atype B WPW pattern.
WPW pathway
The impulse arises normally in the SA node.
Normal P wave
The impulse then travels to the ventricular myocardium by both the accessory pathway and the normal conducting system, via the AV node.
Because there is no delay through the accessory pathway the impulse taking that route will result in premature depolarisation or ‘pre-excitation’ of that part of the ventricular myocardium.
A short or no PR interval is seen:- no delay occurs at the pathway.
An initial slurring of the QRS complex is seen:- premature depolarisation of the ventricle occurs slowly from myocardial cell to myocardial cell.
This slurring of the complex is called a Delta wave.
Normal conduction through the AV node, His bundle and Purkinje system then occurs.
Completion of ventricular depolarisation is therefore narrow and the terminal part of the QRS is normal.
Summary of the ECG findings in WPW
In general terms two types of WPW exist although the position of the pathway can be specifically named.
(1) Left sided WPW –Type AGives a positive QRS in lead V1.
(2) Right sided WPW – Type B Gives a negative QRS in lead V1.
Tachyarrhythmias in WPW
There are only two main forms of tachyarrhythmias that occur in patients with WPW:
Atrial fibrillation or flutter.Due to direct antegrade conduction from atria to ventricles via the AP, bypassing the AV node.
Atrioventricular re-entry tachycardia(AVRT).Due to formation of a re-entry circuit involving retrograde conduction via the AP.
What are Concealed Accessory Pathways?
Commonly the accessory pathway is concealed, that is, it is only capable of conducting only in a retrograde fashion from ventricles to atria.
During normal sinus rhythm, pre-excitation does not occur and the electrocardiogram is normal. All antegrade conduction occurs via the AVN.
Patient can still experience tachyarrhythmias as the accessory pathway can still form part of the re-entry circuit.
Clinical significance of ECG findings for WPW?
In isolation, the ECG appearance of WPW is not significant. However, if seen in patients complaining of palpitations, further investigations will be required i.e. EP study
What is AVRT
AVRT is re-entry circuit that uses the accessory pathway and the AV node as substrates for propagation.
Either the node or the pathway has to be capable of retrograde conduction.
During tachycardia:
Orthodromic: down the node and up the pathway – narrow complex QRS.
Antidromic: down the pathway and up the node – broad complex QRS.
What is AVNRT
Re-entrant circuit within the AV node.
Re-entrant circuit is formed by presence of two pathways – slow and fast pathways.
Micro-circuit stimulating the ventricular myocardium and atrial myocardium.
Almost appears simultaneously on the surface ECG - often no visible retrograde P wave.
Typically paroxysmal and may occur spontaneously or upon provocation with exertion, coffee or alcohol.
What are the pathways in the AVnode during AVNRT?
Two pathways exist within the AV Node.
(1) SLOW pathway – slower conduction velocity but shorter refractory period.
(2) FAST pathway – greater conduction velocity but takes longer to recover from excitation (longer refractory period).
Typically route taken during tachycardia is down the slow pathway and up the fast.
LONG PR - SHORT RP
Atypically the reverse may occur (uncommon), with the impulse travelling down the fast and up the slow pathway.
LONG RP - SHORT PR
AVNRT ECG characteristics
In SR, ECG is normal.
In tachycardia: regular rhythm, narrow QRS, rate 130-250bpm.
Atrial conduction proceeds in a retrograde fashion producing inverted P waves in II,III and aVF.
P wave frequently buried in the QRS complex and may be totally obscured.
P wave distorts last part of QRS complex giving rise to a “pseudo” S wave in the inferior leads and a “pseudo” R wave in V1
RECAP
AVNRT occurs within compact AV node.
AVRT involves an accessory pathway.
AVNRT 130-250bpm.
Simultaneous depolarisation of atria and ventricles so P waves frequently buried in QRS complex or distorting last part of QRS complex.
AVRT 140-250 bpm.
Orthodromic – antegrade conduction via AVN and retrograde conduction via AP (narrow QRS).
Antidromic – antegrade conduction via AP and retrograde conduction via AVN (wide QRS).
