Accessory Pathways and Wolff Parkinson White & Its Link to Arrhythmia
Wolff Parkinson White Syndrome is one of the best known and most common cardiac conduction disorders caused by having an additional pathway between the top of the heart (Atria) and the bottom of the heart (Ventricles). Having an accessory pathway can cause problems!!!
Lets review the heart conduction system first ->
If we cast our eyes to the right, we can see a basic illustration of how a heart beat passes through a healthy heart. The heart beat is initiated by the SA Node (Red Dot/Hearts Natural Pacemaker). This signal passes cell to cell (green arrows) like a Mexican wave passes around a sports arena and the top of the heart contracts. There is a non conductive barrier between the Atrium (top) and the Ventricles (bottom) represented by the red line. The electrical signal does not pass through to the bottom of the heart. However there is a gateway called the AV Node (Atrioventricular Node) where the signal does pass but does so slowly (blue dot). After a delay the AV Node passes the signal on to the bottom of the heart and ‘Hey Presto’ the bottom contracts too and we have 1 normal heart beat.
In some hearts there is a second piece of conductive tissue between the Atria and the Ventricles. These are know as ACCESSORY PATHWAYS. In Wolf Parkinson White the accessory pathway is called the Bundle of Kent (you should remember that for any Cardiology based Pub Quizzes you may be attending).This may look like this….
|An Accessory Pathway Connecting Top
and Bottom of the Heart
Accessory Pathways are normally a hangover from the development of the heart in the womb.
The problem with accessory pathways is that they possess different properties to the AV Node (Blue Dot).
The AV Node has a safety mechanism – it does not allow all signals that arrive at it from the Atrium through into the Ventricles. A bit like a traffic light controlling the rate that traffic is able to pass through to the next village.
Accessory pathways don’t possess such a safety mechanism, more like a motorway all traffic can get through and very rapidly!
So the first major problem with an accessory pathway can be 1-1 conduction from Atrial Arrhythmias.
Consider Atrial Fibrillation or Atrial Flutter these are where the top of the heart can be beating extremely quickly. These generally do not cause too much of an issue because the Traffic Light (AV node) is limiting the number of signals being transferred to the bottom of the heart (Ventricles) and reducing how quickly the heart beats as a result.
In an accessory pathway there is a traffic light but there is another road which all the traffic takes because it is much much faster! for this reason the Ventricles could be made to beat incredibly quickly and not very efficiently either! Therefore blood flow around the body could be compromised and with it your ability to stay conscious.
|AF with Accessory Pathway – Very Fast Rate
Occurs in the Ventricles Potentially Life
Threatening (Approx 1 in 1000)
|AF with No Accessory Pathway – Benign-ish|
So that is one risk of accessory pathways – that Atrial Arrhythmias (that are normally relatively benign) can carry a much graver risk.
Another problem with Accessory Pathways is AVRT (Click to read) which merits its own blog post!
Detecting Wolff Parkinson White on the ECG
Wolff Parkinson White and other accessory pathway disorders are often referred to as Pre-Excitation disorders because they ‘Excite’ the Ventricles before it is ‘Excited’ by the AV Node (Excited=Stimulated, you say Potato I say Potato). This pre-excitation is often picked up on the ECG! It is called a Delta wave and is a slurring of a normal QRS and looks like this….
Why do we get this slurring appearance? Well are you sitting comfortably?…
1) The top of the heart contracts as normal.
2) The Accessory Pathway starts cell to cell conduction in the Ventricle whilst the AV Node is delaying the normal signal.
3) Eventually the AV Node passes on the heart beat to the fast network in the heart and a normally heart beat occurs in the majority of the Ventricles.
|Broad Complex/ Ectopic Beat
Slow Cell to Cell Conduction
|Narrow Complex Normal Heart
Beat Fast Conduction
In Essence you have a combination of an Ectopic Beat (Wide Complex Beat) and a Normal Beat (Narrow Complex Beat)
The combination of the two gives the slurred Delta Wave that makes WPW so recognisable on an ECG.
|Typical Wolff Parkinson White ‘Heart Beat’ with Delta Wave|
So there you have an explanation of Wolff Parkinson White and other Accessory Pathways. Click to read about AVRT’s another problem caused by accessory pathways.
Thanks for Reading
Time to Catch the 2nd Half
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