Atrioventricular Reentrant (Reciprocal) Tachycardia – AVRT – Cardiac Arrhythmias Explained

Atrioventricular Reentrant (Reciprocating) Tachycardia – AVRT

One of the longest named cardiac arrhythmias, it is easy to see why it is only ever referred to by its acronym A.V.R.T.
As we know an arrhythmia is an abnormal heart rhythm and symptoms of AVRT include:-
Palpitation, lightheadedness, dizziness, shortness of breath, reduced exercise capacity, weakness, fatigue, chest discomfort, and sweating episodes as a result of the sudden onset of RAPID heart rates.

So what causes AVRT and what is happening during this Arrhythmia?

For AVRT to occur we need an Accessory Pathway so please read my blog post on WPW and Accessory Pathways first so this will be a lot simpler! In short an accessory pathway is additional gateway between the Atria (top of the heart) and the Ventricles (bottom of the heart) through which electrical signals can pass.

Heart with Accessory Pathway has Two
Gateways Between the Atria and
Normal Heart Only One Gateway Between
Atria and Ventricles
A Typical Wolff Parkinson White Heart Beat
Now this pathway can exist without too many problems, in fact some people with accessory pathways probably go through life without ever having a symptomatic arrhythmia as a result. Like in the example of a typical Wolff Parkinson White ‘Heart Beat’
REMEMBER that once a Heart Cell has contracted (beat) it cannot contract again until it has a short time to ‘recover/recharge’.
So like a car driving down a dead end, the impulse that travels down the accessory pathway meets cells that have already contracted through ‘natural means‘ and both signals do not go any further (See left). You can also see that this ‘protects’ from electrical signals going into the top of the heart (Atria) via the accessory pathway – this will become more significant once you have finished the article 🙂 

Therefore, as long as the ‘heart beat‘ starts with the SA Node (red dot) we do not have too many issues.  Unfortunately this is not always the case. Normally cardiac cells just pass on an electrical signal that originates in the self activating cells found in the SA Node. Though during something called an Ectopic Beat a heart beat spontaneously starts elsewhere in the heart.

If we consider a single ectopic in a normal heart they are benign (not dangerous) here is a picture of a harmless Ventricular Ectopic in a heart without an Accessory Pathway ->
What happens though when an ectopic occurs at the right place and the right time in a heart with an accessory pathway… You can get an AVRT – I will have a bash at explaining this!…. wish me luck!

How AVRT Starts

1. An ectopic heart beat occurs (green dot) and the electrical signal passes up through the accessory pathway (pink bar) the cells in the top of the heart Atria are stimulated and contract. 

2. Because the cells have just contracted they have not had time to recover and the signal impulse coming from the SA Node (red dot) is blocked. 

3. The signal that has come up through the accessory pathway travels from cell to cell and back down the AV Node (blue dot). 

4. The signal passes down the ventricles in a normal fashion but now the accessory pathway does not have a signal coming down from the Atria that would prevent the signal passing up into the top of the heart. 

5. The electrical signal passes back up through into the top of the heart.

6. Any ‘natural heart beat’ activity coming from the SA Node (red dot) is blocked and the signal once more passes down through the AV Node. 

7. A continuous ‘reentrant circuit’ has occurred and this self perpetuating heart rhythm continues and at a fast rate.
So we have a re-entrant circuit that occurs in a loop around the Atria and Ventricles…. Ipso Facto this is called an Atrioventricular Reentrant Tachycardia though often it is referred to as Atrioventricular Reciprocating Tachycardia  across the pond… (Aubergine/Eggplant).
Now for all intents and purposes that is an AVRT explained. There is a little bit more to run through and if you have the energy then carry on reading! 

Types of AVRT.

There are two types of AVRT. The one in previous example is known as Orthodromic AVRT. Orthodromic referring to the signal passing through the AV Node in the right direction. 
Antidromic AVRT
In some people electrical signals can travel in both directions through the AV Node. So this allows a second type of AVRT – Antidromic AVRT. This refers to signal travelling through the AV Node in the Opposite of normal direction. 
1. The ectopic beat occurs. 2. This time the signal passes up through the AV Node (blue dot). 3. The signal passes down through the accessory pathway. 4. The signal travels cell to cell (slower than in orthodromic) through the Ventricle and back up the AV Node. AVRT begins…
Those of you with a keen eye would have spotted a big difference between the two. In Orthodromic the normal ‘Fast’ Conduction pathway of the Ventricles is ‘borrowed’ by the arrhythmia, so the complexes on an ECG are narrow. In Antidromic AVRT the conduction of the signal through the Ventricles travels from Cell to Cell (more slowly) and therefor the complexes on an ECG are Broad.
This can be seen on the ECG examples here….
There you have it, I hope it makes sense as its not the easiest to explain in only words and images! 
Now for some Decorating!
Thanks for reading!
Cardiac Technician

N.B. the majority of AVRT’s are normally started by an Atrial Ectopic and the Accessory Pathway still being refractory – However to explain the concept and Orthodromic and Antidromic, I decided to use a Ventricular Ectopic as the trigger for the SVT.

Image courtesy of rajcreationzs /

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