The most common symptom of AVNRT is fast palpitations that can sometimes be accompanied by lightheadedness or very rarely fainting.
To understand AVNRT I have to first explain a little bit more about the Atrioventicular Node (AV Node)
the gateway between the top and bottom of the heart through which electrical signals pass. As we have learned from How the Heart Beats
– the AV Node
is a gateway through which the electrical signal that triggers a heart beat
passes from the top of the heart (Atria
) to the bottom of the heart (Ventricles
The AV Node itself is a little more complex than a single pathway from top to bottom. Under a microscope it actually looks like this…
|Sagittal histological (Close UP)
section of the AV Node (1).
|The AV Node can have a Fast and Slow Pathway
in a Large Percentage of People.
You can microscope anything and make it complicated! For the purposes of this explanation we can look at the AV Node as a bundle of tissue that (in a large percentage of people) has TWO pathways that a signal can take from the Atria to the Ventricles! Squiggly line and Straight Line in the picture to the left.
SPOILER ALERT – Their are two things you need to know to explain AVNRT.
– One of the Pathways is a relatively FAST Pathway (Carries the message quickly) and One of the Pathways is a relatively SLOW pathway (Carries the message slowly)
– The Faster that Tissue Conducts a Signal (Passes it on) the LONGER it takes to recharge to do it again!
Learn these there will be a test later!
So lets consider the Normal Heart Beat
in when somebody has this ‘Dual Nodal Physiology’.
1) The SA Node starts a heart beat that travels cell to cell through the Atria and arrives at the AV Node (drawn in white this time). The zig zag line shows the Slow pathway and the straight line the Fast Pathway. The pathways are connected at the top and bottom of the AV Node.
2) The Signal travels relatively quickly around the fast pathway and down into the ventricles starting a normal Ventricular Conduction. The signal is taking its tome to travel down the Slow pathway.
3) When the signal travelling down the slow pathway eventually arrives at the bottom of the AV Node, all the tissue has recently been active and is RECHARGING (in a refractory period). Therefor the signal hits a dead end and dissipates. Like a car driving down a dead end – the signal has nowhere to go.
NOTHING exciting happens!!! The DRAMA can unravel when we have an Ectopic beat….
If we remember an ectopic beat is a spontaneous initiation of a heart beat from a misbehaving cell or group of cells. So a person who suffers from AVNRT is going about their normal day with a very normal heart beat when they have an Atrial Ectopic (an early beat originating in the top of the heart) this starts off AVNRT. So what has happened?
How AVNRT Starts!
1) A normal heart beat
has just taken place and both pathways in the AV Node are refractory (recharging) this is shown by the lovely mint green colour. Normally both these pathways have fully recharged (white) before the next heart beat comes along.
2) An early Atrial Ectopic occurs before the AV Node has had a chance to fully recharge. We know that the SLOWER pathway recharges (ready to conduct again) much faster! So the SLOW Pathway IS ready to carry the signal started by the Ectopic beat. The Fast pathway is still recharging however and cannot carry the message.
3) The electrical signal gradually passes down the slow pathway. By the time the signal has reached the bottom of the AV Node the fast pathway is now fully recharged and can carry a signal. Conductive tissue is not fussy about which way it carries a signal so the fast pathway is initiated and conducts from bottom to top (retrograde). As it is doing this the slow pathway is recharging (refractory)
4) By the time that the signal has travelled retrogradely up the fast pathway, the start of the slow pathway is ready to go again (white) and is triggered by the fast pathway. The signal travels down the slow pathway as the fast pathway recharges. You have probably guessed what is going to happen by now…. The signal passes through the slow pathway whilst the fast pathway recharges. Once more the signal reaches the fast pathway when it is ready to carry the signal from top to bottom once more.
5) A re-entrant circuit has been formed (one that is self perpetuating). As the signal passes around and around the node the Atria and Ventricles are triggered rapidly and the node becomes the source (focal point) of the heart rhythm and rate.
How Fast is AVNRT?
Remember earlier I used the words relatively fast and relatively slow? well this was with good reason. All of what has been explained happens very quickly and as a result an AVNRT usually leads to a heart rate of around 150 – 200 beats per minute!
This concept is much easier to explain in person than in writing believe me! In summary –
- You can have a Fast Pathway and Slow Pathway in your AV Node and never have an issue.
- If the TIMING is right, an ectopic can lead to a signal travelling around and around the AV Node.
- This is a re-entrant circuit and therefor self perpetuating.
- This signal causes the Atria and Ventricles to contract at rates of 150 – 200 BPM.
There you have it AVNRT if I am honest I am glad this one is over and done with 🙂
Would really appreciate your comments and for you to share this with your friends and colleagues on Facebook and Twitter Etc etc….
Time for some Croissants!
Thanks for Reading
1.Rev Esp Cardiol. 2010;63:660-7 – Vol. 63 Num.06 DOI: 10.1016/S1885-5857(10)70140-7