Atrial Flutter Explained – Cardiac/Heart Arrhythmias and Palpitations

Four Chambers of the Heart
Chambers of the Heart

Atrial Flutter Explained – Cardiac/Heart Arrhythmias and Palpitations

What is Atrial Flutter?

Atrial Flutter is an irregular rhythm of the heart that originates from the Atria (the top of the heart).
There are a few different types of Atrial Flutter but all usually lead to an unnecessarily fast heart rate around 150 bpm. As with any arrhythmia that originates from above the Ventricles it is referred to as an Supraventricular Tachycardia (SVT). SVT’s are more often than not ‘less dangerous’ than Ventricular arrhythmias.

Symptoms of Atrial Flutter

The main symptom of Atrial Flutter is Palpitations and the arrhythmia is generally tolerated pretty well, however some patients report shortness of breath, chest pains, lightheadedness and dizziness.

Atrial Flutter Explained
The signal passes through the
A.V. Node (Blue Dot) and
causes the Ventricles to contract
in turn.
Atrial Flutter Explained
S.A. Node (red dot) starts a
Heart Beat that spreads through
the Atria

Normal Heart Beat Beat!

Lets first have a very quick recap of the normal conduction system and a normal heart beat. When a healthy heart beats, the contraction is started by the S.A. Node found in the Right Atrium. When this cluster of cells fires a signal propagates through the top of the heart like a Mexican wave (Picture left). This signal passes through to the bottom of the heart via the AV Node (that delays the signal a little) before making the bottom of the heart (Ventricles) contract in turn (Picture right).
When this process happens 60 times in a minute that person will be tottering along at 60bpm.

Atrial Flutter

In Atrial Flutter a re-entrant circuit occurs in the Atrium this circuit. This ‘macro circuit’ go around and around and around causing the Atria to contract at around 300 times per minute. This rate varies from person to person as everyones anatomy is slightly different.
Does this mean the Heart Rate is 300BPM? 

Please do not confuse Atrial Rate with the heart rate itself… 
Heart Rate is defined by how quickly the VENTRICLES are contracting!!! luckily the AV Node (shown to the left as a purple dot) does not transmit every stimulus from the Atria. More commonly every second signal arriving at the AV Node is passed on through to the Ventricles.
This 2:1 relationship between the Atrial contractions and Ventricular contractions is very common and so heart rates in Atrial Flutter are often around 150bpm and regular! 3:1 and 4:1 relationships are often quite common… 
So Flutter is always Regular? NO!  in some cases the transmission of signals transmitted through to the Ventricles is variable giving rise to a less regular arrhythmia.
As with all reentrant circuits this is self perpetuating and will continue until the circuit is broken.

How does Atrial Flutter Start?

*if you haven’t already please read my posts on AVRT and AVNRT arrhythmias as this will help you understand the following*
Their are four types of Atrial Flutter though all the mechanisms are similar Type I Anticlockwise is the most common and this is what I will focus on in this explanation.

As with all Reentrant tachycardias there needs to be a combination of the right anatomy and the right timing of events to trigger the arrhythmia.

Anatomy of the Right Atrium - Atrial Flutter Explained

The Right Anatomy (Slight Pun sorry)

Until now on my blog it has suited us to view the Right Atrium as a simple structure of course that is not the case and in fact it is more complicated than I am about to let on but for Atrial Flutter you have to know this…
The long names sound confusing but they are just names!!

  • The Atrium is essentially in two sections, the posterior (round the backish) smooth wall venous section and the anterior (near the frontish) trabeculated muscular portion. 
  • There is a section of tissue called the Cavo Tricuspid Isthmus that stretches between two land marks in the Atria the Inferior Vena Cava and the Tricuspid Valve that conducts electrical signals relatively slowly.
  • Where these two sections join directly is called the Crista Terminalis which forms a region at the back of the Atrium where electrical signal does not pass.
Regular Heart Beat the Signals Cancel
One Another Out through the Isthmus
This still stimulates the AV Node
In a regular heart beat that starts at the S.A. Node and moves outwards around the Atrium like a mexican wave… This signal moves round the the back-ish of the right Atrium and in doing so also enters the  Cavo Tricuspid Isthmus. Simultaneously the signal is also moving through the front-ish of the Atrium, the signal ‘exiting’ the Isthmus reaches the other side when the tissue has already contracted so the signals cancel each other out… Like driving down a dead end. 

The Right Timing of Events…

Now with the right anatomy Flutter can be instigated by an Early Atrial Ectopic…
For those of you that do not know an Atrial Ectopic is an early spontaneous heart beat initiated by a spontaneous cell or cluster of cells misbehaving in the Atrium. 
We must also REMEMBER that once a Heart Cell has contracted (beat) it cannot contract again until it has a short time to ‘recover/recharge’.  This is true of all cardiac tissue.

Anatomy of the Right Atrium - Atrial Flutter Explained
Atrial Ectopic – the ‘contraction’ signal
reaches the Entrance when it is not refractory
but the ‘exit’ when it is refractory.

So a person who has the DISPOSITION to get Atrial Flutter is going about their day when they have an Atrial Ectopic…. 
The Atrial Ectopic reaches the ‘entrance’ and the ‘exit’ of the Isthmus at relatively similar times (see left). 
However the Exit of the Isthmus is still refractory (recharging) and is unable to carry the electrical signal. The ‘Entrance to the Isthmus has had more time to recharge and is ready to carry the electrical signal started by the Atrial Ectopic.
Atrial Ectopic
Atrial Ectopic

The Isthmus moves the signal quite slowly (pic 1.) and therefore the exit has time to recharge and the signal moves all the way through the Isthmus (remember in a normal heart beat this does not happen). The signal exiting the Isthmus now instigates more Atrial Activity (pic 2.), the signal propagates through the Atrial Tissue (pic 3.) and eventually reaches the ‘Entrance’ to the Isthmus (pic 4.).
The ‘entrance’ to the Isthmus is ready once more to take the signal and the process repeats itself (pic 5.)…. the Reentrant circuit has been formed and repeats this process about 300 times a minute!! (pic 6.)
1.Anatomy of the Right Atrium - Atrial Flutter Explained - How Flutter Starts2.Anatomy of the Right Atrium - Atrial Flutter Explained - How Flutter Starts3.Anatomy of the Right Atrium - Atrial Flutter Explained - How Flutter Starts
4.   Anatomy of the Right Atrium - Atrial Flutter Explained - How Flutter Starts                   5.
So this is how the most typical Flutter starts… This type is known as Type I Anticlockwise Flutter because it moves around the Atrium in an Anticlockwise fashion, for those who are interested… the order of play is down the lateral (side) wall down towards the Isthmus, through to the coronary sinus then up the septum across the roof of the RA and so on and so forth. When the circuit moves through the same pathway but in a Clockwise Direction this is know as Type 1 Clockwise Flutter (clever hey!).
Anatomy of the Right Atrium - Atrial Flutter Explained - How Flutter Starts
What would happen in the absence
of the Crista Terminalis

Now you probably noticed that I mentioned the ‘crest’ of non conductive tissue at the back-ish of the Right Atrium. This Crista Terminalis is vital in sustaining the reentrant tachycardia, if this non conductive tissue was not there chances are the signal would exit the Isthmus pass across into the smooth venous half of the Atrium very rapidly and back into the Isthmus ‘entrance’. This tissue would still be refractory (recharging) and in essence the circuit would have caught itself up and cancelled itself out.

The left Atrium has nothing to do with the re-entrant circuit in Type 1 Flutter, it is just stimulated passively as a result. Type II or left sided Atrial Flutter is really quite different and I will focus on this another time!
Diagnosis and Treatment of Atrial Flutter will feature in a post sometime soon!
Thats all for now though, PLEASE PLEASE PLEASE leave your feedback and share this with anyone you feel may benefit!
Time for Bed!!!
Cardiac Technician

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