Supraventricular Tachycardias - The Narrow Waves of Chaos
In our first discussion about arrhythmias, we spoke about the fact that arrhythmia can be broadly classified into bradyarrythmias and tachyarrhythmias. The tachyarrhythmias are further classified into ventricular and supraventricular arrhythmias. The reason for this distinction is that the genesis, propagation and treatment for the two arrhythmias vary greatly. And in order for us to be able to effectively treat these arrythmias, we need to understand this distinction.
So what does supraventricular tachycardia entail? The electrical impulse that propagates through the electrical wiring of the heart and results in contraction of the ventricles usually originates at the sinoatrial node. If the sinoatrial node malfunctions, other parts of the electrical system can take up the role of generating the necessary impulse. The impulse may come from other parts of the atria, the AV node or the ventricles themselves and all the tachycardias that originate from above the ventricles are called supra-ventricular tachycardias.
Image showing the normal conduction pathway and the ECG deflections in produces.
Reproduced from Goldberger’s Clinical Electrocardiography, 9th edition.
The basic principle behind the formation of an arrhythmia is that there is an improper electrical impulse initiation or a faulty conduction of the electrical impulse. We know that in a supra ventricular tachycardia, the impulse that goes down to the ventricle must come from the atria or the AV node. In order for this to happen, either there needs to be improper firing from somewhere in the atria or there needs to be a messed up conduction. Improper firing can happen from a single point (focal atrial tachycardia) or from multiple points (multifocal atrial tachycardia).
Improper conduction that results in a supra ventricular tachycardia is because of a reentry circuit in which an obstacle causes a hindrance in the normal impulse pathway and forces the impulse in a different direction, leading to a vicious cycle.
These are the two ways in which supraventricular tachycardias are initiated. (A) shows a focal stimulus firing repeatedly. (B) shows the mechanism of a reentry pathway.
Reproduced from Goldberger’s Clinical Electrocardiography, 9th edition.
Now because the sinus node is above the ventricles, even sinus tachycardia due to stress or illness or physical exertion is considered to be a supraventricular tachycardia. But we are more interested in the pathological supraventricular tachycardias aren’t we?
There are three subtypes of pathological suprventricular tachycardia that we must understand.
Tachycardias that originate from the atrium itself.
The atrium is full of tissue that can turn into a temporary pace maker. Some times the SA node fires at inappropriate moments presenting as palpitations in the patient. This condition is due to a dysfunctional autonomic nervous system leading to inappropriate sinus tachycardia.
Image showing origin of atrial tachycardias. Notice the changes in the ECG caused by new source of electrical impulses.
Reproduced from Goldberger’s Clinical Electrocardiography, 9th edition.
In focal atrial tachycardia, the pacemaker is not the SA node but rather another site in the atrium itself. Possible sites of origin for focal atrial tachycardia are the valve annuli of the left or right atrium, pulmonary veins, coronary sinus muscles and superior vena cava.
Two tachycardias that are clinically very important and maybe life threatening are atrial fibrillation and atrial flutter. Atrial flutter is an organized atrial rhythm or more than 200 beats per minute. Atrial fibrillation, on the other hand, is disorganised and its famously irregularly irregular.
ECG showing atrial flutter in lead II and III where as in lead I there is barely any signs of a pathology. The classic “saw tooth appearance” is seen in leads II and III.
Reproduced from Goldberger’s Clinical Electrocardiography, 9th edition.
ECG showing
Reproduced from Goldberger’s Clinical Electrocardiography, 9th edition.
Last of the tachycardia originating in the atria is multi focal atrial tachycardia. In these cases, the impulse arises from multiple places in the atria. Because of this, the morphology of the P waves on the ECG is heterogenous - this means that all the P waves are different even in the same lead. This is commonly seen in patients with long standing pulmonary diseases who have an acute exacerbation.
ECG showing multifocal atrial tachycardia. Note that the morphology of the P waves in the consecutive beats is not the same, suggesting that the beats are originating from different places.
Reproduced from Goldberger’s Clinical Electrocardiography, 9th edition.
Atrioventricular Nodal Re Entry tachycardia.
This sounds complicated I know, but the concept is quite simple. The AV node is a collection of tissues in which the conduction from the SA node to the ventricles is slowed. But this isn’t the whole story. Within the AV node, there are two pathways that an impulse can take - a fast pathway and a slow pathway. Sometimes, due to mismatch between the impulses travelling down the pathway, an impulse might get conducted back into the atria and depolarise the atria. This phenomenon is known as AV nodal reentry and it can lead to a supra ventricular tachycardia.
Image showing the electrical impulse going haywire in the AV node and causing the impulse to be conducted back into the atrial. This phenomenon can cause supraventricular tachycardia.
Reproduced from Goldberger’s Clinical Electrocardiography, 9th edition.
Tachycardias associated with accessory pathways.
Again, it sounds complicated, but its not. Remember when we were talking about introduction to arrhythmias, we spoke about the fibrous ring between the atria and ventricles which prevents electrical impulses from getting transmitted in a haywire fashion? This fibrous ring forces the electrical impulses in the atria to go through the AV node to reach the ventricles. In some cases, this fibrous ring is deficient in some areas and this allows there to be a pathway connecting the atria and ventricles apart form the AV node - such pathways are known as accessory pathways.
One of the diseases with an accessory pathway is Wolff Parkinson White syndrome. And we’ll talk more about these later.
Image showing a reentrant pathway through the ventricles. This is called an accessory pathway. The presence of such accessory pathways can lead to supra ventricular tachycardias as well.
Reproduced from Goldberger’s Clinical Electrocardiography, 9th edition.
So those are the three subtypes of supra ventricular tachycardia. Pretty straight forward right?
Now, let’s see them in detail.
Author: Narendran Sairam (Facebook)
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