Supraventricular Tachycardia (SVT) are a group of tachycardias in which at least a structure above the Bundle of His is necessary for its continuance.
Supraventricular Tachycardias include a number of disorders, Atrioventricular Nodal Reentrant Tachycardia and Atrioventricular Reentry Tachycardia being the most frequent. We also include in this article Atrial Tachycardia and Inappropriate Sinus Tachycardia, which are rare arrhythmias.
SVTs characteristic findings on EKG are high Heart Rates, which are usually rhythmic, and their main distinctive feature is the presence of narrow QRS complexes.
Although less frequent, wide QRS complexes can also turn up in SVT, as for example in AVRT with Antidromic conduction, or if the SVT is accompanied by previous Bundle Branch Block or aberrant conduction (Ashman Phenomenon).
By concept, Atrial Fibrillation and Atrial Flutter are Supraventricular Tachycardias, but they are usually studied separately.
Atrioventricular Nodal Reentrant Tachycardia (AVNRT)
Atrioventricular Nodal Reentrant Tachycardia is the most frequent of paroxysmal SVTs in healthy hearts, accounting for 60% of cases. It is usually exhibited by women in their forties.
It is caused by the presence of two pathways in the AV Node — a slow and a fast one — that at the appearance of an extra stimulus form a reentry mechanism perpetuating tachycardia.
Atrioventricular Nodal Reentrant Tachycardia
From an electrophysiological point of view there are two forms of Atrioventricular Nodal Reentrant Tachycardia: Slow-Fast AVNRT (Common AVNRT) and Fast-Slow AVNRT (Uncommon AVNRT).
AVNRT characteristic findings on Electrocardiogram are:
- Regular narrow QRS complex tachycardia with Heart Rate between 120 and 250 bpm.
- Common AVNRT: absence of P waves (being embedded in the QRS), pseudo r’ wave in lead V1 or pseudo S waves in inferior leads.
- Uncommon AVNRT: negative P wave in inferior leads appears after the corresponding QRS (before T wave).
As an AV node-related Supraventricular Tachycardia, it is usually stopped by the undertaking of vagal manoeuvres or the use of intravenously administered adenosine.
We would like to remind you that the use of intravenous adenosine must be administered by experienced medical staff and require patient monitoring.
Atrioventricular Nodal Reentrant Tachycardia definitive treatment is slow pathway catheter ablation.
Atrioventricular Reentry Tachycardia (AVRT)
Up to a third of the patients diagnosed with pre-excitation syndrome by electrocardiogram can present SVTs related to accessory pathways 1.
In these tachycardias a macro-reentry mechanism is formed, involving the atria, the conduction system, the ventricles and the accessory pathway. The electrical stimulus is conducted continuously through all these parts perpetuating tachycardia.
There are two very different electrocardiographic patterns based on the direction of the stimulus conduction:
It is the most frequent AVRT.
The stimulus is conducted down through the conduction system and then up through the accessory pathway to the atria. As a result of the ventricles being stimulated by the conduction system, the QRS complex is narrow.
Orthodromic Tachycardia at 250 bpm
Narrow QRS complex tachycardia. P waves after the QRS in red.
Electrocardiogram features of Orthodromic Tachycardia:
- Narrow QRS complex tachycardia with HR between 200 and 300 bpm.
- Non-sinus P waves can be found following the QRS complex.
The stimulus is conducted down to the ventricles through the accessory pathway and then up to the atria through the conduction system. As the ventricular depolarisation occurs completely via accessory pathway, the QRS complex is wide.
Antidromic Tachycardia is a wide QRS complex Supraventricular Tachycardia.
Electrocardiogram features of Antidromic Tachycardia:
Atrial Tachycardia is a rare Supraventricular Tachycardia. As suggested by its name, it originates in the atrial muscle and does not require the Atrioventricular Node or the ventricles for its initiation and continuance.
During an Atrial Tachycardia the AV node can present blocks of varying degrees depending on the conduction capacity of the Atrioventricular Node, with the possibility of AV conduction ratio of 1:1, 2:1, etc.
Tachycardia at 125 bpm with ectopic P waves (negative P wave in Lead II)
Atrial Tachycardia characteristic findings on Electrocardiogram:
- Well defined P waves with rates from 100 to 240 bpm.
- P wave has a different morphology and electrical axis than sinus P wave.
- P waves are usually closer to the following QRS complex than to the preceding one (PR interval shorter than RP interval).
- Narrow QRS complex, unless preexisting Bundle Branch Block or aberrant conduction
With very high Heart Rates it is difficult for P waves to be distinguished, making it impossible to differentiate Atrial Tachycardia from other Supraventricular Tachycardia. Vagal manoeuvres or intravenous Adenosine administration can produce a transitory AV block, allowing hidden AT P waves to be seen.
Unlike Supraventricular Tachycardia in which the AV node is involved, adenosine does not usually interrupt the arrhythmia in Atrial Tachycardia.
Atrial Extrasystoles are premature electrical impulses generated in an atrial area other than the Sinus Node.
Characteristic findings on EKG are the occurrence of isolated P waves with different morphology and Electrical Axis than in Sinus Rhythm, followed by a QRS complex similar to previous ones.
As in Ventricular Extrasystoles, the ectopic P wave often arises prematurely (shorter PP interval as compared to previous ones). In Atrial Extrasystoles the PR interval can be normal or shortened.
It may be difficult to identify the Atrial Extrasystole P wave on some Electrocardiograms, as it may be hidden in the preceding QRS or T wave.
Atrial Extrasystoles do not require treatment unless secondary to other pathology or very symptomatic.
Inappropriate Sinus Tachycardia
Inappropriate Sinus Tachycardia is a rare Supraventricular Tachycardia caused by a increase at Sinus Node level in Heart Rate at minimal exercise or at rest.
Sinus Tachycardia at 136 bpm
Inappropriate Sinus Tachycardia Electrocardiogram features are the same as Sinus Tachycardia.
Supraventricular Tachycardia Summary
Supraventricular Tachycardiad are a group of heart rhythm disorders which cause rhythmic narrow QRS complex tachycardias in most cases.
By concept, a continuing SVT needs the involvement of at least an structure above the Bundle of His.
Prognosis is usually benign, although very symptomatic and frequent relapses.
Although medical treatment with antiarrhythmic drugs is usually effective, catheter ablation is usually the treatment of choice as a definitive, high success rate solution.
We hope we have been of some help in expanding your knowledge of Supraventricular Tachycardia. Best regards.
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