Supraventricular Tachycardia (SVT)
Supraventricular Tachycardia is a category rather than specific rhythm. SVTs are tachycardic rhythms which originate above the ventricles.
Supra in Supraventricular Tachycardia is Latin for above and references the origin of the electrical signal "above the ventricles." Tachycardia applies to any rhythm with a Heart Rate (HR) of 100 or more. As a category, Supraventricular Tachycardia technically includes such rhythms as Sinus Tachycardia and Junctional Tachycardia. Both of those examples are tachycardic rhythms with origins above (superior to) the ventricles (Sinus and Junctional origins, respectively.)
However, in clinical practice the term Supraventricular Tachycardia is reserved for EKG tracings which are too fast to differentiate which specific origin is present. While a high HR and narrow QRS Complex (origin above the ventricles) are easy to recognize, SVT is defined by not being able to identify the specific origin. This is because, at high rates, waves become so close together that they can obscure one another.
The origin of an electrical signal is marked by the first feature of the repeating complex on an EKG. In order to recognize this, QRS complexes must be spaced far enough apart to expose details between them. As the HR increases waves become overlapping, sometimes making it impossible to determine specifics.
Once a clinician has identified that an EKG does not have enough information visible to determine which specific tachycardia is present, the tracing can be called the more general Supraventricular Tachycardia. Supraventricular Tachycardia means:
A tachycardic ECG tracing with narrow QRS complexes (non-ventricular origin) on which there is not enough information to make a more specific interpretation.
The factors that define a Supraventricular Tachycardia are:
- Rate: > 99 | R-R Intervals shorter than 0.60sec (15mm) [3 large boxes].
- QRS Complexes: Maximum 0.12sec (3mm) [3 small boxes].
More important than meeting these criteria though is the absence of a clearly defined electrical origin.
The Rates of Supraventricular Tachycardia
As a tachycardia, SVT must have a rate of at least 100. In practice, only excessive rates of tachycardia obscure the origin of a cardiac rhythm, generally over 160. At rates below this the origin of a rhythm is usually visible between complexes and the specific Supraventricular Tachycardia (e.g. Sinus Tachycardia, Atrial Fibrillation with RVR, etc.) can be named.
While a specific rate isn't the determining factor of SVTs it is the high rate (frequent QRS complexes) which obscures other waveforms from interpretation.
QRS Complexes in SVT
The QRS complex in Supraventricular Tachycardia's are normal, as defined in the prototypical NSR:
- QRS Complexes: Maximum 0.12sec (3mm) [3 small boxes]
The narrow QRS complex of Supraventricular Tachycardia is confirmation of the supraventricular origin of the electrical signal. Electrical origins in the ventricles create wide QRS Complexes, so Narrow Complex Tachycardias are always of supraventricular origin.
The Unclear supraventricular Origin of SVTs
The key hallmark of Supraventricular Tachycardia is the indistinct origin. Narrow (< 0.12sec [3mm]) QRS Complexes indicate the origin above the ventricles, but the area where P Waves, fibrillation waves, or baseline would be is obscured by the end of the previous complex (marked in green above.)
The above tracing could be a Sinus Tachycardia, an Atrial Fibrillation with a Rapid Ventricular Response, or a Junctional Tachycardia, there is simply not enough information present to differentiate. This is the case when an EKG would be called only Supraventricular Tachycardia, until a specific underlying rhythm could be identified.
Below are several more examples of Supraventricular Tachycardia presented as monitor captures as well as on EGK graphs.
Once you're comfortable with the category of Supraventricular Tachycardia take a look at at Atrial Fibrillation or Sinus Tachycardia, two rhythms which may present as SVT when they have excessive rates. Or head back to our EKG Rhythm Index to find information on another ECG. Otherwise, practice interpreting novel EKGs with our EKG Generator:
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