An SVT is a rhythm with a ventricular rate greater than or equal to 100 beats per minute (BPM) where the rhythm starts or is maintained by tissue located in either the sinus node, atria, AV node, bundle of His, or some combination of these sites. The atrial rates vary depending on the rhythm involved and can be found anywhere from 100 BPM to the mid-400s, or even higher in some rare cases. The tachycardia can be triggered by either an individual reentry circuit loop (most common mechanism) or an individual focus (influenced by increased automaticity).
As we shall discuss in further detail later in the chapter, the SVTs are broken down into narrow-complex tachycardias and wide-complex tachycardias based on the width of their QRS intervals. If the QRS complexes are less than 0.12 seconds wide, the rhythm is considered narrow complex. If the QRS complexes are greater than or equal to 0.12 seconds, the rhythm is considered wide complex. It is important to point out that the narrow-complex SVTs are narrow because the ventricular depolarization wave travels down to depolarize the ventricles using the normal electrical conduction system.