Supraventricular tachycardias (SVTs) arise at or above the atrioventricular (AV) node due either to abnormal automaticity or abnormal conduction.
AV nodal re-entry tachycardia (AVNRT) makes up 60%-70% of SVT cases due to a functional obstruction with 2 conduction systems within the node.
Ventricular pre-excitation (VPE) occurs when circus movements rely on anatomical accessory pathways from the atria to the ventricle. This is the mechanism of AV re-entry tachycardia (AVRT).
Wolff-Parkinson-White syndrome (WPW) involves conduction through a congenital accessory pathway, leading to pre-excitation to the ventricle. An ECG will reveal a shortened PR wave and up-swerving initiation of the R wave, called a delta wave.
Orthodromic (antegrade) conduction depolarizes through the bundle, resulting in a narrow QRS.
Antidromic (retrograde) conduction causes depolarization through the accessory pathway and retrograde progression through the AV node, leading to a wide QRS.