AVNRT

• Atrioventricular nodal reentry tachycardia (AVNRT) is the most common SVT.
• AVNRT arises from reentry circuit around the AV node, triggered by a premature beat (either PAC or less commonly PJC).
• Because impulses are conducted down the His-Purkinje system, the resulting QRS complexes are normal and narrow.
• Ventricular rate ranges from 120 to 200 bpm.
• AVNRT starts and terminates abruptly, ang is often preceded by a PAC.
• Since the atrial myocardium receives impulses from the AV node down below, P waves are retrograde (negative in inferior leads), but most of the time, P waves are buried in or fused with QRS complexes.
• Types of AVNRT include:
   Slow-fast (typical, common)
   Fast-slow (atypical, uncommon)
   Slow-slow (posterior-type)

• According to dual AV node physiology, electrical impulses are conducted through AV node down to His bundle via 2 pathways (Fig 1):
   Fast pathway: fast conduction but long refractory period
   Slow pathway: slow conduction but short refractory
• Normally, the fast impulse will reach the His bundle first and travel up the slow pathway and cancel out the slow impulse (Fig 2).
• In typical (or slow-fast) atrioventricular nodal reentry tachycardia (AVNRT), the next impulse is sent down prematurely when the fast pathway is still in refractory period (which lasts longer than the slow pathway), the premature impulse is then forced to travel down the slow pathway (Fig 3).
• By the time this new impulse reaches the His bundle, the fast pathway has recovered and now allows the new impulse to travel in a retrograde fashion along the fast pathway (Fig 4).
• As a result, a reentrant loop is created (Fig 5).