• Atrioventricular reentry tachycardia (AVRT) arises from macro-reentrant circuit involving an accessory pathway, triggered by a premature beat (either PAC, PJC, or PVC).
• Ventricular rate ranges from 150 to 250 bpm.
• 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 the QRS complexes.
• Electrical impulses can be conducted from the atria the ventricles via 2 pathways (1):
AV pathway: fast conduction via His bundle-Purkinje fibers
Accessory pathway: slower, less efficient conduction
• Normally, the electrical impulse conducted via AV pathway will travel a longer distance but it reaches the accessory pathway on the other side first, at which time the accessory pathway has already entered refractory period thereby extinguishing any further conduction towards that direction (2).
• In orthodromic AVRT (3-4),
A premature atrial beat is sent down the AV pathway when the accessory pathway is still in refractory period (for whatever reason) (3).
By the time the premature impulse reaches the accessory pathway, which has just recovered from the refractory period, it can now travel across it in a retrograde fashion to the atria, thus creating a clockwise reentrant loop (4).
• In antidromic AVRT (5-6),
A premature atrial beat is sent down the accessory pathway when the AV pathway is still in refractory period (for whatever reason) (5).
By the time the premature impulse reaches the AV pathway, which has just recovered from the refractory period, it can now travel across it in a retrograde fashion to the atria, creating a counter-clockwise reentrant loop (6).
• In orthodromic AVRT, the impulse travels down the AV-His-Purkinje system first then goes back up to cross the accessory pathway in clockwise direction.
• Because it travels down the AV-His-Purkinje pathway, conduction to the ventricles is efficient enough to produce normal, narrow QRS complexes.
• Because conduction to the atria is retrograde and occurs after conduction to the ventricles, P waves are retrograd and end up shortly after QRS complexes or buried in or fused with QRS's.
• This rhythm can be indistinguishable from typical AVNRT since both have RP < PR, however ultrashort RP (< 70 ms) is more consistent with typical AVNRT.
• In antidromic AVRT, the impulse travels down the accessory pathway first then goes back up to cross the AV node in counterclockwise direction.
• Because it does not travel through the AV-His-Purkinje pathway, conduction is less efficient hence widened QRS complexes.
• Because retrograde conduction to the atria is slow across the AV node, retrograde P waves appear after QRS complexes (or right before the next QRS's) with RP > PR, or buried with QRS's.
• This rhythm can be indisinguishable from ventricular tachycardia.
• Permanent juntional reciprocating tachycardia (PJRT) is a form of orthodromic AVRT that involves a concealed accessory pathway in the posteroseptal region.
• Because the retrograde conduction across the concealed accessory pathway is slow, retrograde P waves end up after QRS complexes.
• On ECG, retrograde P wave lags behind QRS complex (right before the next QRS) with RP > PR.
• Like orthodromic AVRT, QRS complexes are normal and narrow because impulses travel down the AV-His-Purkinje system.
• This rhythm can be indisinguishable from atypical AVNRT.
