AVNRT and AVRT

Arrhythmias caused by a surplus conduction circuit between atrial and ventricular anatomy.

They cause the normal electrical circuit to allow an impulse to repeatedly travel around the normal AV node and this new connection.

AVNRT

• Atrioventricular Nodal Re-entrant Tachycardia
• The most common supraventricular arrhythmia in humans
• The most common cause of palpitations in patients with normal heart structure
• Often paroxysmal. Can occur with:
  • • Exertion
    • Coffee/Tea
    • Alcohol

   

• Slow-Fast AVNRT
  • Accounts for 80% of AVNRT
  

  

  

  

Left-Right: Mechanism of re-entry in AVNRT, from normal sinus pathway to the re-entry loop  

• During sinus rhythm, impulses travel simultaneously through both the fast and slow pathways. That which is transmitted via the fast pathway encounters the end of that of the slow, and the two are cancelled out.
• If the fast pathway is in its refractory period, a premature atrial contraction will only travel via the slow pathway
• By its nature, the impulse will not have fully conducted through the slow pathway by the time the fast becomes non-refractory again
• The impulse will travel in a retrograde fashion, up the fast pathway
• This impulse cycles around the two pathways. Its short length gives the accelerated HR
  • The HIS bundle is activated anterogradely
  •  The atria are activated retrogradely

• Fast-Slow AVNRT

  • Occurs in 10% of AVNRT patients
  • Fast AV node pathway
    • Anterograde conduction
  • Slow AV node pathway
    • Retrograde conduction

• Slow-Slow AVNRT
  • Occurs in 1-4% of AVNRT patients
  • Slow AV node pathway
    • Anterograde conduction
  • Slow LA fibres
    • Retrograde conduction

 

ECG features:

• Tachycardia
  • 140-280bpm
• Narrow QRS
  • <120ms
  • BBB & aberrant conduction arrhythmias will negate this, however
• P waves
  • Buried in QRS
  • Can appear after QRS
    • Pseudo R’ in V1, V2
    • Pseudo S in II, III, aVF
• PR distance
  • <100ms
• Onset follows PAC

 

• Fast-Slow
  • QRS-P-T complexes

• Slow-Slow
  • P waves seen before QRS

 

AVRT

• Atrioventricular Re-entrant Tachycardia
• More common in females than males
• Arises due to an accessory pathway
  • A strand of myocardium connects the atria and ventricles, bypassing the AV node
• Two types;
  • Orthodromic- antegrade conduction via the AV node (Left)
  • Antidromic- antegrade conduction via the accessory pathway (Right)

 

ECG features:

Orthodromic:

• Narrow complex tachycardia
  • 200-300bpm
• P waves hidden in QRS complexes or retrograde
  • Inverted inferior and Lead I P waves indicate left sided pathway
• T wave inversion
• ST depression

Antidromic:

• Wide complex tachycardia
  • 200-300bpm