Atrioventricular Nodal Reentrant Tachycardia (AVNRT)

Background

  • Tho most common type of reentrant SVT
  • Caused by the presence of dual AV nodal pathways

Pathophysiology

  • The AV node has two possible pathways for electrical impulses to travel through
    1. A fast pathway (fast conduction)
    2. A slow pathway (slow conduction)
  • Normally, impulses from the atria travel to the ventricles only through the fast pathway (they can begin use the slow pathway but cannot travel further as the fast pathway has already reached the end of the AV node and is in a refractory period)
  • When an extrapremature atrial impulse reaches the AV node, it encounters a refractory fast pathway and must enter the slow pathway.  By the time this reaches the end of the AV node, the fast pathway has reset, allowing the impulse to travel backwards up the fast pathway
  • This creates a reentry circuit which cycles round, activating the ventricles anterogradely and the atria retrogradely.
  • 400px-AVNRT
  • The atria are unable to empty contents into the ventricles and so is backed up into the venous system.

Clinical Diagnosis

  • Fast, regular small complex tachycardia with a rate of 180-250bpm.  Sudden onset.
    • On ECG- RP distance <100ms; P waves often hidden or immediately after QRS and are usually in opposite direction to QRS
      • May have a notched QRS appearance
  • Patient may present with palpitation, syncope, dizziness etc or it may be an incidental finding on ECG/Examination
  • Frog sign- Neck vein (JVP) pulsations occur due to simultaneous contraction of atria and ventricles

Management

  • If the patient presents acutely, manage as adult tachycardia (ABCDE) approach (see here)
  • If the patient is stable, IV adenosine may help acutely to revert the arrhythmia.  If the patient is unstable, synchronised DC shock/amiodarone may be used.
  • In the longer term, prophylactic treatment includes beta blockers and calcium channel blockers.
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