Atrioventricular nodal re-entrant tachycardia

 Atrioventricular nodal re-entrant tachycardia

AVNRT is a type of SVT caused by re-entry in a circuit involving the AV node and its two right atrial input pathways: a superior 'fast' pathway and an inferior 'slow' pathway. This produces a regular tachycardia with a rate of 120-240/min. It tends to occur in the absence of structural heart disease and episodes may last from a few seconds to many hours. The patient is usually aware of a rapid, very forceful, regular heart beat and may experience chest discomfort, lightheadedness or breathlessness. Polyuria, mainly due to the release of ANP, is sometimes a feature. The ECG usually shows a tachycardia with normal QRS complexes but occasionally there may be rate-dependent bundle branch block.

Management

Treatment is not always necessary. However, an acute episode may be terminated by carotid sinus pressure or by the Valsalva maneuver. Adenosine (3-12 mg rapidly IV in incremental doses until tachycardia stops) or verapamil (5 mg IV over 1 min) will restore sinus rhythm in most cases. Intravenous β-blocker or flecainide can also be used. In rare cases, when there is severe haemodynamic compromise, the tachycardia should be terminated by DC cardioversion.

  In patients with recurrent SVT, catheter ablation is the most effective therapy and will permanently prevent SVT in more than 90% of cases. Alternatively, prophylaxis with oral β-blocker, verapamil or flecainide may be used but commits predominantly young patients to long-term drug therapy and can create difficulty in female patients, as these drugs are normally avoided during pregnancy.