Atrioventricular Nodal Reentrant Tachycardia

AV nodal reentrant tachycardia (AVNRT) is the commonest paroxysmal regular SVT, caused by a reentry circuit using dual AV nodal pathways: slow pathway (long conduction time, short refractory period) and fast pathway (rapid conduction, long refractory period).

Typical (slow fast, 90%): anterograde conduction down the slow pathway, retrograde up the fast pathway; P wave buried in or at the end of the QRS (pseudo r' in V1, pseudo S in inferior leads).

Atypical (fast slow, slow slow): P wave after the QRS.

More common in women (70%), age 20 to 50, no structural heart disease.

Triggers: premature atrial complexes, caffeine, alcohol, exercise.

Episodes last minutes to hours; rare tachycardia mediated cardiomyopathy.

• Sudden palpitations at rate 140 to 220, regular, with neck pulsations (cannon a waves), dyspnoea, chest tightness, lightheadedness, polyuria.
• Syncope infrequent.
• Termination often abrupt, sometimes with a pause.

• ECG during tachycardia: regular narrow complex tachycardia with absent visible P waves or pseudo r' in V1 and pseudo S waves in II, III, aVF, RP interval under 70 ms (typical).
• Baseline ECG: normal; exclude pre excitation.
• Response to adenosine: termination with a last QRS ending the tachycardia (reentrant mechanism).
• Ambulatory monitoring or event recorder for documentation.
• Echocardiography and thyroid function.
• Electrophysiology study: dual AV node physiology (jump of more than 50 ms in AH interval), inducible tachycardia, VA interval under 70 ms.

Acute

• Modified Valsalva manoeuvre, carotid sinus massage (after excluding bruit), cold stimulus.
• Adenosine 6 mg IV, then 12 mg, up to two further doses (success about 90 to 95%).
• Diltiazem 0.25 mg/kg IV or verapamil 5 to 10 mg IV; metoprolol 2.5 to 5 mg IV if blood pressure is adequate.
• Synchronised cardioversion for instability.

Prevention

• Catheter ablation of the slow pathway: first line for recurrent symptomatic AVNRT, success 95 to 98%, recurrence 2 to 5%, complete AV block under 1%. Cryoablation reduces block risk.
• Medications for patients declining ablation: metoprolol 25 to 100 mg twice daily or verapamil SR 120 to 240 mg daily, flecainide 100 mg twice daily or propafenone with AV nodal blocker (avoid in structural heart disease).
• Pill in the pocket (diltiazem 120 mg plus propranolol 80 mg, or flecainide 3 mg/kg single dose) for infrequent, well tolerated episodes.
• Pregnancy: vagal manoeuvres and adenosine; metoprolol or verapamil for prophylaxis; ablation without fluoroscopy using electroanatomic mapping if necessary.
• Counsel on trigger avoidance and teaching of vagal manoeuvres.