Atrioventricular Reentrant Tachycardia

Atrioventricular reentrant tachycardia (AVRT) is a macro-reentrant supraventricular tachycardia in which an accessory atrioventricular connection forms one limb of the circuit and the atrioventricular node with the His-Purkinje system forms the other.

In orthodromic AVRT (90 to 95% of cases) the impulse descends through the atrioventricular node and returns to the atria through the accessory connection, giving a regular narrow complex tachycardia. In antidromic AVRT (5 to 10%) the circuit runs the opposite way, giving a regular wide, fully pre-excited tachycardia.

Wolff-Parkinson-White syndrome is manifest pre-excitation (delta wave, short PR interval) with tachyarrhythmia. Pre-excitation alone is present in about 0.1 to 0.3% of the population. A concealed accessory connection conducts only retrogradely and leaves no delta wave.

AVRT accounts for about 30% of paroxysmal supraventricular tachycardia in adults (atrioventricular nodal reentry about 60%) and is the commonest mechanism in infancy.

Associations are Ebstein anomaly (right sided, often multiple connections), hypertrophic cardiomyopathy with PRKAG2 variants, congenitally corrected transposition and familial pre-excitation.

Pathophysiology. Accessory connections are strands of working myocardium that bridge the atrioventricular groove and conduct fast, without decrement. A premature atrial or ventricular beat that meets a refractory connection or a slowed node starts the circuit.

Approximate sites are the left free wall 50 to 60%, posteroseptal 20 to 30%, right free wall 10 to 20% and septal under 10%. Multiple connections occur in about 5 to 10%. A bundle branch block on the same side as the connection lengthens the circuit and slows the rate by 35 ms or more in cycle length (Coumel sign).

The main hazard is atrial fibrillation, which develops in 10 to 30% of patients with Wolff-Parkinson-White syndrome. A connection with a short antegrade refractory period conducts atrial fibrillation rapidly to the ventricles and can degenerate to ventricular fibrillation.

Sudden death is reported in about 0.1 to 0.6% of patients over years of follow up. It is usually the first event in a minority, and is most likely when the shortest pre-excited RR interval during atrial fibrillation is 250 ms or less.

• Abrupt onset and offset palpitations, regular, at 150 to 250/min (up to 300/min in infants), with dyspnoea, chest tightness, presyncope and polyuria after the episode.
• Syncope or near syncope, particularly with pre-excited atrial fibrillation.
• Infants: poor feeding, irritability, pallor, tachypnoea and heart failure after sustained tachycardia, usually at rates over 220/min.
• Resting ECG in manifest pre-excitation: PR interval under 120 ms, delta wave, QRS over 110 ms, secondary ST-T change, sometimes pseudo infarct Q waves. A concealed connection gives a normal sinus rhythm ECG.
• During orthodromic AVRT: regular narrow QRS, retrograde P wave in the ST segment or early T wave (RP interval over 70 ms and shorter than half the RR interval), ST depression and QRS alternation.
• Pre-excited atrial fibrillation: irregular wide complex tachycardia of changing QRS morphology at up to 300/min.

Red flags.

• Syncope, hypotension or heart failure during tachycardia
• Irregular wide complex tachycardia (pre-excited atrial fibrillation)
• Resuscitated arrest
• Shortest pre-excited RR interval of 250 ms or less
• Family history of sudden death or of pre-excitation with cardiomyopathy
• Ebstein anomaly or hypertrophic cardiomyopathy

• 12-lead ECG in sinus rhythm for pre-excitation. A delta wave that is positive in V1 suggests a left sided connection. A negative delta wave in V1 with a left axis suggests a right or septal connection.
• ECG during tachycardia: regular narrow QRS with a retrograde P wave and abrupt termination with adenosine support orthodromic AVRT. A regular, wide, fully pre-excited QRS indicates antidromic AVRT.
• Risk stratification of pre-excitation. Abrupt loss of the delta wave during exercise or intermittent pre-excitation on ambulatory ECG suggests a connection with a long refractory period and lower risk. These findings are supportive and not definitive.
• Electrophysiology study: confirms the mechanism, locates the connection, measures the antegrade effective refractory period (240 ms or less is high risk), shortest pre-excited RR interval during induced atrial fibrillation (250 ms or less is high risk) and the number of connections. Ablation is done in the same session.
• Echocardiography: Ebstein anomaly, hypertrophic cardiomyopathy and ventricular function. Thyroid function and electrolytes.
• Genetic testing (PRKAG2) when there is hypertrophy with pre-excitation or a family history.

Differential diagnosis.

• Atrioventricular nodal reentrant tachycardia (no visible P wave or RP under 70 ms, pseudo r' in V1)
• Focal atrial tachycardia and atrial flutter with 2:1 block (adenosine reveals flutter waves)
• Permanent junctional reciprocating tachycardia (incessant, inverted inferior P waves, long RP)
• Atrioventricular reentry using a Mahaim fibre (left bundle branch block morphology)
• Ventricular tachycardia (atrioventricular dissociation, capture and fusion beats, concordance in the chest leads)
• Atrial fibrillation with bundle branch block
• Sinus tachycardia and inappropriate sinus tachycardia

Acute orthodromic AVRT, stable.

• Modified Valsalva manoeuvre: strain for 15 seconds in a semi-recumbent position (blow into a 10 mL syringe hard enough to move the plunger), then lie flat with the legs raised to 45 degrees for 15 seconds. It terminates about 40% of episodes. Carotid sinus massage for 5 to 10 seconds on one side only, after excluding a bruit and previous stroke or transient ischaemic attack.
• Adenosine 6 mg rapid intravenous bolus into a large proximal vein followed by a 20 mL saline flush. If there is no effect in 1 to 2 minutes, give 12 mg, then 12 mg once more. Maximum total 30 mg. Use 3 mg initially through a central line, with dipyridamole or carbamazepine, and in heart transplant recipients. Higher doses are needed with theophylline or caffeine. Record a 12-lead ECG during the bolus and have a defibrillator ready, because atrial fibrillation is induced in some patients. Contraindicated in asthma, second or third degree atrioventricular block without a pacemaker, sick sinus syndrome, and when pre-excited atrial fibrillation is suspected. Warn the patient of flushing and chest tightness lasting under a minute.
• Verapamil 5 mg intravenously over 2 minutes (3 minutes in older adults), repeated once after 10 minutes if needed. Maximum 10 mg. Contraindicated in heart failure with reduced ejection fraction, hypotension, recent intravenous beta blocker, infants under 1 year, and any wide complex tachycardia of uncertain origin. Monitor blood pressure.
• Haemodynamic instability: synchronised cardioversion starting at 70 to 120 J biphasic, increasing to 150 to 200 J if unsuccessful, under brief sedation if conscious.

Pre-excited atrial fibrillation or antidromic AVRT.

• Avoid adenosine, verapamil, diltiazem, beta blockers, digoxin and intravenous amiodarone, because they block the node but not the connection and can provoke ventricular fibrillation.
• Unstable: synchronised cardioversion 120 to 200 J biphasic under sedation.
• Stable: procainamide 15 to 17 mg/kg intravenously at 20 to 30 mg/min. Maximum 17 mg/kg. Stop for hypotension, QRS widening by 50% or more, or termination. Reduce in renal impairment and heart failure. Alternatives are ibutilide 1 mg intravenously over 10 minutes (0.01 mg/kg if under 60 kg), repeated once after 10 minutes, maximum 2 mg, with 4 hours of ECG monitoring, and contraindicated with QTc over 440 ms, ejection fraction under 30% or uncorrected hypokalaemia or hypomagnesaemia. Flecainide 2 mg/kg intravenously over 10 minutes, maximum 150 mg, contraindicated in structural or ischaemic heart disease.

Definitive treatment.

• Catheter ablation of the accessory connection is first line for symptomatic AVRT and Wolff-Parkinson-White syndrome. Acute success is 93 to 98%, recurrence about 5 to 10%. The main risks are atrioventricular block (about 0.5 to 1%, higher with septal sites, for which cryoablation is used), tamponade and vascular injury. Mortality is about 0.1%.
• Asymptomatic pre-excitation: ablation is advised when electrophysiology study shows high risk features (shortest pre-excited RR interval 250 ms or less, antegrade refractory period 240 ms or less, multiple connections, inducible AVRT or atrial fibrillation), or the person has a high risk occupation (pilot, public transport driver, competitive athlete). Low risk features on non-invasive testing allow observation.
• Children: ablation from about 5 years of age or 15 kg, earlier for drug resistant or life threatening arrhythmia.

Drug therapy when ablation is declined or unavailable.

• Concealed connection or orthodromic AVRT without pre-excited atrial fibrillation: verapamil 40 to 120 mg orally every 8 hours (usual 80 mg), maximum 480 mg daily, or bisoprolol 5 mg orally once daily, increased to 10 mg once daily, maximum 20 mg daily. Do not exceed 10 mg daily at creatinine clearance under 20 mL/min or in severe liver impairment. Long term. Contraindicated in reduced ejection fraction (verapamil), high grade atrioventricular block and severe asthma.
• Manifest pre-excitation: node blocking drugs alone are avoided long term. Use flecainide 50 mg orally twice daily, increased to 100 mg twice daily, maximum 300 mg daily, with ablation planned. Reduce the dose when the QRS widens by over 25% from baseline. At creatinine clearance of 35 mL/min or less, start at 50 mg twice daily and monitor levels. Contraindicated in structural heart disease, ischaemic heart disease and conduction disease.

Special groups.

• Infants and children: adenosine 100 microgram/kg (maximum 6 mg) rapid intravenous bolus, increased by 50 microgram/kg every 1 to 2 minutes to a maximum of 300 microgram/kg (maximum 12 mg). Verapamil is contraindicated under 1 year. Most infant AVRT resolves by 1 year, and maintenance therapy is set by paediatric electrophysiology.
• Pregnancy: adenosine 6 mg and synchronised cardioversion are safe. For maintenance, metoprolol tartrate 25 to 100 mg orally twice daily, maximum 200 mg daily in this setting. Flecainide is reserved for refractory cases under specialist supervision. Ablation with minimal fluoroscopy after the first trimester when refractory.
• Ebstein anomaly: multiple right sided connections make ablation harder and surgery may be needed.

Monitoring. After ablation: ECG at 1 to 3 months, then with symptoms, since recurrence is mostly early. On flecainide: ECG for QRS width at each dose change and after 1 week. On verapamil: heart rate, PR interval and blood pressure. Low risk observed pre-excitation: yearly review.

Referral and escalation. Refer every patient with pre-excitation and symptoms to an electrophysiology service. Urgent for syncope, pre-excited atrial fibrillation, resuscitated arrest, and for athletes and people in high risk occupations.