Junctional Tachycardia

Junctional tachycardia is an automatic or triggered rhythm arising from the atrioventricular junction (the node and His region), with a narrow QRS complex and a rate over 100/min. Junctional rhythm is 40 to 60/min and accelerated junctional rhythm is 60 to 100/min.

Atrioventricular dissociation is common, with the ventricular rate faster than the atrial rate, though 1:1 retrograde conduction can occur. Forms are:

• Postoperative junctional ectopic tachycardia in children after congenital heart surgery (reported in about 2 to 10%), peaking 6 to 72 hours after surgery with rates of 180 to 250/min. Risk factors are surgery near the His bundle (tetralogy of Fallot, atrioventricular septal defect, Fontan), long bypass time, young age, catecholamine infusions and hypomagnesaemia.
• Congenital junctional ectopic tachycardia: rare, presents in infancy with rates of 170 to 260/min and heart failure, has high mortality (about 35%) and may be familial.
• Adult focal junctional tachycardia: usually from digoxin toxicity, myocardial ischaemia or infarction (especially inferior), myocarditis, cardiac surgery, theophylline or chronic lung disease. Some young adults have an idiopathic, exercise sensitive form.

Pathophysiology. Abnormal automaticity or triggered activity in junctional tissue drives the ventricles faster than the sinus node. Loss of atrioventricular synchrony in infants and after surgery reduces ventricular filling and cardiac output. Persistent fast rates cause ventricular dysfunction.

• Adults: palpitations, dyspnoea, presyncope or no symptoms. Accelerated junctional rhythm is often asymptomatic.
• Infants and postoperative patients: tachycardia with hypotension, poor perfusion, low cardiac output and heart failure.
• Examination: regular rapid pulse, variable intensity of the first heart sound and intermittent cannon a waves from atrioventricular dissociation.
• ECG: narrow regular QRS (unless aberrant conduction) at over 100/min, with atrioventricular dissociation (P waves unrelated to QRS, with the atrial rate slower) or retrograde P waves. In postoperative patients, atrial wire electrograms confirm dissociation.

Red flags.

• Hypotension or low cardiac output after cardiac surgery
• Rate over 200/min in an infant
• Digoxin toxicity or hyperkalaemia
• Acute inferior myocardial infarction
• Incessant tachycardia with falling ventricular function
• Atrioventricular block when the tachycardia stops

• 12-lead ECG and rhythm strip with atrial electrogram (postoperative wires) for atrioventricular dissociation. Gradual onset and offset (warm up and cool down) favour automaticity.
• Adenosine: it does not terminate junctional tachycardia, though it may transiently slow it and unmask the dissociation. Termination suggests atrioventricular nodal reentry. Vagal manoeuvres usually have no effect.
• Laboratory: potassium, magnesium, calcium, digoxin level, thyroid function, troponin.
• Echocardiography for ventricular function and structural disease.
• Electrophysiology study in adult or congenital cases for mapping and ablation.

Differential diagnosis.

• Atrioventricular nodal reentrant tachycardia (abrupt onset and offset, terminates with adenosine)
• AVRT
• Focal atrial tachycardia with first degree atrioventricular block
• Fascicular ventricular tachycardia (narrow QRS, atrioventricular dissociation)
• Atrial flutter with 2:1 block
• Sinus tachycardia
• Accelerated idioventricular rhythm
• Digoxin induced rhythm disturbances

General. Correct potassium to 4.0 to 5.0 mmol/L and magnesium to the normal range, treat fever, pain and anxiety, and stop causative drugs. Synchronised cardioversion usually fails because the rhythm is automatic. Asymptomatic accelerated junctional rhythm needs no antiarrhythmic therapy.

Postoperative junctional ectopic tachycardia in children.

• Reduce or stop adrenaline, dopamine and milrinone infusions as haemodynamics allow. Provide sedation and analgesia.
• Target core temperature of 34 to 35 degrees Celsius with surface cooling, sedation and neuromuscular blockade to prevent shivering, and treat any fever.
• Magnesium sulphate 25 to 50 mg/kg intravenously over 30 minutes, maximum 2 g in one dose. Monitor blood pressure, reflexes and serum magnesium. Reduce in renal impairment.
• Atrial pacing through temporary wires at 10 to 20/min over the junctional rate to restore atrioventricular synchrony. It is ineffective at rates over about 200/min, so cool and give amiodarone first.
• Amiodarone intravenously 5 mg/kg over 1 hour (maximum 300 mg), then infusion 5 to 15 microgram/kg/min, maximum 15 microgram/kg/min. Monitor ECG and blood pressure continuously, since hypotension and bradycardia are common. Taper as the rate falls. Check thyroid and liver function with prolonged use.
• Digoxin and calcium channel blockers are avoided. Extracorporeal support is used for refractory low output.
• Atrioventricular block can appear when the junctional rate slows, so pacing facilities must stay available.

Congenital junctional ectopic tachycardia. Oral amiodarone 10 to 20 mg/kg daily in 1 or 2 doses for 7 to 10 days, then 5 to 10 mg/kg once daily, with ECG and thyroid monitoring, in a paediatric electrophysiology centre. Add flecainide or propafenone when refractory. Cryoablation is reserved for refractory cases because of the risk of atrioventricular block.

Adults.

• Digoxin toxicity: stop digoxin and correct potassium. Give digoxin specific antibody fragments for life threatening arrhythmia, with vials = serum digoxin (ng/mL) multiplied by weight (kg), divided by 100 (each 40 mg vial binds about 0.5 mg of digoxin). Empirical dosing is 10 vials for acute ingestion of unknown amount and 3 to 6 vials for chronic toxicity. Avoid cardioversion.
• Ischaemia or myocarditis: treat the cause.
• Symptomatic or persistent tachycardia: bisoprolol 2.5 mg orally once daily, increased to 5 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), or verapamil 40 to 120 mg orally every 8 hours, maximum 480 mg daily. Flecainide 50 mg orally twice daily, increased to 100 mg twice daily, maximum 300 mg daily, without structural or ischaemic heart disease, with QRS monitoring and a dose reduction when the QRS widens by over 25%. Amiodarone 300 mg intravenously over 1 hour, then 900 mg over 23 hours, maximum 1.2 g in 24 hours, for refractory cases.
• Catheter ablation for drug resistant adult focal junctional tachycardia carries a substantial risk of permanent atrioventricular block, which must be discussed before the procedure.

Special groups. Pregnancy is uncommon with this rhythm. Use a beta blocker or verapamil and avoid amiodarone.

Monitoring. Continuous ECG and atrial electrograms in postoperative patients, with electrolytes every 6 to 12 hours. Core temperature during cooling. Heart rate, blood pressure and QT interval on amiodarone. Echocardiography at 3 months when ventricular function was reduced.

Referral and escalation. Refer infants and children to paediatric cardiac intensive care and electrophysiology. Adults with persistent or drug resistant tachycardia go to an electrophysiology service. Urgent for low cardiac output, rate over 200/min in an infant, or new atrioventricular block.