Supraventricular tachycardia and pacing in children

Overview

Supraventricular tachycardia (SVT) is a fast rhythm that starts at or above the AV node. It is the commonest symptomatic tachyarrhythmia in children. The QRS is usually narrow because the ventricles are activated through the normal conduction system.

Infants cannot report palpitations, so SVT is often found late, when the baby is pale, feeding poorly and in heart failure. It must be told apart from sinus tachycardia (fever, dehydration, pain) and from ventricular tachycardia (broad QRS, from the ventricles).

Causes and pathophysiology

Mechanisms

• Re-entry is the usual mechanism: an impulse circles continuously, and in most children the AV node forms part of the circuit (Nelson). This is why slowing AV node conduction (vagal manoeuvres, adenosine) breaks the circuit.
• Some children have Wolff-Parkinson-White (WPW) syndrome or pre-excitation as the cause, with an accessory pathway.
• Less often an ectopic focus is responsible; ablation can abolish either a re-entrant circuit or an ectopic focus (Akbar).

Associations

Clinical features

Infants

• Pallor, irritability, poor feeding, respiratory distress.
• Nelson notes that SVT in infants often runs at 280-300/min, slower in older children.
• Heart failure develops if SVT continues (uncommon over 1 year of age); risk is highest with SVT lasting over 24 hours, rates over 200/min, or underlying heart disease (Akbar; Nelson warns that episodes over 12 hours may affect output).
• Shock: poor perfusion, prolonged capillary refill, hepatomegaly, lethargy.
• In the fetus, SVT is a cause of heart failure and hydrops.

Older children

• Sudden-onset palpitations, often with an equally sudden end.
• With a structurally normal heart, most episodes cause little more than a pounding heartbeat (Nelson).
• Dizziness, chest pain, breathlessness, syncope.

Diagnostic approach and ECG interpretation

The 12-lead ECG is the most important test. Record it before and during treatment, because the response to adenosine helps the diagnosis.

ECG features of SVT

• Rate 150-300/min with 1:1 AV conduction (Akbar). Nelson gives a usual rate over 220/min (range 180-320), and treats a rate under 240/min with normal P waves as sinus tachycardia.
• Very regular, with a fixed RR interval; the rate barely changes through the tracing.
• Narrow QRS (normal shape) in most cases.
• P waves absent, inverted just after the QRS, or abnormal before it.
• Abrupt onset and offset.

SVT versus sinus tachycardia

After conversion

• Repeat a complete 12-lead ECG in sinus rhythm and look for WPW pre-excitation.
• Echocardiography to look for structural disease such as Ebstein anomaly.
• Electrolytes, and thyroid function if thyrotoxicosis is suspected.
• Refer to a paediatric cardiologist.

Management

First steps

• History, examination, and assess circulation (BP, capillary refill, conscious level).
• Continuous ECG and BP monitoring; IV access; resuscitation equipment ready.

Unstable (shock or heart failure)

• Synchronised DC cardioversion at 0.5-2 J/kg immediately (Akbar; Nelson gives 1-2 J/kg).
• Do not delay for IV access, sedation or intubation in a shocked infant.
• In a child already on digoxin, consider lidocaine beforehand to reduce the risk of ventricular fibrillation.
• Nelson's resuscitation table advises considering adenosine first in symptomatic SVT, provided it does not delay the shock.

Stable child

1. Vagal manoeuvres

• Infants: diving reflex - a bag of ice (and water) placed across the nasal bridge; watch for hypothermia with repeated use.
• Older children: Valsalva manoeuvre (the child blows hard into a closed hand with the thumb in the mouth).
• Akbar also lists the gag reflex and carotid massage as vagal manoeuvres.

2. Adenosine (drug of choice)

• Blocks the AV node briefly and breaks the re-entry circuit.
• Very short half-life (about 9 seconds), so give by rapid IV bolus, followed at once by a saline flush.
• Dose range in Akbar: 50-250 micrograms/kg; if the first dose fails, double and repeat; Akbar gives the maximum adult dose as 6 mg. (Nelson starts at 0.1 mg/kg up to 6 mg, then gives 0.2 mg/kg up to 12 mg as the second dose.)
• Too short-acting to prevent recurrence.

3. Other drugs (specialist)

Preventing recurrence

• Infants: maintenance digoxin or propranolol for the first 6-12 months of life.
• Older children: decide by frequency and severity of episodes, age and parental anxiety; some need only education and follow-up (Nelson).
• Before choosing treatment for palpitations, record the heart rate and rhythm during symptoms (Nelson).

Prognosis

• In early infancy, about 90% respond to first treatment; about 30% recur, at an average age of about 8 years.

Recent advances

• Radiofrequency catheter ablation abolishes the re-entrant circuit or ectopic focus and has been very successful (Akbar); it is offered when medical treatment fails, in symptomatic children, or when the child does not want daily medicine (Nelson).
• Fetal SVT is recognised as a cause of hydrops (Nelson), so fetal heart rhythm matters in antenatal scans.
• Capturing the rhythm during symptoms guides the choice between observation, drugs and ablation (Nelson).

Pacemaker use and management

SVT itself is treated with drugs or ablation, not a pacemaker. Pacemakers treat slow rhythms.

Indications in children

Neonatal lupus and heart block in brief

• Conduction defects can be seen on fetal echocardiography after 16 weeks; newborn may have a slow heart rate, facial rash, low platelets or hepatitis.
• Monitor antibody-positive mothers with regular fetal echocardiography from 16 weeks; maternal hydroxychloroquine may protect against congenital heart block.
• After birth, conduction defects are treated by cardiac pacing; with pacing, outlook is excellent (if there is no cardiomyopathy).
• The rash, low counts and hepatitis settle; the heart block does not.
• Without proper management, children develop exercise intolerance and arrhythmias that can be fatal (Akbar).

Quick revision

• SVT: narrow QRS, regular, 150-300/min, fixed RR, P absent or after QRS.
• Infants: pallor, poor feeding, irritability; heart failure if prolonged.
• Stable: vagal manoeuvres (ice to face) then adenosine rapid IV push with flush.
• Unstable: synchronised DC cardioversion 0.5-2 J/kg immediately.
• Verapamil not under 1 year; digoxin not in WPW.
• Prevention in infancy: digoxin or propranolol for 6-12 months; ablation if drugs fail.
• Pacemaker: congenital complete heart block (neonatal lupus), post-surgical block, symptomatic bradycardia.

Written by The House Job from standard references (Basis of Paediatrics by Pervez Akbar; Nelson Essentials of Paediatrics).

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