Supraventricular Tachycardia (SVT)

Why is this topic important?

Supraventricular Tachycardia (SVT) is the most common symptomatic arrhythmia in childhood and one of the highest-yield cardiology topics in MRCPCH examinations.

Candidates are expected to:

Recognise SVT in infants and children
Interpret ECG findings
Understand the underlying mechanisms
Differentiate SVT from sinus tachycardia
Manage stable and unstable patients
Understand the role of vagal manoeuvres and adenosine

SVT frequently appears in MRCPCH questions involving infants with poor feeding, pallor, heart failure or an unexpectedly high heart rate.

Definition

Supraventricular Tachycardia (SVT) is a rapid heart rhythm originating above the ventricles, usually involving an abnormal re-entry circuit.

It is characterised by:

Sudden onset
Sudden termination
Narrow complex tachycardia (usually)
Very rapid heart rate

Key MRCPCH Facts

SVT is the commonest symptomatic arrhythmia in childhood.
Infants may present with heart failure.
Heart rates are usually:
- 220 beats/min in infants
- 180 beats/min in older children
P waves are often absent or difficult to identify.
Vagal manoeuvres are first-line treatment in stable patients.
Adenosine is the first-line drug treatment for stable SVT.
DC cardioversion is required if the child is haemodynamically unstable.

Cardiac Conduction System

Normal Pathway

Electrical impulses travel:

SA Node → AV Node → Bundle of His → Bundle Branches → Ventricles

This produces coordinated cardiac contraction.

Pathophysiology

Re-Entry Mechanism

Most childhood SVTs occur due to a re-entry circuit.

An abnormal pathway allows electrical impulses to circulate repeatedly.

This results in:

Extremely rapid heart rates
Reduced ventricular filling time
Reduced cardiac output

Common Types of SVT

Atrioventricular Re-Entry Tachycardia (AVRT)

Most common in infancy.

Associated with:

Accessory pathways
Wolff-Parkinson-White (WPW) syndrome

Atrioventricular Nodal Re-Entry Tachycardia (AVNRT)

More common in older children and adolescents.

Atrial Tachycardia

Less common.

Wolff-Parkinson-White (WPW) Syndrome

Important MRCPCH topic.

Features

Accessory pathway allows conduction outside the AV node.

ECG Findings

Short PR interval
Delta wave
Broad QRS complex

Patients are at increased risk of SVT.

Clinical Features

Presentation depends on age.

Infants

Symptoms are often non-specific.

Common Features

Poor feeding
Irritability
Pallor
Sweating
Tachypnoea

Delayed Presentation

May develop:

Heart failure
Hepatomegaly
Shock

Older Children

More likely to describe symptoms.

Symptoms

Palpitations
Dizziness
Chest discomfort
Breathlessness
Syncope (rare)

Examination Findings

Heart Rate

Usually:

Infant

220 beats/min

Older Child

180 beats/min

Other Findings

Pallor
Poor perfusion
Tachypnoea

Severe Cases

Heart failure
Hypotension
Reduced consciousness

SVT vs Sinus Tachycardia

This is a favourite MRCPCH examination topic.

Sinus Tachycardia

Usually caused by:

Fever
Dehydration
Pain
Sepsis

Features:

Gradual onset
Gradual resolution
Normal P waves

SVT

Features:

Sudden onset
Sudden termination
Very rapid fixed rate
Often absent P waves

Differential Diagnosis

Tachyarrhythmias

SVT
Atrial flutter
Ventricular tachycardia

Non-Cardiac Causes

Fever
Sepsis
Dehydration
Anxiety
Hyperthyroidism

Investigations

ECG

Most important investigation.

Typical Findings

Narrow complex tachycardia
Regular rhythm
Absent or difficult-to-see P waves

Between Episodes

ECG may be normal.

Look for:

WPW pattern
Delta waves

Echocardiography

Used to:

Exclude structural heart disease
Assess ventricular function

Holter Monitoring

Useful if episodes are intermittent.

Acute Management

Step 1: Assess Stability

Determine if the child is:

Stable

Normal blood pressure and perfusion.

Unstable

Features include:

Shock
Hypotension
Altered consciousness
Severe heart failure

Stable SVT

Vagal Manoeuvres

First-line treatment.

Examples:

Infants

Ice bag to face ("diving reflex")

Older Children

Valsalva manoeuvre

Adenosine

Drug of choice if vagal manoeuvres fail.

Benefits:

Rapid onset
Short half-life
Diagnostic and therapeutic

Important MRCPCH fact:

Adenosine is first-line drug treatment for stable SVT.

Unstable SVT

Immediate Synchronized DC Cardioversion

Required when:

Shock
Severe hypotension
Reduced consciousness
Cardiovascular collapse

This is an important MRCPCH emergency scenario.

Long-Term Management

Observation

Some infants outgrow SVT.

Antiarrhythmic Medication

Examples:

Propranolol
Flecainide
Sotalol

Catheter Ablation

May be considered in:

Recurrent symptomatic SVT
Older children
Drug-resistant cases

Success rates are high.

Complications

Short-Term

Heart failure
Cardiogenic shock
Poor feeding

Long-Term

Recurrent episodes
Reduced quality of life

Rare

Sudden cardiac death (usually associated with specific arrhythmias or WPW complications)

Common Exam Traps

Trap 1

SVT is the commonest symptomatic arrhythmia in childhood.

Trap 2

Infants often present with poor feeding rather than palpitations.

Trap 3

Heart rate is usually >220 bpm in infants.

Trap 4

Adenosine is first-line drug treatment for stable SVT.

Trap 5

Ice-to-face manoeuvre is appropriate in infants.

Trap 6

DC cardioversion is required if unstable.

Trap 7

WPW syndrome is associated with SVT.

Trap 8

SVT has sudden onset and termination.

One Minute Revision

Most common symptomatic arrhythmia in childhood.
Heart rate >220 bpm in infants.
Sudden onset and termination.
Narrow complex tachycardia.
Vagal manoeuvres first.
Adenosine = first-line drug.
Unstable patient = DC cardioversion.
WPW is an important associated condition.

Suggested References

Advanced Paediatric Life Support (APLS)
European Society of Cardiology Paediatric Arrhythmia Guidance
RCPCH Guidance
BNF for Children
Nelson Textbook of Pediatrics
Park's Pediatric Cardiology for Practitioners
Moss & Adams' Heart Disease in Infants, Children and Adolescents

Disclaimer

These notes are intended for MRCPCH revision and educational purposes only. They do not replace local, national or institutional guidelines. Clinical decisions should always be based on current guidance, senior advice and individual patient circumstances.

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