Supraventricular Tachycardia (SVT) in Children
Supraventricular Tachycardia (SVT) is a type of arrhythmia that is the most common regular tachycardia in children. It is characterised by a very fast heart rate that originates from or above the ventricles. In infants, the heart rate is typically over 220 beats/minute, and in children, it’s over 180 beats/minute.
Pathophysiology and Causes
SVT is typically caused by a re-entry circuit that involves the atrioventricular (AV) node. This circuit creates a rapid, self-sustaining electrical signal. The majority of cases have no identifiable cause, but some are associated with:
Wolff-Parkinson-White (WPW) Syndrome: This is a key cause of SVT and involves an extra electrical pathway between the atria and the ventricles. On an ECG, this appears as a delta wave (a slurred upstroke of the QRS complex) and a short PR interval.
Structural Heart Disease: Children with congenital heart defects are at an increased risk of developing SVT.
Post-Cardiac Surgery: SVT can be a complication following cardiac surgery.
Clinical Presentation
The clinical presentation of SVT depends on the child’s age, heart rate, and whether they are haemodynamically stable.
Infants: Due to their small cardiac reserve, infants can quickly become unstable. They may present with non-specific signs like irritability, poor feeding, lethargy, or signs of heart failure (tachypnoea, hepatomegaly).
Older Children: Older children may be more able to describe their symptoms, which can include:
Palpitations or a sensation of a racing heart.
Chest discomfort or pain.
Dizziness or lightheadedness.
Syncope (fainting).
On examination, a child who is still haemodynamically compensated may appear well despite a very high heart rate. However, if uncompensated, they will show signs of shock, such as low blood pressure, poor peripheral perfusion (prolonged capillary refill time), and an increased respiratory rate.
Diagnosis
The diagnosis of SVT is primarily made with a 12-lead ECG, which will show a regular, narrow-complex tachycardia.
Management
Management is based on the child’s haemodynamic status. The first step for any unwell child is to follow the ABCDE approach (Airway, Breathing, Circulation, Disability, Exposure).
Haemodynamically Stable (Child is well-perfused):
Vagal Manoeuvres: These are the first-line treatment. They work by stimulating the vagus nerve to slow conduction through the AV node. In older children, this can include asking them to blow into a syringe. In infants, facial immersion in ice-cold water (ensure the airway is protected) can be effective.
Adenosine: If vagal manoeuvres are unsuccessful, intravenous (IV) adenosine should be administered rapidly via a large-bore cannula. Adenosine has a very short half-life and works by transiently blocking the AV node to break the re-entry circuit.
Adenosine Administration: Due to its rapid half-life, adenosine should be given via a central line or a large-bore cannula in a large vein, followed immediately by a rapid saline flush to ensure it reaches the heart.
Haemodynamically Unstable (Signs of shock or heart failure):
The child requires urgent cardioversion. This is a life-saving measure.
Synchronised Cardioversion: Deliver a synchronised electrical shock at a low dose of 0.5 J/kg initially. If unsuccessful, the dose can be increased.
Cardioversion: Synchronised cardioversion is a critical procedure that should be performed in a monitored setting by trained personnel.
Further Management
Once the child is stabilised, a referral to a paediatric cardiologist is essential. They will investigate the underlying cause, especially to rule out WPW syndrome and structural heart disease. They may also consider long-term management with medications to prevent recurrence.