• Supraventricular tachycardia (SVT) is the most common pathological tachycardia in newborns – can be new presentation or commenced in fetal life

• Sustained, accelerated non-sinus rhythm, regular and narrow-complex, originating above the level of the atrioventricular (AV) junction
• Heart rate >200 bpm 
• May be 1 of 3 tachycardias:
  
  • atrial 
  • atrioventricular nodal re-entry (AVNRT) 
  • atrioventricular re-entrant (AVRT) – most common SVT in fetal and neonatal life 
• Can be presenting feature of a congenital heart defect – but do not wait to exclude this before commencing treatment 

• Can be variable with some common presentations:
  
  • acute onset in a baby in heart failure/shock with no previous signs and symptoms
  • fetal tachycardia during pregnancy
  • baby with irritability, poor feeding, sweating and breathlessness for hours/days before presentation
• SVT can cause reduced cardiac output due to reduced diastolic filling time 
  
  • many babies tolerate SVT well, however if tachycardia is sustained for >6 hr signs of congestive heart failure may develop, with irritability, tachypnoea and pallor

• No known cause in majority of babies
• Idiopathic SVT is more common in neonates than older children
• Wolf-Parkinson-White pre-excitation – only becomes visible after conversion to sinus rhythm
• Congenital heart defect, including Ebstein’s and TGA

• Co-existing infections e.g. LRTI
• Manage all triggers appropriately 

• Heart rate: >200 bpm 
• Capillary refill
• Blood pressure
• Respiratory rate, may be normal/abnormal depending on:
  
  • signs of heart failure
  • co-existing respiratory conditions
  • infections
• SpO₂ may be normal, low, or of poor signal in haemodynamic compromise
• Cardiovascular and respiratory examination; may be normal aside from fast heart rate
• Examine baby for other reasons of tachycardia, including pain and environmental factors e.g. pyrexia (particularly in premature baby in incubator)

• 12-lead ECG to confirm SVT diagnosis in haemodynamically stable cases
  
  • if baby haemodynamically unstable, or if ECG not available, defibrillator can record and print rhythm strips from 3 different leads 
• Once SVT terminated, perform repeat ECG to assist with identification of pre-excitation and any other underlying rhythm abnormality
• Blood gas for:
  
  • acid-base balance
  • electrolytes
  • ionised calcium
• Echocardiogram to assess structural anatomy and cardiac function

If haemodynamic compromise:

If no haemodynamic compromise:

Adenosine

• Give via cannula into large vein in upper limb, followed by rapid sodium chloride 0.9% flush; very short half-life of 10–30 sec – must get to the heart as quickly as possible  
• Acts by slowing conduction time through the AV node
• Intraosseous access ineffective due to time taken for venous return 
• Use 3-way tap with Luer-lock syringes; 1 syringe for adenosine and 1 for sodium chloride 0.9% flush

• Keep defibrillator nearby
• Capture and print rhythm, while adenosine given, via defibrillator rhythm strip or ECG recording
• Starting dose 100 microgram/kg, repeat after 2 min, if no effect increase to maximum dose of 300 microgram/kg as in flowchart above
  
  • if experienced clinician present, maximum dose 500 microgram/kg

Vagal manoeuvres

• Cold stimulation of the trigeminal nerve (afferent branches) leads stimulation of the vagal nerve (efferent branches); slows AV node conduction
  
  • wrap bag of ice in towel and apply to baby’s face or
  • wrap baby in towel and immerse entire head in ice-cold water for 5 sec
• Unilateral carotid massage not recommended – difficult to perform in neonates and has limited effect

DC cardioversion

• Applies direct current of electricity to the heart, synchronised to R wave of QRS complex on ECG
  
  • less risk of inducing ventricular fibrillation than unsynchronised
• Ideally carry out under general anaesthetic, or at least sedation
• If performed outside NNU, will require anaesthetic support 
• Synchronised shock starting at 1 J/kg, if no response increase to 2 J/kg as in flowchart above

Chemical cardioversion

• Discuss with paediatric cardiology if:
  
  • haemodynamically unstable and unresponsive to adenosine IV or DC cardioversion 
  • haemodynamically stable and unresponsive to adenosine IV
• If SVT occurred in-utero consult perinatal plan and discuss with paediatric cardiology

Prophylactic medication

• When SVT has terminated, it is vital to commence medication to prevent further episodes
• Choice of prophylactic medication based on:
  
  • previous history of SVT (including in fetal life)
  • assessment of ECG, both in SVT and once terminated
  • cardiac function 
• Discuss with paediatric cardiology centre and send ECG/echocardiogram for review

• Any episode of SVT – follow-up with paediatrician with expertise in cardiology/paediatric cardiologist 
• Arrange:
  
  • baseline echocardiogram in outpatient clinic (if not already done)
  • Holter monitor