SVT

Supraventricular tachycardia – where abnormal rhythm initiated above atrioventricular node, so narrow complex (with normal conduction down bundles of His). Typically rate over 220, with abrupt onset (can be inferred from history).

ECG showing SVT and effect of adenosine
SVT showing effect of adenosine

Accessory pathway that bypasses the AVN is the usual reason, esp infants, but nodal reentrants quite common in older kids. Get a 12 lead before doing anything, unless absolutely impossible.

An accessory pathway usually has a delta wave (upsloping PR). If retrograde conduction, then inverted P wave seen immediately after the QRS. If a nodal reentrant, P wave is subsumed into QRS complex.

If decompensated then synchronised cardioversion (under IM/IN ketamine).

Vagal manoeuvres do work, such as doing a valsalva manoeuvre (get child to blow through straw!), bag of icy water held to face (count to 5). Older kids can stand on their hands! Carotid sinus massage is the least effective.

But best to go straight to IV adenosine if access available. Adenosine causes sinus arrest, which feels terrifying for child. Within a few seconds, wears off and spontaneous return of rhythm, hopefully a normal one. Can also induce wheeze. Give in antecubital fossa, so as close to heart as possible.

Starting dose lower if over 1yr. Then increase by 50-100mcg/kg each time. If adenosine doesn’t work:

  • check max dose (different for neonates and older children, 12mg max)
  • check rapid bolus and flush
  • check diagnosis is correct.

Digoxin is good for babies and infants but needs 2 loading doses 6 hours apart, and you may not get a result until then. Propanolol is contraindicated in asthma. Amiodarone should be used before 3rd shock. Verapamil is contraindicated under 1 year as can cause arrest.

Prognosis

SVT usually settles down by 6-12 months but may recur around the age of 8yrs. Onset outside infancy will probably not resolve spontaneously. Prevention needs to be discussed: some families will be happy just treating episodes as they occur. Otherwise use digoxin in infancy to prevent, propanolol later.

Rare forms

Rare forms:

  • Permanent form of Junctional Reciprocating Tachycardia (PFJRT) – chronic SVT, so may go unrecognised and then cause cardiomyopathy. The accessory pathway is resistant to DC shock and adenosine. Inverted P wave seen.
  • Ectopic atrial tachycardia – due to an ectopic focus. May also be chronic. Demonstrates warm up and slow down in rate, whereas others tend to have fixed rate. First degree block seen. Resistant to DC shock and adenosine.
  • Atrial flutter usually seen only in congenital heart disease but does occur in neonates with normal hearts! In which case it tends not to recur. Ventricular rate divides neatly into 300.
  • Atrial fibrillation – needs anticoagulation before DC conversion so control rate first with digoxin while loading warfarin.

Leave a Reply

Your email address will not be published. Required fields are marked *