Background Supraventricular tachycardia (SVT) represents the most frequent tachyarrhythmia requiring medical treatment in infants with no underlying cardiac disease. The incidence of SVT in neonates is currently estimated at 1 in 200 to 250².

Aim To study the clinical presentation, treatment and outcomes of narrow complex cardiac arrhythmias in infants. Methods A retrospective observational study included all children under the age of 1 at diagnosis of narrow complex tachyarrhythmia between Jan 2007 and Nov 2015 in Cambridge. Patients were identified from paediatric and neonatal electronic databases: BadgerNet, eMR and EPIC (electronic patient record system).

Results 44 infants diagnosed with narrow complex arrhythmias were identified, of which 35 (81%) were diagnosed with SVT. Other types of supraventricular arrhythmias included atrial flutter (4) and ectopic atrial tachycardia (4). 11 infants were excluded from this study due to transfer to other hospitals (6), death from unrelated causes (4) or lost to follow up (1).

10 infants (30%) were diagnosed antenatally, 7 infants (22%) presented within the first 7 days after birth, 9 (27%) between 1 and 4 weeks of age and the remaining 7 infants (21%) presented after 1 month of age.

8 infants had underlying significant congenital heart disease including Ebstein anomaly (1), congenitally corrected TGA (1), TGA (1), hypoplastic arch and coarctation (2) and large ASD (3).

Five patients with narrow complex tachycardias (12%) did not require maintenance therapy. Two cases of atrial flutter needed DC cardioversion.

Of the 26 infants diagnosed with narrow complex tachycardia who required maintenance therapy, 17 infants (65%) have now successfully completed therapy, whilst 9 (35%) remain on maintenance therapy. 4 (24%) cases had recurrence of SVT requiring reinstatement of treatment.

Average length of therapy was 13.6 months (range 4months to 5 years), with 31% requiring polytherapy. Drugs most commonly prescribed were propanolol, amiodarone and flecainide.

Conclusion Infants with narrow complex arrhythmias have a good prognosis without significant morbidities and most of the infants in our study had maintenance therapy for 6–12 months, although a few difficult cases required medication for longer duration. Around one fourth infants with narrow complex arrhythmia had underlying congenital heart disease.