paediatric supraventricular tachycardias (SVT)

introduction

70-90% of SVT in infants is AVRT and many infant SVTs are due to presence of accessory pathways which close after age 2 yrs as the heart develops
5% of infant SVT are broad complex - BBB or accessory pathways such as WPW
95% of broad complex tachycardias in infants are SVT not ventricular tachycardia (VT)
most have structurally normal hearts
infants with SVT are at risk of cardiac failure: 20% risk if 24-36hrs duration, >50% risk if longer than 48 hrs duration

Mx of the infant with presumed SVT

remember >95% of broad complex tachy in infants is SVT so adenosine is generally a safer option than attempting sedation and DC reversion even if broad complex unless ECG is strongly suggestive of VT or child requires ventilatory support or is in shock
be ready for hypotension, hypoxia, and rarely asystole or VF
verapamil (Isoptin) is C/I in infants as case reports of deaths

move to resus area
cardiac monitor, ECG consider placing R arm lead next to V1
BP and SaO2 monitoring
if stable, attempt physiologic reversion via increased venous return:
- lie with head down and legs up
- 10degC ice water mask or bag applied to face for 30secs
- older kids can blow on sphymomanometer tubing aiming for 40mmHg for 15secs
IV or IO access
if shocked then immediate DC reversion 1J/kg sync and sedate if possible awareness:
- iv midazolam or diazepam
if stable, rapid iv bolus adenosine followed by flush
- should be given only by experienced doctors
- only 35% revert at 0.1mg/kg so many prefer starting at 0.2mg/kg iv or perhaps 0.4mg/kg if via i/o or a foot vein
- hold arm with iv above head for higher efficacy
- use 3-way tap and rapid bolus followed rapidly by 10mL saline flush
- possible outcomes of iv adenosine
  - Reversion and well
  - no response - check the 5 D's:
    - dose - increase dose to max. O.5mg/kg or 18mg whichever is lower
    - delivery - ensure rapid delivery to heart
    - diagnosis - not SVT
  - reversion but reverts back to SVT:
    - check for atrial focus
    - might be incessant re-entry tachycardia (usually HR 180)
    - repeating adenosine won't be useful
    - change drug to either reduce automaticity or rate control:
      - eg. beta adrenergic blockers
  - unmasks an atrial tachycardia such as atrial flutter or atrial focus as evidenced by presence of non-conducting P waves on the transient rhythm strip:
    - if flutter: only need to convert to 2:1 block instead of 1:1 thus try either amiodarone or beta adrenergic blockers
  - reversion but unmasks CCF with fall in BP and SaO2:
    - may need ventilatory support plus inotropes such as dobutamine
  - asystole, VF or VT (rare but need to be prepared for it)
ECG post reversion