When confronted with a tachycardia in a pediatric patient, one will most likely encounter supra ventricular tachycardia (SVT). There are different mechanisms enabling SVT. Of these, reentry tachycardia (i.e. a circulating wavefront) involving atria and ventricles is most common. Up to 90% of SVT in children are reentry based. The majority of these involves an accessory pathway (up to 80%), connecting atria and ventricles and thereby enabling a reentry circuit.
An accessory pathway (AP) is a connection of tissue that allows conduction (antegrade, retrograde or bidirectional) via the atrio-ventricular valve level, that exists additionally to the AV node and bundle of His. APs may have different conduction properties, eventually allowing for rapid conduction (i.e. an effective refractory period of 250ms or less) and fast tachycardia rates. Locations of APs vary. They may be located around the mitral annulus, the tricuspid annulus, or in proximity to the intertribal septum and AVN-His system.
Antegradely conduction APs can be spotted on the 12-lead ECG, as pre excitation of the ventricles happens via the AP. The ventricular myocardium that is depolarized earlier produces a "Delta wave" on the ECG. The sinusbeat is conducted via the AV node which slows conduction (i.e. PR interval). At the same time conduction facilitates via the AP, which does not slow conduction. This results in collision of the wavefronts via the AP and the AVN and produces a fusion of the QRS complex - the delta wave. Also, the PR (or PQ) interval is shortened. This is due to the fact that APs usually do not slow conduction, while the AV node does. The slowing of conduction caused by the AVN (i.e. the PR interval) disappears with AP conduction. The combination of a short PR and a delta wave is called preexcitation on the ECG and is highly suspicious of an AP present.
When an AP is present, this sets the stage for atrio- ventricular reentry tachycardia (AVRT). When an AP is capable of retrograde conduction, an impulse (e.g. from an ectopic beat) may travel from the atria to the ventricles via the AVN. There might be no AP conduction because the AP was still refractory when the impulse of the ectopic beat advanced. The impulse reaches the ventricular myocardium via the AVN only and travels throughout the ventricles, finally reaching the AV valve level. Now, however, the AP is not refractory anymore and the impulse may advance back to the atria, thereby completing the reentry circuit.
ORT
This kind of AVRT using a retrograde conducting AP is called orthodromic reciprocating tachycardia (ORT). If the AP is capable of integrate conduction only (which will give you a pre excitation pattern in the ECG), the reentry circuit may eventually go the other way around. Thus advancing from the atria to the ventricles via the AP and closing the reentry circuit via retrograde AVN conduction. This is called antidromic reentry tachycardia (ART).
ART; QRS pre excited during tachycardia.
One potentially lethal condition (even though rare in childhood) that may appear in patients with rapidly integrate conducting APs is FBI tachycardia. In case of atrial fibrillation (rare in children, however a possible side effect of adenosine application!), rapid atrial rates may be conducted to the ventricles in a 1:1 fashion via the AP. These tachycardias appear Fast (F), Broad (B) and Irregular (I), and should be treated quickly (e.g. electrical cardioversion) as children usually do not tolerate these kinds of rhythm quite well.
FBI tachycardia in a 15 y/o male.