AVRT, the most common type of fetal SVT, may demonstrate sudden onset (paroxysmal) and termination of tachycardia. AVRT is a reentrant or reciprocating (the terms can be used interchangeably) tachycardia involving the atria, ventricles, AV node, and an accessory connection.14
AVRT has a fixed 1:1 relationship between atrial and ventricular contraction with rates of 210 to 320 bpm. Typically, there is little heart rate variability. Onset is most commonly between 18- and 32-weeks gestation. The typical form of AVRT involves ventricular activation occurring through the normal conduction system and atrial re-activation initiated via an accessory AV connection. The accessory connection completes a reentrant circuit, and reciprocating antegrade-retrograde conduction perpetuates the circuit.13
Characteristically, the VA interval is shorter than the AV interval (“short VA” tachycardia). Thus, the VA interval is less than half of the cardiac cycle length and the VA/AV ratio is <1. The tachycardia rate is related to the relative
conduction properties of the accessory connection and the AV node. Tachycardia onset is triggered by an initiating event, usually a PAC,15
and begins abruptly. There may be repeated self-limited episodes or tachycardia may remain incessant. If either the AV node or the accessory connection fails to transmit a single impulse, reentry terminates, which is the reason that antiarrhythmic agents that slow conduction across the AV node often successfully terminate reentrant SVT.