Case 3
A 24-year-old female having manifest preexcitation presented with
frequent but short episodes of palpitation without ECG documentation.
Extrastimuli from high right atrium (HRA) revealed maximal preexcitation
suggesting a right anterolateral pathway, and dual AV nodal phenomenon.
Retrograde conduction over the AP was also confirmed. However, the
tachycardia could not be initiated from either RV or HRA. But stimulus
from left atrium was able to readily induce the tachycardia by
conducting solely over the slow pathway of AV node and causing
concomitant functional right bundle branch block (RBBB), with following
beats showing A-H interval identical to the last paced beat, indicating
perpetuated slow pathway conduction (Figure 3, left panel).
His-refractory PVC during tachycardia confirmed the mechanism of ORT.
When isoproterenol (8mcg/min) was given during tachycardia , fast
pathway and right bundle branch conduction were both improved, followed
by a paradoxically longer tachycardia cycle length (TCL) owing to marked
AV prolongation (Figure 3, right panel), indicating anterograde limb
again switched to the slow pathway. The narrow tachycardia was neither
sustainable nor re-inducible when high-dose isoproterenol was
discontinued. The pathway was eliminated at 11 o’clock on tricuspid
annulus with the help of a deflectable sheath. After AP ablation, slow
pathway conduction remained without echo beat or inducible
atrioventricular nodal reentrant tachycardia. The patient had no
symptoms at 2-year follow-up.