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.