Electrophysiological studies in a patient with paroxysmal supraventricular tachycardia disclosed anterograde dual atrioventricular nodal pathways, and a concealed left-sided anomalous atrioventricular pathway which was used as the retrograde limb of two circus movement tachycardias. Tachycardia No.1, reflecting anterograde fast pathway conduction, was characterised by functional left bundle-branch block and a stable cycle length of 330 ms. Paroxysmal loss of bundle-branch block resulted in tachycardia No.2, which reflected anterograde slow pathway conduction, and was characterised by narrow QRS and a stable cycle length of 355 ms. Tachycardia No.2 had a longer cycle length than tachycardia No.1 because the increment in AH interval (slow pathway instead of fast pathway condition) more than compensated for the decrement in ventriculoatrial interval (narrow QRS instead of bundle-branch block).
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