Concealed antidromic re-entrance during rapid atrial pacing in the Wolff-Parkinson-White syndrome.

In a patient with Wolff-Parkinson-White syndrome, rapid atrial pacing up to rates of 140/min resulted in A-H prolongation; increases in rate up to 200/min, however, failed to lengthen A-H further. The delta-H interval increased as rate rose to 120/min, but remained stable thereafter. These and other findings suggest retrograde activation of the His bundle via the accessory pathway at rates greater than 140/min. Such a phenomenon: 1) prevents initiation of PSVT with atrial pacing; 2) prevents one from determining the anterograde block rate of the AV node; 3) represents concealed antidromic re-entrance, which may theoretically lead to antidromic re-entrant PSVT.