Fatigue Phenomenon in APs. Fatigue phenomenon is transient failure of conduction following a period of repetitive excitation. Fatigue in iiccessory pathways is uncommon, and its electrophysiologic characteristics and clinical implications are unknown. Among the 215 patients who underwent electrophysiology studies from July 1992 to December 1993, 4 (2) were found to exhibit fatigue over accessory pathways. The accessory pathway was posteroseptal in three patients and right free wall in one patient. The mean anterograde effective refractory period of the accessory pathway was 295 ± 26 msec (range 270 to 330, basic drive cycle length 600 msec). Three patients had neither retrograde accessory pathway conduction nor inducible tachycardia even with infusion of isoprotereno). The fatigue phenomenon was observed after both atrial and ventricular stimulation in three patients and only after ventricular stimulation in one patient. Fatigue was dependent on duration more than rate of stimulation. We conclude that pathways exhibiting fatigue have a low margin of safety for conduction and are unlikely to be clinically problematic
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