INTRAVENOUS ADENOSINE REVEALS INTERMITTENT PREEXCITATION BY DIRECT AND INDIRECT EFFECTS ON ACCESSORY PATHWAY CONDUCTION

In some patients with accessory pathways preexcitation occurs intermit tently during sinus rhythm. In these patients the antegrade refractory period of the accessory pathway may either exceed the sinus cycle len gth under some circumstances, or conduction block in the accessory pat hway may be variable. The ability of intravenous adenosine to unmask i ntermittent preexcitation was determined in patients with intermittent preexcitation but absent preexcitation at the time of study. Six pati ents undergoing assessment of the Wolff-Parkinson-White syndrome recei ved incremental doses of intravenous adenosine (3, 6, and 12 mg). Aden osine administration was repeated in three patients after intravenous beta blockade (propranolol 0.2 mg/kg). Adenosine unmasked preexcitatio n in all patients. P delta intervals with preexcited beats were substa ntially shorter than resting PR intervals in all cases (range 40-80 ms ec shorter). In 4/6 patients preexcitation was seen early, coincident with the onset of atrioventricular nodal block. In 416 patients preexc itation was seen late during the secondary sinus tachycardia that foll ows the direct cardiac effects of adenosine. Two patients exhibited ea rly preexcitation and late preexcitation. Beta blockade failed to prev ent early preexcitation (212 patients) but abolished preexcitation rel ated to sinus tachycardia (313 patients). Early preexcitation, coincid ent with the onset of AV nodal block, suggests a direct effect of aden osine on accessory pathway conduction. Late preexcitation, occurring d uring secondary sinus tachycardia, and abolished by beta blockade, sug gests enhanced accessory pathway conduction due to sympathetic activat ion.