ORTHODROMIC CAPTURE OF THE ATRIAL ELECTROGRAM DURING TRANSIENT ENTRAINMENT OF ATRIOVENTRICULAR NODAL REENTRANT TACHYCARDIA

Background. The reentry circuit of atrioventricular nodal reentrant ta chycardia (AVNRT) has not been fully demonstrated. We hypothesized tha t if an upper common pathway was present, the atrial electrogram could not be captured orthodromically during transient entrainment of AVNRT by rapid atrial pacing. Based on this hypothesis, the presence of an upper common pathway was investigated. Methods and Results. The atrial electrogram at the recording site of the His bundle potential was ide ntified during induced AVNRT in 9 patients. To entrain AVNRT transient ly, rapid pacing from the high right atrium and coronary sinus was app lied at a cycle length 10 milliseconds shorter than that of AVNRT and repeated after a decrement of the paced cycle length in steps of 5 mil liseconds until AVNRT was interrupted. In 5 of 7 patients, orthodromic capture of the atrial electrogram at the recording site of the His bu ndle potential was observed during transient entrainment of AVNRT by c oronary sinus pacing, ie, the first postpacing interval of the atrial electrogram at the recording site of the His bundle potential was the same as the paced cycle length. In these 5 patients, the mean minimum paced cycle length capable of orthodromic atrial capture was 349 milli seconds, and the mean difference from the cycle length of AVNRT was on ly 16 milliseconds. During transient entrainment of AVNRT by high righ t atrial pacing, the atrial electrogram could not be captured orthodro mically. Conclusions. Observation of orthodromic capture of the atrial electrogram at the recording site of the His bundle potential by coro nary sinus pacing ruled out the presence of an upper common pathway in AVNRT, and the concept that perinodal atrial tissue is involved in th e reentry circuit of AVNRT was supported.