Incidence, determinants and significance of fixed retrograde conduction in the region of the atrioventricular node. Evidence for retrograde atrioventricular nodal bypass tracts.

Of 104 consecutive patients studied in our laboratory with His bundle electrograms, atrial and ventricular pacing and the atrial and ventricular extrastimulus techniques, 18 patients in whom the existence and utilization of ventriculoatrial (V-A) bypass tracts were excluded demonstrated evidence for fixed and rapid retrograde conduction in the region of the atrioventricular node (A-V) as suggested by the following: (1) short (36 +/- 2 msec [mean +/- standard error of mean]) and constant retrograde H2-A2 intervals during retrograde refractory period studies; (2) significantly (P less than 0.025) better V-A than A-V conduction; (3) significantly (P less than 0.025) shorter retrograde functional refractory period of the V-A conducting system than of the A-V conduction system; and (4) the retrograde effective refractory period of the A=V nodal region was not attainable in any of the 18 patients. Fourteen of the 18 patients (77 percent) had a history of palpitations and 10 (51 percent) had documented paroxysmal supraventricular tachycardia; in 13 (72 percent) single echoes or sustained reentrant supraventricular tachycardia, or both, could be induced during atrial pacing or atrial premature stimulation studies, or both. During tachycardia all these 13 patients had a short (37 +/- 2.4 msec) and constant conduction time in the retrograde limb (H-Ae interval) of the reentrant circuit that was identical to the H2-A2 interval. In conclusion, fixed and rapid retrograde conduction in the region of the A-V node (1) is seen in approximately 17 percent of patients, (2) is associated with a large incidence of reentrant paroxysmal supraventricular tachycardia, and (3) suggests the presence of A-V nodal bypass tracts (intranodal or extranodal functioning in retrograde manner).