The sequence of retrograde atrial activation of premature ventricular beats was studied in 12 patients without pre-excitation syndrome. Forward and retrograde AV nodal conduction times were within normal limits. The septal low right atrial (LRA2) deflection was inscribed 25 to 55 ms (ave: 37.9 ms) before the low left atrial (LLA2) electrogram and 35 to 75 ms (ave: 53.7 ms) ahead of the high right atrial (HRA2) deflection. LLA2 preceded HRA2 in 8 patients, more or less coincided with HRA2 in 3 cases, and was inscribed 10 ms ahead in 1 patient. In all cases a decrease in the coupling (St1-St2) interval resulted in a proportional increase of the septal LRA1-LRA2, LLA1-LLA2, and HRA1-HRA2 intervals. Atrial echoes occurring in 3 cases had a sequence of arrival of excitation at the recording sites which was similar to the ones shown by the same patients during ventricular pacing. This is in keeping with the existence of an upper common pathway located above the area where functional longitudinal dissociation occurred. Thus, it is concluded that impulses emerging from the AV node in a retrograde direction reach the septal low right atrium before they activate the explored low left atrial sites, regardless as to whether they arise in the ventricles or in the atria.
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