Sino-atrial reciprocation in the isolated rabbit heart.

Reciprocal beats between sinus node and atrium were studied in isolated rabbit heart tissue by microelectrode techniques. Early premature atrial responses which entered the sinus node frequently emerged to re-excite the atrium as "echoes." When the premature atrial beats discharged some but not all nodal areas, the atrial echoes, in turn, appeared to re-enter the sinus node. Repetitive reciprocation involving the sino-atrial circuit was occasionally observed. Anatomic and temporal patterns of excitation at multiple recording sites suggest that "entrance" block and slow conduction of premature impulses within the sinus node may explain the observed reciprocation. The results support the hypothesis that self-sustained reciprocation in a sino-atrial circuit might be responsible for some cases of paroxysmal atrial tachycardia. ADDITIONAL