Sinoatrial responses to premature atrial stimulation: clinical observations and experimental study.

Two clinically-observed patterns in the response curve relating coupling intervals of premature atrial stimulation to return cycles were experimentally reproduced and studied by microelectrode techniques in the sinoatrial preparations of rabbit hearts. The progressive prolongation pattern of the return cycles with shortening of coupling intervals was associated with progressive depressions both of atriosinus conduction and sinus node automaticity. The progressive shortening pattern of the return cycles was associated with progressive shortening in the duration of sinus node action potential in two-thirds of the preparations and with pacemaker shift in the remaining one-third of them. Possible "sinus node reentry" was demonstrated. The mechanisms and clinical implications of these phenomena were discussed.