[Changes in the rate of sinoatrial node after high-frequency stimulation of the heart atrium]

Variations in the sinus node rate of the frog heart isolated together with the atria were studied during suprathreshold atrial stimulation at a frequency exceeding the intrinsic sinus node rate. If the stimulation frequency surpassed the intrinsic sinus node rate by 20%, the sinus node rate became equal to the pacing rate. Provided the pacing rate was increased, the sinus node rhythm became irregular. If the pacing rate exceeded the sinus node rate more than two-fold, the latter became half the pacing rate. Variations in the sinus node rate during suprathreshold atrial stimulation were also observed after blockade of possible retrograde conduction by application of the necrotic tissue on the sinoatrial border. The results allow the conclusion that variations in the sinus node rate seen during high-frequency suprathreshold atrial stimulation are not connected with retrograde conduction.