Sinus Node Impulses and Atrial Fibrillation

In atrial muscle strips from rabbits, fibrillation occurred in a solution low in potassium, with or without a high calcium content, and the role of the sinus node in inducing atrial fibrillation was examined by microelectrodes and by direct close bipolar lead electrocardiograms. In right atrial strips that included the sinus node, the sinus node action potential often just preceded spontaneous fibrillation or premature beats. In left atrial strips in a similar condition, the incidence of spontaneous fibrillation or premature beats was very low. However, when electrical stimulation simulating sinus node impulses was applied, fibrillation or the premature beat frequently appeared immediately after the stimulating pulse waves, and its incidence was also remarkably increased, being fairly close to that of the right atrial strip. These findings suggest that the sinus node impulses occasionally caused fibrillation or premature beats during these experiments in a medium low in potassium. In addition, fibrillation was frequently induced even when the impulse fell outside the vulnerable period of the preceding beat.