Activity of the cardiac pacemaker of the frog in relation to atrial fibrillation and to other atrial arrhythmias

¹ Cardiovascular Division, Department of Medicine, Washington University School of Medicine, St. Louis, Missouri

Observations of the activity of the sinus venosus of the frog heart indicate that function of the sinus is regular and sustained during suppression of the atria and ventricle by optimum amounts of acetylcholine. When atrial fibrillation was induced by acetylcholine injection and concurrent atrial distention, the sinus venosus also beat regularly and independently, neither affecting nor being affected by the adjacent fibrillating atria. In a number of observations, sinus impulses appeared to initiate momentary fibrillation in localized atrial areas, leading eventually to perpetual fibrillation of the atria. Furthermore, in occasional hearts, regularly occurring waves of atrial transmission passed through the sino-atrial junction to invoke sinus contractions in regularity with the ectopic focus. These phenomena may be related to the sino-atrial junctional tissue, which prevents transgression of impulses from the fibrillating atria so that sinus rhythm is preserved; however, under certain conditions, the junctional zone will permit impulses to pass from atrium to sinus venosus as an A-S sequence.