Mechanism of the origin of the first extrasystole in short-lived atrial arrhythmia

Using the mapping method (at 80-150 points) on strips of the rabbit left atrium, the authors have studied the spread of excitation in arrhythmias induced by electrical stimulation in the vulnerable phase. The arrhythmias were initiated in normal conditions and against a background of acetylcholine (0.2 mg/l.). The electrical activity was recorded with surface electrodes. It was shown that the first extrasystolic wave, as a rule, is due to the appearance of one or more point or ring-like sources of excitation on the surface of the myocardium. Continuous circulation of the excitation wave over the surface of the heart tissue is absent. The possible mechanisms of the origin of such sources are discussed. Circulations of excitation deep in the myocardium (intramural return) are regarded as the most probable mechanism.