The duration of the monophasic action potential (APD), the interval between atrial beats and the delay time between excitation of the right and left atria of dogs were measured, using coaxial suction electrodes, before and during vagal stimulation.The effect of vagal stimulation on the degree of shortening of APD varied at different recording sites in the atrium, the shortening being more pronounced in the right than in the left atrium. Threshold and rate of recovery after cessation of stimulation varied throughout the atrium. The results indicate that the vagal effect is nonuniformly distributed over the atrium.The time difference between excitation of the right and left atria increased with vagal stimulation under the present experimental condition, suggesting that a reduction of synchrony of excitation occurred in the atrial myocardial fibers as a whole. The same conclusion was derived from analysis of the pattern of the P wave before and during vagal stimulation.
ADDITIONAL KEY WORDSvagal stimulation monophasic atrial action potential averaged P wave shift of pacemaker atrial fibrillation synchronization of atrial excitation coaxial suction electrode data-averaging computer right vs. left atrium• Studies on the distribution of vagal effects in the atrium provide important data regarding the frequency, conduction and recovery of myocardial excitation in normal and pathologic conditions such as tachycardia, flutter, and fibrillation. Measurement of the refractory period in various parts of the right atrium in the dog during vagal stimulation has shown that the vagal effect is nonuniformly distributed. 1 Such nonuniformity of vagal effects is assumed to be one of the possible causes of atrial fibrillation. Recently, the effect of vagal stimulation or acetylcholine on electrically driven or spontaneously active atrial fibers has been studied with intracellular electrodes 4 " 7 and also with From the Department of Physiology, School of Medicine, Hiroshima University, Kasumi-cho, Hiroshima, Japan.This work was supported in part by research grants from the Japanese Ministry of Education, from the U. S. Public Health Service (HE 06968-04), and U. S. Army Research and Development, Far East Office, to H. Irisawa, in whose laboratory the work was done.Accepted for publication May 9, 1966. monophasic records taken from the turtle 8 and dog 9 atrium by recording potentials between injured and normal tissue. These studies have demonstrated that the duration of the action potential is shortened dramatically during vagal stimulation. The duration of the monophasic action potential (APD) can be used as an index of the vagal effect on the myocardial fibers in various parts of the atrium. The purpose of the current experiments is to secure information on the distribution of vagal effect in the canine heart in situ.
MethodsTen mongrel dogs (6 to 10 kg) were used in this study. Each was anesthetized with sodium pentobarbital (Nembutal, 25 mg/kg), the trachea was intubated and the lungs were ventilated with intermitten...