Although many of its effects have not been clarified, it is well known that the carotid sinus reflex has a profound influence on the cardiovascular system. The purpose of this paper is to investigate the effect of the carotid sinus reflex on the canine heart.
The present experiments were performed on the in situ canine heart: 1) during sinus rhythm, 2) during electrically driven rhythm, and 3) during electrically driven rhythm after the administration of atropine. Before and during carotid sinus stimulation, the intracellular and surface electrograms, standard lead II electrocardiograms and aortic blood pressure were simultaneously recorded. The intracellular and surface electrograms were obtained from the epicardial surfaces of the right atrium and the left ventricle.
Atrium. The duration of the membrane action potential (MAP) shortened during sinus rhythm and electrically driven rhythm. There were no significant changes in the amplitude of the MAP and the membrane resting potential (MRP). The PQ interval in the standard lead II electrocardiogram became significantly prolonged during both types of rhythm. The P amplitudes decreased during sinus rhythm. The shortening of the duration of the MAP was almost entirely inhibited by atropine.
Ventricle. The duration of the MAP during sinus rhythm showedsignificant widening. The ST segment in the surface electrogram became significantly elevated during sinus rhythm and during electrically driven rhythm. The QT interval in the standard lead II electrocardiogram prolonged significantly during sinus rhythm, but less so during electrically driven rhythm. The RR interval became significantly prolonged during sinus rhythm. After atropine, the duration of the MAP widened very slightly during electrically driven rhythm.
The following possibilities are discussed: Electrical behavior in atrial muscle is mainly mediated by the cholinergic nervous system. The response of the sinoatrial node is caused by cholinergic activity as well as by the decrease in adrenergic activity.
The effect of carotid sinus stimulation on intracellular electrograms is more apparent in atrial muscle and in the sinoatrial node than in ventricular muscle.