Increased Activity in Left Ventricular Receptors during Hemorrhage or Occlusion of Caval Veins in the Cat. ‐ A Possible Cause of the Vaso‐vagal Reaction

Abstract Impulse activity in vagal afferent fibres and changes in heart rate were simultaneously recorded during rapid hemorrhage or blood pooling in cats in order to elucidate whether the sudden reflex bradycardia, sometimes evoked with the mentioned interventions, was correlated to an increased activity in any type of heart receptors. The results show that the slowing of the heart, resulting from an emptying of the central blood reservoirs was correlated to, and preceded by, an increased activity in receptors located in the left ventricle and signalling in non‐medullated afferents. The receptors were found to be activated also by obstructions of the ascending aorta and mechanical stimulation of the heart, and therefore seem to operate as mechanorecepton, stimulated by a “distortion” of the myocardium. ‐ It is conceivable that with rapid bleeding or pooling of blood, the receptors are excited by an improper squeezing of the myocardium when the ventricles contract vigorously around an almost empty chamber, and then induce a reflex bradycardia. This reflex mechanism, resembling the so‐called vaso‐vagal syncope reaction in man, may therefore serve as a protective system causing a break on the heart and allowing for an improved diastolic filling in situations when venous return is critically reduced.