The vasovagal response.

The vasovagal response is the development of inappropriate cardiac slowing and arteriolar dilatation. Vasovagal responses reflect autonomic neural changes: bradycardia results from sudden augmentation of efferent vagal activity, and hypotension results from sudden reduction or cessation of sympathetic activity and relaxation of arterial resistance vessels. Two different neural pathways are thought to be involved, one originating in the hypothalamus, the other in the heart. Direct hypothalamic activation of the medullary cardiovascular centres triggered by emotional stress or pain causes a vasovagal response (central type). The combination of a reduced central blood volume secondary to venous pooling or blood loss, and an increased inotropic state of the heart, may stimulate ventricular mechanoreceptors and provoke vasodilatation and bradycardia (peripheral type). Cardiovascular afferents originating from stretch receptors in various parts of the vascular tree sometimes induce opposite reflexes when compared with those from ventricular afferents. The depressor reflex involved in the peripheral type of vasovagal response originates in the heart itself and overrides normal baroreflex circulatory control; an antagonism between the control of volume and pressure on the filling side of the heart and the control system of arterial pressure becomes apparent. Vasovagal responses are not necessarily abnormal; the neural pathways involved in the vasovagal response are probably present in all healthy subjects who individually mainly differ in susceptibility.