The origin of vasovagal syncope: to protect the heart or to escape predation?

Abstract

Major lines of evidence suggest that classical (emotional and orthostatic) vasovagal syncope (VVS) is not a disease, but rather a manifestation of a non-pathological trait. It is, therefore, reasonable to investigate the possible factors that may explain its origin and evolution. We reviewed the data available in the literature on the vasovagal reaction in humans and animals in order to identify possible similarities that might provide insight into the evolution of VVS. We found two processes which appear relevant to the investigation of VVS evolution: fear and threat bradycardia in animals, and the vasovagal reflex during hemorrhagic shock in humans and animals. We suggest that VVS in humans involves physiological mechanisms similar to those found in other vertebrates, and that this may indicate a common evolutionary root. The available data seem to suggest that VVS evolved as an advantageous response to inescapable predators or to stressful and possibly dangerous heart conditions. The inhibition of the sympathetic system, together with activation of the vagal system, characterizes VVS. The consequent slowing of the heart rate induced by VVS may constitute a beneficial break of the cardiac pump, thereby reducing myocardial oxygen consumption. We suggest that classical VVS did not evolve recently in the modern human lineage; rather, it should be regarded as a selected response, which probably evolved in the ancient past as a "defense mechanism" of the organism within some ancestral group(s) of vertebrates.

Estradiol potentiates hypothalamic vasopressin and oxytocin neuron activation and hormonal secretion induced by hypovolemic shock

“…Hypotension induced by mild to moderate hemorrhage is usually accompanied by a tachycardic reflex response (29); however, we used moderate to severe hemorrhage in the present study, which is known to produce a bradycardic response as a consequence of the vasovagal reflex, i.e., an acute central hypovolemia with a consequent parasympathetic activation (1,20,40,48). The bradycardic reflex induced by parasympathetic activation is likely to involve activation of the periaqueductal gray substance and serotoninergic system that sends inhibitory projections to the rostral ventrolateral medulla (RVLM), a crucial area implicated in the generation and modulation of sympathetic activity to the cardiovascular system (7, 9, 49, 51).…”

Entropy in Investigation of Vasovagal Syndrome in Passive Head Up Tilt Test

“…While there are some theories explaining the etiology of vasovagal syndrome [3][4][5][6], the main cause remains unknown. Moreover, there is no commonly accepted gold standard diagnostic procedure for vasovagal syncope.…”

To avoid fainting: cross the legs and not the fingers

“…4 Studies in different animal species have demonstrated that vasovagal reactions are common and helpful to discourage predators by simulating death. 5 In humans, VVS could also in some instances be considered an involuntary solution to temporarily escape from an unbearable world (for example, intolerable pain or insupportable vision). In these cases, VVS could be considered 'physiological'.…”