Fainting in animals

Fainting (syncope) isunconsciousness due to insufficientcerebral circulation in the contextof a temporary failure of the systemiccirculation. This paper firstlyaims to discuss fainting in animals,and secondly to discuss animalphysiology to broaden the understandingof human fainting.Of the three major syncopetypes (cardiac, orthostatic and reflexsyncope), only cardiac syncopeoccurs in animals as in man,through arrhythmia or output failure.Mans orthostatic fainting tendencyhas been blamed on his uprightposture. A comparison withanimals shows that giraffes, treeclimbingsnakes, and animals thatquickly raise and lower their headsface more serious gravitational circulatorychallenges than man, butdo not appear to faint. Merely carryingthe brain above the heartdoes not explain a fainting tendency,as the human heart-to-brainheight is smaller than that of manymammals with similar blood pressure.Two evolutionary noveltiesmay be to blame: the proportion ofcardiac output going upwards tothe brain is much larger than inapes, and mans large legs suggestthat the volume lost to venouspooling is also larger.Emotional factors play a role inmany reflex syncope events. Tonicimmobility (feigning death,playingpossum) is not a good model,as it concerns immobility as a survivalstrategy of an attentive brain,rather than unconsciousness due tocirculatory breakdown. Whetherorienting and defense responsesform a valid model remains to beproven. Emotional fainting may beuniquely human; how mentalprocesses can shut down the circulationand thereby the brain needsserious study, as it may hold thekey to syncope prevention.