Fainting in animals.

Fainting (syncope) is unconsciousness due to insufficient cerebral circulation in the context of a temporary failure of the systemic circulation. This paper firstly aims to discuss fainting in animals, and secondly to discuss animal physiology to broaden the understanding of human fainting. Of the three major syncope types (cardiac, orthostatic and reflex syncope), only cardiac syncope occurs in animals as in man, through arrhythmia or output failure. Man's orthostatic fainting tendency has been blamed on his upright posture. A comparison with animals shows that giraffes, tree climbing snakes, and animals that quickly raise and lower their heads face more serious gravitational circulatory challenges than man, but do not appear to faint. Merely carrying the brain above the heart does not explain a fainting tendency, as the human heart-to-brain height is smaller than that of many mammals with similar blood pressure. Two evolutionary novelties may be to blame: the proportion of cardiac output going upwards to the brain is much larger than in apes, and man's large legs suggest that the volume lost to venous pooling is also larger. Emotional factors play a role in many reflex syncope events. Tonic immobility ('feigning death','playing possum') is not a good model, as it concerns immobility as a survival strategy of an attentive brain, rather than unconsciousness due to circulatory breakdown. Whether orienting and defense responses form a valid model remains to be proven. Emotional fainting may be uniquely human; how mental processes can shut down the circulation and thereby the brain needs serious study, as it may hold the key to syncope prevention.