Fainting, or SYNCOPE, is a temporary loss of consciousness caused by inadequate supply of blood to the brain. It may be preceded by nausea, sweating, loss of vision, and ringing in the ears (see TINNITUS). It is most often caused by pooling of blood in the extremities, which reduces venous return (see CIRCULATORY SYSTEM OF THE BLOOD) and thus cardiac output: this may be due to hot weather or prolonged standing. Occasionally, fainting on standing occurs in people with low blood pressure (see HYPOTENSION), autonomic neuropathy (in which normal vasomotor reﬂexes are absent), or those taking antihypertensive drugs. A prolonged rise in intrathoracic pressure caused by coughing, MICTURITION, or VALSALVA’S MANOEUVRE also impedes venous return and may cause fainting. HYPOVOLAEMIA produced by bleeding, prolonged diarrhoea, or vomiting may also cause fainting, and the condition can be produced by severe pain or emotional upset. Cardiac causes, such as severe stenotic valve disease or rhythm disturbances (particularly complete heart block or very rapid tachycardias), may result in fainting (see HEART, DISEASES OF). Treatment must be directed towards the underlying cause, but immediate ﬁrst aid consists of laying the patient down and elevating the legs.
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