Orthostatic intolerance (OI) is defined as "the development of symptoms during upright standing relieved by recumbency," or by sitting back down again. There are many types of orthostatic intolerance. OI can be a subcategory of dysautonomia, a disorder of the autonomic nervous system occurring when an individual stands up.
It affects more women than men (female-to-male ratio is at least 4:1), usually under the age of 35. Up to 97% of those who have chronic fatigue syndrome (CFIDS) have been said been shown in studies to have some form of OI.
Orthostatic intolerance occurs in humans because standing upright is a fundamental stressor and requires rapid and effective circulatory and neurologic compensations to maintain blood pressure, cerebral blood flow, and consciousness. When a human stands, approximately 750 mL of thoracic blood is abruptly translocated downward. People who suffer from OI lack the basic mechanisms to compensate for this deficit. Changes in heart rate, blood pressure, and cerebral blood flow that produce OI "may be related to abnormalities in the interplay between blood volume control, the cardiovascular system, the autonomic nervous system and local circulatory mechanisms that regulate these basic physiological functions."
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