Definitions of Orthostatic Intolerance  

Short: Orthostatic intolerance is a condition of light-headedness and fatigue following bed rest, head-down tilt or microgravity, as a result of inadequate blood flow to the brain.  

Long: Orthostatic intolerance is present when patients experience symptoms such as lightheadedness, palpitations, tremulousness, and poor concentration during standing. Many patients also note other symptoms with upright posture: visual changes, discomfort in the head or neck, throbbing of the head, poor concentration, tiredness, weakness and occasionally fainting. For many years, such patients were felt to have deconditioning and were encouraged to pursue a more vigorous exercise regimen. However, recently it has become clear that many individuals with these symptoms have a more serious problem than mere deconditioning. Indeed, a number of cardiovascular and autonomic abnormalities have recently been detected. These include, in some patients, a reduced blood volume, excessive loss of plasma from the blood vessels on standing, excessive salt loss through the kidney, and abnormalities in the function of the autonomic (involuntary) nervous system. However, not every patient has all these abnormalities. (From http://www.biotech.vt.edu/ncs_homepage/OrthoIntol.html)