Orthostatic Intolerance (OI) occurs when the individual is standing or sitting up. In 1995, a link between OI and Chronic Fatigue Syndrome (CFS) was observed at Johns Hopkins University. In some cases up to 97 percent of Chronic Fatigue Syndrome patients in studies have some form of OI, particularly young people.
Two forms of orthostatic intolerance are neurally mediated hypotension (NMH) and postural orthostatic tachycardia syndrome (POTS). NMH is a dramatic drop in systolic blood pressure while standing. Other symptoms will often increase as well. POTS is increased heart rate within 10 minutes of standing. It's also called chronic orthostatic intolerance (COI).
Symptoms that accompany neurally mediated hypotension and postural tachycardia syndrome include dizziness, tremors, lightheadedness, visual disturbance and breathing difficulties. Swollen, bluish legs will indicate that blood is pooling. A hot environment can increase these symptoms.
"Unlike those with OH, which occurs within the first three minutes of standing, CFS patients with NMH or POTS often have a delayed form of orthostatic intolerance, meaning that heart rate and blood pressure changes don't develop for many minutes after standing, making the standard in-office test for acute orthostatic hypotension ineffective in diagnosis. "
http://www.cfids.org/about-cfids/orthostatic-intolerance.asp