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Many, if not most, patients with ME/CFS report problems standing (orthostatic intolerance). Orthostatic intolerance (OI) is a broad classification referring to exaggerated blood pressure and/or heart rate responses that occur when standing up. When a healthy person stands up, the effects of gravity cause about 10-15% of the blood to settle (pool) in the abdomen, legs, and arms. To make up for the reduced amount of blood returning to the heart, the adrenal glands release adrenaline and noradrenaline (epinephrine and norepinephrine), which cause the heart to beat a little faster and blood vessels to constrict.
People with OI have reversed responses to changes in position. When people with OI stand up, they pool a larger amount of blood in regions below the heart. The longer they remain upright, the more blood pools in their abdomen and legs. The adrenal glands respond by releasing more adrenaline and noradrenaline in an attempt to cause constriction of the blood vessels and boost cardiac output.
However, the blood vessels do not constrict enough to ensure increased back flow into the heart, and the heart races. This, in turn, leads to lower blood volume (hypovolemia), which perpetuates the cycle.
The symptoms produced by this loop are fatigue, nausea, light-headedness, heart palpitations, sweating, and sometimes fainting (syncope). The feedback loop that leads specifically to an increased heart rate of 30 bpm (beats per minute) or more when standing is called POTS – postural orthostatic tachycardia syndrome.
OI/POTS is so common in the patient population that it has been suggested that this response to standing should be a diagnostic marker for ME/CFS.