Postural orthostatic tachycardia syndrome is poorly understood. Many of these patients have elevated levels of plasma norepinephrine, particularly when upright One subgroup of patients has a primary hyperadrenergic state. The majority of patients have a marked deficit in plasma blood volume compared to healthy control subjects. The investigators have previously reported that some patients suffer paradoxically from a partial dysautonomia affecting the lower limbs. Most patients seem to suffer from either a primary or secondary increase in central sympathetic nervous system drive. Recent data have shown that patients with postural orthostatic tachycardia syndrome have a lower cardiac stroke volume than their healthy counterparts and this low stroke volume may drive their orthostatic tachycardia.