The essential guide to living with orthostatic intolerance.Orthostatic intolerance (OI) describes a group of circulatory disorders whose symptoms are characterized by a dramatic drop in blood flow to the brain when people sit up or stand up. It is often associated with other acute issues, such as substantial drops in blood pressure, tachycardia, dizziness, and fainting, or long-term problems, such as chronic fatigue, fibromyalgia, and "long COVID." Living Well ...