Postural orthostatic tachycardia syndrome (POTS) is one of a group of disorders that have orthostatic intolerance (OI) as their primary symptom. OI is a condition in which an excessively reduced volume of blood returns to the heart after an individual stands up from a lying down position.
https://www.ninds.nih.gov/health-information/disorders/postural-tachycardia-syndrome-pots