Orthostatic intolerance occurs when the body fails to properly compensate for moving to an upright position, which is thought to result in reduced blood flow to the brain and symptoms such as lightheadedness, fatigue, and weakness. A subset of those with orthostatic intolerance also meet the criteria for postural orthostatic tachycardia syndrome (PoTS), which includes an excessive increase in heart rate as one of the symptoms.

Many factors can worsen orthostatic intolerance, but heat is a particularly relevant trigger during the summer months.