<qa>  What is associated with TBI?Severe traumatic brain injury TBI is associated with sympathetic hyperactivity resulting in catecholamine excess, abnormal heart rate variability, agitation and sympathetic storms, deep white matter changes, and poor neuropsychological outcomes. Notably, persistent sympathetic hyperactivity after TBI results in higher days of mechanical ventilation and longer intensive care unit ICU length of stay LOS. While there are data describing limited portions of this response, the full spectrum of sympathetic hyperactivity after severe TBI has not been systemically described or methodically intervened upon. 