Inclusion Criteria:  Patients admitted to the PICU of Memorial Hermann Children's Hospital Severe TBI with a Glasgow Coma Scale score 4-8 Intubated and mechanically ventilated with an ICP monitor in place. Continuous invasive monitoring of arterial pressure Hemodynamically Stable, defined by normal blood pressure for age before the ETS. We will not exclude patients with norepinephrine to maintain goal blood pressures to maintain Cerebral perfusion pressure(CPP).  Exclusion Criteria:  Patients with sustained ICP > 25 for more than 30 minutes mm Hg despite medical or neurosurgical intervention Moderate/Severe Acute respiratory distress syndrome (ARDS) with an oxygenation index > 8 Pulmonary hemorrhage Patients receiving neuromuscular blockade Use of other vasopressors for other causes of shock (cardiogenic, hypovolemic and septic)