Traumatic brain injury is a leading cause of death and disability in trauma patients. As the primary injury cannot be reversed; management strategies must focus on preventing secondary injury by avoiding hypotension and hypoxia besides, maintaining appropriate cerebral perfusion pressure, which is a surrogate for cerebral blood flow.

The caval aorta index is an ultrasound element, relatively new, used to assess volume status. Technically, the sonographic assessment of the inferior vena cava /aorta diameter ratio is an easy test to perform and can be performed effectively by doctors who are not experienced in the field of sonography (Kusumastuti et al .2021).

In this study; the caval aorta index will be used to guide fluid dosage in critically ill patients with traumatic brain injury.

Inclusion Criteria:

Traumatic brain injury diagnosed by computed tomography scan Glasgow coma score ≥ 4

Exclusion Criteria:

Inability to get consent. Pregnant and lactating women Traumatic subarachnoid hemorrhage Spinal cord injury Increased intra-abdominal pressure Abdominal aortic aneurysm Inferior vena cava (IVC) thrombus Portal hypertension Acute corpulmonale Congestive heart failure chronic hemodialysis Surgical intervention other than neurological one within the first 7 days Mechanical ventilation need during first 7 days Cardiac arrest during the first 7 days

A vascular US expert will read the number contained in computer generated randomization software and determine group assignments. Daily; he will be responsible on prescribing of the required fluid.