Investigators aim to demonstrate that the evolution of intracranial pressure (ICP) is nonlinear and may be influenced by jugular vein collapse. This collapse is estimated to occur at the angle where the hydrostatic pressure of the blood column matches the central venous pressure, and it can be assessed using ultrasonography. This approach allows for the determination of an individualized tilt angle, optimizing cerebral perfusion pressure without compromising venous drainage.

Cerebral venous drainage is a major determinant of intracranial pressure (ICP), yet the relationship between trunk inclination and ICP remains incompletely understood. Physiological data in healthy subject suggest that venous outflow may behave nonlinearly, with a threshold angle beyond which further elevation only modestly improves drainage while taking the risk of increasingly compromising cerebral perfusion pressure (CPP). The internal jugular vein (IJV) is the primary conduit for upright cerebral venous return; its collapse was estimated in healthy subjects to occurs when jugular hydrostatic pressure exceeds central venous pressure. Understanding the precise angle at which IJV collapse occurs may allow safer patient positioning strategies in critical care. Doppler ultrasonography, ICP measurements and cerebral perfusion pressure provide complementary data on this physiological breakpoint. Accurate characterization of this nonlinearity may improve cerebral perfusion management and reduce secondary brain injury.

After the routine nursing care, during patient repositioning to 30 ° assessments are performed at 0, 10° 20° 30°, and at a calculated threshold angle α. At each angle ICP, CPP and Ultrasound assessment of cerebral blood flow velocities and IJV cross-sectional area at three cervical levels are recorded.

Physiological parameters including arterial blood pressure, ICP, and cerebral perfusion pressure are recorded at each angle. Doppler ultrasonography of the middle cerebral artery is performed for flow assessment. Ventilation parameters, sedation, and vasopressor support are maintained constant. Safety criteria for immediate cessation of the procedure includes ICP >20 mmHg, CPP <60 mmHg, or oxygen desaturation, the same criteria that would have stoped the routine nursing care. Data are analyzed to characterize the nonlinear relationship between trunk angle and cerebral venous outflow, with particular focus on the collapse threshold α.

Inclusion Criteria:

• Patients ≥ 18 years old
• Hospitalized in the neurocritical care unit or the trauma unit
• Scheduled for nursing care requiring a supine position.
• Arterial catheter allowing continuous measurement of mean arterial pressure.
• Central venous catheter for continuous measurement of central venous pressure.
• External ventricular drain or intracranial pressure sensor for intracranial pressure monitoring.
• Bed with controlled inclination capability.
• Clinically stable condition allowing transient positional changes (as judged by the clinician).
• Not included, or planned to be included in another trial.

Exclusion Criteria:

• Contraindication to trunk mobilization.
• Being a minor or placed under supervision.
• Hemodynamic or respiratory instability.
• Uncontrolled intracranial hypertension, refractory arterial hypertension, or severe dysautonomia.
• Pregnancy or breastfeeding.
• Refusal to participate in the study.

Patient hospitalized in the neurocritical care unit or the trauma unit