The present study is an explorative analysis of the relationship between cerebral blood perfusion and oxygenation and lung mechanical variables at different ventilator settings. It is a safety study excluding patients with severe lung injury or brain edema.

The primary goal is to carry out baseline measurements to enable conclusions concerning the safety of lung protective ventilator settings before extending the study to patients with more severe brain and/or lung injury in the future.

The primary objective is to investigate if lung protective ventilator settings (higher Positive end-expiratory pressure and lower tidal volume) as compared with conventional settings.

1. increase intracranial pressure
2. diminish cerebral vasoreactive autoregulation as assessed by pressure reactivity index, ie pressure reactivity index will turn positive, which means that it will change the state from intact to impaired autoregulation.

The secondary objective is an exploratory analysis of the relationship between ventilator settings and other well defined respiratory, cerebral, and cardiovascular variables, including transpulmonary pressure.

Inclusion Criteria:

• • Any tracheally intubated or tracheotomized adult patient with ABI with GCS< 9 on controlled ventilation requiring continuous ICP measurement.
• Proxy informed consent from relatives.

Exclusion Criteria:

• • ICP > 22 mmHg before treatment of high ICP
• Acute respiratory failure defined as partial pressure of oxygen/ inspiratory oxygen fraction (PaO2/FiO2) ratio < 40 kPa and Xray pathology
• History of pulmonary disese: Chronic respiratory failure diagnosis stage III and IV in the GOLD classification, pulmectomy, lobectomy or restrictive lung disease.
• Body mass index (BMI) > 35.
• Known right or biventricular cardiac failure with cardiac index < 2,5 L/min/m2 or ejection fracture < 40 %.
• Refractory hypovolemia as diagnosed with pulse pressure variation > 12 % with tidal volume 8 ml/predicted bodyweight (intubated on controlled ventilation) or passive leg rise test with > 10 % increase in stroke volume measured by VTI echocardiography or PICCO.
• Medulla lesion that affect the autonomic nervous system.
• Patients who has undergone decompressive craniectomy.