Inclusion Criteria:  Age ≥ 18 years Severe TBI (3 < glasgow coma scale (GCS) ≤ 8) Hyperventilation (PaCO2 < 35 mmHg and pondus hydrogenii (pH) > 7.45) Presence of an endotracheal tube Assisted ventilation mode (CPAP/PSV)  Exclusion Criteria:  Induced/iatrogenic hyperventilation No informed consent was signed Transcranial doppler sonography (TCD) data collection cannot be completed due to anatomical structure Severe multiple organ failure, persistent high fever, massive thoraco-peritoneal effusion Medical history of major craniocerebral injury and chronic obstructive pulmonary disease (COPD) Conformed or Suspected history of opioid-related adverse reactions Withdraw from the study due to the change of patient's condition and other methods of treatment and intervention are needed