The Bispectral Index (BIS) is a monitor that converts brain electrical activity from EEG into a simple number from 0 to 100. A higher number means the patient is more awake, while a lower number means deeper sedation or reduced brain activity.

In general, 100 means fully awake, 80 suggests light to moderate sedation, 60 is commonly used as a target for general anesthesia with a low chance of awareness, 40 indicates deep anesthesia, 20 suggests marked brain suppression with burst suppression on EEG, and 0 indicates no detectable cortical electrical activity.

Although BIS was originally developed for use in the operating room, it has also been applied in the ICU to help guide sedation, avoid over- or under-sedation, and assess consciousness in patients who cannot be evaluated reliably using standard clinical scores. BIS has also been studied as a possible tool for predicting outcomes in comatose ICU patients, such as those after cardiac arrest, stroke, encephalitis, or traumatic brain injury. However, evidence is still limited for its use in predicting outcomes among ICU patients with any form of decreased consciousness. Therefore, this study was conducted to explore that role.

The Bispectral Index is the output of a multi-stage process that transforms the brain's electrical activity into a simplified metric. This process begins with non-invasive data acquisition and proceeds through advanced computational analysis to derive the final index.

The BIS monitor translates the EEG data into a number on a scale from 0 to 100. This index is designed to provide a direct measure of a patient's level of consciousness and response to sedation, with specific numeric ranges corresponding to general clinical states.

The generally accepted clinical correlations for the BIS scale are as follows:

• 100: This value indicates a patient who is fully awake and alert, corresponding to a state of responsiveness to a normal voice.
• 80: This range is typically associated with light to moderate sedation or anxiolysis. A patient in this range may respond to loud verbal commands or mild physical stimulation, such as prodding or shaking.
• 60: This value is a critical threshold often targeted for general anesthesia. It represents a low probability of explicit recall and unresponsiveness to verbal stimuli. A BIS value of less than 60 has a high sensitivity for identifying a state of drug-induced unconsciousness, making it a key target in the operating room to prevent awareness.
• 40: This range signifies a deep hypnotic state, with a greater degree of cortical suppression than is typically required for general anesthesia.
• 20: A BIS value in this range indicates the presence of burst suppression on the EEG. This pattern, characterized by periods of electrical activity (bursts) alternating with periods of isoelectricity (suppression), reflects a very deep level of brain suppression seen with high doses of anesthetic agents or in conditions like barbiturate coma or severe anoxic brain injury.
• 0: A BIS value of 0 represents a flatline or isoelectric EEG, indicating the absence of detectable cortical electrical activity.

BIS monitoring was adapted from the operating room to the ICU to help manage the difficult task of sedating critically ill patients.

Its main goals in the ICU are to:

• Prevent the risks of over- or under-sedation.
• Provide an objective number to guide medication dosage.
• Assess consciousness in patients who can't be evaluated by normal methods. BIS monitoring is being used more frequently to assess the depth of sedation in ICU patients, as opposed to relying solely on clinical scoring systems.

The Bispectral Index (BIS) has been used to predict clinical outcomes in ICU patients with coma from various causes, including post-cardiac arrest, cerebrovascular disease, viral encephalitis, and traumatic brain injury. However, there is limited research on using BIS to predict outcomes for patients with any decreased level of consciousness in the ICU. Therefore, this study was initiated.

Inclusion Criteria:

1. Age≥18years
2. RASS score ≤ -3 as case
3. RASS score 0 to -1 as control
4. Expected ICU length of stays ≥ 24hours

Exclusion Criteria:

1. Contraindication for BIS monitoring (wound or infection at forehead)
2. No space for attach BIS monitoring at forehead
3. Patient with sedative drugs (Propofol, Midazolam, Dexmedetomidine)
4. Patient with acute stroke
5. Patient was already on EEG monitoring
6. Denied by patients or patient's surrogates

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

IPD and supporting information will be available once the study has already published and last for 1 year.

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.