BIS Monitoring in General Anesthesia: Evidence-Based Recommendations
Primary Recommendation
For patients over 60 years undergoing general anesthesia, use BIS monitoring to target values of approximately 50 (range 40-60) to reduce postoperative delirium while avoiding excessive anesthetic depth and hypotension. 1, 2
Target BIS Values by Clinical State
Standard General Anesthesia
- Maintain BIS between 40-60 for surgical anesthesia 1, 2
- BIS 60-69 indicates deep sedation 1, 2
- BIS 70-90 represents moderate sedation 1
- BIS <40 denotes deep hypnosis and should be avoided due to increased hemodynamic instability 2
Age-Specific Targets
- Elderly patients (>60 years): Target BIS around 50 rather than deeper levels (BIS 35) to significantly reduce postoperative delirium 1, 2
- Avoid burst suppression patterns on EEG, which increase delirium risk in elderly patients 1, 2
Specific Patient Populations Requiring BIS Monitoring
Strong Indications (Guideline-Supported)
Elderly patients (>60 years) should have BIS monitoring to:
- Prevent relative anesthetic overdose, which is common due to age-related pharmacokinetic changes 1
- Avoid the "triple low" phenomenon (low BIS + hypotension + low MAC), which is associated with higher mortality 1
- Reduce postoperative delirium risk 1
High-risk awareness cases including:
- Emergency laparotomy patients (higher incidence of accidental awareness during emergency surgery) 1
- Patients with significant comorbidities undergoing hip fracture surgery 1
- Cardiac surgery and trauma patients 3
Patients with dementia or alcoholism:
- Initial BIS levels may be abnormally low in these populations 1
- Requires careful interpretation and adjustment of anesthetic dosing 1
Adjusting Anesthetic Agents Based on BIS
Inhalational Agents
When BIS monitoring is unavailable:
- Use age-adjusted MAC values via Lerou nomogram 1
- Modern anesthetic machines have built-in age-adjustment algorithms 1
When BIS is available:
- Titrate sevoflurane or desflurane to maintain BIS 40-60 1, 2
- Critical caveat: Sevoflurane and ketamine may paradoxically increase BIS values despite adequate anesthetic depth 2, 4
- Avoid volatile anesthetic overdose by monitoring age-adjusted MAC alongside BIS 1
Intravenous Agents (TIVA)
Propofol-based TIVA:
- Adjust propofol infusion to maintain BIS 40-60 2, 5
- BIS-guided TIVA reduces awareness risk by 82% compared to routine care 6
- In high-risk patients, BIS-guided TIVA decreased confirmed awareness from 0.65% to 0.14% 5
Important limitation:
- BIS primarily measures hypnotic depth, not analgesia 4
- Opioids (including remifentanil) have minimal effect on BIS values 4
- Do not rely on BIS alone to assess adequacy of analgesia 4
Clinical Implementation Algorithm
Step 1: Risk Stratification
Identify patients requiring BIS monitoring:
- Age >60 years 1
- Emergency surgery 1
- History of awareness or high-risk procedures (cardiac, trauma, cesarean section) 3, 6
- Dementia or chronic alcoholism 1
Step 2: Induction
- Apply BIS sensor to forehead before induction 1
- Titrate induction agents to achieve BIS 40-60 2
- Warning: BIS 50-60 may be inadequate to prevent awareness response to intubation when using propofol/opioid combinations 7
Step 3: Maintenance
- For elderly patients: Target BIS ~50 (avoid <40) 1, 2
- For general population: Maintain BIS 40-60 2
- Monitor for burst suppression and adjust anesthetic depth accordingly 1
Step 4: Multimodal Monitoring
- Never use BIS as sole monitoring method 8
- Combine with hemodynamic monitoring (blood pressure, heart rate) 1
- Use peripheral nerve stimulation when neuromuscular blockers are administered 1
- Consider invasive arterial monitoring in elderly patients with limited cardiac reserve 1
Critical Pitfalls and Caveats
Technical Limitations
BIS cannot distinguish between deep sedation and general anesthesia 4
- Low sensitivity for detecting "asleep" state may lead to unnecessary anesthetic administration 3
- Some patients already asleep may show falsely elevated BIS values 3
Agent-specific paradoxical responses:
- Ketamine and sevoflurane may increase BIS despite adequate depth 2, 4
- Requires clinical judgment alongside BIS values 2
Environmental factors:
- Hypothermia reduces anesthetic requirements by ~1.2 BIS units per 1°C 2
- Electrocautery and other electrical interference can affect readings 1
Clinical Interpretation
BIS does not predict hemodynamic or motor responses to surgical stimulation 4
- A patient with BIS 50-60 may still respond to intubation or incision 7
- Ensure adequate analgesia independent of BIS values 4
In paralyzed patients:
- BIS serves as useful adjunct when clinical assessment is impossible 4
- However, neuromuscular blockade itself does not affect BIS values 1
Special Considerations for High-Risk Populations
Emergency Laparotomy Patients
- Higher baseline risk of awareness and postoperative delirium 1
- Use BIS monitoring in patients >60 years as strong recommendation (moderate evidence) 1
- Avoid extremely low BIS values to reduce delirium risk 1
Hip Fracture Surgery
- BIS monitors may optimize depth and avoid cardiovascular depression 1
- Initial BIS may be abnormally low in patients with dementia 1
- Use alongside cardiac output monitoring and invasive blood pressure in high-risk cases 1
Neurologically Vulnerable Patients
- Cerebral oxygen saturation monitoring may complement BIS in elderly patients 1
- Detection of cerebral desaturation (>15% decrease) may reduce postoperative cognitive dysfunction 1
- Avoid deep anesthesia (BIS <40) to preserve cerebral perfusion 1, 2
Cost-Effectiveness Considerations
For high-risk awareness patients: