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, 3
Target BIS Values by Clinical State
Standard General Anesthesia
- Maintain BIS between 40-60 for surgical anesthesia 4, 3
- BIS 60-69 indicates deep sedation 4, 3
- BIS 70-90 represents moderate sedation 4
- BIS <40 denotes deep hypnosis and should be avoided due to increased hemodynamic instability 3
Age-Specific Targets
- Elderly patients (>60 years): Target BIS around 50 rather than deeper levels (BIS 35) to significantly reduce postoperative delirium 1, 3
- Avoid burst suppression patterns on EEG, which increase delirium risk in elderly patients 1, 2, 3
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 2
- Avoid the "triple low" phenomenon (low BIS + hypotension + low MAC), which is associated with higher mortality 2
- Reduce postoperative delirium risk 1, 2
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 5
- Cardiac surgery and trauma patients 6
Patients with dementia or alcoholism:
- Initial BIS levels may be abnormally low in these populations 5
- Requires careful interpretation and adjustment of anesthetic dosing 5
Adjusting Anesthetic Agents Based on BIS
Inhalational Agents
When BIS monitoring is unavailable:
- Use age-adjusted MAC values via Lerou nomogram 2
- Modern anesthetic machines have built-in age-adjustment algorithms 2
When BIS is available:
- Titrate sevoflurane or desflurane to maintain BIS 40-60 1, 3
- Critical caveat: Sevoflurane and ketamine may paradoxically increase BIS values despite adequate anesthetic depth 3, 7
- 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 3, 8
- BIS-guided TIVA reduces awareness risk by 82% compared to routine care 9
- In high-risk patients, BIS-guided TIVA decreased confirmed awareness from 0.65% to 0.14% 8
Important limitation:
- BIS primarily measures hypnotic depth, not analgesia 7
- Opioids (including remifentanil) have minimal effect on BIS values 7
- Do not rely on BIS alone to assess adequacy of analgesia 7
Clinical Implementation Algorithm
Step 1: Risk Stratification
Identify patients requiring BIS monitoring:
- Age >60 years 1, 2
- Emergency surgery 1
- History of awareness or high-risk procedures (cardiac, trauma, cesarean section) 6, 9
- Dementia or chronic alcoholism 5
Step 2: Induction
- Apply BIS sensor to forehead before induction 4
- Titrate induction agents to achieve BIS 40-60 3
- Warning: BIS 50-60 may be inadequate to prevent awareness response to intubation when using propofol/opioid combinations 10
Step 3: Maintenance
- For elderly patients: Target BIS ~50 (avoid <40) 1, 3
- For general population: Maintain BIS 40-60 3
- Monitor for burst suppression and adjust anesthetic depth accordingly 1, 2
Step 4: Multimodal Monitoring
- Never use BIS as sole monitoring method 11
- Combine with hemodynamic monitoring (blood pressure, heart rate) 5
- Use peripheral nerve stimulation when neuromuscular blockers are administered 2
- Consider invasive arterial monitoring in elderly patients with limited cardiac reserve 5, 2
Critical Pitfalls and Caveats
Technical Limitations
BIS cannot distinguish between deep sedation and general anesthesia 7
- Low sensitivity for detecting "asleep" state may lead to unnecessary anesthetic administration 6
- Some patients already asleep may show falsely elevated BIS values 6
Agent-specific paradoxical responses:
- Ketamine and sevoflurane may increase BIS despite adequate depth 3, 7
- Requires clinical judgment alongside BIS values 3
Environmental factors:
- Hypothermia reduces anesthetic requirements by ~1.2 BIS units per 1°C 3
- Electrocautery and other electrical interference can affect readings 4
Clinical Interpretation
BIS does not predict hemodynamic or motor responses to surgical stimulation 7
- A patient with BIS 50-60 may still respond to intubation or incision 10
- Ensure adequate analgesia independent of BIS values 7
In paralyzed patients:
- BIS serves as useful adjunct when clinical assessment is impossible 7
- However, neuromuscular blockade itself does not affect BIS values 4
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 5
- Initial BIS may be abnormally low in patients with dementia 5
- Use alongside cardiac output monitoring and invasive blood pressure in high-risk cases 5
Neurologically Vulnerable Patients
- Cerebral oxygen saturation monitoring may complement BIS in elderly patients 5, 2
- Detection of cerebral desaturation (>15% decrease) may reduce postoperative cognitive dysfunction 2
- Avoid deep anesthesia (BIS <40) to preserve cerebral perfusion 2, 3
Cost-Effectiveness Considerations
For high-risk awareness patients: