When is Bispectral Index (BIS) monitoring recommended in surgical settings?

BIS Monitoring in Surgical Settings

BIS monitoring may be considered for surgical patients at high risk for intraoperative awareness, but it is not recommended for routine use in all surgical cases. 1

Primary Indication: Prevention of Intraoperative Awareness

BIS monitoring is most strongly indicated for patients at high risk of awareness during general anesthesia, where it should be used to maintain values between 40-60. 1

High-Risk Patient Populations

BIS monitoring should be specifically considered for:

• Patients undergoing total intravenous anesthesia (TIVA) with neuromuscular blockade - This is the strongest indication, as processed EEG monitoring is essential when muscle relaxants prevent clinical assessment of anesthetic depth 2
• Patients with history of awareness or substance abuse - These patients have increased baseline risk 3
• Cardiac surgery patients - Though evidence is mixed, BIS can guide anesthetic titration during cardiopulmonary bypass 4
• Patients requiring light anesthesia due to hemodynamic instability - BIS helps balance adequate depth against cardiovascular depression 1

Target BIS Values for Awareness Prevention

• Maintain BIS between 40-60 for general anesthesia 1, 5
• Avoid BIS values >60, which indicate light anesthesia and increased awareness risk 6, 7
• Set audible alarms if BIS exceeds 60 or falls below 40 7

Secondary Indication: Elderly Patients and Delirium Prevention

For patients over 60 years, target BIS around 50 (lighter anesthesia) to reduce postoperative delirium while maintaining adequate anesthetic depth. 1, 5, 8, 2

Specific Recommendations for Elderly

• Target BIS approximately 50 rather than deeper levels (35-40) - Deeper anesthesia with BIS around 35 significantly increases delirium rates 8
• Avoid burst suppression patterns (very low BIS) - This EEG pattern is strongly associated with postoperative cognitive dysfunction 8
• Use BIS to prevent relative anesthetic overdose - Elderly patients require lower doses but commonly receive standard doses, causing excessive depth and hypotension 1

Evidence Quality and Contradictions

The evidence for BIS monitoring shows important contradictions:

Supporting Evidence

• The B-Aware trial (2004) showed 82% reduction in awareness with BIS monitoring in high-risk patients (0.16% vs 0.91%, p=0.022) 3
• Chinese multicenter trial (2011) demonstrated lower awareness rates with BIS-guided TIVA (0.14% vs 0.65%, p=0.002) 6

Contradictory Evidence

• The BAG-RECALL trial (2011) failed to show BIS superiority - Awareness occurred in 0.24% of BIS group vs 0.07% of end-tidal anesthetic concentration (ETAC) group, opposite the expected direction 7
• The B-Unaware trial (2008) showed no difference between BIS and ETAC protocols (0.21% awareness in both groups) 9

Guideline Consensus Despite Mixed Evidence

The 2019 EACTS/EACTA/EBCP guidelines give a Class IIb recommendation (Level B evidence) for routine processed EEG monitoring to reduce awareness - meaning it "may be considered" but is not strongly recommended 1. This weak recommendation reflects the contradictory trial results.

When BIS Monitoring is NOT Routinely Indicated

• Standard surgical cases with volatile anesthetic monitoring - ETAC-guided anesthesia provides equivalent protection against awareness 1, 9
• Patients without awareness risk factors - The number needed to treat is approximately 138 in high-risk patients, making routine use cost-ineffective 3
• As a replacement for clinical judgment - BIS should complement, not replace, traditional monitoring 5

Practical Implementation Algorithm

Step 1: Risk Stratification

Identify if patient has:

• TIVA with neuromuscular blockade planned → Use BIS 2
• History of awareness or substance abuse → Use BIS 3
• Age >60 years → Use BIS to target ~50 1, 5, 8
• Cardiac surgery with CPB → Consider BIS 4

Step 2: If BIS Used, Set Targets

• General population: BIS 40-60 1, 5
• Elderly patients: BIS ~50 1, 5, 8
• Set audible alarms at <40 and >60 7

Step 3: Alternative if BIS Unavailable

• Use age-adjusted MAC values (0.7-1.3 MAC) 1, 9
• For elderly, use Lerou nomogram for age-adjusted dosing 1

Important Caveats

• BIS values may be unreliable with certain agents - Ketamine and sevoflurane can paradoxically increase BIS despite adequate anesthesia 5
• Initial BIS may be abnormally low in alcoholics and dementia patients 1
• Hypothermia affects BIS - Temperature drops reduce anesthetic requirements by approximately 1.2 BIS units per 1°C 5
• Awareness can occur even with BIS 40-60 - Three of four awareness cases in the B-Unaware trial occurred within target ranges 9
• The "triple low" (low BIS + hypotension + low volatile concentration) is associated with increased mortality 1