BIS Monitoring for Intubation Response Assessment
BIS monitoring is not useful for assessing intubation response and should not be relied upon for this purpose. While BIS values between 40-60 are recommended for general anesthesia, multiple studies demonstrate that these "adequate" BIS levels fail to prevent awareness reactions during intubation, and BIS cannot differentiate between patients who will or will not respond to intubation stimuli 1, 2.
Why BIS Fails During Intubation
The fundamental problem is that BIS measures hypnotic depth but does not assess the analgesic component or predict hemodynamic/motor responses to noxious stimuli like intubation 1.
Evidence of BIS Limitations
In a controlled study of 20 patients maintained at BIS 50-60 during propofol/alfentanil anesthesia, 40% (8/20) showed awareness reactions to intubation despite "adequate" BIS values, squeezing the investigator's hand on command 1
BIS values before and after intubation showed no differences between patients with versus without awareness reactions, demonstrating that BIS cannot predict or identify intubation response 1
A similar study of 51 patients maintained at BIS 40-60 found that 7 patients (14%) tested positive for awareness during LMA-Fastrach insertion or intubation, despite BIS levels indicating adequate anesthesia 2
During rapid sequence induction, BIS values vary significantly between agents (thiopentone vs propofol) but do not reliably predict intubation response or awareness 3
What BIS Actually Monitors
BIS is designed to assess depth of unconsciousness and reduce intraoperative awareness during the maintenance phase of anesthesia, not to predict responses to specific noxious stimuli 4, 5.
Appropriate BIS Applications
BIS monitoring reduces overall awareness incidence during general anesthesia requiring muscle relaxants (0.04% vs 0.18% in unmonitored patients) 4
BIS is best suited for sedative titration during deep sedation or neuromuscular blockade when clinical assessment is impossible 6
BIS monitoring appears most reliable with propofol-based anesthesia, showing good correlation with sedation depth 6
Critical Limitations for Intubation Assessment
BIS values paradoxically increase with ketamine and sevoflurane despite adequate anesthesia depth due to central excitation 6
Opioids and benzodiazepines have minimal and variable effects on BIS, yet these agents are crucial for blunting intubation response 6
BIS cannot distinguish between deep sedation and general anesthesia, nor can it predict hemodynamic or motor responses to stimulation 6
Clinical Implications
For intubation, rely on adequate dosing of both hypnotic and analgesic agents based on pharmacokinetic principles, not BIS values 1.
Practical Approach
Ensure sufficient time for induction agents to reach effect-site concentration before intubation (typically 90-120 seconds for propofol) 3
Administer adequate opioid dosing (e.g., remifentanil, fentanyl, alfentanil) to blunt the sympathetic response to laryngoscopy 1, 2
If using BIS during induction, values >60 for ≥4 minutes are associated with awareness risk, but values 40-60 do not guarantee absence of intubation response 4, 1
Monitor hemodynamic parameters (heart rate, blood pressure) as more reliable indicators of inadequate anesthetic depth during intubation than BIS 1
Common Pitfalls
Do not assume BIS 40-60 provides adequate conditions for intubation - this range indicates hypnotic depth but not analgesic adequacy 1, 2
Do not use BIS as the sole determinant of readiness for intubation - clinical judgment incorporating time since induction, agent pharmacokinetics, and adequate muscle relaxation remains essential 3, 1
Awareness reactions during intubation at "adequate" BIS levels do not necessarily result in explicit recall, but this does not validate BIS as an intubation response monitor 1, 2