Managing Anesthesia Depth Without BIS Monitoring
Use a multimodal clinical approach combining capnography, pulse oximetry, clinical signs (heart rate, blood pressure, movement, lacrimation), and age-adjusted MAC monitoring to assess anesthesia depth when BIS is unavailable. 1
Primary Monitoring Strategy
Capnography is your most critical monitor for detecting inadequate ventilation and airway obstruction several minutes before pulse oximetry would detect desaturation, especially in patients receiving supplemental oxygen. 1 This early warning system is particularly valuable in sedated patients in less-accessible locations like MRI machines or darkened rooms. 1
Essential Monitoring Components:
Capnography: Continuously monitor end-tidal CO2 to detect apnea, airway obstruction, or respiratory depression before oxygen desaturation occurs. 1
Pulse oximetry: Use newer devices less susceptible to motion artifacts; ensure proper probe positioning as clip-on devices easily displace and produce artifactual data. 1
Hemodynamic parameters: Monitor heart rate, systolic/diastolic blood pressure, and mean arterial pressure continuously, though these are not reliable sole indicators of brain anesthetic depth. 2
Clinical signs: Assess for perspiration, lacrimation, and limb movement as indicators of inadequate anesthesia depth. 2
Age-Adjusted MAC Monitoring for Elderly Patients
For patients over 60 years, closely monitor age-adjusted minimum alveolar concentration (MAC) to avoid volatile anesthetic overdose, which causes hypotension and potentially increases postoperative delirium risk. 1 This is critical because elderly patients are at higher risk for both anesthesia-induced hypotension and postoperative delirium. 1
Specific Anesthetic Agent Considerations
Ketamine Monitoring Caveat:
BIS monitoring would paradoxically increase with ketamine despite adequate anesthesia depth due to central excitation, so its absence actually simplifies ketamine management. 1 Monitor ketamine depth using:
- Vital signs (heart rate, blood pressure)
- Clinical response to stimulation
- Purposeless/tonic-clonic movements (which do NOT indicate inadequate depth and do not require additional dosing) 3
Volatile Anesthetics (Sevoflurane/Desflurane):
- Target age-adjusted MAC values
- Monitor for hemodynamic stability
- Assess clinical signs of adequate depth 1
Propofol TIVA:
- Use clinical assessment more frequently
- Monitor hemodynamic response to surgical stimulation
- Consider that propofol can increase vasopressor requirements in hemodynamically challenged patients 1
Procedural Sedation Protocol
Assess patient response to verbal commands at 5-minute intervals during moderate sedation to gauge consciousness level. 4 This systematic approach provides regular checkpoints for depth assessment. 4
Documentation Requirements:
Record the following at regular intervals based on medication administered, procedure length, and patient condition: 4
- Level of consciousness
- Ventilatory status
- Oxygenation status
- Hemodynamic variables
Document before sedative/analgesic administration, after administration, at regular intervals during the procedure, during initial recovery, and just before discharge. 4
High-Risk Situations Requiring Enhanced Vigilance
Neuromuscular Blockade:
When patients receive neuromuscular blocking drugs, clinical assessment becomes impossible, making this the highest-risk scenario without BIS. 5, 6 In this situation:
- Rely heavily on hemodynamic responses to surgical stimulation
- Ensure adequate anesthetic dosing based on weight-based calculations
- Consider that muscle relaxants eliminate movement as a depth indicator 6
Emergency Surgery:
Patients undergoing emergency laparotomy have higher incidence of accidental awareness (compared to elective surgery) and often have hemodynamic instability. 1 Without BIS, use conservative anesthetic dosing while monitoring hemodynamics closely. 1
Common Pitfalls to Avoid
Do not rely solely on hemodynamic parameters (heart rate, blood pressure) as they are unreliable indicators of brain anesthetic depth. 2 Cardiovascular measures can be affected by multiple factors unrelated to consciousness level. 2
Avoid assuming adequate depth based on lack of movement alone, especially with neuromuscular blockade where movement is impossible regardless of consciousness. 6
Do not ignore capnography alarms even if pulse oximetry appears normal—capnography detects problems earlier. 1
Awareness Prevention Strategy
The incidence of intraoperative awareness is 0.1-0.2% in general surgical populations but increases to 1.14% in cardiac surgery and 0.9% in cesarean sections. 2 Without BIS:
- Administer benzodiazepines for prevention of neuropsychological manifestations during emergence, which also provides amnesia if awareness occurs. 3
- Ensure adequate anesthetic dosing throughout the procedure
- Maintain heightened vigilance during high-risk periods (intubation, surgical stimulation, emergence)
- Use multimodal anesthesia when appropriate 1