What are the recommended methods to monitor depth of anaesthesia in adult patients undergoing general anaesthesia (including inhalational agents or total intravenous anaesthesia (TIVA)), such as using processed electroencephalogram (EEG) monitors like Bispectral Index (BIS) or entropy, clinical signs, end‑tidal anaesthetic concentration, and adjuncts like auditory evoked potentials?

Methods to Monitor Depth of Anaesthesia

All patients undergoing general anaesthesia require a multimodal approach combining clinical observation with objective monitoring, with processed EEG monitoring mandatory for TIVA with neuromuscular blockade and end-tidal anaesthetic concentration monitoring essential for inhalational techniques. 1

Clinical Assessment (Foundation for All Cases)

Clinical observation remains fundamental and must never be abandoned regardless of technology available. 1

Essential clinical signs to assess continuously:

• Mucosal colour assessment 1
• Pupil size and reactivity 1
• Lacrimation patterns 1
• Chest wall and reservoir bag movements 1
• Absence of response to surgical stimuli 1
• Pulse palpation and auscultation of breath sounds when indicated 1
• A stethoscope must always be immediately available 1

Technique-Specific Monitoring Requirements

For Inhalational Anaesthesia

End-tidal anaesthetic concentration (ETAC) monitoring is mandatory whenever inhaled anaesthetic drugs are used. 1, 2

Specific requirements:

• Maintain ETAC > 0.7 age-adjusted MAC during maintenance phase to reduce awareness risk 1
• Enable audible alarm for low ETAC (< 0.7 age-adjusted MAC) 1, 3
• Monitors should have age-adjusted MAC alarm capability rather than requiring manual calculation 1
• Continuous monitoring of inspired and end-tidal inhalational anaesthetic drug concentration 1, 2

Important caveat: ETAC monitoring only addresses intra-operative awareness during surgery; nearly two-thirds of awareness cases occur before surgery starts or after it ends, which ETAC cannot prevent. 1 Therefore, consider adding processed EEG monitoring even with inhalational techniques, particularly when neuromuscular blockade is used. 1

For Total Intravenous Anaesthesia (TIVA)

Processed EEG monitoring is absolutely mandatory when TIVA is combined with neuromuscular blockade. 1, 4, 3

Rationale: No monitoring currently exists to confirm anaesthetic drug delivery during TIVA, and clinical signs are completely masked by paralysis. 1 The vast majority of definite or probable awareness cases occurred during anaesthetics involving neuromuscular blocking drugs. 1

Additional recommendations:

• Processed EEG should be considered even when TIVA is used without neuromuscular blockade 1, 4
• Monitoring must start before induction and continue until full recovery from neuromuscular blockade is confirmed 1, 4

Processed EEG Monitoring: Practical Implementation

Target Values and Interpretation

General adult population:

• Maintain BIS or entropy State Entropy (SE) between 40-60 for adequate hypnotic depth 4, 3
• Response Entropy (RE) target: 40-65 3

Geriatric patients (≥60 years):

• Target BIS approximately 50 (lighter plane) 4
• Avoid BIS < 35 in elderly patients due to increased delirium risk 4
• Routine processed EEG monitoring strongly recommended for patients ≥60 years to reduce postoperative cognitive dysfunction and delirium by up to 40% 4

Clinical Benefits

Processed EEG monitoring provides multiple outcome improvements beyond awareness prevention:

• Reduces risk of accidental awareness during general anaesthesia 1, 3
• Improves early recovery times 1, 3
• Reduces incidence of postoperative delirium 1, 3
• Reduces postoperative cognitive dysfunction 1, 3

Critical Limitations to Understand

Do not rely solely on numeric index values. 1, 4, 3 Anaesthetists must develop basic understanding of EEG waveforms and power spectral analysis interpretation. 1, 3

Specific limitations:

• Cannot differentiate deep sedation from general anaesthesia 4
• Ketamine and sevoflurane cause paradoxical index increases despite adequate depth 4
• No direct measurement of analgesia or opioid adequacy 4
• EMG interference can degrade readings, particularly in ICU settings 4

Device Selection

Both BIS and entropy (Masimo SedLine/PSI) are acceptable options with guideline support. 4, 3 BIS has more extensive validation, while Masimo may offer better EMG artifact resistance in high-interference environments. 4

Algorithm for Routine Adult Surgery

Step 1: Identify anaesthetic technique

If TIVA + neuromuscular blockade:

• Processed EEG monitoring is obligatory 1, 4
• Target BIS/SE 40-60 4, 3
• Quantitative neuromuscular monitoring also mandatory 1

If TIVA without neuromuscular blockade:

• Processed EEG monitoring should be strongly considered 1, 4
• Target BIS/SE 40-60 4, 3

If inhalational anaesthesia:

• ETAC monitoring mandatory 1, 2, 3
• Maintain > 0.7 age-adjusted MAC with audible alarm 1, 3
• If neuromuscular blockade used, add processed EEG monitoring 1, 4

Step 2: Adjust for patient age

If patient ≥60 years:

• Strongly recommend processed EEG monitoring regardless of technique 4
• Target BIS ≈ 50 (avoid deep anaesthesia) 4

If patient ≥75 years:

• Maintain BIS values preventing awareness while avoiding BIS < 35 4

Step 3: Integrate with clinical assessment

• Never abandon clinical observation regardless of monitors used 1
• Interpret processed EEG values in conjunction with ETAC (if inhalational), haemodynamics, and clinical signs 3

Essential Baseline Monitoring for All Cases

Regardless of depth monitoring method, these are minimum requirements for every anaesthetic: 1, 2

• Pulse oximeter with plethysmograph 1, 2
• Non-invasive blood pressure (NIBP) 1, 2
• ECG 1, 2
• Temperature (before anaesthesia and every 30 minutes) 1, 2
• Inspired and expired oxygen 1, 2
• Waveform capnography 1, 2

Common Pitfalls and How to Avoid Them

Pitfall 1: Relying on processed EEG numbers alone without clinical context

• Always interpret in conjunction with haemodynamics, surgical stimulation, and clinical signs 1, 4, 3
• Learn to interpret raw EEG waveforms, not just numeric indices 1, 3

Pitfall 2: Failing to monitor ETAC during inhalational anaesthesia

• This is mandatory monitoring, not optional 1, 2, 3
• Set age-adjusted MAC alarms, not just concentration alarms 1

Pitfall 3: Omitting processed EEG during TIVA with paralysis

• This represents the highest risk scenario for awareness 1, 4
• Processed EEG is the only available method to assess anaesthetic effect when clinical signs are masked 1

Pitfall 4: Using excessively deep anaesthesia in elderly patients

• Target lighter planes (BIS ≈ 50) in geriatric patients 4
• Avoid BIS < 35 to reduce delirium risk 4

Pitfall 5: Misinterpreting processed EEG with ketamine or during EMG interference

• Recognize that ketamine causes paradoxical increases in processed EEG indices 4
• Be aware of EMG artifact contamination, particularly with acceleromyography 4