Response of Bispectral Index to Neuromuscular Blockade in Awake Volunteers
Bispectral index (BIS) monitoring can provide information about the depth of anaesthesia based on interpretation of the patient's electroencephalogram (EEG).
Theoretically it can help reduce the complications of excessive or insufficient anaesthetic agents.
There have been concerns about its reliability and the degree to which muscle activity can interact with its readings.
This paper takes a bold approach to looking at the role neuromuscular blockade on its own has on the BIS monitoring.
The original blog post can be read here.
Response of bispectral index to neuromuscular blockade in awake volunteers. British Journal of Anaesthesia. July 2015
Theoretically it can help reduce the complications of excessive or insufficient anaesthetic agents.
There have been concerns about its reliability and the degree to which muscle activity can interact with its readings.
This paper takes a bold approach to looking at the role neuromuscular blockade on its own has on the BIS monitoring.
The original blog post can be read here.
Response of bispectral index to neuromuscular blockade in awake volunteers. British Journal of Anaesthesia. July 2015
What's it about?
Previous small study in 2003 suggesting neuromuscular blockade on its own caused a decrease in BIS values.
No studies since have successfully confirmed or refuted this.
Neuromuscular blockade is implicated in the majority of cases of accidental awareness under general anaesthesia.
Design: Small unblinded case series. 11 volunteers
Intervention: 10 received suxamethonium, 10 received rocuronium. At least 2 week gap between trials.
Observation: Monitored BIS via monitors from 2 different manufacturers. Additional EEG monitoring, including analysis of EMG component. Full AAGBI standards of anaesthetic monitoring.
Inclusion: Anaesthetist, ASA 1/2, aged 25-60,
Exclusion: BMI > 25, gastro-oesophageal reflux, signs of difficult airway, claustrophobia, anxiety disorder.
No studies since have successfully confirmed or refuted this.
Neuromuscular blockade is implicated in the majority of cases of accidental awareness under general anaesthesia.
Design: Small unblinded case series. 11 volunteers
Intervention: 10 received suxamethonium, 10 received rocuronium. At least 2 week gap between trials.
Observation: Monitored BIS via monitors from 2 different manufacturers. Additional EEG monitoring, including analysis of EMG component. Full AAGBI standards of anaesthetic monitoring.
Inclusion: Anaesthetist, ASA 1/2, aged 25-60,
Exclusion: BMI > 25, gastro-oesophageal reflux, signs of difficult airway, claustrophobia, anxiety disorder.
What did they find?
Numbers: 10 volunteers received suxamethonium, 10 received rocuronium.
Results: Most patients demonstrated a biphasic drop in BIS values with neuromuscular blockade.
Suxamethonium: Initial drop to a median of 81 (IQR 79-84). Second drop at 4 minutes (if still paralysed) to as low as 44 (median 66, IQR 60-75). In 3 trials dropped straight to this lower level (median 67, IQR 61-73).
Rocuronium: Initial drop to 75-85. Subsequent drop after 4 minutes to values as low as 46 (median 73, IQR 66-77).
Many volunteers showed periods of time where they were deemed to be anaesthetised based on their BIS score i.e. <60, with one patient having a BIS score <60 for 7mins 36 seconds on one monitor.
Results: Most patients demonstrated a biphasic drop in BIS values with neuromuscular blockade.
Suxamethonium: Initial drop to a median of 81 (IQR 79-84). Second drop at 4 minutes (if still paralysed) to as low as 44 (median 66, IQR 60-75). In 3 trials dropped straight to this lower level (median 67, IQR 61-73).
Rocuronium: Initial drop to 75-85. Subsequent drop after 4 minutes to values as low as 46 (median 73, IQR 66-77).
Many volunteers showed periods of time where they were deemed to be anaesthetised based on their BIS score i.e. <60, with one patient having a BIS score <60 for 7mins 36 seconds on one monitor.
Is it any good?
Overall: Very much so.
Strengths: Simple but effective design. Standardised approach. More than one brand of BIS monitor used. Parallel use of EEG. Rigorous confirmation of awareness throughout.
Weaknesses: Small numbers. No blinding. No 'under anaesthesia' control group.
Strengths: Simple but effective design. Standardised approach. More than one brand of BIS monitor used. Parallel use of EEG. Rigorous confirmation of awareness throughout.
Weaknesses: Small numbers. No blinding. No 'under anaesthesia' control group.
Final Thoughts
A real 'gamechanger'. Powerful evidence that current BIS algorithms don't adequately take into account the impact of muscle activity as part of their algorithms. This would suggest that they lack the reliability needed to be a useful tool for assessing depth of anaesthesia.
Written: Tom Heaton - 18th July 2015
Reviewed: Not done.
Reviewed: Not done.
