Depth of Anaesthesia Monitoring
awareness bis bispectral index depth of anaesthesia entropy monitoring Aug 26, 2024
“The patient is moving!” 😲
We have all heard this comment from a surgeon. But what does it really mean? Awareness under general anaesthesia is quoted to occur as commonly as 1/1000 surgeries. So how do we reduce this risk? 🤔
Depth of Anaesthesia (DOA) monitoring is a broad term encompassing clinical judgement, physiological parameters, gas analysis, and dedicated devices that read EEG to estimate the risk of awareness. 🧠
Here it is in a nutshell:
Clinical judgement:
We commonly maintain general anaesthesia using propofol or volatile agents. 💉 Based on age, weight, frailty, comorbidities, and our intimate understanding of the drugs, we can estimate how much to give a patient. This may be in the form of a target entered into the infusion pump, or the fraction of volatile agent delivered to the patient. Although this method is safe in most patients, interpatient variability and human error can result in excessive depth or awareness. ⚖️
Physiological parameters:
Elevated heart rate & blood pressure, pupillary dilation, and movement are all indicators of light anaesthesia. 🩺 Although these are not reliable DOA monitors, these parameters in the right context may identify a problem with dosing, equipment, or delivery. 🔍
Gas analysis:
We can estimate the concentration of anaesthetic gas in the brain if we know the concentration in the bloodstream. 🧪 And we can estimate the concentration of gas in the bloodstream by measuring the concentration in the alveoli! The little ETSev or ETDes number on the screen is a reasonable estimate of DOA when using volatile agents. 📊 This allows us to estimate the “MAC” based on age, which is a proxy for DOA. 🧩
EEG analysis:
The most advanced DOA monitor is the use of EEG (electroencephalography)-based devices. 🧠📉 The Bispectral Index (BIS) monitor, for example, provides a numerical value between 0 and 100, with a target range of 40-60 during general anaesthesia. Not only does this method reduce the risk of awareness, it also prevents excessive dosing. 🎯
As always, clinical judgement, vigilance, and a deep understanding of the equipment and drugs are the best ways to keep your patient safe.
Build Knowledge ✅
Improve Safety ✅
Ref: Nick Goddard, David Smith, Unintended awareness and monitoring of depth of anaesthesia, Continuing Education in Anaesthesia Critical Care & Pain, Volume 13, Issue 6, December 2013, Pages 213–217, https://doi.org/10.1093/bjaceaccp/mkt016
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