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Adequacy of Anaesthesia

Adequacy of Anaesthesia (AoA) is a concept made up of various parameters to help clinicians assess patients' individual responses to the delivery of inhaled and intravenous hypnotics, opioids, and NMBAs during general anaesthesia.

Learn more about AoA perioperative outcomes.

The AoA concept was designed to address the different components required for general anaesthesia. Indeed, AoA parameters provide continuous non-invasive measurements of:

  • The state of the brain with SPECTRAL ENTROPYTM
  • Patient's response to surgical stimuli and analgesic medications with SURGICAL PLETH INDEX (SPI)TM
  • Muscle relaxation/recovery with NEUROMUSCULAR TRANSMISSION (NMT)

Perioperative Outcomes

The increase in elderly and obese patients undergoing major surgery represents a new challenge for anaesthetists who may struggle to determine the appropriate drug dosing for such complex patients.

As an example, the UK reported the following statistics:

  • 30% increase in the number of bariatric surgical procedures over ten years1
  • 23% of surgical procedures performed in the population aged over 75 years2


The AoA concept was designed to enable clinicians to accurately personalise anaesthesia delivery, helping lower the risk of complications during and after surgery in all types of patients, including in the most vulnerable patients as mentioned above.

Entropy Monitoring

According to the NAP5 report, the incidence of Accidental Awareness during General Anaesthesia (AAGA) is ~1:8,000 when neuromuscular blockage was used and they are associated with psychological consequences for the patients experiencing them as 51% of episodes led to distress and 41% to longer-term psychological harm. 3

In order to prevent such episodes, in particular when neuromuscular blockade is used or in patients who are judges to have high risk of AAGA for other reasons, the use of depth of anaesthesia monitoring, such as the Entropy monitoring, is recommended. 4

The Entropy measurement may be used as an aid in monitoring the effects of certain anaesthetics agents. It is based on data acquisition and processing of raw electroencephalographic signals (State Entropy = SE) and frontal electromyographic signals (Response Entropy = RE).

The recommended range for both RE and SE is from 40-60, therefore a decrease of SE below 40 may indicate a too deep anaesthesia while an increase above 60 may indicate the need for adjusted titration.

In adults, studies have show that Entropy monitoring may help the user titrate anaesthetic drugs according to the individual needs and may be associated with a reduction of anaesthetic use and faster emergency from anaesthesia. 5


SPI Monitoring

Several studies reported fewer unwanted events, reduced opioid consumption and shorter emergence from anaesthesia, when opioid administration was based upon monitoring of the nociceptive-anti-nociceptive balance.6

The Surgical Pleth Index (SPI) is a parameter that reacts to haemodynamic responses caused by surgical stimuli and analgesic medications. SPI is an algorithm that uses two components of the GE photoplethysmographic signal when measured on GE SPO2 finger sensors only.

By observing the SPI value and trend, clinicians can monitor real time adult patient's responses to surgical stimuli and analgesic medications therefore saving vulnerable time for optimisation analgesia delivery.

The optimal SPI target has not been recommended yet as more studies need to prove the clinically relevant range of SPI measurements. However, in several studies, a range of [20;50] has been considered for guiding opioids titration. 7,8

Published literature suggests that SPI-guided anaesthesia may result in lower remifentanil consumption, more stable haemodynamics and lower incidence of unwanted events. 7,9


Adequacy of Anaesthesia