Entropy Interference from Operating Tables

The Association of Anaesthetists (AAGBI) recommends depth of anaesthesia monitoring when patients are anaesthetised with total intravenous anaesthesia (TIVA) and neuromuscular blocking agents to reduce the risk of accidental awareness.1


Entropy Interference from Operating Tables

At our institution, we have become aware of significant interference with entropy processed electroencephalography readings on Datex Ohmeda S5 monitors (GE Healthcare, Chicago, IL, USA), occurring when an Eschmann T20‐1 operating table (Eschmann, Lancing, UK) is plugged in and charging, but disappearing when the table is unplugged. Both response entropy (RE) and state entropy (SE) appear to be affected, producing erroneously high numbers with a divergence in the SE and RE values. On unplugging the table charger, we note that the displayed values for both SE and RE significantly decrease, suggesting the patient has been receiving an inappropriately high dose of anaesthetic (See image to right).

We contacted the table manufacturer, who confirmed that this is a recognised interaction caused by natural leakage (touch current leakage) from the charger. They suggest that it would be seen with any similar equipment connected to both the patient and the mains supply, and that the table should be operated by battery when in use to avoid this. It is our experience, however, that the battery life of the table often does not last the duration of long theatre cases and all-day operating lists. As result, there is a tendency to leave the charger plugged in to enable the table to be moved intra-operatively.

When using the entropy monitor, there is no indication or error message displayed to highlight the fact that this interference is occurring, and so it is not unreasonable to assume that these values could correlate with depth of anaesthesia. Anaesthetists who are not aware of the interaction may increase depth of anaesthesia unnecessarily. Even though a patient may otherwise appear adequately anaesthetised, persistent values corresponding with inadequate depth of anaesthesia are likely to make even the most experienced anaesthetist a little uncomfortable.

Although this interaction has been reported previously 2, it appears to be an ongoing issue. With the increasing popularity of TIVA and the emphasis on minimising accidental awareness following NAP5 3, it is likely that use of depth of anaesthesia monitors will only increase. We suspect that many anaesthetists may not be aware of the problem, and suggest that it could be a potential risk to patients resulting in over treatment with longer wake‐up times, possibly impacting length of stay, list turnover and unnecessary use of critical care beds.



  1. Association of Anaesthetists of Great Britain and Ireland. Recommendations for standards of monitoring during anaesthesia and recovery 2015: association of anaesthetists of Great Britain and Ireland. Anaesthesia 2016; 71: 85–93.
    Wiley Online Library | PubMed | Web of Science® | Google Scholar
  2. Halligan C. How aware is your operating table?
    http://www.respond2articles.com/ANA/forums/1396/ShowThread.aspx (accessed 15/01/2018) | Google Scholar
  3. Pandit JJ, Cook TM, Jonker WR, O'Sullivan E, 5th National Audit Project (NAP5) of the Royal College of Anaesthetists and the Association of Anaesthetists of Great Britain and Ireland. A national survey of anaesthetists (NAP5 Baseline) to estimate an annual incidence of accidental awareness during general anaesthesia in the UK. Anaesthesia 2013; 68: 343–53.
    Wiley Online Library | CAS | PubMed |Web of Science® | Google Scholar


Source: Wiley Online Library


Entropy Interference from Operating Tables

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The Operating Theatre Journal, OTJ, is published monthly and distributed to every hospital operating theatre department in the UK. The distribution includes both the National Health Service and the Private Sector.

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