BIS monitors and EEG – The sound of anaesthesia

BIS monitors and EEG – The sound of anaesthesia

With BIS monitors coming under increasing scrutiny from the anaesthesia community, Josh Stephenson spoke to the developers of the electroencephalophone about transforming EEG signals into soundwaves, the development of new monitoring technology and just how effective BIS monitors actually are.

In its November 2012 costing statements on the use of EEG-based depth of anaesthesia monitors, the National Institute for Health and Clinical Excellence (NICE) indicated that between one and two patients per 1000 would report awareness or recall during general anaesthesia [1]. If we are to extrapolate this to the number of patients who require general anaesthesia each year that means a worrying 3,600 patients in the UK per annum may experience awareness, with the possibility of pain during surgery and post-traumatic stress disorder (PTSD)-like symptoms after it. Thus when the Bispectral Index (BIS) monitor was introduced as an effective means of measuring the depth of anaesthesia, it was wildly heralded as a positive step for the community [2]. Since then, however, there has been a backlash against the technology with Professor Pandit, who is the clinical lead of NAP5, having expressed concerns over the accuracy of BIS monitors [3]. This, coupled with concerns over cost-effectiveness, has meant that many anaesthetists are looking back to more traditional methods of anaesthesia monitoring.

In response to this, Dr Bernd Porr, an engineering professor at the University of Glasgow, and Dr John Glen, an anaesthetist previously based at Southern General Hospital Glasgow, have developed the electroencephalophone (EEP). Porr explained how the device works as follows: “Essentially we are recording the brainwaves from the EEG itself and then making those brainwaves audible. So we use the EEG spectrum and multiply all the frequencies by a factor of 48. This is convenient because the 1KHz sample rate turns into a 48KHz sample rate which is applicable for a soundtrack.” However, this meant that the signal wasn’t playing in real time anymore, so Porr and Glen used a ring buffer to get the EEG settling at a lower rate while the ring buffer plays it back at a higher rate. This makes a repetitive sound in which you can distinguish the EEG. In practice, said Porr, “the awake state brain produces higher frequencies so you hear a high hissing sound. When the person is anaesthetised you hear a much lower frequency sound much like a helicopter, or something similar, and so you can distinguish between awake and anaesthetised.”

The benefit of using the EEP monitor is that it “leaves the actual detection of the EEG to the human listener. The pattern recognition on any automated computer test algorithms haven’t really advanced in the past ten years, so in order to [detect the EEG] detection computers need to have very clean signals.” Those clean signals are something that the BIS monitors require for an accurate reading and Porr doesn’t believe this is happening. “The main issue is that I reckon at least 90% of what we measure at the forehead is muscle activity and the BIS monitor is not able to distinguish between it. Muscle activity is essentially coloured noise and there is no way to tell the difference between that and an EEG signal in this region.” This leads Porr to believe that the BIS is possibly not monitoring brain signals at all but rather muscle activity and so it is not a direct measurer of anaesthetic depth at all.

He believes the EEP monitor also promises tremendous ease of use. In a pilot study, 21 out of 23 anaesthetists who had been trained to use the machine could identify whether a patient was awake or anaesthetised, following a reduction in sound that occurred after induction with anaesthesia with propofol. However, Porr and Glen are aware of the limitations of the pilot study: “We did the initial study on quite a small sample and haven’t yet done it on a larger sample. Plus the ethics we had was only for the anaesthesia induction, basically the period where the patient was in the anaesthesia room.

“We plan to do a whole recording from the induction to the recovery, so we need fuller recordings from the whole procedure with neuromuscular blockade and the isolated forearm technique.” There have been early indications that this could provide the gradually shifting sound scale they are looking for.

In addition, they hope to add a visual element to complement the sonic element, so as to not add too much extra noise to an already busy operating theatre. “This could take the form of a curve on the screen, which is not a BIS index, but something which is essentially going up and down.” They also hope to look deeper at the question of whether the EEG is simply recording muscle activity rather than brain activity: “We need to test on a paralysed patient to see how the signals look without any muscle activity there. So this will hopefully validate our approach but at the same time show the community who are using the BIS monitor how much muscle activity there is.”

Certainly in terms of cost the EEP is highly appealing. Currently the BIS monitor can cost £4350 for the standard monitor with extra cost for the sensors. In contrast the EEP is allegedly compatible with upcoming Bluetooth EEG technology that can be run from an iPad meaning the cost could be kept very low: “The production cost would be in the region of £500, but it would go on sale at a price above that; I would say product price would be about £1000, getting lower when higher quantities are available,” said Porr. The hardware is not that expensive, and the Bluetooth EEG technology is getting better so eventually you could just run this on an iPad or Android device. Therefore, in the long run it could be very, very cheap.”

Covidian, the main manufacturers of BIS monitors, commented: “BIS monitoring has been recommended in all intravenous anaesthesia procedures by the National Institute for Health and Clinical Excellence.” They went on to point to recent statistics provided in the Cochran Review (2010), adding: “Compared with standard clinical practice, adjusting primary anaesthetic dosing to maintain BIS values within a target range (typically BIS values of 45 to 60 during maintenance) has, with certain anaesthetic agents, reduced anaesthetic dosing.”

Porr remains unconvinced, however, and believes the anaesthesia community is looking for an alternative. “The community is excited about the fact there is something new coming out which is not the BIS monitor,” he said. “There is hope that this actually works better and is something that people are interested in.”

“We jump quickly to the conclusion that if you put electrodes on the forehead you are measuring brain activity and many people are convinced by that, but you have muscle activity also and this will be picked up by the EEG.”

It is this problem that will need to be answered before the BIS monitor can become an essential part of any operative theatre. While that is being addressed, testing will continue on the EEP in the hope that it can sound out the answers the anaesthetic community seeks.

Josh Stephenson spoke to Bernd Porr, an engineering professor at the University of Glasgow, and anaesthetist Dr John Glen, previously based at Southern General Hospital, Glasgow.

  1. (2008) NICE diagnostics guidance 6 (2012) paragraph 3.4 www.nice.org.uk/guidance/dg6/resources (accessed 22 September 2014)
  2. Bard, J. (2001) AANA Journal 69(6), 477–483
  3. Pandit, J. (2014) Journal of Anaesthesia Practice 8(1), 14–18
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