̽��ֱ�� of Cambridge - anaesthesia

Opinion: Scientists find way to predict who is likely to wake up during surgery

Anonymous — Fri, 22 Jan 2016 15:52:53 +0000

Measuring certain kinds of brain activity may help doctors track and predict how patients will react to anaesthesia before going under for surgery, our research has found.

Doctors currently have no perfectly reliable way of ensuring patients are adequately unconscious before an operation begins. Although rare, the uncertainty sometimes results in traumatic experiences of patients “waking up” during surgery.

Using a technique that measures electrical impulses in the brain of those in various states of sedation, we discovered network “signatures” that can indicate when loss of consciousness will occur.

Doctors can use similar techniques to accurately identify the concentration of drug needed for a patient to lose consciousness and maintain that loss throughout an operation.

Everyone is different

Every day in Australia, more than 6,000 people are anaesthetised for surgery. Doctors need to figure out exactly how much sedative to give them, and keep them unconscious throughout the operation.

Anaesthetists estimate the concentration of sedative required using calculations mainly based on a patient’s weight. As the patient “goes under”, their level of awareness is monitored by observing indirect – and somewhat crude – measures, such as blood pressure, heart rate or physical movement.

This method works well in most cases, but people’s susceptibility to anaesthesia varies. One to two in every 1,000 people report having some awareness or recall during surgery. This equates to 2,000 to 4,000 such cases each year in Australia.

Recollections include hearing people speaking during surgery, sensations of not being able to breathe, and experiencing pain.

Naturally, experiences such as these are emotionally traumatic. What’s more, many suffer mental distress long after the surgery, resulting in negative memories of their hospital experience. Some have even reported a reduction in their quality of life.

Losing consciousness

Figuring out the best way to sedate someone essentially comes down to understanding how the brain gains and loses consciousness; that is, the inner world of awareness, feelings and sensations. Although a challenging theme in neuroscience, rapid advances have been made in this area.

Some theories suggest that key networks of brain areas communicate with each other to integrate information processing and generate consciousness. This communication network gives off signals that indicate how conscious an individual is.

̽��ֱ��networks come about from brain neurons firing simultaneously at a certain frequency. We can observe them by using a non-invasive technique called electroencephalogram (EEG), where sensors placed on the scalp record the neurons' electrical impulses. These recordings provide us with a brain “signature” indicative of awareness levels.

Brain monitoring such as this is not commonly used in the operating theatre today. One reason is that current devices are unable to deal with the considerable variability in how people respond to sedatives. But our study shows that devices calibrated for accurate monitoring based on the latest neuroscientific advances could help reduce the incidence of awareness during surgery.

Our study

We have previously shown that network signals of consciousness can also be seen in some people in a vegetative state.

This gave us an indication of the types of signals that could be seen in those who experience some awareness during surgery, but are unable to communicate. But we also needed to show that a similar brain-based measure worked well in cases where we could manipulate the level of consciousness.

Our latest study, published in the journal PLOS Computational Biology, helped us better understand the transition to unconsciousness during sedation, and how this transition varies from person to person.

We gave a steadily increasing dose of a commonly used anaesthetic called propofol to 20 people. At the same time, we measured the brain networks known to be associated with consciousness using EEG. ̽��ֱ��drug was administered at different dosages, causing varying degrees of mild to moderate sedation across our participant group, rather than complete unconsciousness in all of them.

We also measured the behavioural responsiveness of the participants with a simple task. They were asked to press one button if they heard a “ping” and a different button if they heard a “pong”.

Alongside this, we recorded the concentration of the drug in their blood at different times. Altogether, we got the information needed to connect the activity of their brain networks to their individual drug responses.

̽��ֱ��right measure

We found that the strength of a participant’s brain network was clearly linked to their behavioural responsiveness. In other words, as the brain network indicating consciousness weakened, behavioural evidence of awareness also diminished.

Interestingly, while some participants showed behavioural evidence of consciousness at moderate levels of the anaesthetic, others remained responsive.

We found that it was actually the strength of their brain networks before sedation that predicted why some eventually lost consciousness while others did not. In other words, people with weaker baseline networks of consciousness were able to lose it more quickly than those with stronger baselines.

Our current findings indicate the change in consciousness due to the sedative was clearly correlated with specific patterns of brain network activity. This gives us confidence in making the “reverse inference” that tracking this network activity can be used to infer the true level of consciousness in the absence of behaviour.

Further engineering and testing could help advance and adapt current brain monitoring technology for use in the operating theatre. It is clear that networks measured by an appropriate EEG can capture and explain why people respond differently to anaesthesia.

This monitoring can help doctors optimise the amount of drug needed for someone to become unconscious without increasing risk of complications or awareness during surgery.

Srivas Chennu, Senior Research Associate, ̽��ֱ�� of Cambridge

This article was originally published on ̽��ֱ��Conversation. Read the original article.

̽��ֱ��opinions expressed in this article are those of the individual author(s) and do not represent the views of the ̽��ֱ�� of Cambridge.

Srivas Chennu (Department of Clinical Neurosciences) discusses how doctors could use brain waves to help predict how patients will respond to general anaesthetics.

UCD School of Medicine

Surgery Image 8

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Brain waves could help predict how we respond to general anaesthetics

cjb250 — Thu, 14 Jan 2016 18:54:14 +0000

Currently, patients due to undergo surgery are given a dose of anaesthetic based on the so-called ‘Marsh model’, which uses factors such as an individual’s body weight to predict the amount of drug needed. As patients ‘go under’, their levels of awareness are monitored in a relatively crude way. If they are still deemed awake, they are simply given more anaesthetic. However, general anaesthetics can carry risks, particularly if an individual has an underlying health condition such as a heart disorder.

As areas of the brain communicate with each other, they give off tell-tale signals that can give an indication of how conscious an individual is. These ‘networks’ of brain activity can be measured using an EEG (electroencephalogram), which measures electric signals as brain cells talk to each other. Cambridge researchers have previously shown that these network signatures can even be seen in some people in a vegetative state and may help doctors identify patients who are aware despite being unable to communicate. These findings build upon advances in the science of networks to tackle the challenge of understanding and measuring human consciousness.

In a study published today in the open access journal PLOS Computational Biology, funded by the Wellcome Trust, the researchers studied how these signals changed in healthy volunteers as they received an infusion of propofol, a commonly used anaesthetic.

Twenty individuals (9 male, 11 female) received a steadily increasing dose of propofol – all up to the same limit – while undergoing a task that involved pressing one button if they heard a ‘ping’ and a different button if they heard a ‘pong’. At the same time, the researchers tracked their brain network activity using an EEG.

By the time the subjects had reached the maximum dose, some individuals were still awake and able to carry out the task, while others were unconscious. As the researchers analysed the EEG readings, they found clear differences between those who were responding to the anaesthetic and those who remained able to carry on with the task. This ‘brain signature’ was evident in the network of communications between brain areas carried by alpha waves (brain cell oscillations in the frequency range of 7.5–12.5 Hz), the normal range of electrical activity of the brain when conscious and relaxed.

In fact, when the researchers looked at the baseline EEG readings before any drug was given, they already saw differences between those who would later succumb to the drug and those who were less responsive to its effects. Dividing the subjects into two groups based on their EEG readings – those with lots of brain network activity at baseline and those with less – the researchers were able to predict who would be more responsive to the drug and who would be less.

̽��ֱ��researchers also measured levels of propofol in the blood to see if this could be used as a measure of how conscious an individual was. Although they found little correlation with the alpha wave readings in general, they did find a correlation with a specific form of brain network activity known as delta-alpha coupling. This may be able to provide a useful, non-invasive measure of the level of drug in the blood.

“A very good way of predicting how an individual responds to our anaesthetic was the state of their brain network activity at the start of the procedure,” says Dr Srivas Chennu from the Department of Clinical Neurosciences, ̽��ֱ�� of Cambridge. “ ̽��ֱ��greater the network activity at the start, the more anaesthetic they are likely to need to put them under.”

Dr Tristan Bekinschtein, senior author from the Department of Psychology, adds: “EEG machines are commonplace in hospitals and relatively inexpensive. With some engineering and further testing, we expect they could be adapted to help doctors optimise the amount of drug an individual needs to receive to become unconscious without increasing their risk of complications.”

Srivas Chennu will be speaking at the Cambridge Science Festival on Wednesday 16 March. During the event, ‘Brain, body and mind: new directions in the neuroscience and philosophy of consciousness’, he will be examining what it means to be conscious.

Reference
Chennu, S et al. Brain connectivity dissociates responsiveness from drug exposure during propofol induced transitions of consciousness. PLOS Computational Biology; 14 Jan 2016

Image
Brain networks during the transition to unconsciousness during propofol sedation (drug infusion timeline shown in red). Participants with robust networks at baseline (left panel) remained resistant to the sedative, while others showed characteristically different, weaker networks during unconsciousness (middle). All participants regained similar networks when the sedative wore off (right).

̽��ֱ��complex pattern of ‘chatter’ between different areas of an individual’s brain while they are awake could help doctors better track and even predict their response to general anaesthesia – and better identify the amount of anaesthetic necessary – according to new research from the ̽��ֱ�� of Cambridge.

A very good way of predicting how an individual responds to our anaesthetic was the state of their brain network activity at the start of the procedure

Srivas Chennu

Brain networks during the transition to unconsciousness during propofol sedation

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