Depth of anaesthesia monitors – Bispectral Index (BIS), E-Entropy and Narcotrend-Compact M

EEG-based depth of anaesthesia monitors are designed to indicate the probability of consciousness with explicit recall in patients receiving general anaesthetics, and to aid the tailoring of anaesthetic dose to the individual patient to avoid inadequate or excessively deep levels of anaesthesia. Measuring a patient's response to anaesthesia is important clinically because individual variation in response to anaesthetics can occasionally lead to inadequate or excessively deep levels of anaesthesia. An inadequate level of anaesthesia can result in patient awareness during surgery, which can cause post-traumatic stress disorder in some patients. Conversely, an excessively deep level of anaesthesia can result in prolonged recovery and has been linked to an increased risk of postoperative adverse outcomes, including myocardial infarction, stroke and cognitive dysfunction in older patients.

The aim of this evaluation is to determine the clinical and cost effectiveness of 3 depth of anaesthesia monitors, in combination with standard clinical monitoring, in patients receiving general anaesthesia.

General anaesthesia is a reversible state of controlled unconsciousness that is achieved with drugs which prevent awareness, pain, recall, distress and movement in patients during surgery. It is estimated that 2.4 million people received general anaesthesia in 2007 in England. Approximately half of those who have a general anaesthetic also receive muscle relaxants.

Some common adverse outcomes of general anaesthesia include nausea, headaches and dizziness. Less common adverse outcomes include neurological and cardiovascular morbidity, and unintended patient awareness and recall. Most studies suggest that between 1 and 2 people in 1,000 experience awareness or recall during general anaesthesia, with a third of these also experiencing pain. For those who experience awareness during anaesthesia, there can be long-term effects such as clinical depression, anxiety, nightmares, flashbacks and, in some cases, severe post-traumatic stress disorder.

Awareness during anaesthesia is more likely during certain types of surgery in which lower levels of anaesthetic are often used. These include cardiac surgery, airway surgery, obstetric surgery or emergency surgery for major trauma. The use of muscle relaxants can also increase the risk of patient awareness because they allow a lower level of anaesthetic to be used. Muscle relaxants also prevent patients from moving. This limits the patient's ability to communicate with the surgical team and means that the anaesthetist has to use other clinical information to judge the patient's state of consciousness.

Anaesthetic agents can affect the body's physiology, in particular, the cardiovascular system. Adverse outcomes of excessively deep general anaesthesia include prolonged recovery, particularly in people with a high BMI. In severe cases or in at-risk patient groups (for example, older patients, patients with liver disease, and patients with poor cardiovascular function), excessively deep anaesthesia can result in haemodynamic instability and respiratory complications (which can be fatal without cardiorespiratory support). Inappropriately deep anaesthesia has also been linked to an increased risk of post-operative complications such as myocardial infarction and stroke in older patients. There is some evidence to suggest a link between longer term morbidity (for example, cognitive dysfunction) and mortality, and the depth of anaesthesia.

Groups of patients who are considered at higher risk of unintended awareness during general anaesthesia include patients with high opiate or high alcohol use, patients with airway problems, and patients with previous experience of accidental awareness during surgery. The risk of unintended awareness is also raised by the concomitant use of muscle relaxants, particularly with total intravenous anaesthesia. Older patients, patients with comorbidities and those undergoing certain types of surgery are also considered at higher risk of unintended awareness because they are at greater risk of haemodynamic instability during surgery. Therefore, lower levels of anaesthetic are often used to prevent adverse effects on the cardiovascular system, which can result in these patient groups receiving inadequate levels of anaesthesia.

Before general anaesthesia, the anaesthetist interviews the patient and reviews the medical records to determine the type and dose of anaesthetic and any monitoring that may be needed. Some patients may receive a premedication before the administration of general anaesthetic. This is to allay anxiety and reduce side effects such as nausea and vomiting. Monitoring devices (for example, to monitor blood pressure and blood oxygen levels) are connected to the patient before general anaesthesia is induced. Monitoring devices are removed after the patient has fully recovered from the effects of the anaesthesia and may be temporarily disconnected when the patient is moved into or out of the operating theatre.

In the UK, anaesthesia is usually induced in an anaesthetic room. General anaesthesia is administered intravenously or by inhalation until the patient loses consciousness. Further anaesthetic procedures (for example, intubation of the trachea) may be carried out before moving the patient into the operating theatre.

During surgery, other drugs may be given with the general anaesthesia. These may include analgesics, regional anaesthesia, antibiotics, anti-emetic drugs and muscle relaxants. In current NHS clinical practice, a patient's response to anaesthesia during surgery is assessed by clinical observation of signs such as excessive tear formation (lacrimation), sweating, pupillary size and reactivity, and the use of supplementary monitoring devices. These devices include an electrocardiograph (ECG) to measure the speed and rhythm of the heart; a non-invasive blood pressure monitor; a pulse oximeter to detect the pulse and estimate the amount of oxygen in the blood; a device to measure the patient's temperature; a device to monitor end-tidal anaesthetic concentration (for inhaled anaesthesia) and provide a minimum alveolar concentration (MAC) value; a nerve stimulator (if a muscle relaxant is used); and a capnograph to monitor the inhaled and exhaled concentration of carbon dioxide. Additional monitoring equipment such as a cardiac output monitor may be used for some patients or certain types of surgery.

Be aware that some pulse oximeters can underestimate or overestimate oxygen saturation levels, especially if the saturation level is borderline. Overestimation has been reported in people with dark skin. See also the NHS England Patient Safety Alert on the risk of harm from inappropriate placement of pulse oximeter probes.

After surgery, the administration of anaesthetic is stopped, muscle relaxant drugs are reversed (if used) and analgesics are given as appropriate. Patients are extubated (if necessary) before being moved to the recovery room and regaining consciousness. Once they have recovered from the anaesthetic and meet the criteria for discharge after anaesthesia, they can be discharged from recovery to a general ward. When patients do not meet the discharge criteria, they remain in the recovery room until assessed by an anaesthetist. After this assessment, any patient not meeting the discharge criteria is transferred to an appropriate unit such as the high dependency unit.