Awareness during anaesthesia

Jonathan G Hardman BMedSci (Hons) BM BS DM FANZCA FRCA CCST (Anaes) MEWI, Associate Professor (Reader) in Anaesthesia & Consultant Anaesthetist, University Department of Anaesthesia, University Hospital, Nottingham NG7 2UH

Tel: 0115 823 1002;  Fax: 0115 970 0739;  Email: j.hardman@nottingham.ac.uk

Summary

Intra-operative awareness is traumatic for patients and is associated with post-operative psychological sequelae. Litigation frequently follows. Awareness occurs during approximately 0.03% of general anaesthetics and is much more likely when muscle relaxants (paralysing drugs) are used. The most common cause of awareness is administration of inadequate anaesthetic agent. Physical signs of awareness include increased heart rate and blood pressure, sweating, tear formation and movement during surgery; these may be masked by drugs or co-existing pathology. Certain situations are “high-risk” with respect to awareness; these include Caesarean section, emergency surgery and major blood loss.

Introduction

Perception of the environment and the formation of memories during anaesthesia is termed awareness. It is a relatively common complication and may be extremely distressing for patients, particularly if pain is recalled. Recall that is spontaneous or easily provoked is termed explicit, while recall that is not consciously perceived (but may affect behaviour or mood at a later time) is termed implicit. Awareness of painful surgery while paralysed is usually distressing and life-altering, while awareness of movement or conversations during anaesthesia may have milder sequelae. Psychological consequences include insomnia, depression, anxiety and post-traumatic stress disorder with distressing flashbacks and alteration in personality. The majority of patients who have experienced intra-operative awareness fear future anaesthesia and surgery. Consequences for the anaesthetist are common; litigation follows frequently, and the majority of claims are successful, with an inadequate standard of care being indicated.

Historically, awareness had an incidence of awareness of up to 7%. However,  in recent times estimates of the incidence of awareness with explicit recall are typically 0.03% of general anaesthetics (1 in 3000). Painless awareness is around 10 times more common than awareness that includes pain. Such painful awareness is at least twice as likely when neuromuscular blocking (paralysing) drugs are used.

Light anaesthesia (i.e. the use of smaller doses of anaesthetic agents) is associated with a greater risk of awareness. Poor anaesthetic technique is frequently to blame, and errors include omission or late commencement of anaesthetic agent, inadequate dosing or failure to recognise the physical signs of awareness. Under-dosing of anaesthetic agent often occurs during attempts to correct a reduced blood pressure. Various types of surgery are associated with increased risk of awareness: cardiac surgery, emergency surgery, surgery associated with substantial blood loss and Caesarean section.

The selection of anaesthetic dose is based upon the patient’s expected requirement. Requirements vary by around 10-20% for a given age group, and by up to 50% from the very young to the very old (with elderly patients requiring less to cause unconsciousness). Variability is probably greater with intravenous than inhaled anaesthetic agents, and it seems likely that awareness is more likely using intravenous anaesthetic techniques.

Resistance to anaesthetic agents is, in general, rare. Pyrexia, hyperthyroidism, obesity, anxiety, tobacco smoking, heavy alcohol use, use of recreational drugs (e.g. opioids, amphetamines, cocaine), chronic use of sedatives (e.g. temazepam) and prior and repeated exposure to anaesthetic agents may incur some increase in dose requirements, but the evidence is patchy. Conversely, pregnancy, hypothyroidism, hypothermia, hypotension and old age reduce dose requirements.

Equipment malfunction has been associated with awareness. Such failures include empty vaporisers (which are used to deliver inhaled anaesthetic drugs), impurities in inhaled agents and disconnection from the anaesthetic machine. Blockage of intravenous infusion pumps or failure of the intravenous cannula (the ‘drip’) risk awareness during intravenous anaesthesia.

The physical signs of awareness are generated through sympathetic nervous system activation; they include: movement during surgery, sweating, increased heart rate and blood pressure, tear formation and pupillary dilatation and reactivity to light. Awareness during anaesthesia may be detected through the use of specialised monitoring such as bispectral index (BIS). The clinical signs of awareness may be masked by some disease states and concurrent medications. These include hypothyroidism, hypothermia and beta blockade.

If intra-operative signs suggest that a patient is experiencing awareness then anaesthesia should be deepened immediately. Administration of a benzodiazepine (e.g. midazolam) may reduce further recall, but does not protect against events already experienced. If a patient complains post-operatively of intra-operative awareness, the anaesthetist should establish the timing of the episode and distinguish between dreaming and awareness. Recalled events (e.g. conversations or pain) should be noted in the medical records. This information will be of great importance and usefulness should litigation follow.

All recall is not necessarily awareness. Fraudulent claims of awareness do occur; such claims often include unrealistically vivid recall of entire operations. Patients may occasionally confuse awareness with memories of events prior to or immediately after anaesthesia, and such confusion is best determined by careful and sensitive questioning. However, the vast majority of claims of awareness have been genuine and denial of the patient’s claim may worsen their psychological outcome. Referral to a psychologist / psychiatrist should be made if the patient is suffering low mood, anxiety, sleep disturbance or flashbacks.

Sedative premedication reduces the incidence of awareness, but is not considered a minimum standard of care because of the potential adverse effects of such premedication. Adjusting the dose of anaesthetic agents for each patient reduces the risk of awareness and of side-effects of anaesthetic agents. Due to the increase in the risk of awareness caused by the use of muscle relaxants, they should be used only when necessary (e.g. where a tracheal tube is required or muscle relaxation is needed to facilitate surgical access). In high-risk situations (e.g. Caesarean section, cardiac surgery, concurrent beta blockers) the use of a monitor of depth of anaesthesia (e.g. BIS) is justified, but is not considered, currently, to be mandatory.

It is important to note that awareness may occur (rarely) despite excellent care. Successful defence against litigation requires thorough medical records that include the timing and dose of anaesthetic agents.