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Awareness trials

Created: 13/7/2005
Updated: 2/2/2007


A number of large-scale clinical trials conducted in the USA and elsewhere evaluated more than 30,000 patients to investigate the incidence of awareness with recall and the impact of bispectral index (BIS) monitoring on predicting and preventing awareness.

AIM Trial

19,575 patients were enrolled in this multi-centre US study. This study established that awareness with recall occurs in the USA at a rate of 1-2 cases per 1000 patients receiving general anaesthesia. Twenty million patients receive general anaesthesia in the USA each year, suggesting that approximately 100 patients experience awareness with recall each operating weekday. Study findings were consistent with international research evaluating the incidence of awareness (Anesth Analg 2004; 99: 833-9).


2503 high awareness-risk patients participated in this clinical trial. Patients evaluated underwent a variety of procedures, including cardiac surgery, trauma surgery, rigid bronchoscopy, and Caesarean section. Patients were randomly assigned to receive either standard general anaesthesia, or general anaesthesia with BIS monitoring. Study findings revealed that BIS-guided anaesthesia reduced the incidence of awareness with recall by approximately 82% (Lancet 2004; 363: 1757-63).

Safe-1 Trial

This was a prospective study, carried out in Sweden, to assess patients' recall of awareness during surgery. The trial recruited 11,785 patients who had undergone general anaesthesia. Patients were interviewed on three occasions:

  • In recovery

  • 1-3 days

  • 7-14 days

The incidence of awareness was 0.18% in those given neuromuscular blocking drugs (NMBs) and 0.10% in the absence of NMBs. There were 18 cases of awareness with explicit recall identified among the 11,785 patients.

Nine of these 18 patients could be located after approximately 2 years and agreed to an interview about possible persisting problems. Four of the nine interviewed patients were still severely disabled as a result of psychiatric/psychological sequelae and fulfilled all criteria of post-traumatic stress syndrome (PTSD). Another three patients were found to suffer from some of the symptoms required for the diagnosis of PTSD, although these were not unbearable in daily life and were of diminishing severity. Only two among nine interviewed patients were free from mental problems related to their unsuccessful anaesthetic approximately 2 years earlier.


[i] Lennmarken C, Bildfors K, Samuelsson P, Sandin R. Victims of awareness. Acta Anaesthesiol Scand 2002; 46: 229-31

[ii] Sandin RH, Enlund G, Samuelsson P, Lennmarken C. Awareness during anesthesia: a prospective case study. Lancet 2000; 355: 707-11

SAFE2 Trial

This study was conducted in Sweden and investigated the incidence of awareness in two groups of consecutively treated patients. 7826 patients receiving standard general anaesthesia were compared with 4945 patients who received BIS-guided anaesthesia. In this study, routine use of BIS monitoring reduced the incidence of awareness in the general patient population by 77% compared with historical control.

Further studies:

Intraoperative Awareness in a Regional Medical System: A Review of 3 Years' Data.
Pollard, Richard J. M.D. *; Coyle, Joseph P. M.D. +; Gilbert, Richard L. M.D., M.B.A. ++; Beck, Janet E. C.P.H.Q. [S]; Intraoperative Awareness in a Regional Medical System: A Review of 3 Years' Data, Anesthesiology. 106(2):269-274, February 2007.


[i] Ekman A, Lindholm ML, Lennmarken C, Sandin R. Reduction in the incidence of awareness using BIS monitoring. Acta Anaesthesiol Scand 2004; 48: 20-6

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