|The isolated forearm technique was originally described by Tunstall in obstetric anaesthesia. A tourniquet is applied to the patient's upper arm and inflated above systolic blood pressure, before the administration of muscle relaxants. Movement of the arm, either spontaneously or to command, indicated wakefulness, although not necessarily explicit awareness. At 15 - 20 minutes the anaesthetist lets the tourniquet down, and may then reinflate it if further muscle relaxant is required. Some would argue that response to command during surgery is a late sign when attempting to prevent awareness; however, not all patients responding have any recall.
One study assessed response to command during deep sedation targeted using bispectral index monitoring. Fifty-six patients in the study were repeatedly commanded to squeeze the observer's hand and 37 patients gave an unequivocal response at some point. Of these patients, only nine had any explicit recall of the events. This demonstrates that postoperative recall and reporting by patients underestimates the incidence of wakefulness and 'near-awareness' during anaesthesia.
Another study from 1986 compared an etomidate intravenous anaesthetic with nitrous oxide and fentanyl-based anaesthesia. 44% of the nitrous oxide group showed signs of wakefulness while anaesthetised.
[i] Monitoring versus remembering what happened. Anesthesiology 2003; 99: 570-5
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[iii] Midazolam-alfentanil: an anaesthetic? An investigation using the isolated forearm. Br J Anaesth 1993; 70: 42-6
[iv] Recovery of consciousness after thiopental or propofol. Bispectral index and the isolated forearm technique. Anesthesiology 1997; 86: 613-9
[v] Detecting wakefullness during GA for caesarian sections. BMJ 1977; 1: 1321.