The article on subjective assessment of anaesthesia by Hadzidiakos and colleagues  is a fascinating glimpse into the ‘art’ of assessing ‘depth of anaesthesia’. As the authors  point out, ‘hypnosis’ (i.e. unconsciousness) is the major aim of general anaesthesia but we anaesthetists continue to try and quantify this state by using surrogate markers such as the clinical signs of anaesthesia or, more recently, some numerical surrogate provided by an electronic ‘Anaesthesia Brain Monitor’. Such monitors provide an output which, because of very wide inter-individual variability, can only be understood in terms of a patient's probability of being conscious. Whatever number one chooses to aim for will represent either too much anaesthetic drug for some patients or not enough for others. I doubt that the average patient would relish the idea that they are about to be paralysed and undergo major surgery with even a 1% probability of being conscious, let alone a 10–20% probability!
While I agree that there is no ‘gold standard’ for defining depth of anaesthesia (we are not even clear if such an entity as ‘depth’ of anaesthesia exists) but there is a ‘gold standard’ for detecting the onset of consciousness when muscle relaxants are being used – the Isolated Forearm Technique (IFT) . It is thus extremely disappointing to note that the authors continue to perpetuate the myth that the IFT can only be used for 30 min . The IFT was first described by Tunstall  as a method of directly assessing the presence of consciousness during Caesarean section, up until delivery of the baby. After this point the tourniquet was deflated and anaesthesia was deepened. Since Dr Tunstall used a suxamethonium infusion, when the tourniquet was released the arm became paralysed. I had the privilege of training under Dr Tunstall in the 1970s and with his encouragement I investigated the IFT during non-obstetric surgery using non-depolarizing relaxants and reported in 1979  that the IFT could be used very effectively during prolonged surgery (in one of these early cases I used the IFT for over 4 h in a patient having surgery for oesophageal varices). Since then many publications have described the technique in detail [5-8]. I would urge the authors, and readers in general, to read further about the technique. If any wish to try it for themselves I would add the caveat that the IFT does not work well with pancuronium and doses of rocuronium still need to be investigated.
1. Hadzidiakos D, Nowak A, Laudahn N, Baars J, Herold K, Rehberg B. Subjective assessment of depth of anaesthesia by experienced and inexperienced anaesthetists. EurJ Anaesth
2006; 23: 292–299.
2. Jessop J, Jones JG. Conscious awareness during general anaesthesia – what are we attempting to monitor? BrJ Anaesth
1991; 66: 635–637.
3. Tunstall ME. Detecting wakefulness during general anaesthesia for caesarean section. BMJ
1977; 1: 1321.
4. Russell IF. Auditory perception under anaesthesia. Anaesthesia
1979; 34: 211.
5. Russell IF. Conscious awareness during general anaesthesia: relevance of autonomic signs and isolated arm movements as guides to depth of anaesthesia. In: Jones JG, ed. Depth of Anaesthesia. Bailliéres Clinical Anaesthesiology.
London: Bailliére Tindall, 1989: 511–532.
6. Russell IF, Wang M. Absence of memory for intraoperative information during surgery under adequate general anaesthesia. Br J Anaesth
1997; 78: 3–9.
7. Russell IF. Memory when the state of consciousness is known: studies of anaesthesia with the isolated forearm technique. In: Ghoneim MM, ed. Awareness during Anesthesia.
Oxford, UK: Butterworth Heinemann, 2001: 129–143.
8. Russell IF. The Narcotrend ‘depth of anaesthesia’ monitor cannot reliably detect consciousness during general anaesthesia: an investigation using the isolated forearm technique. Br J Anaesth
2006; 96: 346–352.