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Awareness in general anaesthesia.

As the concentrations of anaesthetics in the brain increase the patient initially loses some sensations, including pain, and then becomes unconscious, not recognising or remembering what happens. At higher concentrations muscle relaxation occurs and surgery becomes easy. Thus if anaesthetics only are used and conditions are acceptable for surgery, the patient cannot be aware of what is happening. Neuromuscular blocking drugs have changed this, since patients can be paralysed and surgery proceed while the concentrations of the anaesthetics in the brain are insufficient to produce unconsciousness or even analgesia. The combination of neuromuscular blockade and light anaesthesia is well known to leave some patients aware of what is happening-and some, frighteningly, in pain.' Waters2 has discussed many of the factors in the techniques of anaesthesia that could produce this state. Awareness is most commonly reported as occurring during general anaesthesia in operative obstetrics. Here the anaesthetist has two problems. Firstly, he has to anaesthetise the mother and, secondly, he has to produce the minimum depression of the baby, who may well be already in trouble. Additional anaesthetic and opiate drugs crossing the placenta will depress respiration and circulation further. In most series of women given anaesthesia for caesarean section awareness has been reported in about one case in 12 when only nitrous oxide is used to maintain anaesthesia, with an incidence of one in 50 or less when the nitrous oxide is supplemented by halothane, methoxyflurane, trichlorethylene, or enflurane.3-7 Such supplementation does not appear to add to the risks for the baby, and even if it did the depression should be treatable with normal methods of neonatal resuscitation-it would be of short duration. But the anaesthetist must balance the two risks. Two papers have described techniques which should allow anaesthetists to assess whether individuals are aware during anaesthesia or whether a particular technique is likely to be associated with too light a plane of anaesthesia. Tunstall8 9 has described the isolated-forearm technique to assess wakefulness during surgery. After induction of anaesthesia but before any suxamethonium is given a sphygmomanometer cuff is inflated to 250 mm Hg on one arm and kept at that pressure. The patient's hand is clasped by the anaesthetist and the patient is asked to squeeze or grip the hand and subsequently to let go. If the patient responds to both commands Tunstall considers her to be wakeful. Using suxamethonium