Correspondence
211
Auditory perception under anaesthesia A recent editorial (Anaesthesia, 1978, 33, 131) on this topic discussed the recall of events occurring under anaesthesia when patients were subsequently hypnotised. It then drew attention to the importance of Tunstall’s unconfmned findings during anaesthesia for Caesarean section. Tunstall found that, during the first few minutes of anaesthesia for Caesarean section, patients can obey commands to move the unparalysed hand. He uses this to determine a level of light anaesthesia he now calls ‘amnesic awareness’. I should like to confirm his findings and describe a modification of the technique for use with total intravenous anaesthesia. Following a sleep dose of thiopentone Tunstall occludes the circulation to one arm with a wide, padded pneumatic tourniquet, then administers suxamethonium to facilitate intubation. Anaesthesia is maintained with nitrous oxide, oxygen, halothane and a suxamethonium drip. Once the baby is delivered the cuff is deflated, halothane is discontinued and anaesthesia is deepened by an intravenous narcotic. A slight modification of the technique allows it to be used for prolonged surgical procedures in conjunction with a total intravenous anaesthetic technique. A cuff is inflated immediately before administering the muscle relaxant and deflated 1015 minutes later. This procedure is followed throughout the anaesthetic whenever increments of relaxant are required. Neuromuscular function of the ‘isolated‘ arm is checked at regular intervals with a nerve stimulator. When a patient’s conscious level lightens he responds to the command (recorded on a continuous loop cassette) and a further increment of Althesin is given. With this technique and very little experience the prolonged wake up times found in other studies2v3of intravenous anaesthesia can be avoided. Previously only clinical criteria for gauging anaesthetic depth with an intravenous technique have been used, and these are difficult to evaluate without considerable experience. The absence of spontaneous movement and regular respiration cannot be used in conjunction with muscle relaxants and the behaviour of the pupils is confusing-Althesin causes dilatation and narcotics cause constriction. Sweating may only indicate a physiological response to a warm environment while tachycardia will vary with the use of vagolytics. By using this technique of deepening
anaesthesia from the ‘amnesic awareness’ level the anaesthetist knows fairly accurately the patient’s conscious level, without the fear of true awareness occurring. This is true for most patients but there is the occasional patient who shows signs of light anaesthesia, e.g. brow wrinkling or head shaking, with no visible response to the commands. One patient who behaved in this ‘aberrant’ way early in the series had distinct recall of both intra abdominal traction and of a recorded voice asking her to move her left hand. The left hand was not paralysed but the patient says she was unable to move it. After 2-3 min of head and eyebrow movement a further increment of Althesin was given which lasted to the end of surgery ten minutes later. Since this occurrence all patients who wrinkle their eye brows or shake their heads get an increment of Althesin within one minute and there have been no further cases of awareness. However, this case of awareness is interesting in as much as it occurred without significant muscle relaxation in the left hand, yet the patient could not move the hand. A possible cause could be confusion between right and left hands while consciousness was depressed. Alternatively a nerve block could occur due to uneven pressure from the sphygmomanometer cuff used. This latter is unlikely since the period of awareness probably occurred some 50 min after cuff deflation and 10 min later the patient was extubated and moving all limbs. Previously when a patient was aware and unable to communicate, the lack of movement has always been blamed on muscle relaxants. This case would suggest that some other mechanism may also be involved. Aberdeen Royal Infirmary, Aberdeen
IANF. RUSSELL
References I . TUNSTALL, M.E. (1977) Detecting wakefulness during general anaesthesia for Caesarian Section. British Medical Journal, 1, 1321.
T.M., RAMSAY, M.A.E.,CURRAN, J.P.J., 2. SAVEGE, COTTER, J.. WALLING, P.T. & SIMPSON, B.R. (1975) Intravenous anaesthesia by infusion. Anaesthesia, 30, 757. 3 . JAGO,R.H. & RESTALL, J. (1977) Total Intravenous anaesthesia. A technique based on alphaxalone/ alphadalone and pentazocine. Anaesthesia, 32, 904.
Manual ventilation and the E M 0 system As one who developed a great fondness for his E M 0 during three years at an up-country African Mission Hospital, I read with interest the article by Drs Towey, Jacobacci and Ganapathy (Anaesthesia, 1978,
33, 772). This raised certain caveats in my mind. It is stated that their technique accepts some rebreathing. This is not so, as, unlike the bag in the Mapleson A circuit, the Oxford Inflating Bellows
211
Auditory perception under anaesthesia A recent editorial (Anaesthesia, 1978, 33, 131) on this topic discussed the recall of events occurring under anaesthesia when patients were subsequently hypnotised. It then drew attention to the importance of Tunstall’s unconfmned findings during anaesthesia for Caesarean section. Tunstall found that, during the first few minutes of anaesthesia for Caesarean section, patients can obey commands to move the unparalysed hand. He uses this to determine a level of light anaesthesia he now calls ‘amnesic awareness’. I should like to confirm his findings and describe a modification of the technique for use with total intravenous anaesthesia. Following a sleep dose of thiopentone Tunstall occludes the circulation to one arm with a wide, padded pneumatic tourniquet, then administers suxamethonium to facilitate intubation. Anaesthesia is maintained with nitrous oxide, oxygen, halothane and a suxamethonium drip. Once the baby is delivered the cuff is deflated, halothane is discontinued and anaesthesia is deepened by an intravenous narcotic. A slight modification of the technique allows it to be used for prolonged surgical procedures in conjunction with a total intravenous anaesthetic technique. A cuff is inflated immediately before administering the muscle relaxant and deflated 1015 minutes later. This procedure is followed throughout the anaesthetic whenever increments of relaxant are required. Neuromuscular function of the ‘isolated‘ arm is checked at regular intervals with a nerve stimulator. When a patient’s conscious level lightens he responds to the command (recorded on a continuous loop cassette) and a further increment of Althesin is given. With this technique and very little experience the prolonged wake up times found in other studies2v3of intravenous anaesthesia can be avoided. Previously only clinical criteria for gauging anaesthetic depth with an intravenous technique have been used, and these are difficult to evaluate without considerable experience. The absence of spontaneous movement and regular respiration cannot be used in conjunction with muscle relaxants and the behaviour of the pupils is confusing-Althesin causes dilatation and narcotics cause constriction. Sweating may only indicate a physiological response to a warm environment while tachycardia will vary with the use of vagolytics. By using this technique of deepening
anaesthesia from the ‘amnesic awareness’ level the anaesthetist knows fairly accurately the patient’s conscious level, without the fear of true awareness occurring. This is true for most patients but there is the occasional patient who shows signs of light anaesthesia, e.g. brow wrinkling or head shaking, with no visible response to the commands. One patient who behaved in this ‘aberrant’ way early in the series had distinct recall of both intra abdominal traction and of a recorded voice asking her to move her left hand. The left hand was not paralysed but the patient says she was unable to move it. After 2-3 min of head and eyebrow movement a further increment of Althesin was given which lasted to the end of surgery ten minutes later. Since this occurrence all patients who wrinkle their eye brows or shake their heads get an increment of Althesin within one minute and there have been no further cases of awareness. However, this case of awareness is interesting in as much as it occurred without significant muscle relaxation in the left hand, yet the patient could not move the hand. A possible cause could be confusion between right and left hands while consciousness was depressed. Alternatively a nerve block could occur due to uneven pressure from the sphygmomanometer cuff used. This latter is unlikely since the period of awareness probably occurred some 50 min after cuff deflation and 10 min later the patient was extubated and moving all limbs. Previously when a patient was aware and unable to communicate, the lack of movement has always been blamed on muscle relaxants. This case would suggest that some other mechanism may also be involved. Aberdeen Royal Infirmary, Aberdeen
IANF. RUSSELL
References I . TUNSTALL, M.E. (1977) Detecting wakefulness during general anaesthesia for Caesarian Section. British Medical Journal, 1, 1321.
T.M., RAMSAY, M.A.E.,CURRAN, J.P.J., 2. SAVEGE, COTTER, J.. WALLING, P.T. & SIMPSON, B.R. (1975) Intravenous anaesthesia by infusion. Anaesthesia, 30, 757. 3 . JAGO,R.H. & RESTALL, J. (1977) Total Intravenous anaesthesia. A technique based on alphaxalone/ alphadalone and pentazocine. Anaesthesia, 32, 904.
Manual ventilation and the E M 0 system As one who developed a great fondness for his E M 0 during three years at an up-country African Mission Hospital, I read with interest the article by Drs Towey, Jacobacci and Ganapathy (Anaesthesia, 1978,
33, 772). This raised certain caveats in my mind. It is stated that their technique accepts some rebreathing. This is not so, as, unlike the bag in the Mapleson A circuit, the Oxford Inflating Bellows
