Summary We have studied the effect of nitrous oxide on the cuff pressure of a laryngeal mask both in vitro and in vivo. In laboratory tests, we showed that nitrous oxide and carbon dioxide diffuse across the cuff wall much more rapidly than nitrogen and oxygen. Differing partial pressures of these gases across the cuff wall therefore give rise to changes in volume and pressure within the cuff. We then studied 18 patients undergoing general anaesthesia with nitrous oxide, and found a consistent and linear increase in cuff pressure in all patients. After 30 min, the mean pressure had increased by 30 mmHg, and there was approximately 10% nitrous oxide in the cuff. It is difficult to relate these findings to pressure on pharyngeal structures, but methods of limiting the rise in intracuff pressure are discussed.

In v i m and in vivo studies A. B. LUMB AND M.W. WRIGLEY SummaryWe have studied the effect of nitrous oxide on the cufpressure o f a laryngeal mask both in vitro and in vivo. In laboratory tests. we showed that nitrous oxide and carbon dioxide diffuse across the cuff wall much more rapidly than nitrogen and oxygen. Diflkring partial pressures of these gases across the cuff wall therefore give rise to changes in volume and pressure within the cuff. We then studied 18 patients undergoing general anaesthesia with nitrous oxide, and found a consistent and linear increase in cufl pressure in all patients. After 30 min. the mean pressure had increased by 30 mmHg, and there was approximately 10% nitrous oxide in the cuff: It is dificult to relate these findings to pressure on pharyngeal structures, but methods of limiting the rise in intracufl pressure are discussed.

We have examined a combination of two local anaesthetics to see if the resultant solution is superior to the agents individually. This study shows that a mixture of bupivacaine and lignocaine provided an excellent alternative to bupivacaine alone, and was superior to 2% lignocaine with adrenaline for elective Caesarean section. By reducing the dose of bupivacaine used, the combination may reduce the risk of cardiotoxicity.

Propopol in myotonic dystrophyA 37-year-old, slightly mentally retarded woman with Steinert myotonic dystrophy was admitted for cholecystectomy. Premedication consisted of midazolam 4 mg intramuscularly and I hour later an epidural catheter was inserted at the T,,-Tll interspace and 15 ml 0.5% bupivacaine injected. Ringer's lactate solution 500 ml and two doses of ephedrine 15 mg were required to maintain a normal arterial blood pressure. When the block was fixed and the haemodynamic status had stabilised, general anaesthesia was induced with propofol 2 mg/kg.Immediately following the propofol injection myoclonic movements occurred in the extremities and appeared to provoke a myotonic state which began in the upper limbs and extended to the trunk. It was possible to intubate the trachea easily without muscle relaxants and the myotonia resolved with the introduction of isoflurane. Anaesthesia was maintained thereafter with isoflurane in 50% nitrous oxide and oxygen; the lungs were mechanically ventilated without the use of muscle relaxants. Recovery from anaesthesia was uneventful; analgesia was provided by an infusion of fentanyl into the epidural space.Propofol has been used previously in a patient with myotonic dystrophy without a problem,' although in another case exaggerated physiological responses were reported.* In our patient, the myotonic response followed an episode of myoclonia and may have been due to a centrally mediated increase in muscle stimulation.