British journal of anaesthesia
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The performance of a system to control the alveolar concentration of halothane in patients undergoing halothane and nitrous oxide or halothane anaesthesia with controlled ventilation has been evaluated. The method involved the identification and quantification of the uptake characteristics of patients from their early response to the anaesthetic and implements the vaporizer control necessary to achieve and maintain a desired alveolar halothane concentration. Initial targets are based on the concept of MAC, but modifications to the desired alveolar concentration may be effected readily by the anaesthetist at any time during the procedure if evaluation of the normal clinical signs indicates inappropriate depth of anaesthesia. The results obtained during anaesthesia for routine surgery in 80 patients demonstrated that the system was accurate, stable, robust and able to adapt for variability between patients in the uptake of halothane.
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The pharmacokinetics of ketamine 2 mg kg-1 i.v. and 6 mg kg-1 i.m. were investigated in nine children undergoing minor surgery. After either route of administration plasma ketamine concentrations were similar to those found in adult patients receiving the same dose, except at later times after i.v. injection, when concentrations were smaller in children. Also, absorption after i.m. injection appeared to be more rapid in children. ⋯ Concentrations of ketamine upon awakening in a further group of nine children receiving ketamine as the sole anaesthetic showed large inter-individual variation. The concentrations were greater than those previously reported for adults. The greater dose requirements in children, compared with adults, are probably attributable to pharmacodynamic rather than pharmacokinetic factors.
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Ten patients with cerebral tumours were anaesthetized with thiopentone, 70% nitrous oxide in oxygen and fentanyl. Ventilation was controlled to give mean PaCO2 4.8 (range 3.6-6.7) kPa. Enflurane 2% was administered and ICP and MAP were recorded continuously for 10-15 min. ⋯ There were significant decreases in MAP (P less than 0.001) and CPP (P less than 0.001) during the administration of enflurane. In four patients the administration of enflurane had to be terminated prematurely because of a low CPP. Thus, enflurane has very little effect on ICP in patients with cerebral tumours and low concentrations of enflurane can safely be used during anaesthesia for intracranial operations, provided that the arterial pressure is monitored carefully.
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Comparative Study
Neuromuscular blocking and autonomic effects of vecuronium and atracurium in the anaesthetized cat.
The effects of vecuronium and atracurium on neuromuscular transmission, on the responses of the heart rate to vagal stimulation and on the responses to preganglionic stimulation of the nictitating membrane were compared in the chloralose-anaesthetized cat. Vecuronium was four times more potent than atracurium as a neuromuscular blocking agent, whereas the two compounds had similar potencies in blocking the effects of stimulation of the cardiac vagus. The vagal/neuromuscular ratios measured at 50% inhibition were 96 for vecuronium and 25 for atracurium. ⋯ Both compounds had longer time-courses of action than suxamethonium. Very large doses of vecuronium decreased the responses of the preganglionic stimulation of the nictitating membrane, suggesting that at high doses the compound possesses ganglion blocking activity. Large doses of atracurium also decrease the nictitating membrane responses and, in some cats, contractions of the nictitating membrane associated with increases in heart rate and arterial pressure were observed.
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A method of closed-circuit anaesthesia has been developed in which the end-tidal concentration of the volatile anaesthetic agent used is controlled automatically using a closed-loop servo system. End-tidal anaesthetic concentrations, measured by the Engstrom EMMA, were maintained in the closed circuit by direct liquid injection. The system was tested in the laboratory and in clinical use (12 subjects). ⋯ The major sources of error in the method were the result of zero offset in the Engstrom EMMA which in turn was caused by humidity and the intrinsic characteristic of the simple proportional controller used. These errors were easily correctable, and end-tidal halothane concentration could be controlled to within 0.1%. Mean halothane vapour uptake at a constant end-tidal concentration of 0.8% was 114 ml min-1 at 1 min, 36 ml min-1 at 5 min, 29 ml min-1 at 10 min and between 22 and 18 ml min-1 at 20-35 min.