Anaesthesia
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This paper describes practical aspects relating to the safe use of target controlled infusion systems in anaesthesia. Consideration is given to the correct use of syringes and infusion lines for any target controlled infusion system. ⋯ Particular features of these systems are discussed. Finally, the practical consequences of possible misuse of infusion systems incorporating pharmacokinetic models are considered.
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A technique of balanced anaesthesia involving propofol infusions and a rapidly metabolised opioid offers advantages over traditional anaesthetic techniques for eye surgery. The most appropriate mode of delivery of propofol is by a 'Diprifusor' target controlled infusion system. This paper discusses the use of a 'Diprifusor' target controlled infusion system for eye surgery. Published data are reviewed and the results of a retrospective analysis of 138 successive eye surgery patients are presented.
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The 'Diprifusor' target controlled infusion system has been developed as a standardised infusion system for the administration of propofol by target controlled infusion. A preferred set of pharmacokinetic parameters for propofol was selected using computer simulation of a known infusion scheme with pharmacokinetic parameters described in published literature. The selected model was included in a 'Diprifusor' module that was interfaced with, and later incorporated into, a computer-compatible infusion pump. ⋯ Standardisation of the delivery performance (+/- 5%) of commercial systems has been achieved with a laboratory performance specification. Clinical studies indicate that the actual blood concentrations achieved were about 16% greater than the calculated values displayed by the system. In an individual patient, titration of the target concentration is required in the same manner as an anaesthetic vapouriser is adjusted to obtain a specific pharmacodynamic effect.
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The development of new short-acting anaesthetic drugs, improved drug assay techniques and the availability of reliable infusion systems opened the field of clinical pharmacokinetics and pharmacodynamics. The tri-exponential drug concentration decay complicates the definition of therapeutic dosage regiments and prevents straightforward prediction of recovery from drug effects. The context-sensitive half-time, the time required for drug blood concentration to decrease to half its value, provides a useful comparative predictor of drug concentration decline after infusion. ⋯ The rationale for drug infusion is reduction of fluctuating drug concentrations and drug effects. A variability similar to that observed with the use of inhalation agents, must be achieved by the choice of an appropriate pharmacokinetic model. The use of a target controlled infusion device, delivering proportional changes based on pharmacokinetic principles, allows titration of the concentration against clinical effect in individual patients.
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The pharmacokinetic profile of propofol is an advantage in neurosurgery, where the rapid return of cognitive function is essential for an early postoperative assessment of neurological status. Administration of propofol by 'Diprifusor' target controlled infusion allows induction of anaesthesia in neurosurgical patients without significant reduction of mean arterial pressure or occurrence of apnoeic episodes. This short paper describes our experience of 'Diprifusor' target controlled infusion for neuroanaesthesia in a series of 20 patients undergoing craniotomy for excision of epileptic foci. The results have been compared with ten similar operations where propofol infusion was controlled manually.