Anaesthesia
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Review Comparative Study
Intravenous anaesthesia: manual infusion schemes versus TCI systems.
Propofol was first used to induce and maintain anaesthesia in the early 1980s. Several infusion regimens were described, some based on pharmacokinetic predictions. Advances in computing technology subsequently have allowed the development of portable target controlled infusion devices, with drugs delivered to achieve specific predicted target blood propofol concentrations. Assessments of propofol target controlled infusion systems in clinical practice and comparisons of 'Diprifusor' target controlled infusion systems with manual infusion are reviewed here.
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Comparative Study Clinical Trial Controlled Clinical Trial
Evaluation of the HemoCue for measuring intra-operative haemoglobin concentrations: a comparison with the Coulter Max-M.
We compared haemoglobin concentration values obtained using a portable haemoglobinometer, the HemoCue, in the operating theatre with the results obtained by the Coulter Max-M in the laboratory. Haemoglobin concentrations were measured on 52 arterial blood samples obtained from 13 patients during aortic surgery, in theatre with the HemoCue and again by the Coulter Max-M. Twenty routine samples from the laboratory were also analysed by both methods. ⋯ The coefficients of repeatability of the 20 samples analysed in duplicate on each device were 0.26 g.dl-1 and 0.33 g.dl-1, respectively. The coefficients of variance were 0.74% (HemoCue) and 0.93% (Coulter). With adequate training and monitoring, the HemoCue provides comparable haemoglobin results for near-patient testing in theatre.
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Infusion pumps incorporating 'Diprifusor' for the administration of propofol by target controlled infusion are now commercially available and are becoming more widely used. This paper considers possible future applications of target controlled infusion and summarises results obtained using prototype systems as a component of other control techniques and with other drugs. These include studies with patient-controlled systems for the administration of analgesia or sedation and a closed loop control system for the administration of propofol. Among currently available analgesic drugs, alfentanil and remifentanil are considered to be the most suitable for administration by target controlled infusion, but commercial systems for these agents are not yet available.
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The pharmacokinetic and pharmacodynamic interactions between propofol and adjuvant agents have increasingly been recognised as clinically important and the improved knowledge of these interactions is being used to optimise the quality of intravenous anaesthesia. It is now known that propofol interferes with opioid metabolism, thereby increasing the plasma concentrations of the opioids, while opioids such as alfentanil increase propofol concentrations by reducing both the distribution and clearance of propofol. ⋯ From the pharmacodynamic interaction data, optimal target propofol-opioid concentrations and optimal infusion regimens have been developed that ensure adequate anaesthesia in 50% and 95% of patients with the most rapid recovery possible. These optimal target propofol concentrations and infusion regimens are affected by the opioid with which propofol is combined, as well as the duration of infusion.
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The availability of fast-acting intravenous drugs for anaesthesia has encouraged anaesthetists to gain a practical understanding of the pharmacodynamics and pharmacokinetics of these drugs to optimise their administration. 'Diprifusor' technology uses pharmacokinetic modelling to control the infusion rate of a pump, providing the anaesthetist with direct control of the blood concentration rather than indirect control by the infusion rate. The 'Diprifusor' electronics module is a dual microprocessor component that can be incorporated into an infusion pump to provide this mode of administration. This paper describes the module's infusion control system, prefilled syringe recognition system and the safety aspects addressed by the design.