Anaesthesia
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Several factors have to be considered in determining the cost of applying a new inhalational anaesthetic such as desflurane into clinical practice. Factors beyond the immediate control of the anaesthetic practitioner include the price set by the manufacturer (although this may be influenced by economic and political pressures), and the physical-pharmacological properties of the anaesthetic (e.g. vaporization, potency, solubility). The anaesthetic practitioner can minimise cost by applying lower inflow rates. ⋯ The use of lower inflow rates presupposes that such rates do not allow the production of toxic compounds in recirculating gases. Modern equipment makes low-flow anaesthesia reliable and easy to control, and as desflurane is not degraded by the standard carbon dioxide absorbents, its use in low-flow systems is effective and economical. These cost considerations do not take into account the savings that may result from a more rapid recovery from anaesthesia, nor do they take into account the increased expense of capital equipment needed to apply a new anaesthetic.
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Increased duration of anaesthetic administration has implications for recovery from anaesthesia, has cardiovascular effects, and potential for toxicity through metabolism and breakdown of the anaesthetics. Recovery of function after desflurane or sevoflurane anaesthesia, because of the low blood gas and tissue solubilities of these agents, is more rapid than after halothane, isoflurane or enflurane, with recovery being most rapid after desflurane. Increased duration of anaesthesia amplifies the differences in rate of recovery because of the additional anaesthetic (greater with more soluble agents) dissolved in tissues. ⋯ Sevoflurane undergoes considerable metabolism, producing free fluoride ion, with plasma concentrations proportional to dose and duration of anaesthesia exceeding 50 microM in approximately 7% of patients. In rats, the effects of a toxic breakdown product of sevoflurane, CF2 = C(CF3)OCH2F (compound A), are also dose- and duration-dependent, with lower concentrations producing toxic effects as duration of exposure increases. The clinical importance of the metabolism and in vitro breakdown of sevoflurane has still to be adequately tested.
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Although many anaesthesia machines are equipped with circle rebreathing systems, inhalational anaesthesia remains frequently performed using relatively high fresh-gas flows. The major advantages of rebreathing techniques can be achieved only if the fresh-gas flow is reduced to 1 l.min-1 or less. Although there are potential risks associated with low-flow anaesthesia, modern anaesthesia machines meet all the technical requirements for the safe use of low-flow techniques if they are used in conjunction with equipment for monitoring inhaled and exhaled gas concentrations; these monitors are already increasingly available and, in the near future, are likely to become an obligatory safety standard in many countries. For both economic and ecological reasons, the use of new inhalational anaesthetics, with low tissue solubility and low anaesthetic potency, can be justified only if the efficiency of administration is optimised by using low-flow anaesthetic techniques.
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The relevant physiological and pharmacokinetic differences between adults and infants and children are considered, and the advantages and disadvantages of the available inhalational anaesthetics are discussed. Desflurane shows promise as a new paediatric anaesthetic as it allows rapid changes in the depth of anaesthesia. However, irritant effects on the airways make desflurane unsuitable for induction. Desflurane anaesthesia, following halothane induction, seems to be well tolerated.
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Desflurane is a new volatile, inhaled anaesthetic that differs significantly from presently available inhaled agents in being halogenated solely with fluorine. This fluorination produces a lower solubility and increased resistance to biodegradation. ⋯ Limitations include a lesser potency and greater pungency at concentrations exceeding 1 minimum alveolar concentration (MAC). Other pharmacological properties are similar to those of isoflurane.