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© The Board of Management and Trustees of the British Journal of Anaesthesia [2008]. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org
Low-flow anaesthesia
Geoffrey Nunn BA (Hons) FRCA
Consultant Anaesthetist
United Leeds Teaching Hospitals
Leeds LS1 3EX UK
Tel: +44 (0)113 392 6672 Fax: +44 (0)113 392 2645 E-mail: geoffrey.nunn@leedsth.nhs.uk
Key Words: The equipment required to deliver safe anaesthesia with low fresh gas flow rates is already standard in the UK. • Low-flow anaesthesia is characterized by economy in the use of anaesthetic agents and reduced atmospheric pollution. • As the carrier gas flow rate is reduced, an increasing disparity develops between the fresh gas and the inspired gas mixtures. The fresh gas composition increasingly needs to reflect the patient's uptake of its components. • A progressive reduction in a practitioner's gas flow rates permits safe and relaxed self-learning of this technique. • Both compound A and carbon monoxide are produced to a significant extent only in CO2 absorbents containing potassium hydroxide. No such absorbents remain in commercial production.
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Modern inhalational anaesthetic agents are metabolized to a small extent only and are largely exhaled unchanged. The use of breathing systems fitted with carbon dioxide absorption units and comprehensive gas monitoring permits the exploitation of this to perform economical and safe ‘low-flow anaesthesia’.
There is no universally accepted definition of low-flow anaesthesia, though it certainly implies a carrier gas flow less than that attainable with a non-absorber breathing system. Baum's suggestion of ‘a rebreathing fraction of greater than 50%’ gives too high a figure,1 since it is at a rebreathing fraction above 75% that the special characteristics of low-flow anaesthesia become apparent. A technique for nitrous oxide–oxygen anaesthesia with a gas flow of 1 litre min–1 was described by Foldes in 1952.2 He sought to define safe nitrous oxide–oxygen mixtures for patients of different weights in the absence of oxygen monitoring. He drew attention to the fact that as the
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Characteristics of low fresh gas flow techniques |
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Requirements for the use of low-flow techniques |
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Advantages of low-flow techniques |
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Economy
Climatization
Pollution
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Disadvantages of low-flow techniques |
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Limitations of currently available vaporizers
Accumulation of unwanted gases in the breathing system
Substances exhaled by the patient
Contaminants of medical gases
Products of reactions with absorbents
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Future developments |
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