An evaluation of the Universal PAC and Oxford Miniature Vaporizers for paediatric field anaesthesia

Vapourisers: physical principles and classification

Vapourisers have evolved from rudimentary inhalers to the microprocessor controlled, temperature compensated and flow sensing devices, which are universal today. The improvements in the design was influenced by the development of potent inhalational anaesthetics, unique properties of some agents, a deeper understanding of their mechanism of action, inherent flaws in the older vapourisers, mechanical problems due to thymol deposition, factors influencing their output such as temperature and pressure variations. It is important to review the principles governing the design of the vapouriser to gain insight into their working. It is fascinating to know how some of the older vapourisers, popularly used in the past, functioned. The descendant of Oxford Miniature Vapourizer, the Triservice vapouriser is still a part of the military anaesthesia draw over equipment meant for field use whereas the Copper Kettle the first precision device is the fore-runner of the Tec 6 and Aladdin cassette vapouriser. Anaesthesia trainees if exposed to draw over techniques get a deeper understanding of equipment and improved skills for disaster situations. In the recent advanced versions of the vapouriser a central processing unit in the anaesthetic machine controls the operation by continuously monitoring and adjusting fresh gas flow through the vapouriser to maintain desired concentration of the vapour.

Sevoflurane output from the Oxford Miniature Vaporizer in drawover mode

The sevoflurane output from the Oxford Miniature Vaporizer (OMV) was measured in a series of bench tests in a drawover system. Using a range of settings on two OMVs in series, output ranged from 0.5 to 5.9% with a 600 ml tidal volume at a rate of 12 breaths/min produced by a Penlon Multivent. The OMVs' output was steady over 40 minutes. In tests with tidal volumes ranging from 300 ml to 1200 ml, the OMVs' output was lower at higher minute volumes. When continuous flow of carrier gas at 7.2 l/min was used, the output was lower than at the equivalent intermittent flow rate. Using two OMVs with a drawover system appears to be a feasible technique for the induction and maintenance of sevoflurane anaesthesia. Clinical tests of sevoflurane drawover anaesthesia are now required so that the advantages of sevoflurane can gain wider application in field and military anaesthesia.