Sevoflurane for analgesia-testing a modified vaporiser for delivery.

The Diamedica Sevoflurane Inhaler (Diamedica UK Ltd, Bratton Fleming, UK) (DSI) is a breathing system which includes a modification of an existing vaporiser (Diamedica Draw-over Vaporiser, Diamedica UK Ltd, Bratton Fleming, UK), to enable the delivery of 0.8% sevoflurane. Previous studies have suggested that self-administered sevoflurane at sub-anaesthetic concentration can provide useful pain relief during the first stage of labour and that it may be more effective than Entonox. Further research and potential clinical use have been impeded by the lack of a practical delivery system. In this study, the performance of two versions of the DSI (DSI-1 and DSI-2) was investigated. DSI-1 was tested over a range of minute volumes (1 to 30 l/min) and ambient temperatures (10°C to 40°C). The sevoflurane output increased unacceptably with rising ambient temperature, therefore the design was modified to create the DSI-2. The results from testing this revised version are also described. Mean sevoflurane output from the DSI-2 was found to be within a clinically acceptable range at the minute volumes tested (0.78% to 0.88%) and ambient temperatures tested (0.69% to 0.9%). Based upon these results, the authors propose to undertake further studies of sevoflurane analgesia using the DSI-2.

Evaluation of oxygen concentrators for use in countries with limited resources.

Seven different models of oxygen concentrators were purchased. The manufacturer's data were evaluated by a ranking method for operation at high temperature and high relative humidity, power consumption, warranty and cost. Measurements were then made of the oxygen concentration produced at maximum operating temperature. All the concentrators were CE marked and claimed compliance with the relevant Standard ISO 8359:1996. Only two models complied with their specification. On examination of the concentrators and the accompanying documents we found that compliance with 61 points listed in ISO 8359 ranged from 85% to 98%. Oxygen concentration was measured with the machines running simultaneously under both high temperature and high humidity. All models delivered low oxygen concentrations at 40 °C and 95% relative humidity. Only two models delivered >82% at 35 °C and 50% relative humidity. Concentrators intended for use in countries with limited resources should be evaluated before they are purchased, by independent experts, using the methods described herein.

Modification of a draw-over vaporizer for use with sevoflurane.

BACKGROUND: Draw-over anaesthesia is widely used throughout the developing world, in disaster areas and in military anaesthesia when the supply of pressurized oxygen is unreliable. To date, no draw-over vaporizer has been able to deliver sufficient concentrations of sevoflurane for use in inhalation induction of anaesthesia. A laboratory study to assess the performance of a new vaporizer (DDV2) to deliver sevoflurane in a wide range of situations is described. METHODS: In this study, the concentration of sevoflurane delivered at different dial settings (1-4%) and at different temperatures (20-40°C) in a draw-over mode was measured. The concentration of sevoflurane delivered at different dial settings with continuous flow (6 and 8 litre min(-1)) at 20°C was measured. The maximum possible concentration of sevoflurane that can be delivered by the DDV2 was measured at a continuous flow rate of 8 litre min(-1) at 20, 30, and 40°C. RESULTS: Concentrations of sevoflurane delivered in the draw-over mode were within 0.5% of dialled setting up to 30°C. Above this temperature, higher levels of vapour were delivered. With continuous flow, concentrations of sevoflurane at 20°C were within 0.5% of dialled setting and were stable throughout the duration of the experiment. On the 'induction' setting, concentrations of sevoflurane of between 6.4% and 10.1% could be delivered with continuous flow. CONCLUSIONS: The modifications to the DDV2 allow stable concentrations of sevoflurane to be delivered in draw-over and continuous flow modes over a range of temperatures. With continuous flow, concentrations of sevoflurane sufficient for induction of anaesthesia can be achieved.

A new valve for draw-over anaesthesia.

The Diamedica non-rebreathing valve has been developed for use in draw-over anaesthesia and can be positioned at the common gas outlet. Its performance was evaluated against the Laerdal, Ruben and Ambu valves under laboratory conditions. Valve resistance during inspiration and expiration was simulated over a range of constant flow conditions. During flows ranging from 5 to 45 l.min(-1) , the Diamedica and Ruben valves exhibited a negligible resistance of < 150 Pa, with the Laerdal and Ambu valves achieving resistance of < 200 Pa. To assess the effects of sterilisation, this procedure was repeated following autoclaving, after which the Diamedica valve exhibited resistance to flow of < 150 Pa at 25 l.min(-1) for inlet resistance and 35 l.min(-1) for outlet resistance. The Diamedica, Ambu and Laerdal valves demonstrated resistance of < 100 Pa when saturated with water, while the Ruben valve exhibited resistance > 500 Pa.

The portable Glostavent: a new anaesthetic machine for use in difficult situations.

A portable version of the Glostavent anaesthetic machine is described in which recent developments in draw-over technology are incorporated into a traditional draw-over anaesthetic system. The additional features include a more efficient reservoir and an improved vaporiser which have enhanced the performance and versatility. The portable Glostavent weighs less than 10 kg and is transported in a container the size of a small suitcase. It can be used to provide inhalational anaesthesia safely and economically in situations where there are no support facilities. It is ideal for use in battlefield or disaster situations and in isolated hospitals in disadvantaged regions of the world.

An assessment of the efficiency of the Glostavent ventilator.

In parts of the world where supplies of oxygen and electricity are erratic, ventilating patients' lungs can be problematic. Should the electricity supply fail, gas driven ventilators have an advantage as they can continue functioning. However, many are extravagant in their requirement for the driving gas. The Glostavent ventilator was designed to minimise these requirements. We measured the duration of ventilation achieved by the Glostavent ventilator using an E-size oxygen cylinder at a range of minute volumes, and the inspired oxygen concentration achieved by recycling the driving gas. The period of mechanical ventilation from a single E-size cylinder ranged from 11 h 8 min (SD 4 min) with a minute volume of 7 l min(-1) to 18 h 15 min (SD 7 min) with a minute volume of 3 l min(-1). The mean fractional inspired oxygen concentration achieved by recycling the driving gas without further inspired oxygen supplementation was 0.33. We conclude that the Glostavent ventilator performs as efficiently and cost effectively as predicted.

The Diamedica Draw-Over Vaporizer: a comparison of a new vaporizer with the Oxford Miniature Vaporizer.

The Diamedica Draw-Over Vaporizer (DDV) has been developed as an alternative to the Oxford Miniature Vaporizer (OMV). Both can function as draw-over or plenum vaporizers. The performances of these two vaporizers were compared under conditions simulating intermittent positive pressure ventilation (IPPV) and continuous flow (CF). Series 1 experiments were conducted with the vaporizers in water baths at 20, 25 and 30 degrees C. Vaporizers were tested at dial settings of 1-4% over a range of minute volumes (1.75-6 l.min(-1)) and flow rates (3-8 l.min(-1)). Series 2 experiments compared output of the vaporizers over time at ambient temperatures of 20, 25 and 30 degrees C. A minute volume of 6 l.min(-1) (IPPV) and a gas flow of 8 l.min(-1) (CF) were used with a vaporizer setting of 2%. Vapour concentrations were recorded at 5-min intervals. In series 1 IPPV experiments, the DDV vaporizer was more accurate, producing significantly fewer vapour concentrations 0.5% more than or less than setting (p = 0.013). The OMV tended to produce more favourable results under continuous flow (p = 0.42). In series 2 experiments, the accuracy of both vaporizers was similar but consistency of output over time was better for the DDV and consistency of output according to differences in ambient temperature was better for the DDV. The OMV produced more vapour concentrations that were markedly higher than dial setting, particularly at high ambient temperatures. The DDV is a suitable alternative to the OMV with some distinct advantages. These include a larger reservoir, tendency towards greater accuracy during IPPV and improved consistency of output.

The Glostavent: evolution of an anaesthetic machine for developing countries.

The sophisticated anaesthetic machines designed for use in modem hospitals are not appropriate for many parts of the developing world, as they are reliant on regular servicing by skilled engineers and an uninterrupted supply of electricity and compressed gases, which are not always available. The Glostavent has been designed specifically to meet the challenges faced by anaesthetists working in these countries. It is robust, simple to use, economical, easy to service and will continue to run during an interruption of the supply of oxygen or electricity. Feedback from widespread use throughout the developing world over the last 10 years has led to significant improvements to the original design. This article describes the basic components of the original version and the modifications which have been introduced as a result of practical experience in the developing world.

Anaesthesia equipment for resource-poor environments.

The design of anaesthesia equipment for use in hospitals in the developing world must take into account the local conditions, particularly whether reliable supplies of compressed oxygen and electricity are available. Designs should ensure that maintenance is feasible locally. International standards should encourage the design of suitable equipment to ensure safe anaesthesia for patients worldwide.

Cost-effectiveness of basal flow sevoflurane anaesthesia using the Komesaroff vaporizer inside the circle system.

After ethics committee approval, 51 consenting ASA physical status 1 or 2 adult patients were given basal flow sevoflurane anaesthesia using fresh gas flows of 150 to 300 ml x min(-1) oxygen. A Komesaroff vaporizer was placed on the inspiratory limb of the circle system. Basal flows were introduced immediately following intravenous induction of anaesthesia. The vaporizer was set to deliver the maximum concentration until the inspired sevoflurane concentration (FSI) reached 3%. The dial was then adjusted to maintain the FSI at 3%. After every 60 minutes, the circuit was washed out with 100% oxygen at a flow rate of 10 l x min(-1) for one minute. The FSI reached 3% after an average of 8.5 (3.8) [mean (SD)] minutes. The trends in FSI and the expired sevoflurane concentrations were significantly different (P<0.05) between the mechanically ventilated patients (n=21) and the spontaneously ventilating patients (n =30) and demonstrated a more gradual build-up in the former group. The consumption of sevoflurane was found to be 9.2 (2.8) ml x h(-1). This represented a 52.5% cost saving over the clinical application of the Mapleson's ideal fresh gas flow sequence for low-flow anaesthesia.

Closed circuit anaesthesia in ventilated patients using the Komesaroff vaporizer within the circle.

A study was undertaken to assess the performance of the Komesaroff vaporizer, placed within the circuit, in ventilated patients during maintenance of closed circuit anaesthesia with halothane or isoflurane. Following intravenous induction, anaesthesia was maintained by inhalation. This was achieved using a conventional vaporizer outside the circle for the first 10 minutes to manage the fast uptake phase. The fresh gas flow was then reduced to the basal oxygen requirement with the Komesaroff vaporizer within the circle maintaining inhalational anaesthesia. Complete isolation of the circuit was achieved by returning all anaesthetic gases to the circuit following analysis and using a bag-in-bottle ventilator. The Komesaroff vaporizer dial was positioned at between the first and second division and end-tidal volatile anaesthetic agent levels were measured. This study demonstrated that at dial positions 1 or 1.5 with either agent, the end-tidal volatile concentration plateaued at clinically acceptable levels. The Komesaroff vaporizer can therefore be used safely in ventilated patients to maintain closed circuit anaesthesia provided clinical observation and monitoring are meticulous.

The role of clonidine in anaesthesia.

Clonidine is an alpha 2-adrenoceptor agonist which has been used for over 20 years to treat hypertension. It has recently, however, found a new and possibly significant role in anaesthesia and the treatment of pain. This article discusses the premedicant, perioperative and postoperative uses of clonidine.

The Oxyvent. An anaesthetic machine designed to be used in developing countries and difficult situations.

The Oxyvent is an anaesthetic machine designed specifically for use in the developing world and difficult situations. It is made up of four components, each of which has, in its own right, already proved to be of great value in difficult situations. These are the drawover system, the Penlon Manley Multivent Ventilator, the DeVilbiss Oxygen Concentrator and the air compressor. The four components are mounted on a simple trolley carrying two oxygen cylinders. The Oxyvent can be used to provide anaesthesia in the absence of electricity or oxygen or both. It is simple, robust and easily serviceable. It is versatile and can be used both as an anaesthetic machine in the operating theatre and as a ventilator in an intensive care unit.