Evaluation of a new range of air drawover vaporizers. The 'PAC' series--laboratory and 'field' studies

FIELD USE OF ISOFLURANE AND AIR ANESTHETIC EQUIPMENT IN WILDLIFE

Conventional inhalation anesthesia of wildlife species within natural habitats presents significant practical problems. Heavy cylinders of medical grade oxygen are often unavailable in field situations. Equipment has been modified to permit the delivery of isoflurane in ambient air as the carrier and to be fitted with circuitry adaptable for different species and anesthetic situations. Preliminary empirical studies at low altitude in a range of small mammalian and avian species demonstrate the suitability of this combination and these techniques for inducing and maintaining anesthesia in clinically normal patients undergoing relatively minor procedures. The equipment has also been used to deepen and prolong anesthesia in several larger species, including great apes and large cats, after induction with injectable agents. These techniques, in combination with pulse oximetry to detect hypoxemia, provide a cheap, robust, and portable inhalation anesthetic system for field situations that is not dependent on compressed gasses.

A protocol for safe anasthesia for cleft lip and palate surgery in developing countries

A project to perform surgical correction of cleft lips and palates was carried out in Uganda in 1998. Twenty centres were visited and 336 cleft lips and 41 cleft palates were repaired. The age of the patients ranged from 2 weeks to 60 years. Many of the centres visited were remote and lacked even the most basic equipment. Patients were anasthetised using ketamine, ether or halothane according to a protocol that we developed. There was no anasthetic mortality and only one case of significant morbidity. We report our experience and discuss recommendations regarding the provision of anasthesia in remote circumstances based on our outcome. An additional benefit of the project was that we provided training to local medical personnel in anasthesia and surgery for cleft lips and palates.

Inspired oxygen concentrations with or without an oxygen economizer during ether draw-over anaesthesia

An oxygen economizer tube is attached to draw-over vaporizers and acts as a reservoir of supplemental oxygen. The clinical importance of the presence or absence of the economizer tube (volume 130 ml) has not been adequately studied in manually ventilated patients using ether from an Ohmeda Cyprane Portable Anesthesia Complete (PAC) draw-over vaporizer. A total of sixteen patients ASA 1-2, undergoing elective surgery for peripheral orthopaedic procedures were studied with and without an economizer tube. Each patient acted as his or her own control. Standard procedures were used for anaesthetic induction with muscle relaxant, endotracheal intubation and anaesthetic maintenance. Supplemental oxygen was supplied by an oxygen concentrator. Using the draw-over vaporizer without an oxygen economizer tube, there was a slight increase in FiO2 of 20%, 23%, 27%, 30%, 33% and 33%, with increasing oxygen supplementation of 0 to 5 l/min, respectively. With an economizer tube, the FiO2 values increased to 20%, 26%, 35%, 46%, 54% and 66% at 0 to 5 l/min of oxygen respectively. The FiO2 values were significantly different at 3, 4, and 5 l/min (P < 0.05), showing the potential advantages of an oxygen economizer tube attached to a draw-over vaporizer in this setting. No significant differences were seen in the oxygen saturations of these healthy patients with or without an oxygen economizer.

The Ohmeda Universal PAC drawover apparatus. A technical and clinical evaluation

The Ohmeda Universal PAC drawover apparatus, a modification of an earlier series of vapour-specific vaporizers, was assessed with regard to its output under varying gas flows and temperatures and its clinical usage with isoflurane and enflurane. The device tended to over-deliver vapour, particularly at low flows and at high temperatures. Its clinical performance during spontaneous and positive pressure ventilation was satisfactory, although adequate concentrations were not achieved using enflurane in the spontaneous ventilation mode. The device was not assessed with ether or halothane. The robust construction, relatively large capacity and thermocompensation make it suitable for field or military anaesthesia.

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

A laboratory study has demonstrated that the Universal PAC and Oxford Miniature Vaporizers can be used for paediatric anaesthesia in drawover mode at varying ambient temperatures. Changes in tidal volume have minimal effect on the delivered concentration of halothane from either vaporizer but varying ambient temperature significantly affects the output of the Oxford Miniature Vaporizer. The Oxford performs well in continuous flow mode, which is of particular use for T-piece anaesthesia, whereas the Universal PAC does not perform adequately in this way.

Can a drawover vaporizer be a pushover?

Bench testing was carried out to establish whether the vapour output from an OMV50 vaporizer, as used in the Triservice apparatus, differs according to whether the carrier gas is either drawn or pushed through the vaporizer. Results show that the differences in output concentration between the two modes were clinically insignificant.

Performance evaluation of a draw-over vaporizer with a nonbreathing circuit during simulated adverse conditions

STUDY OBJECTIVE

Performance evaluation of a draw-over vaporizer with a nonrebreathing circuit during simulated adverse conditions.

DESIGN

Open laboratory study.

SETTING

University hospital laboratory.

MATERIALS AND INTERVENTIONS

The output (%) of a temperature-compensated Ohmeda Cyprane PAC (portable anesthesia complete) draw-over vaporizer (Ohmeda, Madison, WI) using isoflurane attached to a nonrebreathing circuit was tested in the laboratory during manual ventilation under normal and simulated adverse conditions. The adverse conditions tested were high ambient temperature and static and dynamic positional variation.

MEASUREMENTS AND MAIN RESULTS

The results show that the output bias of the PAC vaporizer has a generally consistent profile as tested, except when placed in the 180 degrees full-inversion or 90 degrees anterior-tilt position. We also conducted trials with the Penlon ventilator (Ohmeda, Abingdon, Oxford, U.K.) attached to the circuit at each of its input ports. Ventilation at one input port produced dangerously high airway pressures (within the circuit), thus supporting the manufacturer's recommendation against the use of this port for positive-pressure ventilation. Using the recommended port, the test lung was seen to inflate and deflate appropriately, but, surprisingly, no vapor output was detected by the agent monitor at any vaporizer setting when using the ventilator at the recommended port.

CONCLUSION

Anesthetists should be aware of the pitfalls and possible problems that may be associated with this type of anesthetic delivery system.

Anaesthesia in the field. Spontaneous ventilation--a new technique

In recent years the British Army has used the Triservice Anaesthetic Apparatus in the field. Trichloroethylene is no longer manufactured in the United Kingdom and halothane is not recommended for closely repeated anaesthetics. A method based on existing equipment is described for patients breathing spontaneously. A background infusion of ketamine, midazolam and alfentanil supplements the inhalation of isoflurane in oxygen-enriched air.

Drawover anaesthetic systems. Factors determining the inspired oxygen concentration

The factors that determine the inspired oxygen concentration delivered by drawover anaesthetic systems were studied in the laboratory and in anaesthetised patients. The inspired oxygen concentration is dependent upon oxygen flow, reservoir volume and total ventilation; respiratory rate and inspiration:expiration ratio have a small influence. A reservoir of at least of 1 m corrugated tube (internal volume 415 ml) makes optimal use of a 4 litres/minute oxygen flow. Delivery of a safe inspired oxygen concentration can be ensured with economical oxygen flows.

Inhalational anaesthesia in developing countries. Part II. Review of existing apparatus

We have reviewed the availability of apparatus which permits the maximum use to be made of available supplies and services in developing countries. In particular we have stressed the use of electrically operated air compressors and oxygen concentrators to drive sophisticated anaesthetic apparatus which is not dependent on supplies of compressed gases in cylinders.