FIELD INHALATION ANAESTHESIA

Safe and effective field anaesthesia can be achieved with the use of injectable agents alone; however, the use of inhalational anaesthesia alone or as supplementation or maintenance after injectable agent induction can be bene­ficial.

Several relatively light-weight, portable field anaes­thetic machines are available, some of which can be fitted into a backpack carry case. However, their size and weight, and the need for oxygen cylinders, may still limit their use in some circumstances. Several lighter, more compact and portable alternatives are available to replace standard vaporises and/or the need for oxygen cylinders.

15.1 Oxygen concentrators

Oxygen concentrators are used widely for oxygen provi­sion in human healthcare applications, especially where liquid or pressurised oxygen is too dangerous or incon­venient. Weighing 2-4 kg and powered by either 240V AC or in-built rechargeable battery, they concentrate oxygen in atmospheric air using a chemical filter and molecular sieve to filter out nitrogen and produce oxy­gen-enriched air. In field anaesthesia they replace the need for oxygen cylinders. Normally, the flowmeter on the anaesthetic machine is bypassed with the oxygen con­centrator connected directly to the vaporiser inlet and the outlet feeds into the anaesthetic circuit as normal. This type of system has been used successfully for field anaes­thesia of a range of Australian native mammals (W Boardman pers. comm.; I Smith pers. comm).

15.2 Draw-over anaesthesia

Draw-over inhalation anaesthetic systems provide anaes­thesia without the need for compressed gases. Atmos­pheric air is used as the main carrier gas. The vaporiser has a very low resistance to airflow and air is drawn through the vaporiser either by the patient’s inspiratory effort or the use of a self-inflating bag. The patient inspires and expires via a non-rebreathing valve downstream from the vaporiser outlet. Air entering the system from the atmosphere may be supplemented with oxygen using an oxygen cylinder or concentrator (Fig. 9.15). A self-inflat­ing bag is useful when anaesthetising small animals as it acts as a reservoir of volatile anaesthetic gas when the

Fig. 9.15. Draw-over anaesthetic system. The patient (P) inspires and expires via a non-rebreathing valve (V). Air (A) enters the system from the atmosphere and may be supplemented with oxygen. The tubes and self-inflating bag (B) provide reservoirs of oxygen (if being used) and anaesthetic-containing gas. The self-inflating bag can be used to control and assist ventilation if necessary. Adapted with permission: Guy Watney, Anaesthesia Service and Equipment, Overland Park, KS, USA.

Fig. 9.16. Simple bubble-through isoflurane vaporiser system for brief field anaesthesia of small mammals. The animal is placed in a cloth bag within a plastic bag. Vaporised isoflurane in air is pumped into the bag. Induction is rapid and ~3-4 min of anaesthesia is achieved to facilitate brief examination, measuring, marking or sample collections such as ectoparasites, hair and blood.

inspiratory effort of the patient or the seal of the circuit (particularly if a face mask is used) is not sufficient to maintain gas flow; in these cases it is recommended to dump the reservoir and refill at regular intervals.

Table 9.7. Construction and use of a field bubble-through vaporiser (Fig. 9.16)

Equipment

• 250-mL Nalgene plastic bottle containing no more than 100 mL of isoflurane

• Vaporiser-pump apparatus:

> Blood pressure-type squeeze bulb

> 2 pieces of plastic hose (300 mm)

> 1 rubber cork stopper with 2 holes

> 1 L-shaped piece of metal tubing (200 mm) with several small holes drilled laterally through the base of the 'L'. This tube and squeeze bulb are connected by rubber hose

> 1 smaller piece of metal tubing (80 mm) connected to outflow tube

• Heavy-duty clear plastic bag (300 ? 440 mm)

Set up

• Remove cap of plastic bottle containing isoflurane and firmly insert rubber stopper with metal tubing to create an airtight seal

• Make sure that the base of the L-shaped metal tube is fully submersed in isoflurane

• Keep the bottle upright

• Have plastic bag open and ready

• Have marking tags, measurement tools or other equipment ready

• Perform procedure in the open air or well-ventilated area

Use

• Place animal in the plastic bag

• Constrict the bag opening around the outflow tube

• Deliver isoflurane in air into the bag by pumping the squeeze bulb (2-10 pumps depending on the size of the animal

• Observe the animal for signs of effect (loss or righting reflex, recumbency, relaxation, immobility)

• Administer further isoflurane in air if required

• Remove animal from bag immediately after a satisfactory level of anaesthesia is achieved and perform procedures

• Monitor respiration and response to stimuli closely

• If animal starts recovering before completing procedures, place back into bag and re-anaesthetise with 1-3 pumps from vaporiser

• On completion of procedure, place animal in a cloth bag or other suitable container to recover. Monitor closely and release once fully recovered

• Remover rubber stopper and recap bottle with isoflurane

for a range of Australian mammals, including flying­foxes, small macropods, bandicoots and possums. It should be used with caution in koalas (W Boardman pers. comm.; I Smith pers. comm.).

15.3 Bubble-through vaporiser

A simple bubble-through vaporiser has been used for very brief field anaesthesia of bandicoots and rodents (Fig. 9.16, Table 9.7).

ACKNOWLEDGEMENTS

I thank Kate Bodley for providing information and reviewing the pinniped section, Dave Blyde for providing cetacean and dugong information, Tim Portas for provid­ing macropod remote chemical restraint information, Wayne Boardman, Paul Eden, Lisa Steindler and Jayne Weller for information they provided and Wayne Board­man for proof reading the chapter.