Whichever kind of general anaesthetic you use, the patient must have a secure airway and adequate breathing. There is no reason why you should not use the draw over system to provide these in conjunction with a total intravenous anaesthetic. The invention of electronically control led infusion pumps leads some to suggest that these may one day replace vaporisers for most anaesthetics - but in many situations a carefully regulated and monitored intravenous infusion of a drug such as ketamine can be given using an ordinary intravenous drip and a watch with a second hand.

Prepare a solution of intravenous anaesthetic to a standard concentration (e.g. ketamine 1000mg in a 500 ml bottle or bag of normal saline, equal to ketamine 2mg/ml). You will need to know the number of drops/ml of your giving set. Prepare your apparatus, give oxygen by facemask, and induce anaesthesia with a fast running infusion (you will need about 120mg of ketamine - 60ml of the above dilution). When the patient has lost consciousness give a muscle relaxant and intubate the trachea. Reduce the infusion of ketamine to about 2mg/min of the above dilution according to clinical signs for maintenance, and give further doses of relaxant as necessary. You must monitor your infusion continuously - if it stops or becomes "tissued" the patient may become aware. At the end of anaesthesia reverse the relaxant, stop the ketamine, make sure the patient is breathing well and put them in the recovery position. Benzodiazepine premedication or postmedication will prevent dreaming and emergence reactions. The addition of atropine will reduce excessive secretions.

You can use a similar technique with other intravenous agents, but be warned that recovery after the use of barbiturate infusions may be very prolonged.

 
Oxygen Supplies

The use of a T-piece (see Fig 1) to enrich a draw-over system is very economical and allows you to make the most of your supplies.

A flow of 1 litre/min provides an inspired concentration of 30-40% oxygen; 4 litres/min provides 60-80%. To make the best use of oxygen post-opera tively, or in cases of breathing difficulty due to respiratory infections etc, use a nasopharyngeal catheter (eg a 8-10FG rubber or plastic catheter) inserted into the nasopharynx with a flow of 1 litre/min for a child or 2 litres/min for an adult, giving an inspired concentration of about 40% oxygen. It is desirable to humidify the flow of oxygen and vital to check that the catheter is not inserted too far (e.g. into the oesophagus) or gastric dilatation could result. As well as its economy, this method is preferred by many patients as it allows them to talk, cough, expectorate and eat - all difficult to do with a conventional facemask!

Other sources of oxygen are worth considering. Industrial (welding) oxygen is normally made by the same process as "Medical oxygen" - and indeed industrial oxygen is often made to a higher degree of purity! You must check your own local specification!

Oxygen concentrators (see Update No.1) can also provide a supply for draw-over or ward use. Concentrators compress room air to a pressure of 4 bar, then pass it though a zeolite column which absorbs the nitrogen, leaving up to 96% oxygen (the rest is argon). If excessive flows are demanded the concentration delivered falls off. Small concentrators, which meet the World Health Organisation's (WHO) standards can deliver 4 litres/min of oxygen (>90%) with a power consumption of around 350 watts (mains electricity or AC generator required). Concentrators are usually the cheapest way of getting oxygen - often 30-50% of the cost of cylinders. They require simple servicing every 5000 hours and an overhaul every 20,000 (equivalent in running time to about half a million miles for a car!). For details of WHO approved concentrators write to the author.

 
Table 1: Suggested plan for General Anaesthesia

Is general anaesthesia required for this case?	--> No -->	Use regional technique
|
Yes
|
Is the anaesthetist trained in endotracheal intubation?	--> No -->	Use ketamine or spinal
|
Yes
|
Has the patient a difficult airway?	--> Yes -->	Use regional or seek expert help
|
No
|
Proceed as follows:

• Check your apparatus and drugs

• Obtain intravenous access and preoxygenate the patient

• Give a sleep dose of thiopentone or ketamine

• Give 1mg/kg suxamethonium i.v.

• Intubate the trachea

• Ventilate the lungs manually with 3% ether in air. Increase this during the first 5 minutes to 6-10% to settle the patient. (Halothane 1-1.5% can be used instead of ether).

• When breathing returns (usually after 3-5 minutes) allow the patient to breathe 6-8% ether in air or 1-1.5% halothane in oxygen-enriched air or if available give a long term relaxant (eg alcuronium or gallamine) and continue to ventilate the lungs manually, at an appropriate concentration of volatile agent (This technique allows rapid recovery).

• At the end of surgery reverse long acting muscle relaxants (if given) with neostigmine and atropine, continuing to ventilate the lungs until breathing resumes; turn the patient on his side, and remove the tube when the patient is awake.

This article contained links to the following additional information:

Issue 1 - Draw-over Anaesthesia Part 1 - Theory
Issue 3 - Draw-over Anaesthesia Part 3 - Looking After Your Own Apparatus

©World Federation of Societies of Anaesthesiologists WWW implementation by the NDA Web Team, Oxford

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