Dr M B Dobson,
Consultant Anaesthetist, John Radcliffe Hospital, Oxford OX3 9DU.

Introduction		Other volatile agents
Ether as an anaesthetic		Without volatile anaesthetics
Separation in time		Oxygen Supplies
Separation in space		Suggested plan

In the first article on draw-over, Georg Kamm described the apparatus used in draw-over anaesthesia. In this issue, I want to describe the ways in which draw-over apparatus can be used to provide safe, high quality anaesthesia. We must first recognise that safe anaesthesia is not produced by apparatus alone!

The essentials for safe anaesthesia in any situation include adequate pre-operative assess ment, preparation and resuscitation of patients, reliable intravenous access, a pleasant and safe induction, a secure airway, adequate tissue oxygenation, appropriate monitoring, and rapid recovery. None of these depends on high cost or high technology equipment, but all require properly trained and reliable people, on whom primary safety depends. If you are responsible for anaesthesia in a district hospital, and do not have extensive specialist training, you are well advised to use a small number of safe, widely applicable clinical techniques. This will allow you to become thoroughly familiar and confident with them through regular practice. No-one can be safe or confident when they use an unfamiliar technique. Restricting yourself to a small number of techniques also means that you use a limited number of drugs, and it is easier to make sure that adequate stocks are held in the hospital which do not run out.

A draw-over system is most suitable for the needs of district hospitals. It is simple to under stand, robust, independent of compressed gases, and can be repaired on site if necessary. In a draw-over system the carrier gas (air, with or without oxygen enrichment) passes through a low resistance vaporiser, through a self-inflating bag or bellows and reaches the patient via a univer sal breathing valve which ensures that expired gases are directed into the atmosphere and do not re-enter the anaesthetic system.

 
Ether as an Anaesthetic Agent.

Most of the disadvantages of ether are seen when it is used alone for induction of anaesthe sia. They include a slow onset, an unpleasant smell for the patient, with coughing, breath-hold ing and laryngeal irritability, salivation, swallowing and sometimes vomiting. In the vast majority of patients, these problems can be easily and safely avoided by intravenous induction of anaes thesia with drugs such as thiopentone or ketamine. Insertion of an endotracheal tube secures the airway, and allows the concentration of ether to be increased rapidly to maintenance levels with no risk of laryngospasm. If a muscle relaxant is used, inhaled ether concentrations of 3-4% are enough to ensure unconsciousness, and wake up reasonably rapidly at the end of surgery. Ether also provides a considerably longer period of post-operative analgesia than other volatile anaes thetics. Thus, the clinical disadvantages of ether can be overcome, allowing us to make use of its advantages in safety, availability and economy. (A summary of a suitable technique for ether anaesthesia is shown in Table 1). The only problem remaining is the question of its flammability.

Ether is flammable (will burn, but not explode) when mixed with air. In this respect you can compare it with alcohol, but not with petrol, which will explode when ignited in air. The addition of oxygen (or nitrous oxide) to ether does produce an explosive mixture, in which ignition could be caused by a source such as surgical diathermy, a sparking electric socket, or (especially in a dry climate) static electricity. Flammable or explosive mixtures must therefore be separated from possible ignition sources - there are two ways of doing this:-

 
(1) Separation in time

Healthy patients are most likely to need added oxygen at the beginning of anaesthesia (before and just after intubation) and at its end (before extubation). At these times surgical diathermy is not in use. During the operation use ether/air without added oxygen unless the patient is very sick, old, very young or anaemic, or there are other indications such as pregnancy, cardio-respiratory disease or high altitude.

 
(2) Separation in space

During the use of flammable/explosive mixtures no source of ignition is permitted in a "zone of risk" which extends 30 cm from all points of the breathing system where gas might escape - thus no diathermy in the thoracic cavity, head and neck, or mouth is permitted, but diathermy in the bladder or abdominal cavity is considered safe. A simple scav enging system - a length of tubing to direct the expired gases away from the site of surgery, is helpful. In dry climates (including air conditioning) anti-static precautions should also be used.

It is worth remembering that Western operating theatres are still mostly built with antistatic precautions, since even without ether there is a risk of fires and explosions with other substances mixed with oxygen (enflurane, trichloroethylene, alcohol etc.).

 
Using Other Volatile Anaesthetic Agents

Halothane is widely available, and has a number of advantages. It is non-flammable, has a pleasant, non-irritant smell, and induces unconsciousness more quickly than ether. Its disadvan tages are that it depresses the cardiovascular and respiratory systems, resulting in hypotension and hypoxia. It is more potent than ether, and must never be given by "open drop" techniques. Halothane should never be put in an EMO vaporiser, as it attacks the metal from which these vaporisers are made, and the vaporiser will be wrecked! The most suitable draw-over vaporiser for halothane is the Oxford Miniature Vaporiser, which can also be used for other volatile anaes thetics if these are available (e.g. trichloroethylene, enflurane etc.).

Halothane can be used alone for anaesthesia; its main disadvantages in this situation is the respiratory depression which it causes, and supplementary oxygen should always be used throughout the procedure. If oxygen is not available, ventilation should be assisted or controlled to prevent hypoxia. Halothane sensitises the heart to adrenaline, and you should warn the surgeon not to infiltrate the wound with adrenaline-containing solutions when halothane is in use. (Some anaesthetists allow infiltration of up to 20 ml of 1:200,000 adrenaline provided the pulse is closely monitored).

Halothane may be very useful in combination with other volatile anaesthetics. If you plan an inhalational induction, begin with halothane, then change to ether once the patient becomes unconscious - this is much quicker and more pleasant for the patient - and you will see the con trast between the respiratory depression of halothane and the stimulation of ether!

Halothane has also been used successfully for military anaesthesia in combination with trich-loroethylene, using two Oxford Miniature Vaporisers in the "Tri-service" apparatus. The agents complement each other, since halothane is a good hypnotic but a poor analgesic, while the reverse is true of trichloroethylene. Use 0.5% trichloro-ethylene as a "baseline" and vary the concentration of halothane to obtain the required depth of anaesthesia. Turn the trichloroethylene off a few minutes before the end of the operation as it takes a while to wear off. Once again, added oxygen is necessary.

(Continued...)

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