Using Volatile Anaesthetic Agents (page 1)

Issue 5 (1995) Article 5: Page 1 of 3

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Using Volatile Anaesthetic Agents

Dr P Fenton,
Queen Elizabeth Central Hospital, Blantyre, Malawi.

Introduction		Trichloroethylene
Basic pharmacology		Nitrous oxide
Ether		Use of agents
Halothane

There are a variety of anaesthetic techniques available and various clinical situations demand different techniques of anaesthesia. A trained anaesthetist should be able to decide the most appropriate method for each case as an anaesthetic decision (not a surgical decision) and have the necessary skills to use the technique of choice. Anaesthetists should practise their skills as widely as possible and not give the same anaesthetic to every single patient. This is of particular importance where supplies of drugs and equipment are unreliable.

Volatile anaesthetic agents are commonly used and have an important safety feature in that agents which enter the circulation via the lungs can leave by the same route. Therefore the concentration of anaesthetic at the brain can be rapidly reduced as long as the patient is breathing adequately.

Basic pharmacology of volatile agents

An agent inhaled into the lungs will first enter the circulation and is then carried to all tissues of the body. We are primarily interested in the concentration reaching the brain because this produces the state of anaesthesia. The exact mechanism of anaesthesia is poorly understood but it seems that the nerve cells absorb the agent and in so doing their ability to conduct impulses to each other is reduced.

The more soluble the agent is in blood the longer it takes to build up an effective concentration in the brain and the slower the onset of unconsciousness. Thus with a very soluble agent such as ether, the induction of anaesthesia is prolonged. On the other hand an agent such as nitrous oxide is relatively insoluble in blood; the blood becomes saturated quickly, the brain concentration rises quickly and the effect is seen rapidly. The degree of solubility of an agent in blood is indicated by its blood gas solubility coefficient (see

table 1).

The inhalation agents that are commonly used in Africa and other places where resources are limited are ether and halothane. When it is available, trichloroethylene is also used.

Patterns of anaesthesia differ from country to country. Halothane, once the mainstay of modern inhalation anaesthesia and an agent that we in Malawi still regard as a "luxury" anaesthetic is now hardly used in the USA. In adult anaesthesia in the UK it has been largely displaced by two newer agents, enflurane ("Ethrane") and Isoflurane ("Forane") both of which are far more costly than halothane. Ether, of course, is never used in the western world and trichloroethylene has a diminishing number of users worldwide as production has now virtually ceased.

ETHER (Diethyl ether)

This is an inexpensive agent made from sugar cane (ethanol). Ether has been known since the 16th century as "sweet vitriol" but only when W.T.G.Morton demonstrated its effects in Boston in 1846 did its anaesthetic properties become known worldwide. This "first anaesthetic" took place on 16th October 1846.

Ether is stored in dark bottles with corks/caps as light may decompose it. If it is taken to high altitude its boiling point is lowered (for example where atmospheric pressure is 425 mmHg ether will boil at 20°C).

Advantages: Ether stimulates respiration and blood flow due to its sympathomimetic effect mediated by adrenaline release. When too much ether is given respiration becomes depressed before the heart. These effects make ether a "safe" anaesthetic agent. It is a bronchodilator and produces analgesia. It may be used as the sole anaesthetic agent and is capable of producing good abdominal muscle relaxation. Ether causes little uterine relaxation.

Disadvantages: Ether is associated with a slow onset and a slow recovery. It stimulates salivation and is best used with atropine premedication. The vapour is unpleasant to breathe initially and causes irritation of the bronchial tree which may slow down the induction of anaesthesia. The incidence of nausea and vomiting is higher with ether than with other agents. The frequency is related to the concentration of ether used and is lower when ether is given via an endotracheal tube during relaxant anaesthesia.

Indications: Any general anaesthetic. It is especially useful for caesarean section (because the baby tolerates it and the uterus contracts well), major operations requiring intubation and poor risk cases (using a low dose). It is the volatile agent of choice when general anaesthesia is needed but no oxygen is available.

Contraindications: There are no absolute contra-indications to ether. It is better avoided in moderate or severe pre-eclampsia because of its sympathomimetic activity. Likewise, liver or renal

failure and phaeochromocytoma are relative contra-indications.

Ether is explosive when mixed with oxygen and is inflammable in air. It may be ignited by a flame or an electrical spark such as those produced by diathermy or static electricity. The ether vapour is inflammable within the patient (lungs, airway or stomach full of vapour) or outside the patient within 25cm of the anaesthetic circuit. Scavenging must always be carried out (where possible) to avoid contact between heavy inflammable ether vapour and diathermy apparatus or other electrical devices that may spark. If the end of the scavenging tube is placed on the floor (away from any possible sources of ignition) then the heavy ether vapour will remain at floor level and the smell of the agent to the surgical and anaesthetic team reduced.

If in doubt about the safety of ether with diathermy, don't use them together.

Dosage and technique: The easiest method is to give ether to the patient after they have been intubated following atropine, thiopentone and suxamethonium. IPPV is commenced initially with 10-15% ether and then according to the patient's requirements, the ether is cut back after 2-8 minutes to 4-8% (usually 6-8%). Poor risk, septic or shocked patients can be kept just insensible with 2% ether. Discontinue the ether well before the end of the operation to avoid a prolonged recovery. With skill you can have your patients almost awake as they are moved off the table. If the patient is given a long acting muscle relaxant and the ventilation controlled, then the ether may be reduced to around 3-4%.

Vaporisers: The small Ether TEC vaporiser will deliver a lower dose than the EMO for the same setting. When using the EMO vaporiser do not give 15-20% ether for more than a few minutes because it is a very efficient vaporiser and will rapidly overdose the patient.

Practical points: If you are not used to giving ether, you will be surprised at its slow onset of action. Don't let the surgeon start until the patient is really deep. People working in theatre may complain about the smell of ether and nurses may claim that it gives them headaches. Symptoms improve after a while and may be reduced by efficient scavenging.

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