During the 19th century, nitrous oxide, ether, chloroform and ethyl chloride were being used experimentally to alleviate pain experienced during surgery.

The method by which these anaesthetic agents were administered is now considered to be very primitive by modern standards. A major breakthrough was made when apparatus was devised that could control the flow of nitrous oxide and oxygen in adjustable proportions.

Anaesthetic agents that were administered over prolonged periods, in particular chloroform, caused considerable physiological damage to the patients due to their toxicity. A further discovery, trichlorethylene (trilene), made the use of chloroform much less popular. Although much safer it still had one major disadvantage, in that it can not be used in a 'closed circuit' method of anaesthesia. Phosgene, a very toxic gas is produced when trilene flows through the CO2 absorbing chemical soda lime.

Since 1945, a radically different method of anaesthetising patients has become widely practised, based on the discovery that a purified extract of a naturally occurring poison 'curare' can be safely injected intravenously. It produces a muscular relaxation by acting on the patients neuromuscular system.

What does this mean? Various depths of physiological paralysis can be achieved to enable more complex surgery to be carried out. If deep anaesthesia is required utilising this technique, the patients lungs become paralysed and the anaesthetist has to keep the patient alive by controlled artificial ventilation of the lungs.

Modern surgery techniques can sometimes require that a patient is anaesthetised for many hours at the extreme levels of anaesthesia. Previously, this had been a most daunting task for the anaesthetist, and most dangerous to the patients physiologically. The anaesthetist now, has a vast array of sophisticated, reliable, patient and gas monitoring equipment. Closer control in the administration of anaesthetic agents, has now been further enhanced by the widely available compact gas analysers. These can monitor the gases to the patient and also expired gases from the patients lungs.



Surgical anaesthesia is a reverseable state of unconsciousness produced by drugs, with sufficient depression of the reflexes to allow an operation to be performed.

It can be seen from the above statement that two conditions are necessary before the state of surgical anaesthesia can be achieved. Firstly there should be unconsciousness, which after all, is all that the patient is interested in! Secondly, some depression of muscular reflex activity, which the surgeon needs to enable delicate work.

If an anaesthetic agent is administered in sufficient quantity for long periods, overdose levels are reached which can lead to death. From the time that the first noticeable effect is produced until fatal overdose occurs, the progressive reflex depression of the central nervous system may be divided into four stages.

Stage one

During childbirth, some women in labour are administered an analgesic agent (pain relief) and remain conscious and co-operative. Accident victims also need to have some degree of pain relief to lessen the effects of shock. Specifically designed equipment delivers analgesic gas, usually in the proportion 50-50 nitrous oxide and oxygen (entonox).

Stage two

(Unconscious, without reflex depression or delerium)
This is too deep for a woman in labour, as she is required to remain conscious. It is not deep enough for a surgeon as it cannot provide the conditions he requires. This stage is therefore passed through or bypassed altogether.

Stage three

(Unconscious, with reflex depression or surgical anaesthesia)
This provides at its lightest level, sufficient reflex depression to prevent movement of a limb or muscle if the skin is cut.

More anaesthetic agent, and hence more anaesthesia, must be used to depress the more sensitive reflexes, such as those from handling the parietal peritoneum (the abdominal cavity), where inadequate anaesthesia would cause a reflex tightening of the abdominal muscles and prevent the surgeon reaching his objective.

Stage four

(Respiratory paralysis)
Respiration becomes progressively paralysed in the deeper planes of stage three, and when it finally ceases, stage four has been reached. Sometimes the anaesthetist has to take the patients near to the brink of this stage in order to meet the surgeon's requirements.

General Anaesthetic Agents

Nitrous oxide

Gas which can be compressed into liquid form. Non explosive, non inflammable. Low toxicity, weak anaesthetic agent. It was used as the sole anaesthetic agent for dental or outpatient procedures. It can be used to maintain anaesthesia during major surgery in combination with other anaesthetic agents; but it is not used on its own to produce a deep level of anaesthesia. Usually administered to the patient in the proportion of two-thirds nitrous oxide to one-third oxygen.

Nitrous oxide mixed in equal proportions with oxygen is used as an analgesic agent for dental surgery or maternity work.


Non explosive, non inflammable. Heavy vapour, an organic compound, very potent. Expensive, normally used in closed circuit systems. It is vaporised in special devices that are capable of delivering a calibrated, known concentration. Reduces blood pressure, even at light levels of anaesthesia.

Non anaesthetic gases


Present in the air at a concentration of approximately 21 percent. It cannot be ignited, but its presence will aid combustion. Explosive whilst under pressure and brought into contact with oil or grease. Should not be at administered to patients in concentrations above 40 percent.

Carbon dioxide

It will stimulate respiration making it deeper, but not increasing the rate. Sometimes used when spontaneous respiration does not occur after an operation.

It can also be used, in low proportions to the total gas mixture being administered, as an aid to the smooth induction of anaesthetic agents. (ie. Used with nitrous oxide and oxygen for a short period before using the anaesthetic agent, reduces the patients resistance to breathing in the agent)

Anaesthetic Equipment

The Boyles apparatus was first developed for use in 1917. It was one of the most common types of anaesthetic equipment used in the operating theatre. It operates on the continuous flow principle whereby gas flows all the time during the inspiritory and expiritory phases of patient respiration, being temporarily stored during expiration in a reservoir bag.

Most gas apparatus in use today is based on the boyles apparatus, and although dated, the boyles apparatus is still used in many hospitals.

The basic principles of gas anaesthesia have been known for over a hundred years and are still used. An anaesthetising agent is delivered to the patient via flow controllers and mix controllers. Normally a mix of nitrous oxide and oxygen would act as a carrier for the main agent (i.e. Halothane).



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