GENERAL ANAESTHESIA (NARCOSIS)

General anaesthesia is a period of reversible unconsciousness, the absence of pain, reflexes and relaxation of skeletal muscles as a result of the effect of narcotic substances on the central nervous system. Depending on the way the drug is given into the body, the two types of general anaesthesia are identified: inhalation and non-inhalation anaesthesia.

Inhalation anaesthesia

Inhalation anaesthesia is achieved by breathing gaseous or volatile narcotic agents. Ether, halothane, methoxyflurane and trichloroethylene are volatile forms, while gaseous forms include nitrous oxide, cyclopropane.

Narcotic agents work to cause characteristic changes throughout the whole body. During the period of saturation with the substance a certain trend of events (stages) is observed, i.e. in changes in consciousness, breathing and circulation. The period narcosis is hence divided into stages that correspond to the depth of anaesthesia. These stages are mainly obvious when ether is used.

The four stages are as follows: I - analgesia, II - excitement/agitation, III - surgical stage of 4 levels, and IV - awakening.

Analgesia (I). The patient is conscious but confused and answers questions reluctantly. He/she is not sensitive to superficial pain, whereas touch and heat sensitivity are intact. Minor operations (e.g. incision and drainage of abscesses and diagnostic procedures) can be performed during this period. This stage is very short as it persists for 3-4 minutes.

Excitement (agitation) (II). Inhibition of the cortical centres occurs in this stage while the subcortical ones are excited: unconsciousness, increased motor and speech reactions. Patients shout, attempt to get off the operating table. The skin is hyperaemic, pulse accelerated and blood pressure level increased. The pupils are dilated but react to light, tears appear in the eyes. Patients normally cough at this stage, their bronchial secretion is increased and vomit may even occur. Operating on at this stage is prohibited. It is necessary to continue saturating the body with the drug to deepen the narcotic effect. The maintenance period depends on the patient's condition and anaesthetist's qualification. Excitement normally lasts for 7-15 minutes.

Surgical stage (III). With the onset of this period the patient calms down, breathes smoothly, the pulse and blood pressure are near the basal values. Surgical interventions may be performed in this stage. Depending on the depth of anaesthesia, this stage is divided into 4 levels:

• level 1: the patient is calm, breathes smoothly, blood pressure and pulse approach the initial values. The pupils begin to constrict, still react to light. The eyeballs are placed eccentrically and move rolling. Corneal and swallow reflexes are maintained. The muscle tone is maintained, so it is difficult to perform operations on visceral organs;

• level 2: the eyeballs stop moving and are located centrally. Pupils start to gradually dilate; reaction of the pupils to light abates. Corneal and swallow reflexes weaken and at the end of the second level are absent. Breathing is quiet and smooth. Blood pressure and pulse rate are normal. Muscle tone starts to decrease, which allows for operations on visceral organs;

• level 3: the patient is in deep narcosis. The pupilsare dilated, do not react to light, swallow reflex is absent. The skeletal muscles including the intercostal ones are fully relaxed. Breathing is shallow and diaphragmatic. As a result of relaxation of the muscles of the mandible which can hang, the tongue can fall back to block the entrance of the larynx and cause respiratory arrest. To prevent this complication the mandible should be held forward and kept there. The pulse is fast and weak. Blood pressure falls. It is noteworthy that keeping anaesthesia in this condition may be fatal;

• level 4: maximal dilation of the pupils with no reaction to light, the cornea is dry. Breathing is shallow, taking place through the movements of the diaphragm since the intercostal muscles are paralysed. Pulse is faint and fast; blood pressure level is low and can hardly be assessed. To deepen anaesthesia to level 4 may be fatal since respiratory and circulatory arrests can easily set in. Stage of awakening (IV). The moment the supply of the narcotic is stopped, its blood concentration falls, the patient in a reverse way goes through all the stages passed and wakes up.

The following substances are used for inhalation narcosis.

Ether is a colourless liquid with a specific odour and a boiling temperature of 36, 5 °C. It dissolves well in fat and alcohol. It should be kept in dark well-cocked bottles since it is oxidized on exposure to light to form a toxic compound. As it is inflammable, it has to be handled with care when used in modern time theatres. Ether is a strong narcotic substance, which causes deep anaesthesia. It is eliminated through the respiratory tract, irritates it and therefore stimulates bronchial secretion. It may also impair hepatic functions.

Halothane (fluothane, narcotan) is a colourless fat-soluble liquid with sweet odour and a boiling temperature of 50, 2 °C, which should be kept in dark bottles. It is fire-resistant. Halothane is a potent narcotic: acts fast (within 3-4 minutes), the excitement stage is either very short or does not manifest at all, patients wake up very fast. Transfer from one stage of narcosis to another is very immediate and because of this over dosage can occur. The drug inhibits the cardiovascular system that leads to a reduction in the heartbeat and blood pressure level. The drug is toxic to the liver. It does not irritate the respiratory tract, dilates the bronchi hence can be used for patients with respiratory system diseases. Halothane may increase the sensitivity of cardiac muscles to epinephrine and norepinephrine. Thus, if a patient is under this kind of anaesthesia, such drugs should be avoided.

Methoxyflurane (pentran) is a colourless transparent liquid with a characteristic fruity odour and a boiling temperature of 104 °C. Its narcotic potency is higher than that of ether and chloroform; the narcotic sleep is slow, after 8-10 minutes of its introduction. The excitement stage is more pronounced and awakening is slow. The drug like halothane can reduce blood pressure and increase the sensitivity of the myocardium to the catecholamines (epinephrine and norepinephrine).

Nitrous oxide, or «laughing gas», is a colourless gas without odour, not explosive, but, if combined with ether and oxygen, it can burn. The gas should be kept in grey metallic cylinders in a liquid form under the pressure of 50 atmospheres. Nitrous oxide is an inert gas, in the body it does not affect functions of any organ, it is eliminated via the lungs unchanged. As a narcotic it is only used in combination with oxygen since pure nitrous oxide is toxic. Nitrous oxide and oxygen are used in the ratio of 1: 1, 2: 1, 3: 1 and 4: 1. The latter ratio contains 80% nitrous oxide and 20% oxygen. Decreasing the concentration of oxygen in the compound below 20% is prohibited, otherwise it may cause severe hypoxia. Nitrous oxide causes quick and quiet sleep without the excitement stage. Awakening is immediate, just at the time the anaesthesia is stopped. The disadvantage of nitrous oxide is its low anaesthetic potency: even in its maximum concentration of 80% it gives a slight anaesthetic effect (levels 1-2 of the surgical stage). Muscle relaxation is absent. Nowadays nitrous oxide is used in combination with barbiturates, halothane - in so called «combined anaesthesia».

Cyclopropane is a colourless gas with mild petroleum odour. It is prepared into red cylinders in liquid form under the pressure of 5 atmospheres. It is only used when mixed with oxygen. Its disadvantage is that it is highly inflammable. Cyclopropane is a potent anaesthetic agent: inhalation of only 10-15% of the compound with oxygen causes surgical stage of anaesthesia. Sleep as well as awakening is very fast. It does not irritate the respiratory tract, but its action on the vagus nerve can provoke cardiac arrhythmias. Cyclopropane increases the myocardial sensitivity to epinephrine and norepinephrine, maintains and stabilizes blood pressure and is therefore indicated for bleeding and shock. It does not affect hepatic or renal function.

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