Chapter 18. CNS 1. General anaesthesia, local anaesthetics and resuscitation
Learning objectives
At the end of this chapter, the reader should be able to:
• outline preoperative assessment procedures taken before general anaesthesia and surgery
• give an account of the role that drugs have in the preoperative preparation of the patient for general anaesthesia and surgery
• describe the main elements of general anaesthesia and have an understanding of the groups of drugs involved
• explain how muscle relaxants work and how their effects may be reversed using drugs
• describe the ways in which local anaesthetics may be used and of some of the differences between the various drugs available
• describe which drugs may be used in cardiopulmonary resuscitation (CPR) and what value they have in this role
History
Three important drugs, capable of producing general anaesthesia when inhaled in sufficient quantity, were introduced to medicine within a time span of 5 years in the mid-19th century. These inhalational anaesthetic agents are:
• ether
• nitrous oxide
• chloroform.
In 1842, William E. Clark of Rochester, New York, used ether to provide general anaesthesia for the extraction of a tooth. It is a drug that is still in use in parts of the world where more recent and expensive agents are unavailable. Nitrous oxide was first used as a general anaesthetic in 1844 by Horace Wells, a dentist from Hartford, Connecticut. He had first persuaded a travelling lecturer in chemistry to give him nitrous oxide while a fellow dentist took out one of his teeth before he then used it successfully on his own patients. James Y. Simpson of Edinburgh introduced the use of chloroform in 1847 for general surgery and obstetrics and he administered it to Queen Victoria in 1853 at the birth of Prince Leopold. Chloroform remained popular for over 100 years but is no longer used as a general anaesthetic.
Preoperative assessment of the patient
The patient’s medical history must be thoroughly investigated, and existing drug use (especially use of steroids) and any pre-existing medical conditions (e.g. diabetes mellitus, heart, liver or respiratory disease, or problems with blood coagulation) identified before elective surgery. Examination should include:
• measurement of blood pressure and heart rate
• check the veins for ease of canulation
• examination of the lungs, including chest X-rays if indicated
• ECG if indicated
• examination of any carotid bruit (murmur)
• identification of heart murmurs
• test for haemoglobinopathy as appropriate
• check for impaired mobility of the temporomandibular joints (TMJ; connect the lower jaw to the skull)) and for rheumatoid arthritis (RA)-associated mobility problems of the neck
• blood tests, including liver function tests.
Premedication
Premedication is the administration of drugs to patients an hour or two before anaesthesia and surgery. Nowadays it is used much less than previously. The objectives of premedication are:
• to relieve anxiety
• to reduce the production of saliva
• to reduce the volume, and increase the pH, of the gastric contents.
Premedication is rarely essential nowadays, and it is common practice not to administer any drugs to patients for the above purposes before anaesthesia and surgery. If drugs are used, they are generally given orally and rarely by intramuscular or intravenous injection. Intramuscular premedication, in particular, is not tolerated well by children. Some drugs may still sometimes be necessary before surgery – for example, steroids and antibiotics.
Drugs used for premedication include:
• anxiolytic drugs
• antimuscarinic drugs to reduce secretions
• drugs to raise gastric pH.
Relief from anxiety
Most patients are anxious before anaesthesia and surgery. Many feel a general sense of nervousness or apprehension. Others have more specific fears which may be of:
• injections
• pain after surgery
• waking up in the middle of the operation
• dying and not waking up at all
• the embarrassment of nakedness
• talking aloud while asleep (and perhaps giving away personal secrets)
• the loss of control over themselves and their environment
• having the wrong operation
• what the surgeon may find wrong with them.
The best treatment for anxiety is to listen to patients carefully; to give clear, simple and accurate explanations in response to their concerns and questions; and to give reassurance whenever possible.
Anxiolytic drugs
Several of the benzodiazepine group of drugs may be used to help reduce anxiety before anaesthesia and surgery. Temazepam is the most commonly used. It is well absorbed and effective given orally (there is no injectable preparation). Midazolam is only available as an injection solution, but this can be used orally. It is, however, more effective if given intranasally or sublingually, and is sometimes given by these unusual routes in children. The injection solution is extremely bitter and needs to be diluted with something like neat blackcurrant squash immediately before use by all of these routes. It has a shorter duration of action than temazepam. Other drugs in this group that can be used are diazepam and lorazepam. Alimemazine (trimeprazine) may still sometimes be used for oral premedication in children. It can cause marked pallor and a tachycardia. It is a phenothiazine.
Opioids
Morphine and other opioid analgesics, although good anxiolytics, are only used for premedication for patients who are in pain before surgery.
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Always give patients the opportunity and time to ask all the questions they wish to ask, and to express as fully as possible any anxieties they may have before they undergo anaesthesia and surgery.
Reduction of the production of saliva
Excessive salivation can present problems in safe airway management, particularly during the induction of anaesthesia. It is a more common problem in children than in adults. Premedication with drugs that reduce these oral secretions is occasionally used, especially in preparation for an awake fibreoptic intubation, before certain complex examinations of the upper airway in children, and before the use of ketamine. Antimuscarinic anticholinergic drugs are used for this purpose.
Atropine
Atropine markedly reduces salivary secretions. It is usually given by intramuscular injection but it can also be effective when given orally and this is the preferred route for administration in children. The oral dose is about twice the intramuscular dose. The intravenous route is not used for premedication. It crosses the blood–brain barrier and causes central nervous system (CNS) stimulation, although this is only a mild effect.
Glycopyrronium (glycopyrrolate) may be given intravenously minutes before the induction of anaesthesia, to reduce secretions immediately. It is not absorbed if given orally. Hyoscine, although effective in reducing secretions, is no longer used for premedication because of its marked sedative action. It may also cause confusion and restlessness, particularly in the elderly.
Reduction of the volume, and increase in the PH, of gastric contents
A reduction in the volume of and an increase in the pH of gastric contents reduces some of the risks of vomiting, regurgitation and of subsequent inhalation of gastric contents during general anaesthesia. Drugs are commonly used to achieve these effects during labour when women face the possibility of elective or emergency surgery under general anaesthesia. They may also be used in other patients known to have significant gastro-oesophageal reflux.
Metoclopramide hastens gastric emptying. It is given on the morning of, or a few hours before, delivery in obstetric patients or general anaesthesia. It may be given orally or intravenously. Sodium citrate raises the pH of gastric contents by neutralizing acid in the stomach. It is given orally about 30 minutes before general anaesthesia.
Ranitidine also raises the pH of gastric secretions. It is an H 2-receptor antagonist that reduces gastric acid production and so increases the pH of gastric contents. It is usually given on the night before, and the morning of, delivery in obstetric patients or general anaesthesia. It can be given orally or intravenously.
Intravenous induction agents
Thiopental
Thiopental was first used in 1934 and is still in widespread use. It is a barbiturate.
Onset and duration of action
Unconsciousness occurs about 20 seconds after injection and continues for several (5–10) minutes. The termination of its action occurs as the drug is redistributed away from the brain into other tissues, particularly muscle and fat. It is metabolized very slowly (several hours) and so cannot be used as a continuous intravenous infusion as it would then accumulate and lead to prolonged sleepiness or unconsciousness when discontinued (compare with propofol).
Effects on muscle, respiration and blood pressure
Loss of muscle tone and therefore of normal airway control occurs immediately after injection, as does a short period of hypoventilation (respiratory depression) and sometimes apnoea. It is therefore essential to have facilities for pulmonary ventilation and the delivery of oxygen immediately at hand whenever it is used. Thiopental causes a small drop in blood pressure mainly due to a reduction in peripheral resistance. A marked fall in blood pressure may occur if the injection is given too rapidly, the dose is too large, or the patient is elderly, has significant cardiac disease or is hypovolaemic.
Accidental intra-arterial injection
This results in immediate and severe pain in the arm distal to the site of injection and, if concentrations of greater than 2.5% are used, this may be followed by arterial spasm, loss of peripheral pulses and permanent ischaemic damage to parts of the arm. Injecting the drug into a vein on the dorsum of the hand reduces the risk of this occurrence. Extravascular injection can also result in tissue damage.
Etomidate
Etomidate was first used in 1973. It is not a barbiturate. It is metabolized more quickly than either of the two barbiturates and recovery is probably faster than from either barbiturate. It has little effect on blood pressure and for this reason is sometimes chosen for use in patients with cardiac problems. It is otherwise not commonly used.
Propofol
Propofol is a widely used agent. It was first used in 1977, although it was not released for general use until 1986. It is dissolved in the oil phase of an emulsion of soybean oil and purified egg phosphatide and is an opaque white fluid that looks like milk (like Diazemuls; see p. 272). Recovery from its effect is more rapid and complete than from any other intravenous induction agent. It is therefore particularly suited to use in day surgery units and for short procedures. It is very rapidly metabolized (in a few minutes) and can be used as a continuous low-dose intravenous infusion to provide prolonged periods of anaesthesia or to sedate patients for hours or days in intensive care units. Injection of propofol is followed by a short period of apnoea and a small drop in blood pressure. Like etomidate, propofol causes pain on injection, which can be markedly reduced by mixing it with lidocaine.
Ketamine
Ketamine is unique among the induction agents. Some of the differences between ketamine and the other induction agents are:
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• It can be given intramuscularly as well as intravenously.
• It has potent analgesic activity and produces a state known as dissociative analgesia in which the patient looks dreamily awake and may move around but is, in fact, unaware of his or her surroundings and is free of any pain.
• Muscle tone is maintained and therefore the patient retains the ability to maintain his or her own airway despite being unconscious. Respiration is generally not depressed. It is thus of particular value when access to the head and neck is not possible, as occurs in children receiving radiotherapy, some civilian transport disasters and in casualties in the field of battle.
• It causes a rise in blood pressure and is therefore popular for use in children with cardiac disease.
• During recovery, nightmares and hallucinations, referred to as emergence phenomena, are common, except, apparently, in children. These effects are so unpleasant in adults that the drug is rarely used in adults except in the unusual circumstances referred to above.
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After ketamine has been used, let the patient wake up peacefully, preferably in a quiet room with subdued lighting, and do not prod or shout at the patient to wake him or her up more quickly. This will reduce the incidence and severity of emergence phenomena –nightmares and unpleasant hallucinations.
Maintenance of anaesthesia
Inhalational anaesthetics
Nitrous oxide is the original ‘gas’ or ‘laughing gas’, so called because it causes some patients to laugh during the induction of anaesthesia if it is used on its own. It is a faintly smelling gas that is compressed and stored as a liquid in cylinders (coloured blue in the UK). Even in concentrations of 80% it is only a weak anaesthetic and it needs to be combined with other inhalational agents or intravenous drugs. Unlike the other inhalational anaesthetics, it has a strong analgesic effect in concentrations less than those required to produce unconsciousness.
Entonox takes advantage of the analgesic properties of nitrous oxide. It is a 50:50 mixture of nitrous oxide and oxygen, stored as a compressed gas in cylinders (coloured blue and white in the UK). It is used for pain relief in labour, and by ambulance crews and others who treat pain outside hospital. It is also useful for rapid but potentially painful procedures, such as when very painful dressings are applied.
Halothane, enflurane, isoflurane and sevoflurane are potent halogenated hydrocarbons and have similar structures and effects. Some of the differences between them are listed in Table 18.1. They are also referred to as volatile anaesthetic agents as they are liquids at room temperature and it is the vapour from the liquid that is inhaled as the anaesthetic. They require a carrier gas, usually oxygen and nitrous oxide, to deliver them, and vaporizers capable of delivering accurate concentrations in the range of 0.25–8.0%. Unlike nitrous oxide, they have no analgesic properties in sub-anaesthetic concentrations. Halothane is the oldest, but is now little used as it causes cardiac arrhythmias and, very rarely, severe hepatitis. Isoflurane is the most commonly used. It has little effect on cardiac output and is associated with a rapid recovery from anaesthesia. Sevoflurane is the most expensive. It is particularly suitable for inducing anaesthesia in children as it has a weak and not unpleasant smell and, in a high concentration of 8%, induces anaesthesia within a few respirations.
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Nurses should make a point of knowing when an inflammable inhalational anaesthetic is going to be used, as may happen in less developed countries where ether and other inflammable gases may still be used, and ensure that qualified electricians have checked for properly earthed equipment. This is much less of a problem in the UK, where inflammable anaesthetics are avoided where possible. It is also important to know that some anaesthetics accumulate in rubber tubing and accidental overdose can occur. In the UK these problems are more of historical interest (or should be).
| Parameter | Halothane | Enflurane | Isoflurane | Sevoflurane | Desflurane |
| First use in humans | 1956 | 1966 | 1970 | 1981 | 1988 |
| Boiling point | 50°C | 56°C | 49°C | 58.9°C | 23.5°C |
| Equipotent | 0.8% | 1.70% | 1.2% | 2.0% | 7.3% |
| Cardiac output | Falls | Falls | Falls a little | Falls a little | Little |
| Recovery | Slow | Moderate | Fast | Fast | Very fast |
| Metabolism | 20% | 2.5% | 0.2% | 0.02% | 4% |
| Relative cost | Low | Moderate | High | Very high | Very high |
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