Chapter 19 Anaesthesia and neuromuscular block
General anaesthesia
Until the mid-19th century such surgery as was possible had to be undertaken at tremendous speed. Surgeons did their best for terrified patients by using alcohol, opium, cannabis, hemlock or hyoscine.1 With the introduction of general anaesthesia, surgeons could operate for the first time with careful deliberation. The problem of inducing quick, safe and easily reversible unconsciousness for any desired length of time in humans began to be solved only in the 1840s when the long-known substances nitrous oxide, ether and chloroform were introduced in rapid succession.
• 1842 – W E Clarke of Rochester, New York, administered ether for a dental extraction; however, the event was not made widely known at the time.
• 1844 – Horace Wells, a dentist in Hartford, Connecticut, introduced nitrous oxide to produce anaesthesia during dental extraction.
• 1846 – On 16 October William Morton, a Boston dentist, successfully demonstrated the anaesthetic properties of ether.
• 1846 – On 21 December Robert Liston performed the first surgical operation in England under ether anaesthesia.2
• 1847 – James Y Simpson, professor of midwifery at the University of Edinburgh, introduced chloroform for the relief of labour pain.
The next important developments in anaesthesia were in the 20th century when the appearance of new drugs, both as primary general anaesthetics and as adjuvants (muscle relaxants), new apparatus and clinical expertise in rendering prolonged anaesthesia safe enabled surgeons to increase their range. No longer was the duration and type of surgery determined by patients’ capacity to endure pain.
Phases of general anaesthesia
Before surgery,
All of these may influence the choice of anaesthetic technique and anaesthetic drugs.
Before surgery (premedication)
During surgery
Induction
1. Usually intravenous: pre-oxygenation followed by a small dose of an opioid, e.g. fentanyl or alfentanil to provide analgesia and sedation, followed by propofol or, less commonly, thiopental, etomidate or ketamine to induce anaesthesia. Airway patency is maintained with a supraglottic airway device (e.g. laryngeal mask airway (LMA)), a tracheal tube or, for very short procedures, an oral airway and facemask. Insertion of a tracheal tube usually requires paralysis with a neuromuscular blocker and is undertaken if there is a risk of pulmonary aspiration from regurgitated gastric contents or from blood.
2. Inhalational induction, usually with sevoflurane, is undertaken less commonly. It is used in children, particularly if intravenous access is difficult, and in patients at risk from upper airway obstruction.
Maintenance
1. Most commonly with oxygen and air, or nitrous oxide and oxygen, plus a volatile agent, e.g. sevoflurane, desflurane or isoflurane. Additional doses of a neuromuscular blocker or opioid are given as required.
2. A continuous intravenous infusion of propofol can be used to maintain anaesthesia. This technique of total intravenous anaesthesia is relatively common because the quality of recovery may be better than after inhalational anaesthesia. The propofol infusion is often combined with an infusion of remifentanil, an ultra-short-acting opioid.
When appropriate, peripheral nerve block with a local anaesthetic, or neural axis block, e.g. spinal or epidural, provides intraoperative analgesia and muscle relaxation. These local anaesthetic techniques provide excellent postoperative analgesia.
Some special techniques
Dissociative anaesthesia
is a state of profound analgesia and anterograde amnesia with minimal hypnosis during which the eyes may remain open; it can be produced by ketamine (see p. 301). It is particularly useful where modern equipment is lacking or where access to the patient is limited, e.g. in prehospital or military settings.
Sedation and amnesia
without analgesia is provided by intravenous midazolam or, less commonly nowadays, by diazepam. These drugs can be used alone for procedures causing mild discomfort, e.g. endoscopy, and with a local anaesthetic where more pain is expected, e.g. removal of impacted wisdom teeth. Benzodiazepines produce anterograde, but not retrograde, amnesia. By definition, the sedated patient remains responsive and cooperative. (For a general account of benzodiazepines and the competitive antagonist flumazenil, see Ch. 20.)
Inhalation anaesthetics
Nitrous oxide
Halogenated anaesthetics
Halothane
Halothane has the highest blood/gas partition coefficient of the volatile anaesthetic agents and recovery from halothane anaesthesia is comparatively slow. It is pleasant to breathe. Halothane reduces cardiac output more than any of the other volatile anaesthetics. It sensitises the heart to the arrhythmic effects of catecholamines and hypercapnia; arrhythmias are common, in particular atrioventricular dissociation, nodal rhythm and ventricular extrasystoles. Halothane can trigger malignant hyperthermia in those who are genetically predisposed (see p. 309).
Intravenous anaesthetics
Pharmacokinetics
Intravenous anaesthetics enable an extremely rapid induction because the blood concentration can be raised quickly, establishing a steep concentration gradient and expediting diffusion into the brain. The rate of transfer depends on the lipid solubility and arterial concentration of the unbound, non-ionised fraction of the drug. After a single induction dose of an intravenous anaesthetic, recovery occurs quite rapidly as the drug is redistributed around the body and the plasma concentration reduces. Recovery from a single dose of intravenous anaesthetic is thus dependent on redistribution rather than rate of metabolic breakdown. With the exception of propofol, repeated doses or infusions of intravenous anaesthetics will cause considerable accumulation and prolong recovery. Attempts to use thiopental as the sole anaesthetic in war casualties led to it being described as an ideal form of euthanasia.3
