Anaesthesia and neuromuscular block

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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.

The details surrounding the first use of surgical anaesthesia were submerged in bitter disputes on priority following an attempt to take out a patent for ether. The key events around this time were:

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

Balanced surgical anaesthesia (hypnosis, analgesia and muscle relaxation) with a single drug would require high doses that would cause adverse effects such as slow and unpleasant recovery, and depression of cardiovascular and respiratory function. In modern practice, different drugs are used to attain each objective so that adverse effects are minimised.

The perioperative period may be divided into three phases, and several factors determine the choice of drugs given in each of these. In brief:

Before surgery (premedication)

The principal aims are to provide:

During surgery

The aim is to induce unconsciousness, analgesia and muscle relaxation – the anaesthetic triad. Total muscular relaxation (paralysis) is required for some surgical procedures, e.g. intra-abdominal surgery, but most surgery can be undertaken without neuromuscular blockade. A typical general anaesthetic consists of:

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.

Pharmacology of anaesthetics

All successful general anaesthetics are given intravenously or by inhalation because these routes enable closest control over blood concentrations and thus of effect on the brain.

Inhalation anaesthetics

The preferred inhalation anaesthetics are those that are minimally irritant and non-flammable, and comprise nitrous oxide and the fluorinated hydrocarbons, e.g. sevoflurane.

Pharmacokinetics (volatile liquids, gases)

The depth of anaesthesia is correlated with the tension (partial pressure) of anaesthetic drug in brain tissue. This is driven by the development of a series of tension gradients from the high partial pressure delivered to the alveoli and decreasing through the blood to the brain and other tissues. The gradients are dependent on the blood/gas and tissue/gas solubility coefficients, as well as on alveolar ventilation and organ blood flow.

An anaesthetic that has high solubility in blood, i.e. a high blood/gas partition coefficient, will provide a slow induction and adjustment of the depth of anaesthesia. Here, the blood acts as a reservoir (store) for the drug so that it does not enter the brain readily until the blood reservoir is filled. A rapid induction can be obtained by increasing the concentration of drug inhaled initially and by hyperventilating the patient.

Anaesthetics with low solubility in blood, i.e. a low blood/gas partition coefficient (nitrous oxide, desflurane, sevoflurane), provide rapid induction of anaesthesia because the blood reservoir is small and anaesthetic is available to pass into the brain sooner.

During induction of anaesthesia the blood is taking up anaesthetic selectively and rapidly, and the resulting loss of volume in the alveoli leads to a flow of anaesthetic into the lungs that is independent of respiratory activity. When the anaesthetic is discontinued the reverse occurs and it moves from the blood into the alveoli. In the case of nitrous oxide, this can account for as much as 10% of the expired volume and so can significantly lower the alveolar oxygen concentration. Mild hypoxia occurs and lasts for as long as 10 min. Oxygen is given to these patients during the last few minutes of anaesthesia and the early post-anaesthetic period. This phenomenon, diffusion hypoxia, occurs with all gaseous anaesthetics, but is most prominent with gases that are relatively insoluble in blood, for they will diffuse out most rapidly when the drug is no longer inhaled, i.e. just as induction is faster, so is elimination. Nitrous oxide is especially powerful in this respect because it is used at concentrations of up to 70%.

Nitrous oxide

Nitrous oxide (1844) is a gas with a slightly sweetish smell that is neither flammable nor explosive. It produces light anaesthesia without demonstrably depressing the respiratory or vasomotor centre provided that normal oxygen tension is maintained.

Halogenated anaesthetics

Halothane was the first halogenated agent to be used widely, but in the developed world it has been largely superseded by isoflurane, sevoflurane and desflurane. A description of isoflurane is provided, and of the others in so far as they differ. The MAC in oxygen of some volatile anaesthetics is:

Isoflurane

Isoflurane is a volatile colourless liquid that is not flammable at normal anaesthetic concentrations. It is relatively insoluble and has a lower blood/gas coefficient than halothane or enflurane, which enables rapid adjustment of the depth of anaesthesia. It has a pungent odour and can cause bronchial irritation, making inhalational induction unpleasant. Isoflurane is minimally metabolised (0.2%), and none of the breakdown products has been related to anaesthetic toxicity.