Muscle Function and Neuromuscular Blockade
In the last 70 years, neuromuscular blocking drugs have become an established part of anaesthetic practice. They were first administered in 1942, when Griffith and Johnson in Montreal used Intocostrin, a biologically standardized mixture of the alkaloids of the Indian rubber plant Chondrodendron tomentosum, to facilitate relaxation during cyclopropane anaesthesia. Previously, only inhalational agents (nitrous oxide, diethyl ether, cyclopropane and chloroform) had been used during general anaesthesia, making surgical access for some procedures difficult because of lack of muscle relaxation. To achieve significant muscle relaxation, it was necessary to deepen anaesthesia, which often had adverse cardiac and respiratory effects. Local analgesia was the only alternative.
At first, muscle relaxants were used only occasionally, in small doses, as an adjuvant to aid in the management of a difficult case; they were not used routinely. A tracheal tube was not always used, the lungs were not ventilated artificially and residual block was not routinely reversed; all of these caused significant morbidity and mortality, as demonstrated in the retrospective study by Beecher & Todd (1954). By 1946, however, it was appreciated that using drugs such as curare in larger doses allowed the depth of anaesthesia to be lightened, and it was suggested that incremental doses should also be used during prolonged surgery, rather than deepening anaesthesia – an entirely new concept at that time. The use of routine tracheal intubation and artificial ventilation then evolved.
In 1946, Gray & Halton in Liverpool reported their experience of using the pure alkaloid tubocurarine in more than 1000 patients receiving various anaesthetic agents. Over the following 6 years, they developed a concise description of the necessary ingredients of any anaesthetic technique; narcosis, analgesia and muscle relaxation were essential – the triad of anaesthesia. A fourth ingredient, controlled apnoea, was added at a later stage to emphasize the need for fully controlled ventilation, reducing the amount of relaxant required.
This concept is the basis of the use of neuromuscular blocking drugs in modern anaesthetic practice. In particular, it has allowed seriously ill patients undergoing complex surgery to be anaesthetized safely and to be cared for postoperatively in the intensive therapy unit.
Acetylcholine, the neurotransmitter at the neuromuscular junction, is released from presynaptic nerve endings on passage of a nerve impulse (an action potential) down the axon to the nerve terminal. The neurotransmitter is synthesized from choline and acetylcoenzyme A by the enzyme choline acetyltransferase and stored in vesicles in the nerve terminal. The action potential depolarizes the nerve terminal to release the neurotransmitter; entry of Ca2 + ions into the nerve terminal is a necessary part of this process, promoting further acetylcholine release. On the arrival of an action potential, the storage vesicles are transferred to the active zones on the edge of the axonal membrane, where they fuse with the terminal wall to release the acetylcholine (Fig. 6.1). Three proteins, synaptobrevin, syntaxin and synaptosome-associated protein SNAP-25, are involved in this process. These proteins along with vesicle membrane-associated synaptotagmins cause the docking, fusion and release (exocytosis) of acetylcholine from the vesicles. There are about 1000 active sites at each nerve ending and any one nerve action potential leads to the release of 200–300 vesicles. In addition, small quanta of acetylcholine, equivalent to the contents of one vesicle, are released at the neuromuscular junction spontaneously, causing miniature end-plate potentials (MEPPs) on the postsynaptic membrane, but these are insufficient to generate a muscle action potential.
FIGURE 6.1 The neuromuscular junction with an axon terminal, containing vesicles of acetylcholine. The neurotransmitter is released on arrival of an action potential and crosses the junctional cleft to stimulate the postjunctional receptors on the shoulders of the secondary clefts. (Reproduced with kind permission of Professor WC Bowman.)
The active sites of release are aligned directly opposite the acetylcholine receptors on the junctional folds of the postsynaptic membrane, lying on the muscle surface. The junctional cleft, the gap between the nerve terminal and the muscle membrane, has a width of only 60 nm. It contains the enzyme acetylcholinesterase, which is responsible for the ultimate breakdown of acetylcholine. This enzyme is also present, in higher concentrations, in the junctional folds in the postsynaptic membrane (Fig. 6.1). The choline produced by the breakdown of acetylcholine is taken up across the nerve membrane to be reused in the synthesis of the transmitter.
The nicotinic acetylcholine receptors on the postsynaptic membrane are organized in discrete clusters on the shoulders of the junctional folds (Fig. 6.1). Each cluster is about 0.1 μm in diameter and contains a few hundred receptors. Each receptor consists of five subunits, two of which, the alpha (α; MW = 40 000 Da), are identical. The other three, slightly larger subunits, are the beta (β), delta (δ) and epsilon (ε). In fetal muscle, the epsilon is replaced by a gamma (γ) subunit. Each subunit of the receptor is a glycosated protein – a chain of amino acids – coded by a different gene. The receptors are arranged as a cylinder which spans the membrane, with a central, normally closed, channel – the ionophore (Fig. 6.2). Each of the α subunits carries a single acetylcholine binding region on its extracellular surface. They also bind neuromuscular blocking drugs.
FIGURE 6.2 Two postjunctional receptors, embedded in the lipid layer of the postsynaptic muscle membrane. The α, β, ε and δ subunits are demonstrated on the surface of one receptor and the ionophore is seen in cross-section on the other receptor. On stimulation of the two α subunits by two molecules of acetylcholine, the ionophore opens to allow the passage of the end-plate current. (Reproduced with kind permission of Professor WC Bowman.)
Activation of the receptor requires both α sites to be occupied, producing a structural change in the receptor complex that opens the central channel running between the receptors for a very short period, about 1 ms (Fig. 6.2). This allows movement of cations such as Na+, K+, Ca2 + and Mg2 + along their concentration gradients. The main change is influx of Na+ ions, the end-plate current, followed by efflux of K+ ions. The summation of this current through a large number of receptor channels lowers the transmembrane potential of the end-plate region sufficiently to depolarize it and generate a muscle action potential sufficient to allow muscle contraction.
At rest, the transmembrane potential is about −90 mV (inside negative). Under normal physiological conditions, a depolarization of about 40 mV occurs, lowering the potential from −90 to −50 mV. When the end-plate potential reaches this critical threshold, it triggers an all-or-nothing action potential that passes around the sarcolemma to activate muscle contraction via a mechanism involving Ca2 + release from the sarcoplasmic reticulum.
Each acetylcholine molecule is involved in opening one ion channel only before it is broken down rapidly by acetylcholinesterase; it does not interact with any of the other receptors. There is a large safety factor in the transmission process, in respect of both the amount of acetylcholine released and the number of postsynaptic receptors. Much more acetylcholine is released than is necessary to trigger the action potential. The end-plate region is depolarized for only a very short period (a few milliseconds) before it rapidly repolarizes and is ready to transmit another impulse.
Acetylcholine receptors are also present on the presynaptic area of the nerve terminal. These are of a slightly different structure to the postsynaptic nicotinic receptors (α3β2). It is thought that a positive feedback mechanism exists for the further release of acetylcholine, such that some of the released molecules of acetylcholine stimulate these presynaptic receptors, producing further mobilization of the neurotransmitter to the readily releasable sites, ready for the arrival of the next nerve stimulus (Fig. 6.3). Acetylcholine activates sodium channels on the prejunctional nerve membrane, which in turn activate voltage-dependent calcium channels (P-type fast channels) on the motor neurone causing an influx of calcium into the nerve cytoplasm to promote further acetylcholine release.
FIGURE 6.3 Acetylcholine receptors are present on the shoulders of the axon terminal, as well as on the postjunctional membrane. Stimulation of the prejunctional receptors mobilizes (MOB) the vesicles of acetylcholine to move into the active zone, ready for release on arrival of another nerve impulse. The mechanism requires Ca2 + ions. (Reproduced with kind permission of Professor WC Bowman.)
In health, postsynaptic acetylcholine receptors are restricted to the neuromuscular junction by a mechanism involving the presence of an active nerve terminal. In many disease states affecting the neuromuscular junction, this control is lost and acetylcholine receptors of the fetal type develop on the adjacent muscle surface. The excessive release of K+ ions from diseased or swollen muscle on administration of succinylcholine is probably the result of stimulation of these extrajunctional receptors. They develop in many conditions, including polyneuropathies, severe burns and muscle disorders.
This quaternary ammonium compound is comparable to two molecules of acetylcholine linked together (Fig. 6.4). The two quaternary ammonium radicals, N+(CH3)3 have the capacity to cling to each of the α units of the postsynaptic acetylcholine receptor, altering its structural conformation and opening the ion channel, but for a longer period than does a molecule of acetylcholine. Administration of succinylcholine therefore results in an initial depolarization and muscle contraction, termed fasciculation. As this effect persists, however, further action potentials cannot pass down the ion channels and the muscle becomes flaccid; repolarization does not occur.
FIGURE 6.4 The chemical structures of acetylcholine and succinylcholine. The similarity between the structure of succinylcholine and two molecules of acetylcholine can be seen. The structure of decamethonium is also shown. The quaternary ammonium radicals N+(CH3)3 cling to the α subunits of the postsynaptic receptor.
The dose of succinylcholine necessary for tracheal intubation in adults is 1.0–1.5 mg kg–1. This dose has the most rapid and reliable onset of action of any of the muscle relaxants presently available, producing profound block within 1 min. Succinylcholine is therefore of particular benefit when it is essential to achieve tracheal intubation rapidly, e.g. in a patient with a full stomach or an obstetric patient. It is also indicated if tracheal intubation is expected to be difficult for anatomical reasons, because it produces optimal intubating conditions.
The drug is metabolized predominantly in the plasma by the enzyme plasma cholinesterase, at one time termed pseudocholinesterase, at a very rapid rate. Recovery from neuromuscular block may start to occur within 3 min and is complete within 12–15 min. The use of an anticholinesterase such as neostigmine, which would inhibit such enzyme activity, is contraindicated (see below). About 10% of the drug is excreted in the urine; there is very little metabolism in the liver although some breakdown by non-specific esterases occurs in the plasma.
If plasma cholinesterase is structurally abnormal because of inherited factors, or if its concentration is reduced by acquired factors, then the duration of action of the drug may be altered significantly.
Inherited Factors: The exact structure of plasma cholinesterase is determined genetically, by autosomal genes, and this has been completely defined. Several abnormalities in the amino acid sequence of the normal enzyme, usually designated , are recognized. The most common is produced by the atypical gene, , which occurs in about 4% of the Caucasian population. Thus a patient who is a heterozygote for the atypical gene demonstrates a longer effect from a standard dose of succinylcholine (about 30 min). If the individual is a homozygote for the atypical gene , the duration of action of succinylcholine may exceed 2 h. Other, rarer, abnormalities in the structure of plasma cholinesterase are also recognized, e.g. the fluoride and silent genes. The latter has very little capacity to metabolize succinylcholine and thus neuromuscular block in the homozygous state lasts for at least 3 h. In such patients, non-specific esterases gradually clear the drug from plasma.
It has been suggested that a source of cholinesterase, such as fresh frozen plasma, should be administered in such cases, or an anticholinesterase such as neostigmine be used to reverse what has usually developed into a dual block (see below). However, it is wiser to:
This condition is not life-threatening, but the risk of awareness is considerable, especially after the end of surgery, when the anaesthetist, who may not yet have made the diagnosis, is attempting to waken the patient. Anaesthesia must be continued until full recovery from neuromuscular block is demonstrable.
As plasma cholinesterase activity is reduced by the presence of succinylcholine, a plasma sample to measure the patient’s cholinesterase activity should not be taken for several days after prolonged block has been experienced, by which time new enzyme has been synthesized. A patient who is found to have reduced enzyme activity and structurally abnormal enzyme should be given a warning card or alarm bracelet, detailing his or her genetic status. Examining the genetic status of the patient’s immediate relatives should be considered.
In 1957, Kalow & Genest first described a method for detecting structurally abnormal cholinesterase. If plasma from a patient of normal genotype is added to a water bath containing a substrate such as benzoylcholine, a chemical reaction occurs with plasma cholinesterase, emitting light of a given wavelength, which may be detected spectrophotometrically. If dibucaine is also added to the water bath, this reaction is inhibited; no light is produced. The percentage inhibition is referred to as the dibucaine number. A patient with normal plasma cholinesterase has a high dibucaine number of 77–83. A heterozygote for the atypical gene has a dibucaine number of 45–68; in a homozygote, the dibucaine number is less than 30.
Acquired Factors: In these instances, the structure of plasma cholinesterase is normal but its activity is reduced. Thus, neuromuscular block is prolonged by only minutes rather than hours. Causes of reduced plasma cholinesterase activity include:
anticholinesterases, including those used to reverse residual neuromuscular block after a non-depolarizing muscle relaxant (e.g. neostigmine or edrophonium); these drugs inhibit plasma cholinesterase in addition to acetylcholinesterase. The organophosphorus compound ecothiopate, once used topically as a miotic in ophthalmology, is also an anticholinesterase.
other drugs which are metabolized by plasma cholinesterase, and which therefore decrease its availability, include etomidate, propanidid, ester local anaesthetics, anti-cancer drugs such as methotrexate, monoamine oxidase inhibitors and esmolol (the short-acting β-blocker).
Muscle Pains: These occur especially in the patient who is ambulant soon after surgery, such as the day-case patient. The pains, thought possibly to be caused by the initial fasciculations, are more common in young, healthy patients with a large muscle mass. They occur in unusual sites, such as the diaphragm and between the scapulae, and are not relieved easily by conventional analgesics. The incidence and severity may be reduced by the use of a small dose of a non-depolarizing muscle relaxant given immediately before administration of succinylcholine, e.g. gallamine 10 mg (which is thought to be most efficacious in this respect) or atracurium 2.5 mg. However, this technique, termed pre-curarization or pretreatment, reduces the potency of succinylcholine, necessitating administration of a larger dose to produce the same effect. Many other drugs have been used in an attempt to reduce the muscle pains, including lidocaine, calcium, magnesium and repeated doses of thiopental, but none is completely reliable.
Increased Intraocular Pressure: This is thought to be caused partly by the initial contraction of the external ocular muscles and contracture of the internal ocular muscles after administration of succinylcholine. It is not reduced by pre-curarization. The effect lasts for as long as the neuromuscular block and concern has been expressed that it may be sufficient to cause expulsion of the vitreal contents in the patient with an open eye injury. This is unlikely. Protection of the airway from gastric contents must take priority in the patient with a full stomach in addition to an eye injury, as inhalation of gastric contents may threaten life.
Increased Intragastric Pressure: In the presence of a normal lower oesophageal sphincter, the increase in intragastric pressure produced by succinylcholine should be insufficient to produce regurgitation of gastric contents. However, in the patient with incompetence of this sphincter from, for example, hiatus hernia, regurgitation may occur.
Hyperkalaemia: It has long been recognized that administration of succinylcholine during halothane anaesthesia increases the serum potassium concentration by 0.5 mmol L–1. This effect is thought to be caused by muscle fasciculation. It is probable that the effect is less marked with the newer potent inhalational agents, e.g. isoflurane, sevoflurane. A similar increase occurs in patients with renal failure, but as these patients may already have an elevated serum potassium concentration, such an increase may precipitate cardiac irregularities and even cardiac arrest.
In some conditions in which the muscle cells are swollen or damaged, or in which there is proliferation of extrajunctional receptors, this release of potassium may be exaggerated. This is most marked in the burned patient, in whom potassium concentrations up to 13 mmol L–1 have been reported. In such patients, pre-curarization is of no benefit. Succinylcholine should be avoided in this condition. In diseases of the muscle cell, or its nerve supply, hyperkalaemia after succinylcholine may also be exaggerated. These include the muscular dystrophies, dystrophia myotonica and paraplegia. Hyperkalaemia has been reported to cause death in such patients. Succinylcholine may also precipitate prolonged contracture of the masseter muscles in patients with these disorders, making tracheal intubation impossible. The drug should be avoided in any patient with a neuromuscular disorder, including the patient with malignant hyperthermia, in whom the drug is a recognized trigger factor (see p. 879).