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Significance of plasma cholinesterase for the anaesthetist
Significance of Plasma Cholinesterase for the Anaesthetist
F. S. J e n s e n , J. V i b y - M o g e n s e n a n d D. O s t e r g a a r d
Definition of plasma cholinesterase
day after delivery. 3 The pChe activity will return to pre-pregnancy value within 6-8 weeks after delivery. The newborn has about 50% of normal pChe activity and reaches normal levels at puberty. Thereafter the enzyme activity slowly decreases with age, reaching about 75% at the age of 78-80 years. The enzyme activity is depressed in many diseases, primarily those affecting liver function. Diseases which affect the synthesis of cholinesterase in the liver, as for example acute hepatitis, liver metastasis, and liver cirrhosis, often reduce pChe activity to 50%. Reduced activity has been reported in cholecystitis, but it seems that biliary obstruction does not affect the activity unless there is damage to the liver cells as well. 7 In liver transplant patients assays of pChe activity are of value as a prognostic parameter as a decrease in activity indicates a deteriorating condition of the transplant and an increase in improvement, s Plasma cholinesterase activity can be reduced to a level of 30% in patients with renal disease, including uraemia. Many other diseases will lower the pChe activity, but to a lesser degree. 3 Patients with malignant tumours often have a pChe activity of 60-65% of healthy normal adult values. A more pronounced reduction in activity below 25% is seen in patients with gastrointestinal tumours or bronchogenic carcinoma. 9 In burned patients activity can fall to 20% of normal. A correlation exists between the degree and extent of burn injury and the activity, s0 Many different drugs reduce the pChe activity either by an inhibition of the enzyme in plasma or by a reduced synthesis of the enzyme in the liver. Inhibitors are usually classified as reversible or irreversible. Organophosphates, such as pesticides (paraoxon) and certain anticancer drugs (cyclophosphamide,
The plasma cholinesterase (pChe) molecule is a tetramer of four identical subunits each containing 574 amino acids with a total weight of approximately 342000 Daltons. The tetrameric subunits are held together by two interchain disulfide bridges and by hydrophobic, non-covalent forces. 1 The enzyme has two active sites, the anionic site acting on positively charged quaternary ammonium groups on the substrate and the esteratic site, breaking ester bonds. The enzyme is a glycoprotein synthesised in the liver, containing about 24% carbohydrate. Cholinesterase is found in both the central and peripheral nervous system of all animals, in cerebrospinal fluid in small amounts 2 and especially in liver and in blood plasma. 3 The physiological function of pChe is still not known. Since 1932, however, it has been recognised as an enzyme that hydrolyses choline esters. 4 The half-life of pChe is about 12 days. 5
Decreased plasma cholinesterase activity Plasma cholinesterase deficiency could be due to physiological variation, disease, iatrogenic changes, and genetic defects. Plasma cholinesterase activity is known to vary with age, sex, body fat and with other parameters such as plasma lipids or lipoprotein fractions. 3 Males have higher pChe activities than females. 6 During pregnancy pChe activity can decrease 20-30% partly due to a haemodilution and partly to reduced synthesis by the liver, the activity being lowest on the 3rd F. S. Jensen, J. Viby-Mogensen, D. Ostergaard, Department of Anaesthesia, Rigshospitalet, University of Copenhagen, DK-2100 Copenhagen, Denmark Current Anaesthesia and Critical Care
(~)1991LongmanGroupUK Ltd
(1991)2, 232-237 232
SIGNIFICANCE OF PLASMA CHOLINESTERASE FOR THE ANAESTHETIST
thiotepa) irreversibly inhibit cholinesterase. Patients treated with cyclophosphamide or thiotepa have 35-70% of normal activity. After discontinuation of treatment with this type of drug it may take 6-8 weeks to regain full activity, because new cholinesterase has to be synthesised by the liver. Ecothiopate eye drops may reduce activity to nearly zero 11. Reversible cholinesterase inhibitors depressing activity transiently include bambuterol, a new bronchodilator drug 12 (see later), anaesthetic agents, neuromuscular blocking drugs, anticholinesterases, antibiotics, cytotoxic drugs, and several hormones. The biosynthesis of pChe is controlled by one gene locus located on chromosome 3.13 The four well established genes are the E u (usual), the E a (atypical), the E f (fluoride resistant), and the E S (silent) with no detectable enzymatic activity. During the last decade, several additional variants of the enzyme have been discovered: E j14, E klS, and E hi6. The frequency of the different pChe variants varies in different populations. 3 In Caucasian populations 95-97% have normal pChe phenotype (E~Ey) and only 2.5-4% (1:25) will be heterozygous for the usual and the atypical gene (E~E~). The incidence of other heterozygousgenotypesislower: EIE1 u f in 0.4% of the population (1:250), and E~E~ in 0.5% of the population (1:200). The clinically most important one is the homozygous atypical genotype (E~E~) with an incidence of 0.04% (1:2500). In other ethnic groups the gene frequencies vary. In certain Eskimo groups the E~ variant is common, and the E~ variant is very rare. Three pChe variants with increased activity are known. One is called Cynthiana after its place of origin, 3'17 and patients with this variant are resistant to succinylcholine. Another, found in a South African family (Johannesburg variant) have twice the normal activity, is The third, the C5 + variant is associated with up to 30% increase in activity. 19 The C5 + variant is not important to the anaesthetist as it does not affect response to succinylcholine.a3
Determination of plasma cholinesterase activity and genotypes Plasma cholinesterase activity is determined by measuring the rate of hydrolysis of a substrate (an ester) catalysed by pChe. Most commonly used today is UV-spectrophotometry using benzoylcholine or propionylthiocholine as a substrate. 3'a° It is not possible to discriminate between the various cholinesterase genotypes by measuring the pChe activity alone because the activity of different genotypes overlaps. Kalow & Genest 21 developed a simple method for determining who was the carrier of an atypical cholinesterase. By introducing the 'dibucaine number' (DN), which is the percent inhibition of hydrolysis of substrate in the presence of 10-5M dibucaine, they classified the types of esterases as
233
usual, intermediate or atypical according to differences in sensitivity to inhibition. Usual pChe is very sensitive to dibucaine which inhibits 80% of its activity. Atypical pChe is resistant to dibucaine being inhibited only 20%, and the heterozygote has intermediate sensitivity so that 60% of its activity is inhibited. It is not possible to differentiate all genotypes solely by the DN, and in order to identify the fluoride, h, j, and k variants 16'22 (Table 1) other inhibitors including sodium fluoride, Ro 2-0683 (the dimethylcarbamate of (2-hydroxy-5-phenylbenzyl)trimethylcarbamate), and urea are used. 23'24. The k and j genes can only be identified with certainty when they occur together with the atypical variant. A reduction in pChe activity by 33% and 66% is seen due to the k and j genes, respectively. The new h-variant is a quantitative variant and when segregating with the atypical gene, it reduces the enzymatic activity by approximately 90%. The activity can be as low as that found in some patients homozygous for the silent genes. 16
Structural analyses of variants at DNA-level The increasing number of possible phenotypes has complicated the identification of phenotypes using only biochemical analysis. Molecular biological studies are the basis for new, specific, diagnostic methods to identify pChe variants. The D N A sequence of human pChe including the complete coding sequence as well as intron sequences near exon/intron boundaries was found by using the polymerase chain reaction (PCR).25 Each of the four subunits was found to contain 574 amino acids and the active site serine to be the 198th residue from the amino terminus. Knowledge of the DNA sequence of the normal enzyme is a prerequisite for finding the coding for other variants of the enzyme and hence to deduce the amino acid alterations in the protein (Table 2). In molecular biology new specific diagnostic methods are now being developed, using the D N A from peripheral blood cells, the PCR amplification, allele-specific oligonucleotide probes, and calorimetric (non-radioactive) detection methods. There is no doubt that future studies using gene technology will increase our understanding of the physiological and clinical significance of pChe and its variants.
Plasma cholinesterase activity and the reaction with succinylcholine The depolarising neuromuscular blocking agent succinylcholine was introduced as a muscle relaxant in 1951, and is still the drug of choice for acute tracheal intubation because of its rapid onset and recovery. After an intravenous injection, succinylcholine is hydrolysed in plasma by pChe to form succinylmonocholine a n d choline in a few minutes.
234
CURRENT ANAESTHESIA AND CRITICAL CARE
Table 1 - - The biochemical characteristics of the various cholinesterase variants (locus El). From the Danish Cholinesterase Research Unit 7 No of patients
pChe activity (u.1 1)
Dibucaine number
Fluoride number
Urea n u m b e r
E~E~'* EyE~* E~E~*
970 745 207
690-1560 433-1197 190-732
79-87 55-72 14-27
55-66 40-53 16-30
41-53 54-69 86-100
E u1E1s * E~E~* E~E[
78 33 9
329-870 146-450 0-48
78-86 16-27 -
56-66 19-30 -
42-52 86-100 -
u f** EIE1 E~E~** EaeE~* *
21 12 2
514-1150 318-777 351-509
74-80 45-52 63-64
44-53 25-33 26-38
61-71 75-100 90-91
Genotype
* indicates that 2.5 and 97.5 percentiles are given ** indicates that ranges are given
While the diester is a powerful muscle relaxant, the monoester has little pharmacologic activity. Only 5-10% of the injected drug actually reaches the neuromuscular endplates where succinylcholine binds to the receptor and depolarises the endplate. People with decreased pChe activity show a prolonged duration of action to succinylcholine, resulting in a substantial prolongation of muscle paralysis. 3'26'27 In the absence of hydrolysis by cholinesterase a total muscle paralysis of about 30-60 minutes can occur. Recovery from apnoea does not begin until most of the succinylcholine has been hydrolysed or cleared from blood and extracellular fluid. After the blood and extracellular fluid have been cleared of succinylch01ine the rate of recovery depends on the concentration of succinylcholine remaining at the nerve endplate, i.e., on the number of receptors occupied, and on the rate of diffusion of succinylcholine away from the nerve endplate. The prolonged response by people with abnormal cholinesterase is explained by the inability of atypical cholinesterase to hydrolyse succinylcholine. Atypical cholinesterase has a low affinity for succinylcholine, and as a consequence none or very little of the drug is hydrolysed in blood. Therefore, the endplate receives a 50-100 fold overdose of succinylcholine. 2s
When discussing the correlation between phenotypes and recovery of neuromuscular function following succinylcholine the phenotypes can conveniently be divided into four groups, i.e., homozygotes for the usual gene, heterozygotes for the usual and one of the abnormal genes, heterozygotes for two abnormal genes, and finally homozygotes for the two abnormal genes. In phenotypically normal patients (E~Ey) there is a correlation between the patients' pChe activity and the time to first evoked response to nerve stimulation. 27 Plasma cholinesterase activities as low as 400U 1-1, however, only cause a moderate prolongation of the block. Only when the enzyme activity is severely depressed will the time to first evoked response be significantly prolonged (Fig. 1). Following an intubating dose of succinylcholine ( l m g kg -a) the first response to peripheral nerve stimulation is seen 5-10 min later, and the time to 90% twitch height recovery is 6-13 min 29 (Table 3). In patients with normal pChe activity and genotype a typically depolarising block is seen following succinylcholine injection with for instance equal inhibition of all four responses in the train-of-four (TOF). Very low enzyme activity, or repeated or larger doses of succinylcholine may, however, cause a change in
Table 2 - - Current status of identification of human pChe variants at D N A level. The following symbols are used for the D N A bases: adenine (A), cytosine (C), guanine (G), and thymine (T), nt = nucleotide Cholinesterase variant
Amino acid change
D N A change
Usual Atypical
None 70 Asp ~ Gly
None nt 209 ( G A T --~ G G T )
Silent
117 Gly--* frameshift
Fluoride 1
243 Thr ~ Met
nt 351 ( G G T ~ G G A G ) (37) nt 728 (ACG ~ A T G )
Fluoride 2
390 Gly ~ Thr
nt 1169 ( G G T ~ GTT)
K-variant
539 Ala ~ Thr
H-variant
142 Val ~ Met
J-variant
497 Glu ~ Val
nt 1615 (GCA ~ A C A ) (39) nt 424 ( G T G -+ ATG) (40) nt 1490 ( G G A ~ GTA)
(25)
(38) (38)
Asp = aspartate; Glu = glutamine; Gly = glycine; Met = methionine; Thr = threonine; Val = valine
235
SIGNIFICANCE OF PLASMA CHOLINESTERASE FOR THE ANAESTHETIST
following succinylcholine 1 mg kg-1. The duration of apnoea (or time to first response to nerve stimulation) is 35-40 min following this dose. Time to adequate clinical recovery is 140-150 min 31 (Table 3 and Fig.
First evoked response (min) 50
40.
2). 30-
In patients homozygous for the silent gene (E~E~) or fluoride gene (E~E~) no controlled studies using a nerve stimulator have been published, but the former have no active enzyme and will probably react like patients homozygous for the atypical gene (E~E~).
20-
10
Plasma cholinasterase
activity (U/I)
Fig. 1 - - Relationship between enzyme activity and time to f i r s t e v o k e d r e s p o n s e to train-of-four nerve stimulation following administration of succinylcholine 1 mg kg -~ i.v. in patients with normal plasma cholinesterase genotype. Mean curve and 95% confidence intervals are given. Arrows i n d i c a t e normal values of plasma cholinesterase activity. 33 (Reproduced by kind permission of the publishers.)
neuromuscular block. The original depolarising block (phase I block) changes to a non-depolarisinglike block (or phase II block), characterised by fade in the TOF response. The development of a phase II block depends on the amount of succinylcholine that reaches the neuromuscular endplate and this of course depends on both the pChe activity and the dose of succinylcholine. Patients heterozygous for the usual and one of the abnormal genes, i.e. ERE1, u a E1E1, u s o r E1E1, u f normally show a depolarising block following a normal dose of succinylcholine of i mg kg -1. The duration of action may be normal or slightly prolonged) °. Pre-treatment with a small dose of a non-depolarising relaxant and a corresponding increase in the dose of succinylcholine to 1.5 mg kg-1 to facilitate tracheal intubation may, however, cause a phase II block and prolonged respiratory insufficiency. (Personal observation) In patients heterozygous for two abnormal genes, i.e., EIE1 a f f s or EIE1 a phase II block is seen already following a normal dose of succinylcholine 1 mg kg-1, and the time to 90% twitch height recovery is moderately prolonged 3° (Table 3). In patients homozygous for the atypical gene (E~E~) and patients heterozygous for the atypical and the silent gene (E~E~) a phase II block is always seen
Diagnosis and treatment of prolonged response to succinylcholine If prolonged apnoea is suspected to be caused by succinylcholine, the first part of treatment is patience on the part of the anaesthetist. While the patient is kept anaesthetised and ventilated, respiratory insufficiency unconnected with cholinesterase levels and genotypes has to be excluded by applying a nerve stimulator. If a normal reaction to nerve stimulation is seen the prolonged apnoea is of course not caused by the neuromuscular blocking drug. However, the presence of a phase II block following a single normal dose of succinylcholine indicates an abnormal pChe genotype or very low pChe activity. 7 Some cases of prolonged apnoea may be due to low cholinesterase activity caused by disease or anticholinesterase drugs. Because these patients have some pChe activity there will be no succinylcholine left in plasma 10-15 min after an i.v. injection. Only succinylcholine bound to the receptors persists. There is a 'pure' phase II block with fade in the response to TOF. This block is reversible by anticholinesterases. 7'32 In contrast, in patients with abnormal genotypes, the presence of a phase II block does not
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Table 3 - - Time to first response to train-of-four nerve stimulation and to 90% twitch height recovery following succinylcholine 1 mg kg -1 (29-31). Mean and ranges are given
Genotype
No. of patients
EyEy
41
u a u s u f EIEx, E1E1, E1Ea
39
E~E~, E~E~ E~E~
4 12
First evoked response (min)
90% twitch height recovery (min)
5.6 (4.0-8.0) 8.6 (4.0-15.0) 24.5 (22.5-25.0) 38.0 (35.0-40.0)
9.3 (6.0-13.0) 13.0 (7.5-22.0) 29.5 (29.0-30.0) 160.0 (120.0-180.0)
120 Min
160
Min
Fig. 2 - - The reaction to train-of-four (TOF) nerve stimulation after intravenous administration of succinylcholine 1 mg kg 1 (arrow) in a patient homozygous for the atypical gene (E~E~). Anaesthesia: thiopentone, N20/02, halothane. The neuromuscular blockade is markedly prolonged compared with that seen in normal patients and there is pronounced fade in the TOF response (phase II block) 7 (Reproduced by kind permission of t h e publishers.)
236
C U R R E N T A N A E S T H E S I A A N D CRITICAL C A R E
Twitch height in % of control 100
New drugs hydrolysed by plasma cholinesterase
~ A
Bambuterol 8O c
60
40
20
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10
¢
30
1
50
i
|
70
i
|
90
I
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110
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130 Minutes
Fi9. 3 - - Recovery of thumb twitch (percent of control) following intravenous administration of succinylcholine 1 mg kg -1. (A) Spontaneous recovery in genotypically normal patients. (B) and (C) Recovery in patients homozygous for the atypical enzyme following i.v. administration of human plasma cholinesterase 30 and 90 rain following succinylcholine, respectively.43 See text for further explanation. (Reproduced by kind permission of the publishers.)
indicate that reversal of the block is possible with a cholinesterase inhibitor. The effect of the anticholinesterase drug is unpredictable and depends upon the genotype, the dose of the inhibitor, and on the dose of succinylcholine. In patients homozygous for the atypical gene (E~E~) the quantity as well as the quality of pChe are changed and as a result the succinylcholine will not be metabolised but will persist for longer in plasma and at the neuromuscular junction. Immediately after administration of succinyl-choline the depolarising block dominates, later the non-depolarising part of the block becomes more prominentfl a The depolarising block can be treated with human pChe and the non-depolarising part with anticholinesterase. If purified human pChe is injected later than 35-40 min after injection of succinylcholine, recovery will still be prolonged (Fig. 3). If the TOF ratio is below 0.70 following the injected pChe, neostigmine or edrophonium may be administered to treat the remaining non-depolarising part of the block. 31 In daily clinical practice the patient's pChe activity and genotype are normally not known when a prolonged response to succinylcholine is seen. If an abnormal pChe genotype cannot be excluded the block should not be treated solely with an anticholinesterase drug. This may prolong the block. The initial dose of succinylcholine should not exceed 1.0-1.5mg kg -1 and no other relaxant drug should be administered until there is response to peripheral nerve stimulation or some other signs of recovery of neuromuscular function.
Bambuterol has recently been introduced in the treatment of bronchial asthma. It is a dicarbamate pro-drug designed to inhibit cholinesterase, thus allowing slow release of the active compound, terbutaline. Bambuterol, however, causes a clinically significant prolongation of the duration of action of succinylcholine due to the drug's very pronounced pChe inhibiting effect.12'33 The effect of bambuterol on the pChe activity has been found to be maximal 2-3h after oral administration and to remain for about 8h. Bang et al. 34 have found that in patients with normal genotype the maximal inhibition of enzyme activity was seen 2 h after bambuterol 30 mg orally. Following an injection of succinylcholine 1 mg kg-1 at that time the neuromuscular recovery was 3-4 times prolonged. In some patients with very low pChe activity a phase II block developed resulting in a clinically significant prolonged neuromuscular blockade. In patients heterozygous for the usual and the atypical gene, bambuterol 20mg causes a 2-3 fold prolongation of the duration of action of succinylcholine. In some patients a clinically very significantly prolonged neuromsucular blockade may be seen and a respiratory insufficiency of 2-3h duration may follow. 34
Mivacurium
Mivacurium chloride is a new short-acting non-depolarising neuromuscular blocking drug presently undergoing clinical trials. It has been designed to undergo hydrolysis by pChe. In vitro the hydrolysis rate of mivacurium by purified human pChe is about 70-80% of the rate of hydrolysis of succinylcholine. Other routes of metabolism do exist, but the clinical significance of these are not known. 35 Clinical studies in patients with normal genotype and a low or normal pChe activity have shown an inverse correlation between pChe activity and the duration of action of mivacurium.36 In patients heterozygous for the atypical gene for plasma pChe the duration of action of " mivacurium is prolonged approximately 30%, compared to normal patients (personal observation). Preliminary results in patients homozygous for the atypical gene indicate that these patients are extremely sensitive. Thus, one patient given a very low dose of mivacurium, equivalent to an ED10, was paralysed for more than 2h (personal observation).
Conclusion Prolonged respiratory insufficiency after succinylcholine is comparatively rare. It is, however, a very unpleasant experience for the patient and for the
SIGNIFICANCE OF PLASMA CHOLINESTERASE FOR THE ANAESTHETIST
anaesthetist. In spite of the decreasing use of succinylcholine, the introduciton of bambuterol and mivacurium into clinical practice may increase the frequency with which an anaesthetist will face a patient with a prolonged neuromuscular response caused by an abnormal pChe activity. Knowledge of pChe activity is therefore still essential for the anaesthetist.
References 1. Lockridge O, Eckerson HW, La Du NN. Interchain disulfide bonds and subunit organization in human serum cholinesterase. J Biol Chem 1979; 254:8324-8330 2. Kambam JR, Horton B, Parris WCV, Human SA, Berman ML, Sastry BVR. Pseudocholinesterase activity in human CNS. Anesth Analg 1989; 69:486-488 3. Whittaker M. Cholinesterase. Ed. L. Beckman, Karger, Exeter, 1986 4. Stedman E, Stedman E, Easson LH. Cholinesterase. An enzyme present in the blood of the horse. Biochem J 1932; 26:2056 5. Ostergaard D, Viby-Morgensen J, Hanel HK, Skovgaard LT. Half life of plasma cholinesterase. Acta Anaesth Scand 1988; 32:266-269 6. Propert DN, Backenridge CJ. The relation of sex, age, smoking, birth rank and parental ages to pseudocholinesterase activity and phenotypes in a sample of Australian Caucasian adults. Hum Genet 1976; 32:181-188 7. Viby-Mogensen J. Cholinesterase and succinylcholine. Dan Med Bull 1983; 30:129-150 8. Evans DB, Lehman H. Pseudocholinesterase activity in liver transplant. Lancet 1971; i: 1040-1044. 9. Kaniaris P, Fassoulaki A, Liarmakopoulou K, Dermizakis E. Serum cholinesterase levels in patients with cancer. Anesth Analg 1979; 58:82-84 10. Vihy°Mogensen J, Hanel HK, Hansen E, SOrensen B, Graae J. Serum cholinesterase activity in burned patients. I. Biochemical findings. Acta Anaesth Scand 1975; 19: 159-168. 11. Cavallaro ILl, Krumperman LW, Kugler F. Effect of ecothiopate therapy on the metabolism of succinylcholine in man. Anesth Analg 1968; 47:570-574 12. Fischer DM, Caldwell JE, Sharma M, WirEn JE. The influence of Bambuterol (carbamylated terbutaline) on the duration of action of succinylcholine-induced paralysis in humans. Anaesthesiology 1988; 69:757-759 13. Lockridge O. Genetic variants of human serum cholinesterase influence metabolism of the muscle relaxant succinylcholine. Pharmac Ther 1990; 47:35-60 14, Garry PJ, Dietz AA, Lubrano T, Fode PC, James K, Rubinstein HM. New allele at cholinesterase locus 1. J Med Genet 1976; 13:38-42 15. Rubinstein HM, Dietz AA, Lubrano T. E1k, another quantitative variant at cholinesterase locus 1. J Med Genet 1978; 15:27-29 16. Whittaker M, Britten J. A new allele at cholinesterase locus 1. Hum Hered 1987; 37:54-58 17. Neitlich HW. Increased plasma cholinesterase activity and succinylcholine resistance: a genetic variant. J Clin Invest 1966; 45:380-387 18. Krause A, Lane AB, Jenkins T. A new high activity plasma cholinesterase variant. J Med Genet 1988; 25:677-681 19. Robson EB, Harris H. Further data on the incidence and genetics of the serum cholinesterase phenotypes C5+. Am J Hum Genet 1966; 29:403-408 20. Kalow W, Lindsay HA. A comparison of optical and manometric methods for the assay of human serum cholinesterase. Can J Biochem Physiol 1955; 35:568-574 21. Kalow W, Genest K. A methoc for the detection of atypical forms of human serum cholinesterase. Determination of dibucaine numbers. Can J Biochem 1957; 35: 339-346. 22. Evans RT, Wardell J. On the identification and frequency of
237
the J and K cholinesterase phenotypes in a Caucasian population. J Med Genet 1984; 21:99-102 23. Hanel HK, Viby-Mogensen J. The inhibition of serum cholinesterase by urea. Br J Anaesth 1977; 49:1251-1257 24. Ostergaard D, Viby-Mogensen J. Prolonged apnea after succinylcholine. Problems in Anesthesia 1989; 3:455-464 25. McGuire MC, Nogneira CP, Bartels CF, Lightstone H, Hajra A, Van der Spek AFL, Lockridge O, La Du BN. Identification of the structural mutation responsible for the dibucaine-resistant (atypical) variant form of human serum cholinesterase. Proc Natl Acad Sci USA 1989; 86:953-957 26. Viby-Mogensen J. Interaction of other drugs with muscle relaxants. In: Norman J, ed. Clinics in Anesthesiology, London: WB Saunders Company, 1985:467-482 27. Pantuck E J, Pantuck CB. Prolonged apnea following succinylcholine administration, side effects and rational approach to relaxation. In: I. Azar, ed. Muscle Relaxants. Side effects and a rational approach to selection. New York: M. Decker Inc., 1987:205-229 28. Kalow W. Pharmacokinetics, heredity and the response to drugs. W.B. Saunders, Philadelphia, 1962; 69-93 29. Viby-Mogensen J. Correlation of succinylcholine duration of action with plasma cholinesterase activity in subjects with genotypically normal enzyme. Anesthesiology 1980; 53: 517520 30. Viby-Mogensen J. Succinylcholine neuromuscular blockade in subjects heterozygous for abnormal plasma cholinesterase. Anesthesiology 1981; 55:231-235 31. Viby-Mogensen J. Succinylcholine neuromuscular blockade in subjects homozygous for atypical plasma cholinesterase. Anesthesiology 1981; 55:429-434 32. Donati F, Bevan DR. Antagonism of phase II succinylcholine block by neostigmine. Anesth Analg 1985; 64:773-776 33. Bang, U, Viby-Mogensen J, Wir6n JE, Skovgaard LT. The effect of Barnbuterol (carbamylated terbutaline) on plasma cholinesterase activity and succinylcholine-induced neuromuscular blockade in genotypically normal patients. Acta Anaesth Scand 1990; 34:596-599 34. Bang U, Viby-Mogensen J, Wir6n JE. The effect of Bambuterol on plasma cholinesterase activity and succinylcholine-induced neuromuscular blockade in subjects heterozygous for abnormal plasma cholinesterase. Acta Anaesth Scand 1990; 34:600-604 35. Savarase JJ, Ali HH, Basta S-J, Embree PB, Scott RPF, Sunder N, W~akley, JN, Wastila WB, EI-Sayad HA. The clinical neuromuscular pharmacology of Mivacurium chloride (BW B1090U). Anesthesiology 1988; 68:723-732 36. Ostergaard D, Jensen FS, Jensen E, Viby-Mogensen J. Influence of plasma cholinesterase activity on recovery from Mivacurium-induced neuromuscular blockade. Acta Anaesth Scand 1989; suppl. 191; 33:164 37. Nogueira CP, McGuire MC, Bartels C, Arpagaus M, Van der Spek AFL, Lightstone H, Lockriodge O, La Du Bn. Identification of a frameshift mutation responsible for the silent phenotype of human buturylcholinesterase in serum (variant Ann Arbor). Third International Meeting on Cholinesterase. La Grande Motte, France, 1990; 102 38. Bartels CF, Nogueira CP, McGuire MC, Adkins S, Rubinstein HM, Lubrano T, Van der Spek AFL, Lightstone H, Lockridge O, La Du BN. Identification of two different mutations associated with human buturylcholinesterase fluoride resistance in serum. Third International Meeting on Cholinesterase. La Grand Motte, France, 1990; 103. 39. Bartels CF, Van der Spek A, Rubinstein H, Lubrano T, Lockridge O, La Du BN. DNA coding for the K polymorphism in linkage equilibrium with atypical human buturyl-cholinesterase complicates phenotyping. Third International Meeting on Cholinesterase. La Grande Motte, France, 1990; 104 40. Jensen FS, Bartels CF, La Du BN. A DNA mutation associated with the H-variant of human buturylcholinesterase. Third International Meeting on Cholinesterase. La Grande Motte, France, 1990; 101 41. Harris H, Whittaker M. Differential inhibition of human serum cholinesterase with fluoride. Recognition of two new phenotypes. Nature 1961; 191:496-498
F. S. J e n s e n , J. V i b y - M o g e n s e n a n d D. O s t e r g a a r d
Definition of plasma cholinesterase
day after delivery. 3 The pChe activity will return to pre-pregnancy value within 6-8 weeks after delivery. The newborn has about 50% of normal pChe activity and reaches normal levels at puberty. Thereafter the enzyme activity slowly decreases with age, reaching about 75% at the age of 78-80 years. The enzyme activity is depressed in many diseases, primarily those affecting liver function. Diseases which affect the synthesis of cholinesterase in the liver, as for example acute hepatitis, liver metastasis, and liver cirrhosis, often reduce pChe activity to 50%. Reduced activity has been reported in cholecystitis, but it seems that biliary obstruction does not affect the activity unless there is damage to the liver cells as well. 7 In liver transplant patients assays of pChe activity are of value as a prognostic parameter as a decrease in activity indicates a deteriorating condition of the transplant and an increase in improvement, s Plasma cholinesterase activity can be reduced to a level of 30% in patients with renal disease, including uraemia. Many other diseases will lower the pChe activity, but to a lesser degree. 3 Patients with malignant tumours often have a pChe activity of 60-65% of healthy normal adult values. A more pronounced reduction in activity below 25% is seen in patients with gastrointestinal tumours or bronchogenic carcinoma. 9 In burned patients activity can fall to 20% of normal. A correlation exists between the degree and extent of burn injury and the activity, s0 Many different drugs reduce the pChe activity either by an inhibition of the enzyme in plasma or by a reduced synthesis of the enzyme in the liver. Inhibitors are usually classified as reversible or irreversible. Organophosphates, such as pesticides (paraoxon) and certain anticancer drugs (cyclophosphamide,
The plasma cholinesterase (pChe) molecule is a tetramer of four identical subunits each containing 574 amino acids with a total weight of approximately 342000 Daltons. The tetrameric subunits are held together by two interchain disulfide bridges and by hydrophobic, non-covalent forces. 1 The enzyme has two active sites, the anionic site acting on positively charged quaternary ammonium groups on the substrate and the esteratic site, breaking ester bonds. The enzyme is a glycoprotein synthesised in the liver, containing about 24% carbohydrate. Cholinesterase is found in both the central and peripheral nervous system of all animals, in cerebrospinal fluid in small amounts 2 and especially in liver and in blood plasma. 3 The physiological function of pChe is still not known. Since 1932, however, it has been recognised as an enzyme that hydrolyses choline esters. 4 The half-life of pChe is about 12 days. 5
Decreased plasma cholinesterase activity Plasma cholinesterase deficiency could be due to physiological variation, disease, iatrogenic changes, and genetic defects. Plasma cholinesterase activity is known to vary with age, sex, body fat and with other parameters such as plasma lipids or lipoprotein fractions. 3 Males have higher pChe activities than females. 6 During pregnancy pChe activity can decrease 20-30% partly due to a haemodilution and partly to reduced synthesis by the liver, the activity being lowest on the 3rd F. S. Jensen, J. Viby-Mogensen, D. Ostergaard, Department of Anaesthesia, Rigshospitalet, University of Copenhagen, DK-2100 Copenhagen, Denmark Current Anaesthesia and Critical Care
(~)1991LongmanGroupUK Ltd
(1991)2, 232-237 232
SIGNIFICANCE OF PLASMA CHOLINESTERASE FOR THE ANAESTHETIST
thiotepa) irreversibly inhibit cholinesterase. Patients treated with cyclophosphamide or thiotepa have 35-70% of normal activity. After discontinuation of treatment with this type of drug it may take 6-8 weeks to regain full activity, because new cholinesterase has to be synthesised by the liver. Ecothiopate eye drops may reduce activity to nearly zero 11. Reversible cholinesterase inhibitors depressing activity transiently include bambuterol, a new bronchodilator drug 12 (see later), anaesthetic agents, neuromuscular blocking drugs, anticholinesterases, antibiotics, cytotoxic drugs, and several hormones. The biosynthesis of pChe is controlled by one gene locus located on chromosome 3.13 The four well established genes are the E u (usual), the E a (atypical), the E f (fluoride resistant), and the E S (silent) with no detectable enzymatic activity. During the last decade, several additional variants of the enzyme have been discovered: E j14, E klS, and E hi6. The frequency of the different pChe variants varies in different populations. 3 In Caucasian populations 95-97% have normal pChe phenotype (E~Ey) and only 2.5-4% (1:25) will be heterozygous for the usual and the atypical gene (E~E~). The incidence of other heterozygousgenotypesislower: EIE1 u f in 0.4% of the population (1:250), and E~E~ in 0.5% of the population (1:200). The clinically most important one is the homozygous atypical genotype (E~E~) with an incidence of 0.04% (1:2500). In other ethnic groups the gene frequencies vary. In certain Eskimo groups the E~ variant is common, and the E~ variant is very rare. Three pChe variants with increased activity are known. One is called Cynthiana after its place of origin, 3'17 and patients with this variant are resistant to succinylcholine. Another, found in a South African family (Johannesburg variant) have twice the normal activity, is The third, the C5 + variant is associated with up to 30% increase in activity. 19 The C5 + variant is not important to the anaesthetist as it does not affect response to succinylcholine.a3
Determination of plasma cholinesterase activity and genotypes Plasma cholinesterase activity is determined by measuring the rate of hydrolysis of a substrate (an ester) catalysed by pChe. Most commonly used today is UV-spectrophotometry using benzoylcholine or propionylthiocholine as a substrate. 3'a° It is not possible to discriminate between the various cholinesterase genotypes by measuring the pChe activity alone because the activity of different genotypes overlaps. Kalow & Genest 21 developed a simple method for determining who was the carrier of an atypical cholinesterase. By introducing the 'dibucaine number' (DN), which is the percent inhibition of hydrolysis of substrate in the presence of 10-5M dibucaine, they classified the types of esterases as
233
usual, intermediate or atypical according to differences in sensitivity to inhibition. Usual pChe is very sensitive to dibucaine which inhibits 80% of its activity. Atypical pChe is resistant to dibucaine being inhibited only 20%, and the heterozygote has intermediate sensitivity so that 60% of its activity is inhibited. It is not possible to differentiate all genotypes solely by the DN, and in order to identify the fluoride, h, j, and k variants 16'22 (Table 1) other inhibitors including sodium fluoride, Ro 2-0683 (the dimethylcarbamate of (2-hydroxy-5-phenylbenzyl)trimethylcarbamate), and urea are used. 23'24. The k and j genes can only be identified with certainty when they occur together with the atypical variant. A reduction in pChe activity by 33% and 66% is seen due to the k and j genes, respectively. The new h-variant is a quantitative variant and when segregating with the atypical gene, it reduces the enzymatic activity by approximately 90%. The activity can be as low as that found in some patients homozygous for the silent genes. 16
Structural analyses of variants at DNA-level The increasing number of possible phenotypes has complicated the identification of phenotypes using only biochemical analysis. Molecular biological studies are the basis for new, specific, diagnostic methods to identify pChe variants. The D N A sequence of human pChe including the complete coding sequence as well as intron sequences near exon/intron boundaries was found by using the polymerase chain reaction (PCR).25 Each of the four subunits was found to contain 574 amino acids and the active site serine to be the 198th residue from the amino terminus. Knowledge of the DNA sequence of the normal enzyme is a prerequisite for finding the coding for other variants of the enzyme and hence to deduce the amino acid alterations in the protein (Table 2). In molecular biology new specific diagnostic methods are now being developed, using the D N A from peripheral blood cells, the PCR amplification, allele-specific oligonucleotide probes, and calorimetric (non-radioactive) detection methods. There is no doubt that future studies using gene technology will increase our understanding of the physiological and clinical significance of pChe and its variants.
Plasma cholinesterase activity and the reaction with succinylcholine The depolarising neuromuscular blocking agent succinylcholine was introduced as a muscle relaxant in 1951, and is still the drug of choice for acute tracheal intubation because of its rapid onset and recovery. After an intravenous injection, succinylcholine is hydrolysed in plasma by pChe to form succinylmonocholine a n d choline in a few minutes.
234
CURRENT ANAESTHESIA AND CRITICAL CARE
Table 1 - - The biochemical characteristics of the various cholinesterase variants (locus El). From the Danish Cholinesterase Research Unit 7 No of patients
pChe activity (u.1 1)
Dibucaine number
Fluoride number
Urea n u m b e r
E~E~'* EyE~* E~E~*
970 745 207
690-1560 433-1197 190-732
79-87 55-72 14-27
55-66 40-53 16-30
41-53 54-69 86-100
E u1E1s * E~E~* E~E[
78 33 9
329-870 146-450 0-48
78-86 16-27 -
56-66 19-30 -
42-52 86-100 -
u f** EIE1 E~E~** EaeE~* *
21 12 2
514-1150 318-777 351-509
74-80 45-52 63-64
44-53 25-33 26-38
61-71 75-100 90-91
Genotype
* indicates that 2.5 and 97.5 percentiles are given ** indicates that ranges are given
While the diester is a powerful muscle relaxant, the monoester has little pharmacologic activity. Only 5-10% of the injected drug actually reaches the neuromuscular endplates where succinylcholine binds to the receptor and depolarises the endplate. People with decreased pChe activity show a prolonged duration of action to succinylcholine, resulting in a substantial prolongation of muscle paralysis. 3'26'27 In the absence of hydrolysis by cholinesterase a total muscle paralysis of about 30-60 minutes can occur. Recovery from apnoea does not begin until most of the succinylcholine has been hydrolysed or cleared from blood and extracellular fluid. After the blood and extracellular fluid have been cleared of succinylch01ine the rate of recovery depends on the concentration of succinylcholine remaining at the nerve endplate, i.e., on the number of receptors occupied, and on the rate of diffusion of succinylcholine away from the nerve endplate. The prolonged response by people with abnormal cholinesterase is explained by the inability of atypical cholinesterase to hydrolyse succinylcholine. Atypical cholinesterase has a low affinity for succinylcholine, and as a consequence none or very little of the drug is hydrolysed in blood. Therefore, the endplate receives a 50-100 fold overdose of succinylcholine. 2s
When discussing the correlation between phenotypes and recovery of neuromuscular function following succinylcholine the phenotypes can conveniently be divided into four groups, i.e., homozygotes for the usual gene, heterozygotes for the usual and one of the abnormal genes, heterozygotes for two abnormal genes, and finally homozygotes for the two abnormal genes. In phenotypically normal patients (E~Ey) there is a correlation between the patients' pChe activity and the time to first evoked response to nerve stimulation. 27 Plasma cholinesterase activities as low as 400U 1-1, however, only cause a moderate prolongation of the block. Only when the enzyme activity is severely depressed will the time to first evoked response be significantly prolonged (Fig. 1). Following an intubating dose of succinylcholine ( l m g kg -a) the first response to peripheral nerve stimulation is seen 5-10 min later, and the time to 90% twitch height recovery is 6-13 min 29 (Table 3). In patients with normal pChe activity and genotype a typically depolarising block is seen following succinylcholine injection with for instance equal inhibition of all four responses in the train-of-four (TOF). Very low enzyme activity, or repeated or larger doses of succinylcholine may, however, cause a change in
Table 2 - - Current status of identification of human pChe variants at D N A level. The following symbols are used for the D N A bases: adenine (A), cytosine (C), guanine (G), and thymine (T), nt = nucleotide Cholinesterase variant
Amino acid change
D N A change
Usual Atypical
None 70 Asp ~ Gly
None nt 209 ( G A T --~ G G T )
Silent
117 Gly--* frameshift
Fluoride 1
243 Thr ~ Met
nt 351 ( G G T ~ G G A G ) (37) nt 728 (ACG ~ A T G )
Fluoride 2
390 Gly ~ Thr
nt 1169 ( G G T ~ GTT)
K-variant
539 Ala ~ Thr
H-variant
142 Val ~ Met
J-variant
497 Glu ~ Val
nt 1615 (GCA ~ A C A ) (39) nt 424 ( G T G -+ ATG) (40) nt 1490 ( G G A ~ GTA)
(25)
(38) (38)
Asp = aspartate; Glu = glutamine; Gly = glycine; Met = methionine; Thr = threonine; Val = valine
235
SIGNIFICANCE OF PLASMA CHOLINESTERASE FOR THE ANAESTHETIST
following succinylcholine 1 mg kg-1. The duration of apnoea (or time to first response to nerve stimulation) is 35-40 min following this dose. Time to adequate clinical recovery is 140-150 min 31 (Table 3 and Fig.
First evoked response (min) 50
40.
2). 30-
In patients homozygous for the silent gene (E~E~) or fluoride gene (E~E~) no controlled studies using a nerve stimulator have been published, but the former have no active enzyme and will probably react like patients homozygous for the atypical gene (E~E~).
20-
10
Plasma cholinasterase
activity (U/I)
Fig. 1 - - Relationship between enzyme activity and time to f i r s t e v o k e d r e s p o n s e to train-of-four nerve stimulation following administration of succinylcholine 1 mg kg -~ i.v. in patients with normal plasma cholinesterase genotype. Mean curve and 95% confidence intervals are given. Arrows i n d i c a t e normal values of plasma cholinesterase activity. 33 (Reproduced by kind permission of the publishers.)
neuromuscular block. The original depolarising block (phase I block) changes to a non-depolarisinglike block (or phase II block), characterised by fade in the TOF response. The development of a phase II block depends on the amount of succinylcholine that reaches the neuromuscular endplate and this of course depends on both the pChe activity and the dose of succinylcholine. Patients heterozygous for the usual and one of the abnormal genes, i.e. ERE1, u a E1E1, u s o r E1E1, u f normally show a depolarising block following a normal dose of succinylcholine of i mg kg -1. The duration of action may be normal or slightly prolonged) °. Pre-treatment with a small dose of a non-depolarising relaxant and a corresponding increase in the dose of succinylcholine to 1.5 mg kg-1 to facilitate tracheal intubation may, however, cause a phase II block and prolonged respiratory insufficiency. (Personal observation) In patients heterozygous for two abnormal genes, i.e., EIE1 a f f s or EIE1 a phase II block is seen already following a normal dose of succinylcholine 1 mg kg-1, and the time to 90% twitch height recovery is moderately prolonged 3° (Table 3). In patients homozygous for the atypical gene (E~E~) and patients heterozygous for the atypical and the silent gene (E~E~) a phase II block is always seen
Diagnosis and treatment of prolonged response to succinylcholine If prolonged apnoea is suspected to be caused by succinylcholine, the first part of treatment is patience on the part of the anaesthetist. While the patient is kept anaesthetised and ventilated, respiratory insufficiency unconnected with cholinesterase levels and genotypes has to be excluded by applying a nerve stimulator. If a normal reaction to nerve stimulation is seen the prolonged apnoea is of course not caused by the neuromuscular blocking drug. However, the presence of a phase II block following a single normal dose of succinylcholine indicates an abnormal pChe genotype or very low pChe activity. 7 Some cases of prolonged apnoea may be due to low cholinesterase activity caused by disease or anticholinesterase drugs. Because these patients have some pChe activity there will be no succinylcholine left in plasma 10-15 min after an i.v. injection. Only succinylcholine bound to the receptors persists. There is a 'pure' phase II block with fade in the response to TOF. This block is reversible by anticholinesterases. 7'32 In contrast, in patients with abnormal genotypes, the presence of a phase II block does not
-
i
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i
I
I
!
....... ' ..........
[ I i i I I 50 Min
I
i
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'
~
I
i
E
ii"!!-! 60
i
i
~!-:!~ Min
i
' ......
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Table 3 - - Time to first response to train-of-four nerve stimulation and to 90% twitch height recovery following succinylcholine 1 mg kg -1 (29-31). Mean and ranges are given
Genotype
No. of patients
EyEy
41
u a u s u f EIEx, E1E1, E1Ea
39
E~E~, E~E~ E~E~
4 12
First evoked response (min)
90% twitch height recovery (min)
5.6 (4.0-8.0) 8.6 (4.0-15.0) 24.5 (22.5-25.0) 38.0 (35.0-40.0)
9.3 (6.0-13.0) 13.0 (7.5-22.0) 29.5 (29.0-30.0) 160.0 (120.0-180.0)
120 Min
160
Min
Fig. 2 - - The reaction to train-of-four (TOF) nerve stimulation after intravenous administration of succinylcholine 1 mg kg 1 (arrow) in a patient homozygous for the atypical gene (E~E~). Anaesthesia: thiopentone, N20/02, halothane. The neuromuscular blockade is markedly prolonged compared with that seen in normal patients and there is pronounced fade in the TOF response (phase II block) 7 (Reproduced by kind permission of t h e publishers.)
236
C U R R E N T A N A E S T H E S I A A N D CRITICAL C A R E
Twitch height in % of control 100
New drugs hydrolysed by plasma cholinesterase
~ A
Bambuterol 8O c
60
40
20
i
10
¢
30
1
50
i
|
70
i
|
90
I
|
110
¢
i
130 Minutes
Fi9. 3 - - Recovery of thumb twitch (percent of control) following intravenous administration of succinylcholine 1 mg kg -1. (A) Spontaneous recovery in genotypically normal patients. (B) and (C) Recovery in patients homozygous for the atypical enzyme following i.v. administration of human plasma cholinesterase 30 and 90 rain following succinylcholine, respectively.43 See text for further explanation. (Reproduced by kind permission of the publishers.)
indicate that reversal of the block is possible with a cholinesterase inhibitor. The effect of the anticholinesterase drug is unpredictable and depends upon the genotype, the dose of the inhibitor, and on the dose of succinylcholine. In patients homozygous for the atypical gene (E~E~) the quantity as well as the quality of pChe are changed and as a result the succinylcholine will not be metabolised but will persist for longer in plasma and at the neuromuscular junction. Immediately after administration of succinyl-choline the depolarising block dominates, later the non-depolarising part of the block becomes more prominentfl a The depolarising block can be treated with human pChe and the non-depolarising part with anticholinesterase. If purified human pChe is injected later than 35-40 min after injection of succinylcholine, recovery will still be prolonged (Fig. 3). If the TOF ratio is below 0.70 following the injected pChe, neostigmine or edrophonium may be administered to treat the remaining non-depolarising part of the block. 31 In daily clinical practice the patient's pChe activity and genotype are normally not known when a prolonged response to succinylcholine is seen. If an abnormal pChe genotype cannot be excluded the block should not be treated solely with an anticholinesterase drug. This may prolong the block. The initial dose of succinylcholine should not exceed 1.0-1.5mg kg -1 and no other relaxant drug should be administered until there is response to peripheral nerve stimulation or some other signs of recovery of neuromuscular function.
Bambuterol has recently been introduced in the treatment of bronchial asthma. It is a dicarbamate pro-drug designed to inhibit cholinesterase, thus allowing slow release of the active compound, terbutaline. Bambuterol, however, causes a clinically significant prolongation of the duration of action of succinylcholine due to the drug's very pronounced pChe inhibiting effect.12'33 The effect of bambuterol on the pChe activity has been found to be maximal 2-3h after oral administration and to remain for about 8h. Bang et al. 34 have found that in patients with normal genotype the maximal inhibition of enzyme activity was seen 2 h after bambuterol 30 mg orally. Following an injection of succinylcholine 1 mg kg-1 at that time the neuromuscular recovery was 3-4 times prolonged. In some patients with very low pChe activity a phase II block developed resulting in a clinically significant prolonged neuromuscular blockade. In patients heterozygous for the usual and the atypical gene, bambuterol 20mg causes a 2-3 fold prolongation of the duration of action of succinylcholine. In some patients a clinically very significantly prolonged neuromsucular blockade may be seen and a respiratory insufficiency of 2-3h duration may follow. 34
Mivacurium
Mivacurium chloride is a new short-acting non-depolarising neuromuscular blocking drug presently undergoing clinical trials. It has been designed to undergo hydrolysis by pChe. In vitro the hydrolysis rate of mivacurium by purified human pChe is about 70-80% of the rate of hydrolysis of succinylcholine. Other routes of metabolism do exist, but the clinical significance of these are not known. 35 Clinical studies in patients with normal genotype and a low or normal pChe activity have shown an inverse correlation between pChe activity and the duration of action of mivacurium.36 In patients heterozygous for the atypical gene for plasma pChe the duration of action of " mivacurium is prolonged approximately 30%, compared to normal patients (personal observation). Preliminary results in patients homozygous for the atypical gene indicate that these patients are extremely sensitive. Thus, one patient given a very low dose of mivacurium, equivalent to an ED10, was paralysed for more than 2h (personal observation).
Conclusion Prolonged respiratory insufficiency after succinylcholine is comparatively rare. It is, however, a very unpleasant experience for the patient and for the
SIGNIFICANCE OF PLASMA CHOLINESTERASE FOR THE ANAESTHETIST
anaesthetist. In spite of the decreasing use of succinylcholine, the introduciton of bambuterol and mivacurium into clinical practice may increase the frequency with which an anaesthetist will face a patient with a prolonged neuromuscular response caused by an abnormal pChe activity. Knowledge of pChe activity is therefore still essential for the anaesthetist.
References 1. Lockridge O, Eckerson HW, La Du NN. Interchain disulfide bonds and subunit organization in human serum cholinesterase. J Biol Chem 1979; 254:8324-8330 2. Kambam JR, Horton B, Parris WCV, Human SA, Berman ML, Sastry BVR. Pseudocholinesterase activity in human CNS. Anesth Analg 1989; 69:486-488 3. Whittaker M. Cholinesterase. Ed. L. Beckman, Karger, Exeter, 1986 4. Stedman E, Stedman E, Easson LH. Cholinesterase. An enzyme present in the blood of the horse. Biochem J 1932; 26:2056 5. Ostergaard D, Viby-Morgensen J, Hanel HK, Skovgaard LT. Half life of plasma cholinesterase. Acta Anaesth Scand 1988; 32:266-269 6. Propert DN, Backenridge CJ. The relation of sex, age, smoking, birth rank and parental ages to pseudocholinesterase activity and phenotypes in a sample of Australian Caucasian adults. Hum Genet 1976; 32:181-188 7. Viby-Mogensen J. Cholinesterase and succinylcholine. Dan Med Bull 1983; 30:129-150 8. Evans DB, Lehman H. Pseudocholinesterase activity in liver transplant. Lancet 1971; i: 1040-1044. 9. Kaniaris P, Fassoulaki A, Liarmakopoulou K, Dermizakis E. Serum cholinesterase levels in patients with cancer. Anesth Analg 1979; 58:82-84 10. Vihy°Mogensen J, Hanel HK, Hansen E, SOrensen B, Graae J. Serum cholinesterase activity in burned patients. I. Biochemical findings. Acta Anaesth Scand 1975; 19: 159-168. 11. Cavallaro ILl, Krumperman LW, Kugler F. Effect of ecothiopate therapy on the metabolism of succinylcholine in man. Anesth Analg 1968; 47:570-574 12. Fischer DM, Caldwell JE, Sharma M, WirEn JE. The influence of Bambuterol (carbamylated terbutaline) on the duration of action of succinylcholine-induced paralysis in humans. Anaesthesiology 1988; 69:757-759 13. Lockridge O. Genetic variants of human serum cholinesterase influence metabolism of the muscle relaxant succinylcholine. Pharmac Ther 1990; 47:35-60 14, Garry PJ, Dietz AA, Lubrano T, Fode PC, James K, Rubinstein HM. New allele at cholinesterase locus 1. J Med Genet 1976; 13:38-42 15. Rubinstein HM, Dietz AA, Lubrano T. E1k, another quantitative variant at cholinesterase locus 1. J Med Genet 1978; 15:27-29 16. Whittaker M, Britten J. A new allele at cholinesterase locus 1. Hum Hered 1987; 37:54-58 17. Neitlich HW. Increased plasma cholinesterase activity and succinylcholine resistance: a genetic variant. J Clin Invest 1966; 45:380-387 18. Krause A, Lane AB, Jenkins T. A new high activity plasma cholinesterase variant. J Med Genet 1988; 25:677-681 19. Robson EB, Harris H. Further data on the incidence and genetics of the serum cholinesterase phenotypes C5+. Am J Hum Genet 1966; 29:403-408 20. Kalow W, Lindsay HA. A comparison of optical and manometric methods for the assay of human serum cholinesterase. Can J Biochem Physiol 1955; 35:568-574 21. Kalow W, Genest K. A methoc for the detection of atypical forms of human serum cholinesterase. Determination of dibucaine numbers. Can J Biochem 1957; 35: 339-346. 22. Evans RT, Wardell J. On the identification and frequency of
237
the J and K cholinesterase phenotypes in a Caucasian population. J Med Genet 1984; 21:99-102 23. Hanel HK, Viby-Mogensen J. The inhibition of serum cholinesterase by urea. Br J Anaesth 1977; 49:1251-1257 24. Ostergaard D, Viby-Mogensen J. Prolonged apnea after succinylcholine. Problems in Anesthesia 1989; 3:455-464 25. McGuire MC, Nogneira CP, Bartels CF, Lightstone H, Hajra A, Van der Spek AFL, Lockridge O, La Du BN. Identification of the structural mutation responsible for the dibucaine-resistant (atypical) variant form of human serum cholinesterase. Proc Natl Acad Sci USA 1989; 86:953-957 26. Viby-Mogensen J. Interaction of other drugs with muscle relaxants. In: Norman J, ed. Clinics in Anesthesiology, London: WB Saunders Company, 1985:467-482 27. Pantuck E J, Pantuck CB. Prolonged apnea following succinylcholine administration, side effects and rational approach to relaxation. In: I. Azar, ed. Muscle Relaxants. Side effects and a rational approach to selection. New York: M. Decker Inc., 1987:205-229 28. Kalow W. Pharmacokinetics, heredity and the response to drugs. W.B. Saunders, Philadelphia, 1962; 69-93 29. Viby-Mogensen J. Correlation of succinylcholine duration of action with plasma cholinesterase activity in subjects with genotypically normal enzyme. Anesthesiology 1980; 53: 517520 30. Viby-Mogensen J. Succinylcholine neuromuscular blockade in subjects heterozygous for abnormal plasma cholinesterase. Anesthesiology 1981; 55:231-235 31. Viby-Mogensen J. Succinylcholine neuromuscular blockade in subjects homozygous for atypical plasma cholinesterase. Anesthesiology 1981; 55:429-434 32. Donati F, Bevan DR. Antagonism of phase II succinylcholine block by neostigmine. Anesth Analg 1985; 64:773-776 33. Bang, U, Viby-Mogensen J, Wir6n JE, Skovgaard LT. The effect of Barnbuterol (carbamylated terbutaline) on plasma cholinesterase activity and succinylcholine-induced neuromuscular blockade in genotypically normal patients. Acta Anaesth Scand 1990; 34:596-599 34. Bang U, Viby-Mogensen J, Wir6n JE. The effect of Bambuterol on plasma cholinesterase activity and succinylcholine-induced neuromuscular blockade in subjects heterozygous for abnormal plasma cholinesterase. Acta Anaesth Scand 1990; 34:600-604 35. Savarase JJ, Ali HH, Basta S-J, Embree PB, Scott RPF, Sunder N, W~akley, JN, Wastila WB, EI-Sayad HA. The clinical neuromuscular pharmacology of Mivacurium chloride (BW B1090U). Anesthesiology 1988; 68:723-732 36. Ostergaard D, Jensen FS, Jensen E, Viby-Mogensen J. Influence of plasma cholinesterase activity on recovery from Mivacurium-induced neuromuscular blockade. Acta Anaesth Scand 1989; suppl. 191; 33:164 37. Nogueira CP, McGuire MC, Bartels C, Arpagaus M, Van der Spek AFL, Lightstone H, Lockriodge O, La Du Bn. Identification of a frameshift mutation responsible for the silent phenotype of human buturylcholinesterase in serum (variant Ann Arbor). Third International Meeting on Cholinesterase. La Grande Motte, France, 1990; 102 38. Bartels CF, Nogueira CP, McGuire MC, Adkins S, Rubinstein HM, Lubrano T, Van der Spek AFL, Lightstone H, Lockridge O, La Du BN. Identification of two different mutations associated with human buturylcholinesterase fluoride resistance in serum. Third International Meeting on Cholinesterase. La Grand Motte, France, 1990; 103. 39. Bartels CF, Van der Spek A, Rubinstein H, Lubrano T, Lockridge O, La Du BN. DNA coding for the K polymorphism in linkage equilibrium with atypical human buturyl-cholinesterase complicates phenotyping. Third International Meeting on Cholinesterase. La Grande Motte, France, 1990; 104 40. Jensen FS, Bartels CF, La Du BN. A DNA mutation associated with the H-variant of human buturylcholinesterase. Third International Meeting on Cholinesterase. La Grande Motte, France, 1990; 101 41. Harris H, Whittaker M. Differential inhibition of human serum cholinesterase with fluoride. Recognition of two new phenotypes. Nature 1961; 191:496-498