Up-and-down regulation of skeletal muscle acetylcholine receptors. Effects on neuromuscular blockers

Increased cis-atracurium requirements during prolonged administration to a child

PURPOSE

To report increase infusion requirements of cis-atracurium during prolonged infusion (six weeks) to provide neuromuscular blockade in a child during prolonged mechanical ventilation. Despite a previous study in adult patients which demonstrated no increase in infusion requirements over five days, we noted a considerable increase over six weeks.

CLINICAL FEATURES

A seven month old infant required prolonged mechanical ventilation and neuromuscular blockade following an episode of multi-system organ failure from pseudomembranous colitis. The infusion of cis-atracurium was adjusted according to the train-of-four response obtained with a peripheral nerve stimulator using standard train-of-four monitoring. Initial infusion requirements which were 2.8 micrograms.kg-1 min-1 on day #1 increased to 22.3 micrograms.kg-1 min-1 on day #40.

CONCLUSION

Increased infusion requirements were necessary during the prolonged administration of cis-atracurium to a critically ill infant. Titration of the dose based on monitoring with a peripheral nerve stimulator is recommended.

Succinylcholine-induced hyperkalemia following prolonged pharmacologic neuromuscular blockade

While being treated for the acute respiratory distress syndrome, a 27-year-old woman developed profound hyperkalemia and cardiac arrest following the administration of succinylcholine chloride (SCh). She had none of the risk factors previously described for development of severe hyperkalemia following SCh administrations; however, she had been intermittently treated with nondepolarizing neuromuscular blocking drugs throughout the course of her illness. We suggest that immobilization of critically ill patients with pharmacologic neuromuscular blockade may predispose them to severe hyperkalemia and cardiac arrest following administration of SCh. SCh should be used with great caution in such patients.

Neurological sequelae following a high voltage electrical burn

Neurological complications secondary to electrical injury can manifest themselves either early or late. A small percentage of these patients develop delayed peripheral neuropathy. However, patients experiencing transient spinal cord symptoms have been described. We describe the development of a lower motor neurone syndrome affecting all the limbs in a patient following a significant electrical injury, which although expected to cause spinal cord necrosis showed no evidence of this on magnetic resonance imaging. The fact that neurophysiology was unable to localize the problem made this case all the more perplexing. Similar cases reported in the literature are discussed.

Airway management in neuroanaesthesia

PURPOSE

Airway management in neurosurgical patients presents unique challenges to the anaesthetist. This review will consider specific approaches to numerous problems in airway management related to logistical, physiological and anatomical concerns. The goal is to provide a clinically oriented and practical discussion regarding issues of airway management in neurosurgical patients.

SOURCE

The recent literature has been reviewed regarding airway management options and related perioperative complications in the neurosurgical population. This is interlaced with approaches to many of the problems and their solutions based on experience gained in a very busy university neurosurgical practice over the past decade.

PRINCIPAL FINDINGS

Specific pathophysiological alterations in the neurosurgical patient influence the technique chosen for securing an airway. These relate to the presence of increased intracranial pressure, intracranial aneurysms or arteriovenous malformations. Other important disorders influencing airway management include severe coronary artery disease, acromegaly and congenital airway difficulties. Stereotactic neurosurgery and conscious sedation for various neurosurgical procedures also provide unique challenges. There are other considerations unique to the neurosurgical patient such as intra-and postoperative airway obstruction and the timing of postoperative extubation.

CONCLUSION

The demands for airway management in neuroanaesthesia require expertise in the various modes of securing the airway while considering the patient's physiological requirements as well as the unique surgical demands.

Properties of fibres, endplates and acetylcholine receptors in the diaphragm, masseter, laryngeal, abdominal and limb muscles in the goat

PURPOSE

Although differences in fibre composition, fibre size or acetylcholine receptor (AChR) density between muscles have often been proposed to explain the unequal sensitivities of muscles to muscle relaxant drugs, it is not clear whether or how these parameters differ among muscles or are related to one another, In this study, several muscles were examined to determine the composition and cross-sectional area (CSA) of types I and II fibres, the surface area of their motor endplates (ESA), and their AChR density.

METHODS

Biopsies from the thyroarytenoideus, cricoarytenoideus dorsalis, masseter, diaphragm, transversus abdominis, rectus abdominis, gastrocnemius and soleus muscles of goats were processed by muscle histochemistry and morphometry and the ESA:CSA ratio was computed. The number and density of AChRs per endplate were estimated by 125I-alpha-bungarotoxin binding studies.

RESULTS

The mean type I fibre composition (range: 0-100%), fibre diameter (28-50 microns) and the ESA:CSA ratio (0.27-1.01) differed among muscles (P = 0.0001), but there were no significant differences (P > 0.05) in the mean endplate size (577-725 microns 2) AChR number (6.6-14.5 x 10(6)) or AChR density (8,900-22,300 microns-2) probably because of marked individual variations. Fibre size increased and the ESA:CSA ratio decreased in the order laryngeal, diaphragm, jaw, limb and abdominal muscles.

CONCLUSION

It is concluded that between muscles fibre size varies more than endplate size or AChR number.

An extended pharmacokinetic/pharmacodynamic model describing quantitatively the influence of plasma protein binding, tissue binding, and receptor binding on the potency and time course of action of drugs

An extended pharmacokinetic/pharmacodynamic (PK/PD) model is presented, in which the effect of binding of the drug to plasma proteins and to tissue binding sites in a peripheral compartment, and nonspecific and receptor binding in the effect compartment are taken into account. It represents an extension of the classical Sheiner model, and the model proposed by Donati and Meistelman. The present model is characterized by the following parameters: Kue (exit rate constant of unbound drug from the effect compartment), Pue (ratio of the unbound clearances to and from the effect compartment), fue (fraction of drug in effect compartment that is not bound to nonspecific binding sites), Kd (equilibrium dissociation constant of drug-receptor binding), and Rtot (concentration of receptor binding sites in effect compartment). The rate of association and dissociation of the drug-receptor complex can be incorporated in the model. The influence of the pharmacokinetic parameters (V1, V2, fu, fu2, CLu10, CLu20, CLu12, CLu21) and the PK/PD model parameters (kue, Pue, fue, Kd, Rtot) on various dynamic parameters is analyzed. These include potency (single dose needed to produce 90% effect, ED90), constant infusion dosing rate needed to maintain a constant effect of 90%, time to maximum effect (onset time), and duration to 90% recovery. The neuromuscular blocking agent vecuronium is used as an example. It is shown that both potency and time course of action are strongly dependent on the ratio V1/fu, CLu10, kue, Pue (at equipotent doses the time course is not affected by Pue), fue, Kd, and Rtot (only if Rtot is high), whereas they are less affected by the ratio V2/fu2, CLu20, CLu12, and CLu21. In general, the model parameters affect the ED90 and the time course of action in the same direction, e.g., an increase of V1 results in an increase of ED90 and an increase of onset time and duration. However, the unbound clearance CLu10, the intercompartmental unbound clearance CLu12 and the receptor affinity Kd have an opposite effect on ED90 and the time course parameters, e.g., an increase of CLu10 results in an increase of ED90 and a decrease of onset time and duration. This effect may be responsible for the inverse relationship between onset time and potency of neuromuscular blocking drugs observed in animal experiments and clinical studies. We demonstrate that PK/PD analysis using the traditional effect compartment model (Sheiner model) results in an apparent value of keo, which is a function of kue, fue, Kd, Rtot, as well as the unbound drug concentration in the effect compartment Cue. On the other hand, the model proposed by Donati and Meistelman gives correct values of keo (equal to the product fue.kue), but the receptor affinity Kd and the receptor density Rtot obtained by this method are apparent values, which depend on fu, fue, and Pue.

[Resistance to vecuronium in burnt patients. Influence of the burnt surface on the effectiveness of the dose 95]

OBJECTIVE

To assess the neuromuscular blocking effect of vecuronium in adult burn patients, to draw dose-response curves, to determine the ED 95 according to burn surface area, to analyze the time course of this pattern in order to recognize the development of a resistance according to the length of postinjury period.

STUDY DESIGN

Prospective open study, extending over a 12 month period.

PATIENTS

Sixty-three consecutive adult burn patients in an acute phase and 13 control patients who had been thermally injured at least 500 days before their inclusion in the study.

METHODS

Anaesthesia was achieved with thiopentone, fentanyl and vecuronium in patients undergoing excision and autograft surgery. Neuromuscular blockade was assessed by thumb adduction, measured by electromyography using evoked train of four responses to ulnar nerve stimulation. Dose-response curves were determined using the single dose method from only one predetermined dose of vecuronium per patient on each day of the study. Dose-response curves were compared using linear regression and ED 95 were calculated from log-probit data.

RESULTS

In the control group, ED 95 was 53 mg.kg-1. Before the 7th postinjury day, patients did not differ from controls. Between the 7th and the 70th postinjury day the ED 95 increased significantly. Patients with a burn surface area (BSA) of less than 20% had a ED 95 of 69 mg.kg-1, between 20% and 40% of BSA the ED 95 was 103 mg.kg-1, between 40% and 60% BSA the ED 95 was 134 mg.kg-1 and patients with a BSA over 60% had a ED 95 at 154 mg.kg-1. The onset of action increased in all groups and was significantly different from control group.

CONCLUSION

Acutely burn patients become resistant to the neuromuscular blocking effect of vecuronium. This resistance is related to the magnitude of burn injury. The mechanism of resistance is related to an increase in nicotonic acetylcholine receptors. In these patients, the dose of vecuronium must be titrated to achieve effective muscular paralysis: the correcting factor is 1.3 for a BSA under 20%, 1.9 for a BSA between 20 and 40%, 2.5 for a BSA between 40 and 60%, and 2.9 for a BSA above 60%.

Metocurine pharmacokinetics and pharmacodynamics in goats

Non-depolarizing muscle relaxants can facilitate surgery and anaesthesia in numerous species. and volatile inhalational anaesthetics such as isoflurane potentiate their action. We studied the effect of isoflurane on the pharmacodynamics and pharmacokinetics of metocurine in six goats. Each was studied twice: once during barbiturate-opiate anaesthesia and once during isoflurane anaesthesia. The evoked response to sciatic nerve stimulation was measured using a force transducer attached to the hoof. Metocurine was infused until approximately 80-90% blockade. Plasma metocurine concentration was determined by high-performance liquid chromatography. Isoflurane increased the potency of metocurine significantly; IC50 (the concentration in the effect compartment at 50% paralysis) was 70 +/- 15 ng/mL during isoflurane anaesthesia and 129 +/- 42 ng/mL during barbiturate-opiate anaesthesia (P < 0.03). Volume of distribution (63 +/- 18 mL/kg), clearance (1.6 +/- 0.4 mL/min.kg) and elimination half-life (99 +/- 9 min) during barbiturate-opiate anaesthesia were not significantly different during isoflurane anaesthesia: 64 +/- 25 mL/kg, 1.5 +/- 0.7 mL/kg.min, 116 +/- 16 min respectively. We conclude that, relative to barbiturate-opiate anaesthesia, isoflurane potentiates metocurine in goats.

Hyperkalaemia associated with haemorrhagic shock in rabbits: modification by succinylcholine, vecuronium and blood transfusion

In haemorrhagic patients, hyperkalaemia may occur after succinylcholine administration. We investigated in haemorrhagic rabbits whether vecuronium caused hyperkalaemia and if blood transfusion prevented succinylcholine-induced hyperkalaemia. Rabbits were lightly anaesthetized with halothane/N2O and 30-35 ml/kg blood were withdrawn to cause arterial pH to decrease to approximately 7.0-7.1. Group C (n = 12) received 3 ml saline, Group S (n = 13) received succinylcholine 1 mg/kg, Group V (n = 7) received vecuronium 0.25 mg/kg and Group T (n = 9) was transfused with 10 ml/kg blood 10 min prior to receiving succinylcholine 1 mg/kg. K+ analysis was repeated at 5 min, and at 10 min in most animals. Haemorrhage increased K+ (nmol.1-1, mean +/- SD): Group C, 3.4 +/- 0.3 to 6.8 +/- 1.8; Group S, 3.8 +/- 0.5 to 6.9 +/- 2.3; Group V, 3.8 +/- 0.3 to 7.1 +/- 1.9; Group T, 3.6 +/- 0.5 to 7.6 +/- 2.9. K+ decreased in Group T (to 5.4 +/- 1.8) after blood transfusion. K+ increased at 5 min in Group S (to 8.7 +/- 2.6) and at 10 min for Group C (to 8.4 +/- 1.7) and Group T (to 7.2 +/- 2.3). The K+ increase at 5 min for Group S (1.8 +/- 0.8) was significantly higher than those for Group C (1.0 +/- 0.6) or Group V (0.9 +/- 0.4), but was not different from that of Group T (1.5 +/- 0.7). We conclude that haemorrhage and succinylcholine cause hyperkalaemia in rabbits. Muscle relaxation itself does not appear to be a factor, but transfusion may lessen the hyperkalaemia.

Resistance to atracurium in rats with experimental inflammation: role of protein binding

The influence of altered protein binding on the neuromuscular effect of atracurium has been studied in rats with experimental inflammation induced by subcutaneous injection of turpentine oil. Doses of atracurium ranging from 0.45 to 1.5 mg.kg-1 were administered to control (n = 30) and to experimental inflammation induced rats (n = 30). Neuromuscular transmission was monitored by recording the twitch tension of the tibialis-anterior muscle elicited by stimulation of the sciatic nerve. Three effect parameters were recorded: (i) intensity of the effect, measured as percentage depression of baseline twitch tension, (ii) duration of drug action (min) and (iii) recovery time (min). The dose-intensity of the effect relationship was modelled using a sigmoid Emax model. The ED50 (effective dose eliciting 50% of the maximum effect) was significantly increased (P < 0.01) in the inflammation group as compared to the control group (0.94 vs. 0.68 mg.kg-1). This change was reflected in a shift of the dose-response curve to the right in the pretreated rats. For equipotent doses ED95 (defined as the effective dose eliciting 95% of maximum effect), no differences were found in recovery time and duration of action between the two groups of rats. Mucoproteins levels (index of alpha 1-acid glycoprotein (AAG) and protein binding were significantly increased in rats with experimental inflammation as compared to control rats. Based on these results, altered serum protein binding of atracurium appears to be responsible, at least in part, for the resistance to atracurium.

Immunocytochemical localization of nicotinic acetylcholine receptor in rat cerebral cortex

Localization of nicotinic acetylcholine receptor (nAChR) alpha 4 subunits was investigated in rat cerebral cortex using a monoclonal antibody against alpha 4 subunits. The antibody depleted more than 70% of the [3H]methylcarbamylcholine choline binding activity of the solubilized membrane fraction. By light microscopy alpha 4-like immunoreactivity (alpha 4-LI) was found through layers II to VI. The immunostaining was the most prominent in cell bodies and apical dendrites of pyramidal cells in layer V. By electron microscopy most immunoreaction products were observed in the rough endoplasmic reticulum, cytoplasmic matrix and synaptic membranes. Alpha 4-LI was detected in the postsynaptic membranes of neuronal cell bodies and apical dendrites. These findings suggest that alpha 4-containing subtypes serve as one possibly the postsynaptic nAChR in rat cerebral cortex.

Practice parameters for sustained neuromuscular blockade in the adult critically ill patient: an executive summary. Society of Critical Care Medicine

OBJECTIVE

The development of practice parameters for achieving sustained neuromuscular blockade in the adult critically ill patient for the purpose of guiding clinical practice.

PARTICIPANTS

A task force of more than 40 experts in disciplines related to the use of neuromuscular blocking agents in the intensive care unit was convened from the membership of the American College of Critical Care Medicine (ACCM) and the Society of Critical Care Medicine (SCCM).

EVIDENCE

The task force members provided the personal experience and determined the published literature (MEDLINE articles, textbooks, pharmacopeias, etc.) from which consensus would be sought. Published literature was reviewed and classified into one of four predetermined categories, according to study design and scientific value.

CONSENSUS PROCESS

The task force met several times as a whole, and numerous times in smaller groups by teleconference, over a 1-yr period to identify the pertinent literature and arrive at consensus recommendations for the whole task force to discuss. Consideration was given to the relationship between the weight of scientific information and the experts' viewpoints. Over the next year, draft documents were composed by a task force steering committee and debated by the task force members until consensus was reached by nominal group process. The task force draft was then reviewed, assessed, and edited by the Board of Regents of the ACCM. After steering committee approval, the draft document was reviewed and approved by the SCCM Council.

DATA SYNTHESIS

To facilitate rapid communication of the three recommendations contained within the complete and unabridged practice parameter document, an executive summary was prepared for publication by the ACCM Board of Regents, and this executive summary was approved by the task force steering committee and the SCCM Executive Council.

CONCLUSIONS

A consensus of experts provided three recommendations with supporting data for achieving sustained neuromuscular blockade in critically ill patients: a) pancuronium is the preferred neuromuscular blocking agent for most critically ill patients; b) vecuronium is the preferred neuromuscular blocking agent for those patients with cardiac disease or hemodynamic instability in whom tachycardia may be deleterious; c) patients receiving neuromuscular blocking agents should be appropriately assessed for the degree of blockade that is being sustained. This executive summary selectively presents supporting information and is not intended as a substitute for the complete document.

The use of neuromuscular blocking drugs in intensive care practice

Critically ill patients represent a very different population from that of the operating theatre, but much of our knowledge of many of the neuromuscular blocking drugs is derived from intraoperative use. The diversity of clinical-practice and case-mix differences in intensive care are probably responsible for the absence of a formal consensus about the use of neuromuscular blocking drugs in the intensive care unit (ICU). Various surveys suggest that these drugs are used comparatively infrequently, but we do not know whether current usage is either safe or appropriate. In addition to the adverse effects which inevitably accompany prolonged paralysis and immobility, the steroidal relaxants, pancuronium and vecuronium, have also been associated with myopathy. This seems to be aggravated by concurrent use of pharmacologic doses of corticosteroids or the aminoglycoside antibiotics. Neither the mechanism nor the validity of the association with steroidal relaxants is known at present. Muscle dysfunction is a common feature of critical illness, and it is possible that neuromuscular blocking drugs interfere with muscle repair and regrowth. Patients with multiple organ failure present a particular challenge both because of the extent of tissue injury and because drug clearance via the liver or kidneys is generally impaired.

Pancuronium infusion for neuromuscular block in children in the pediatric intensive care unit

When neuromuscular blockade becomes necessary in the intensive care unit, there are several options available in regard to both the drug and the mode of delivery (continuous versus intermittent administration). Despite extensive experience with intermediate acting drugs such as atracurium or vecuronium, these muscle relaxants are costly and may account for a significant portion of the pharmacy charges. We undertook an open label study to evaluate the efficacy and dosing requirements for a less costly drug, pancuronium. The study group included 25 patients ranging in age from 3 mo to 17 yr and in weight from 3.2 to 68 kg. If the patient had not previously received neuromuscular blocking agents (NMBAs), pancuronium was administered as a bolus dose of 0.1 mg/kg followed by a continuous infusion of 0.05 mg.kg-1.h-1. A nerve stimulator was applied to either the ulnar or peroneal nerve and a standard train-of-four (TOF) was monitored every 2 h. In patients that had previously received other NMBAs, no bolus dose of pancuronium was administered and the infusion was started at 0.05 mg.kg-1.h-1. The pancuronium infusion was increased or decreased by increments of 0.01 mg.kg-1.h-1 to maintain one to two twitches of the TOF. In patients that required an increase in the infusion rate, an additional bolus dose equivalent to the current hourly rate was administered and then followed by the increase in the infusion rate.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in acetylcholine receptor number in muscle from critically ill patients receiving muscle relaxants: an investigation of the molecular mechanism of prolonged paralysis

OBJECTIVE

Previous reports have described prolonged paralysis after the administration of muscle relaxants in critically ill patients. The purpose of this study was to examine possible pathophysiologic causes for this paralysis by measuring muscle-type, nicotinic acetylcholine receptor number in necropsy muscle specimens from patients who had received muscle relaxants to facilitate mechanical ventilation before death.

DESIGN

Prospective laboratory study of human muscle collected at autopsy.

SETTING

Medical and surgical intensive care units (ICUs) at a university hospital and a research laboratory.

PATIENTS

Fourteen critically ill patients, with a variety of diagnoses, all of whom required mechanical ventilatory support before their deaths in the ICU and who underwent post mortem examination. Patients were arbitrarily divided into three groups, according to their total vecuronium dose and number of days mechanically ventilated before death. Three patients were in the control group (defined as dying within 72 hrs of initiation of ventilatory support and receiving a total dose of < 5 mg of vecuronium). Six patients were in the low-dose group (defined as requiring ventilatory support for > 3 days before death and receiving a total vecuronium dose of < or = 200 mg). Five patients were in the high-dose group (defined as requiring ventilatory support for > 3 days before death and receiving a total vecuronium dose of > 200 mg).

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Nicotinic acetylcholine receptor numbers as measured by specific 125I-alpha-bungarotoxin binding to human rectus abdominis muscle obtained at autopsy were determined. In general, receptor number reflected the clinical requirements for the muscle relaxants of each patient. Patients who had increasing requirements for muscle relaxants before death had increases in receptor number, as compared with control values.

CONCLUSIONS

The increase in nicotinic acetylcholine receptor number in muscle from patients with an increasing requirement for muscle relaxants before death suggests that nicotinic acetylcholine receptor up-regulation may underlie the increased requirements for muscle relaxants seen in some patients. Furthermore, these findings suggest that muscle relaxant-induced, denervation-like changes may at least be partially responsible for prolonged muscle paralysis after the long-term administration of muscle relaxants. This study may provide the first information into the molecular mechanisms underlying prolonged paralysis.

Adverse effects of depolarising neuromuscular blocking agents. Incidence, prevention and management

Muscle relaxants block neuromuscular transmission, acting at nicotinic acetylcholine receptors of the neuromuscular junction. Suxamethonium (succinylcholine) is a depolarising agent, whereas all other relaxants in clinical use are nondepolarising. The desired neuromuscular block results from the structural similarity of muscle relaxants to acetylcholine, enabling the interaction with receptors at the neuromuscular junction. Adverse effects of suxamethonium are generally related to its agonist mode of action. Autonomic cardiovascular effects may result. Other adverse effects include anaphylactic or anaphylactoid reactions, and histamine release. Various disease states may present specific considerations in the use of muscle relaxants. Although many complications of muscle relaxants (such as prolonged block or resistance) are easily treated, others may require immediate intervention and vigorous therapy. Careful selection of appropriate relaxants for particular patients will usually prevent the occurrence of complications.

Neuromuscular blocking drugs in infants and children

Neuromuscular blocking drugs are valuable adjuncts to the practice of pediatrics. Monitoring of drug effects is technically more difficult in the younger patient. Nevertheless, careful observation of drug effects will improve the usefulness of NMB and safeguard the patient from prolonged weakness. Although there are differences in neuromuscular function with age in the pediatric age range, the differences between the NMB currently available are greater than the differences between the patients. Thus, the only uniform finding across age and all drugs is that onset of drug effect is more rapid in the infant than in the child when circulatory function is normal. In general, children require more of all NMB on a mg/kg basis than do infants or adults to obtain the same effect. Children recover from NMB more rapidly than do patients of other ages. Infants, however, may recover more rapidly than do any other patients from the effects of drugs such as mivacurium which are metabolized in the plasma. Tables 4 and 5 summarize doses, onset of action, and duration of NMB. Please note in Table 4 that succinylcholine is only used for endotracheal intubation, whereas the other nondepolarizing muscle relaxants can be used for endotracheal intubation or to maintain some degree of muscle paralysis in the child whose trachea is already intubated. Nondepolarizing muscle relaxants (e.g., mivacurium, ORG 9426, atracurium, vecuronium) are used both for initial bolus for endotracheal intubation and maintenance of muscle relaxation. Long-acting drugs (e.g., pancuronium, pipecuronium, and doxacurium), however, are used more commonly in small incremental doses to maintain muscle paralysis in patients already intubated. The advantages of these long-acting drugs are minimal cardiovascular side effects (i.e., tachycardia or hypotension from histamine release) and longer dosing interval. In all children, the dosing interval should be adjusted to the needs of the individual. In children with renal insufficiency or in those receiving drugs which impair neuromuscular function (e.g., aminoglycosides), the interval at which supplemental doses are required is longer than normal.

[Curare and burns]

Burns of more than 15% of the body surface area result in major physiological changes with an alteration of cardiovascular, pulmonary, hepatic and renal functions, as well as modifications in the pharmacokinetics and the pharmacodynamics of many drugs. Among these, a major change in the activity of muscle relaxants occurs which can be specific to this pathology. Succinylcholine is contra-indicated during recovery from a burn trauma because of a possible hyperkaliemic response, directly related to the dose, the post-burn delay and the area of burned body surface. The kaliemic response and the related cardiac complications remain unpredictable. The height of twich depression with small doses of succinylcholine such as 0.1 to 0.2 mg.kg-1, demonstrates the hypersensitivity to this agent and does cause neither metabolic disturbances nor cardiac arrest. Nevertheless, the administration of succinylcholine is contra-indicated for from the 5th day on at least two years after the burn injury. Conversely, the action of non-depolarizing muscle relaxants is characterized by a resistance, which is correlated to both the post-traumatic delay and the extent of the burned area. It starts on about the seventh day, reaches peak intensity between day 15 and day 40 and can persist up two years after the thermal injury. In the course of a burn, the so-called "immature" acetylcholine receptors, characterized by the substitution of the sub-unit epsilon by a protein gamma, increase at the level of the end plate areas and the extra-synaptic muscle membrane. These receptors explain both the hyperkaliemic response and the hypersensitivity to succinlycholine.(ABSTRACT TRUNCATED AT 250 WORDS)

Resistance to vecuronium

A 19-year-old epileptic patient taking carbamazepine was anaesthetised for a sigmoid colectomy. Such patients are reported to show a resistance to the actions of competitive neuromuscular blocking drugs, and a conventional dose of vecuronium (0.1 mg.kg-1) produced relaxation for only 18 min. Subsequently she received vecuronium 6.7 mg.h-1 which kept the first twitch of the train-of-four response at 30% of the control value. Antagonism with neostigmine 1.25 mg produced a rapid and full recovery. The report illustrates the usefulness of monitoring neuromuscular transmission whenever there is an anticipation of an altered response.

Immunocytochemical localization of nicotinic acetylcholine receptor in rat hypothalamus

Immunocytochemical localization of neuronal nicotinic acetylcholine receptor (nAChR) was examined in rat hypothalamus. Monoclonal antibody against alpha 4 ACh-binding subunits of nAChR was used in the avidin-biotin-peroxidase complex (ABC) immunocytochemical method at both the light and electron microscopic levels. By light microscopy nAChR-like immunoreactivity was found in many neuronal cell bodies and their fibers in the paraventricular nucleus (PVN) and in many axons and axon terminals in the median eminence (ME). The immunoreactivity of nAChR was the most intense in the ME. By electron microscopy immunoreaction products occurred on the rough endoplasmic reticulum, nuclear envelope, cytoplasmic matrices and postsynaptic densities of synaptic junctions in some neurons in the parvocellular part of the PVN. In the external layer of the ME, nAChR-like immunoreactivity was found over the entire plasma membranes of many axon terminals. Involvement of nAChRs in the release of neurotransmitters and neuropeptides both in the PVN and the ME is discussed.

Prolonged duration of succinylcholine in patients receiving anticonvulsants: evidence for mild up-regulation of acetylcholine receptors?

Succinylcholine (SCh) normally causes a small increase in serum potassium concentration, but certain conditions may predispose to severe hyperkalaemia. This is due to "up-regulation" of skeletal muscle acetylcholine receptors (AChR), which also results in resistance to non-depolarizing muscle relaxants (NDMR). Anticonvulsant therapy causes NDMR resistance because of sub-clinical blockade, and diminished release, of acetylcholine. We studied nine patients chronically receiving anticonvulsants (phenytoin and/or carbamazepine) and nine control patients. Anaesthesia was induced typically with thiopentone or propofol; isoflurane and N2O were used for maintenance. The ulnar nerve was supramaximally stimulated and mechanical twitch height was measured with a force transducer at the adductor pollicis, before and after SCh 1 mg.kg-1, until return to baseline height. Plasma potassium concentration was measured before and at three, five, and ten minutes following SCh. Mean maximum potassium rise was 0.2 mEq.L-1 in each group. The time for return to baseline twitch height was 14.3 +/- 2.3 min (mean +/- SD) in the anticonvulsant group and 10.0 +/- 1.6 min in the control group, P = 0.001. The recovery index (time for 25% to 75% recovery) was 2.6 +/- 0.9 min in the anticonvulsant group and 1.4 +/- 0.3 min in the control group, P < 0.01. The normal potassium response coupled with prolonged duration suggests a hypersensitivity to SCh that is consistent with an anticonvulsant-induced mild up-regulation of AChR.

Burn injury-induced nicotinic acetylcholine receptor changes on muscle membrane

Patients with thermal injury show pharmacological responses akin to those seen following denervation of motor nerve where there is an increase in the number of muscle-type acetylcholine receptors (AChR). This study in the rat, examined the effect of 20%, 30%, and 50% body surface area burn injury on gastrocnemius and diaphragm AChR number, quantitated by [125I]alpha-bungarotoxin (alpha-BGT). Pharmacodynamic responses to D-tubocurarine were evaluated in the gastrocnemius muscle. AChR from gastrocnemius muscle were significantly increased in all burn groups at 14 days after burn. After 28 days, the AChR had returned to control in the 20% and 30% burn groups, but persisted in the 50% burn group. AChR from diaphragm did not show significant changes between groups. Resistance to D-tubocurarine was not seen in burn groups compared with controls. Burn trauma causes increases of AChR at sites distant from the area of injury. Diaphragm muscle may be less sensitive than gastrocnemius to mediators which upregulate AChR.

The use of muscle relaxants in the intensive care unit

Neuromuscular blocking agents are frequently used in the Intensive Care Unit to facilitate tracheal intubation and the application of continuous paralysis. This review will focus on various conditions of the critically ill patient such as multi-organ dysfunction, acid-base and electrolyte imbalance, prolonged immobility, multiple drug interactions and specific disease/injury processes that may affect the pharmacokinetic and pharmacodynamic behaviour of muscle relaxants. As such, due to the complex nature of the critically ill patients, the effects of neuromuscular blocking agents are unpredictable. Therefore, guidelines regarding their administration and the methodology and requirement for continuous bedside monitoring of neuromuscular function will be presented.