Artículo de Reflexión

Neuromuscular Blocking Agents: an Argument for Their Adequate Use

Roberto Carlo Rivera Díaz*, Johan Sebastián Rivera Díaz**

* Anestesiólogo. Docente de Anestesia y Dolor, Universidad CES, Instituto Colombiano del Dolor. Correspondencia: Cra 48 No. 19A-40 Unidad 1205 Torre Médica Ciudad del Río, Medellín, Colombia. Correo electrónico: robertorivera@incodol.com

** Residente de Anestesia, Universidad CES, Medellín, Colombia. Correo electrónico: jsebastian827@hotmail.com

Recibido: abril 4 de 2011. Enviado para modificaciones: junio 9 de 2011. Aceptado: junio 14 de 2011.

SUMMARY

Introduction. The debate regarding the use of muscle relaxants is still ongoing, with arguments against such as their adverse effects and published risks, and arguments in favor such as the need to ensure rapid and adequate intubation in emergency cases, the benefit of an easy surgical field for the surgeon, and other reported benefits.

Objective. To review aspects associated with the adequate use of neuromuscular blocking agents in anesthesia.

Materials and methods. Narrative review of the scientific literature available on the subject that focused on indications, risks, benefits and monitoring.

Results. The absolute indication of muscle relaxants is rapid-sequence intubation, but there are other situations where they are recommended: Intubation in elective surgery, certain types of special surgical procedures and situations (ARDS, electroconvulsive therapy, intra-abdominal hypertension, intracranial hypertension), and they may be avoided in many general anesthesia cases. Most important is to be aware of their indications, risks and adequate use.

Conclusion. In conclusion, there are situations where the use of muscle relaxants may be avoided, but there are also instances were they are a requirement and it is in those cases where they are recommended. Therefore, if they are to be used, they must be the ideal choice for the individual patient at the right time and the right dose, and always under appropriate monitoring.

Key Words: Neuromuscular blocking agents, intubation, medical emergencies, risk. (Source: MeSH, NLM).

INTRODUCTION

The mortality outcome in anesthesia was reported in 1954 for 599,548 procedures performed in ten American hospitals over a four-year period. One of the most striking findings was a mortality rate six times higher in the group receiving muscle relaxants. Based on these data, the group of researchers published the following recommendations: to improve neuromuscular blocking agents, to follow good practices, and to design monitoring devices (1). Six decades later, the debate between using or not using muscle relaxants is still open.

Criticisms against the use of these drugs have been based on the risks they entail:

• They are responsible for 60% to 70% of anaphylaxis cases in anesthesia (2), mainly with rocuronium and succinylcholine (3).

• Neuropathy in critically-ill patients, most commonly with aminosteroids.

• Two-fold increase in the incidence of intraoperative recall.

• The use of succinylcholine is associated with malignant hyperthermia, hypercalcemia, arrhythmias and cardiac arrest.

• Residual blockade reported in up to 64% of cases with intermediate-action drugs, giving rise to additional problems such as hypoxemia, atelectasis, pneumonia and delayed discharge in outpatient surgery (4).

• Risks associated with reverting drugs (5).

The question is: should muscle relaxants be used or not? With the wide variety of anesthetic drugs available at present, there is a significant number of patients who do not require muscle relaxants, in particular in several cases of elective surgery, use of laryngeal mask, anesthesia maintenance with remifentanil infusion; additionally, there even are techniques for orotracheal intubation without using muscle relaxants, with excellent results (6). It must be made clear that studies in this regard have been conducted in young patients coming for elective surgery, with a low anesthetic risk, and using high doses of remifentanil and propofol; consequently, no inferences should be made for the vast majority of patients or for cases of emergency surgery (7,8).

The controversy pertains to those cases in which there are clear indications for the use of muscle relaxants, such as rapid-sequence induction. This is the absolute indication for their use, and the debate revolves around the technique in particular: which muscle relaxant should be used? What is the ideal timing? What is the appropriate dose? Should cricoid pressure be used? Should ventilation be used during induction?

There is no argument regarding the fact that patients on a full stomach must be intubated with the help of a muscle relaxant (9). In such a case, the recommendation is to do it properly, using succinylcholine as the first option because of its excellent characteristics during the first minute of induction. The recommended dose is between 0.6 and 1.5 mg/kg. Higher and lower doses must be avoided because of increased severity of adverse events and suboptimal 60-second intubation conditions, respectively. In those cases where succinylcholine is contraindicated, the next option is a dose of 0.9 - 1.2 mg/kg of rocuronium, which provides adequate first-minute induction conditions (10).

Additional uses are only recommended, and they include:

• Intubation for elective surgery: traditionally, two or three times the effective dose (ED95) of a paralyzing agent has been used for endotracheal intubation. When analyzing the definition of ED95, it is defined as the dose of muscle relaxant required to reduce 95 % of the response to a stimulus. That definition is consistent with the adequate timing for intubation, and that is why intubation may be done under a single ED95, provided the right time has elapsed. The use of two to three ED95 for intubation is required only in order to reduce the drug's latency. Moreover, the administration of rapid onset drugs such as propofol and remifentanil, is of additional help (11).

• Abdominal surgery: it is important to ensure an adequate surgical field and this is achieved not only with the help of muscle relaxants, but also with optimal depth of anesthesia, which reduces the need for relaxants. Moreover, the goal is to maintain the patient with a surgical blockade, that is, with one or two train-of-four (TOF) responses, thus ensuring adequate abdominal wall relaxation. There is no need to give high doses or to bring the patient to a non-reponse phase, and intense or deep blockade must be avoided. Regardless of the length of the surgical procedure intermediate- acting agents must be used; it is better to use repeated doses instead of a single long-acting agent. The use of pancuronium is currently considered as bad practice because of the associated threefold increase in the probability of a TOF ratio less than 0.7 in the postoperative period and the increased incidence of hypoxemic events during recovery, and of pulmonary complications (12,13).

• Critical situations: recent studies have shown reduced mortality with the use of muscle relaxants during the early phase of acute respiratory distress syndrome. In patients with abdominal compartment syndrome, intra-abdominal and airway pressure values were reduced while the final treatment was provided. In cases of intracranial hypertension, muscle relaxants are part of the pharmacological armamentarium, as is also the case in the management of tetanus, during electroconvulsive therapy with anesthesia, among others. (14,15)

There are other situations where the surgical procedure may be made easier, for example in airway, laryngeal, middle ear, chest and anterior chamber surgical procedures (16).

Neuromuscular relaxation monitoring is mandatory as part of good practices with the use of these drugs. Monitoring helps avoid unnecessary additional doses that may cause intense or deep blockade, and it also helps determine the need to revert, the timing and the dose. If there is a lasting ratio of 1, measured by acceleration electromyography (or greater than 0.9 when measured by mechanomyography), there is no need to revert. It is recommended to make sure the patient recovers the four responses before using neostigmine (this recommendation does not apply for sugammadex). Adjusted doses of anticholinesterase must be used in accordance with the ratio: 20 mcg/kg of neostigmine if the ratio is higher than 0.4, or 40 mcg/kg if the ratio is less than 0.4, and it should be given 15 minutes before extubation (17). It is important to bear in mind that clinical assessment does not prevent residual blockade.

In conclusion, there are situations where the use of neuromuscular blocking agents may be avoided, but there are also other situations where they are mandatory, and other cases where they may be considered as an option. Whenever they are used, they must be given at the right time, the appropriate dose, and always under monitoring.

REFERENCES

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16. Miller RD. Miller's Anesthesia 6th ed. Philadelphia, PA: Elsevier Churchill Livingstone, 2005. p. 481-572

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