Neuromuscular Monitoring in Anesthesia

Clinical use of muscle relaxants in children

Olli A. Meretoja, MD, PhD
Department of Anesthesia
Hospital for Children and Adolescents
University of Helsinki
FIN-00029 HUS, Finland

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The most important indication for using muscle relaxants in children is the use of endotracheal intubation. The well-defined characteristics of muscle relaxants for this indication include fast onset of action, reliable potency and duration of effect. However, due to wide individual variation in response to all muscle relaxants, adequate monitoring of the neuromuscular block is an essential part of good clinical pediatric anesthesia practice.

Factors affecting the onset of action of muscle relaxants

The onset time of a muscle relaxant is the time from intravenous administration of the drug to its maximum effect. Several factors affect the onset time.

First, age-dependent differences of onset times are clinically significant for all muscle relaxants. Young children have a shorter onset of action of muscle relaxants compared to older children owing to their much faster circulation time. Infants circulate their blood volume in 24 seconds compared to one minute in adolescents. Table 1 shows that infants have onset times that are only 40-60% of the times needed in adolescents or young adults. Evidently, no non-depolarizing muscle relaxant has as short an onset time as the traditional depolarizing drug suxamethonium (1).

Second, the dose of a muscle relaxant has an important effect on the speed of onset. The greater the dose, the faster the drug’s molecules reach the receptor and produce their effect. Several muscle relaxants show a 25% reduction in onset time if the dose is doubled from the normal intubation dose. A dose that produces a 95% neuromuscular block is called ED95 (effective dose 95). It is important to note that the potency of any non-depolarizing muscle relaxant is higher for infants and lower for children than for adolescents (Table 2) (2,3,4). Therefore a constant dose of a muscle relaxant in mg/kg has significantly greater and longer effects in infants than in children. An extreme example is vecuronium, which is said to be a long-acting agent in infants (5).

Third, the drugs used to induce anesthesia have important effects on onset times. Drugs that enhance the effects of muscle relaxants (like volatile anesthetics) in fact shorten onset times. Accordingly, onset times of muscle relaxants are generally some 25% shorter during volatile anesthetic use than during intravenous anesthetic use (6).

Intubation conditions

Intubation conditions can be graded as excellent, good, poor or impossible. The first two grades are regarded as clinically acceptable. Several studies have assessed that a clinically acceptable intubation technique should provide at least 95% of patients with excellent or good intubation conditions. Usually the anesthesiologist is in no hurry to provide these conditions, but there are times when the conditions should be reached very fast.

Following intravenous induction of anesthesia, a dose of a muscle relaxant should usually be 1.5 to 2 times the dose that produces a 95% neuromuscular block (Table 3). This dose can safely be larger for muscle relaxants that have a short duration of effect since even these larger doses may not compromise the recovery of neuromuscular function at the end of surgery. Atracurium and cisatracurium, which are eliminated at a constant rate in all age groups (7), are especially reliable in this respect.

It is important to note that excellent intubation conditions can also be provided without using any muscle relaxant. Deep sevoflurane anesthesia, or a combination of propofol with either alfentanil or remifentanil, can produce clinically acceptable intubation conditions within 2-3 minutes in infants and children (8,9).

Which muscle relaxant for intubation?

When it was realized that any muscle relaxant can be used for routine endotracheal intubation anesthesiologists started to avoid polypharmacy and selected only one muscle relaxant to cover all the needs of a child for a particular operation. If airway management is easy and there is no need for crash induction, then any intermediate-acting muscle relaxant can be used for cases with a >30 min. duration of surgery. If the duration of surgery is only 10-15 minutes, then mivacurium is the preferred muscle relaxant (Table 3).

Rocuronium provides adequate intubation conditions faster than atracurium, vecuronium or cistaracurium in infants, children and adolescents (Table 1) (10). Cisatracurium, due to its low potency, has a slightly longer onset time than other intermediate-acting muscle relaxants in pediatric patients (11). Normal intubation doses of all these intermediate-acting agents have a clinical duration of effect of 20-30 minutes and the effects can be reversed approximately 20 minutes after intravenous administration (12).

Mivacurium has a similar onset time compared to atracurium and vecuronium whereas its clinical duration of effect is only half to one-third of the duration of intermediate-acting agents (13). This makes mivacurium the preferred agent for short surgery requiring muscle relaxation, such as bronchoscopy, gastroenteroscopy or adenoidectomy.

If there is a need to create excellent intubation conditions very quikly, then a technique that combines appropriate anesthetics and muscle relaxants should be used. This type of technique may consist of propofol, alfentanil or remifentanil and rocuronium. Excellent intubation conditions may be provided in less than 60 seconds when these agents are used optimally in infants and children (14).

Which muscle relaxant for maintenance of surgery?

Muscle relaxants were first introduced more than 50 years ago just to obtain muscle flaccidity for surgery. In modern general anesthesia, however, dosing of muscle relaxants may be better optimized according to the patient's needs. Today our knowledge of pharmacology is better and monitoring of muscle relaxant response has been greatly improved. Selection of the agent for repeated dosing should be based on consideration of the possible residual effects of muscle relaxants. Long-acting agents, like pancuronium, have a much higher incidence of residual curarization postoperatively than intermediate-acting agents in children (15). Therefore, the use of pancuronium has been limited to patients undergoing specific surgery (like cardiovascular surgery) or to patients on ventilator therapy postoperatively. Even if mivacurium is used following pancuronium at the end of surgery, the duration of muscle relaxation cannot be shortened (16).

All intermediate-acting muscle relaxants can be administered as repeated doses amounting to one-third of the intubation dose or as a continuous infusion. Since their hourly requirement to maintain a surgical muscle relaxation averages 1.5 times the intubation dose (Table 4), these incremental doses are required at approximately 10-15 min. intervals (1). However, repeated doses of muscle relaxants should not be administered based solely on the time but based on adequate monitoring of the response (17).

Monitoring the effect of muscle relaxants

Neuromuscular block can be monitored by several methods. A simple but rather inaccurate way is visual or tactile assessment of thumb, big toe or eyelid movement following controlled stimulation of the ulnar, posterior tibial or facial nerve, respectively. A much more accurate measurement consists of assessing the electromyographic (18), accelerographic (19) or mechanomyographic (20) response to nerve stimulation. These measurements can be carried out as part of routine patient monitoring (17).

The nerve stimuli can be delivered as single stimuli, train-of-four or double-burst series of stimuli, or as tetanic stimuli. In clinical practice, train-of-four and double-burst stimuli are used before intubation and during maintenance of anesthesia and recovery. Tetanic stimuli are used to assess a profound level of neuromuscular block at times when no response can be detected by other modes of stimuli (21).

A child should never be left with residual curarization in the recovery room. Hence, if non-depolarizing muscle relaxants are used, it is necessary to monitor the response carefully. As there are wide individual variations in responses to even constant doses of muscle relaxants, monitoring is mandatory. Accurate monitoring helps to determine the correct timing of intubation and to titrate the maintenance dose of muscle relaxants, and to affirm the need for and result of reversing residual neuromuscular block. Thus, adequate monitoring of the response to a muscle relaxant is one way to maintain a high clinical standard of pediatric anesthesia care.

References

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16. Erkola O, Rautoma P, Meretoja OA. Mivacurium when preceded by pancuronium becomes a long-acting muscle relaxant. Anesthesiology 1996; 84:562
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18. Kalli I. Effect of surface electrode positioning on the compound action potential evoked by ulnar nerve stimulation in anaestehtised infants and children. Br J Anesth 1989; 62:188
19. Meretoja OA et al. Comparison of thumb acceleration and thenar EMG in a pharmacodynamic study of alcuronium. Acta Anesthesiol Scand 1989; 39:545
20. McCluskey A et al. A comparison of acceleromyography and mechanomyography for determination of the dose-response curve of rocuronium in children. Anesthesia 1997; 52:345
21. Ridley SA, Hatch DJ. Post-tetanic count and profound neuromuscular blockade with atracurium infusion in pediatric patients. Br J Anesth 1988; 60:31

Table 1. Onset time of muscle relaxants in pediatric patients (1)

Table 2. ED95 dose of muscle relaxants in pediatric patients (1)

Table 3. Intubation dose and clinical duration of effect of muscle relaxants in children (1)

Table 4. Maintenance requirement of muscle relaxants in children (1)

Last updated: 1 July 2003Created
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