Neuromuscular blocking drugs in pediatric anesthesia

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AGE-RELATED DIFFERENCES IN POTENCY

The potency of a neuromuscular blocker is defined for clinical purposes in terms of the ED95, the dose in milligrams per kilogram that produces an average block of 95% in neuromuscular function of the ulnar nerve-adductor pollicis. In many patients, the average maximal effect is that the evoked function of the thumb is only 5% of what it was at baseline after this dose of drug. Because of interpatient variability in effect and the desire to block the diaphragm as well as muscles in the

DURATION OF ACTION

How long a dose of drug lasts varies with the clearance of that drug and the magnitude of the dose, as well as the age of the patient. After 0.15 mg/kg of cisatracurium, for example, duration of block was longer in younger patients. An average of 43 minutes plus or minus a standard deviation (SD) of 6 minutes passed before return of four twitches in infants, whereas recovery to the same level of function took only an average of 36±5 minutes in children during nitrous oxide-narcotic anesthesia.50

KINETICS AND DYNAMICS

If a drug has a linear plasma decay rather than bi-exponential or tri-exponential decline in plasma concentration, then doubling the dose administered results in increased duration of action by approximately the elimination half-life of the drug (Table 3). Increasing the dose of cisatracurium from 0.1 to 0.2 or from 0.2 to 0.4 mg/kg, therefore, in creases the duration of action by about 20 minutes. Doubling the dose of mivacurium in children increases its duration of action by less than 5

INTRAMUSCULAR INJECTION

There is interest in the effects of intramuscular administration of neuromuscular blockers in pediatric patients because establishment of an intravenous catheter can be difficult in these patients. If intravenous access is available, it is preferable to administer blockers intravenously, which eliminates variability caused by absorption into the blood. Several blockers have been studied after intramuscular administration. The bioavailability of intramuscular rapacuronium is 56% and peak plasma

Depolarizing Effects of Succinylcholine

Although succinylcholine has been used since its introduction to “break” laryngospasm in pediatric patients, it should be remembered that this neuromuscular blocker has many adverse side effects related to its depolarizing action. In some patients, succinylcholine produces noticeable increased tension in the masseter, thereby closing the mouth and making laryngoscopy more difficult. In the extreme case, this is a contracture, known as masseter muscle spasm.19, 55 Two conditions—fever49 and

DISEASE STATES

There has been increasing documentation of the effects of neuromuscular blockers in the presence of disease. It might be expected that resistance to nondepolarizing neuromuscular blockers would occur in patients who have been burned, for example. Decreased plasma cholinesterase activity resulting from the metabolic effects of burn injury, however, is associated with a delay in recovery of up to 25% of baseline function in patients with burns of greater than 30% of body surface area.35 In

REVERSAL

After administration of a nondepolarizing neuromuscular blocker it is necessary to consider whether or not the administration of anticholinesterase is warranted prior to awakening the patient. In general, children recover from blockers more rapidly than adults, so residual block is less of a problem in children than adults. This does not imply that residual block cannot occur or that it is not important when it does occur in a child. There is no clinical advantage in attempting to reverse

INTENSIVE CARE UNIT SETTING

There is little agreement between clinical observations of movement and the results of peripheral nerve stimulation using a TOF (2 Hz for 2 seconds) stimulation in pediatric intensive care unit patients.40 This is no surprise, because several studies have documented that the diaphragm is resistant to neuromuscular blockers compared with the hand and that the muscles innervated by the facial nerve are also resistant compared with the hand or foot. Lack of movement of the thumb or first toe in

SUMMARY

There are differences between pediatric patients and adults in the responses to neuromuscular blockers. The results of new studies of neuromuscular blockers in pediatric patients are generally consistent with earlier studies in the type of age-related differences documented. Surprising new results are the side effects, including anaphylaxis, that have been observed, and studied directly, in pediatric patients. Continued evaluation of these drugs in pediatric patients will provide better

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