McKenna's Pharmacology for Nursing, 2e

435

C H A P T E R 2 8 Neuromuscular junction blocking agents

Drug therapy across the lifespan

BOX 28.1

NMJ-blocking agents CHILDREN

contraction does not occur. Because they are not broken down by acetylcholinesterase, their effect is longer lasting than that of ACh. The non-depolarising NMJ blockers are hydrophilic instead of lipophilic, so they do not readily cross the blood–brain barrier and have little effect on the ACh receptors in the brain. Non-depolarising NMJs are metabolised in the serum, although metabolism is dependent on the liver to produce the needed plasma cholinesterases. Most of the metabolites are excreted in the urine. Each non-depolarising NMJ blocker differs in terms of time of onset and duration (Figure 28.3). The drug of choice in any given situation is determined by the pro- cedure being performed, including the estimated time involved. Contraindications and cautions Non-depolarising NMJ blockers are contraindicated in the following conditions: known allergy to any of these drugs to prevent hypersensitivity reactions ; myas- thenia gravis because blocking of the ACh cholinergic receptors aggravates the neuromuscular disease (which results from destruction of the ACh-receptor sites) and increases the muscular effects (see Chapter 32); renal or hepatic disease, which could interfere with the metabo- lism or excretion of these drugs, leading to toxic effects ; and pregnancy. Caution should be used in people with any family or personal history of malignant hyperthermia , a serious adverse effect associated with these drugs that is charac- terised by extreme muscle rigidity, severe hyperpyrexia (fever), acidosis and death in some cases, because malig- nant hyperthermia can occur with the use of these drugs. Caution should also be used in the following circumstances: pulmonary or cardiovascular dysfunc- tion, which could be exacerbated by the paralysis of the respiratory muscles and resulting changes in perfusion and respiratory function ; altered fluid and electrolyte Because older people often also have renal or hepatic impairment, they are more likely to have toxic levels of the drug related to changes in metabolism and excretion. The older person should receive special efforts to provide skin care to prevent skin breakdown, which is more likely with older skin.The older person may require longer monitoring and regular orienting and reassuring. to be told that they will experience muscle pain and discomfort when the procedure is over. PREGNANCY AND BREASTFEEDING The NMJs are used during pregnancy and breastfeeding only if the benefit to the mother outweighs the potential risk to the fetus or neonate. OLDER ADULTS

gastric acid if the meat were eaten raw, it was safe for humans. Curare was first purified for clinical use as the NMJ blocker tubocurarine, which has since been replaced with more refined drugs that can control onset and duration of effect. Non-depolarising NMJ blockers include atracurium ( Tracrium ), cisatracurium ( Nimbex ), mivacurium ( Mivacron ), pancuronium (generic), rocu- ronium ( Esmeron ) and vecuronium ( Norcuron, Vecure ). Therapeutic actions and indications Non-depolarising NMJ blockers are used when clinical situations require or desire muscle paralysis (see Table 28.1 for preferred uses). Therapeutically, non- depolarising NMJ blockers: • Serve as an adjunct to general anaesthetics during surgery when reflex muscle movement could interfere with the surgical procedure or the delivery of gas anaesthesia. • Facilitate mechanical intubation by preventing resistance to passing of the endotracheal tube and in situations in which people “fight” or resist the respirator. • Facilitate various endoscopic diagnostic procedures when reflex muscle reaction could interfere with the procedure. • Facilitate electroconvulsive therapy when intense skeletal muscle contractions as a result of electric shock could cause the person broken bones or other injury. Pharmacokinetics All non-depolarising NMJ blockers are similar in struc- ture to ACh and compete with ACh for the muscle Ach-receptor site (Figure 28.2). As a result, they occupy the muscular cholinergic receptor site and do not allow stimulation to occur. These agents do not cause the activation of muscle cells, and consequently muscle Children require very careful monitoring and support after the use of NMJ blockers.These agents are used by anaesthetists who are skilled in their use and with full support services available. The non-depolarising NMJs are preferable because of the lack of muscle contraction with its resultant discomfort on recovery. Suxamethonium is usually preferred when a very short-acting, rapid-onset blocker is needed (e.g. for intubation). ADULTS Adults need to be monitored closely for full return of muscle function. If suxamethonium is used, they need