McKenna's Pharmacology for Nursing, 2e

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P A R T 4  Drugs acting on the central and peripheral nervous systems

Extreme caution is necessary in the presence of genetic or disease-related conditions causing low plasma cholinesterase levels, such as cirrhosis, metabolic dis- orders, carcinoma, burns, dehydration, malnutrition, hyperpyrexia, thyroid toxicosis, collagen diseases and exposure to neurotoxic insecticides. Low plasma cholinesterase levels may result in a very prolonged paralysis because suxamethonium is not broken down in the plasma and continues to stimulate the receptor site, leading to a need for prolonged support after use of the drug is discontinued. Adverse effects The adverse effects of suxamethonium are the same as those for non-depolarising NMJ blockers. In addition, suxamethonium is associated with muscle pain related to the initial muscle contraction reaction. A non- depolarising NMJ blocker may be given first to prevent some of these contractions and the associated discom- fort. Aspirin also alleviates much of this pain after the procedure. Malignant hyperthermia, which may occur in susceptible individuals, is a very serious con- dition characterised by massive muscle contraction, sharply elevated body temperature, severe acidosis and, if uncontrolled, death. This reaction is most likely with suxamethonium, and treatment involves dantrolene (see Chapter 25) to inhibit the muscle effects of the NMJ blocker. Clinically important drug–drug interactions Potential drug–drug interactions for suxamethonium are the same as for the non-depolarising NMJ blockers. Prototype summary: Suxamethonium Indications: As an adjunct to general anaesthesia; to facilitate endotracheal intubation; to induce skeletal muscle relaxation during surgery or mechanical ventilation. Actions: Combines with ACh receptors at the motor endplate to produce depolarisation; this inhibits neuromuscular transmission, causing a flaccid paralysis. Pharmacokinetics: Route Onset Duration IV 30–60 secs 4–6 mins T 1/2 : 2 to 3 minutes; metabolised in the tissues, excreted unchanged in the urine. Adverse effects: Muscle pain, related to the contraction of the muscles as a first reaction; respiratory depression, apnoea.

Cultural considerations

BOX 28.2

in the cell membrane close. Certain ethnic groups may have a genetic predisposition for a prolongation of paral- ysis (Box 28.2). Pharmacokinetics Suxamethonium, like the non-depolarising NMJ blockers, is metabolised in the serum, although metab- olism is dependent on the liver to produce the needed plasma cholinesterases. Individuals with hepatic impair- ment may experience prolonged effects of this drug. Onset of action is usually within 1 minute, and duration of effect is 10 to 12 minutes. Most of the metabolites are excreted in the urine. Individuals with renal impairment may be at risk for increased toxicity from the drugs. Suxamethonium crosses the placenta. Effects on breast- feeding are not known. Contraindications and cautions The contraindications and cautions for suxamethonium are the same as for non-depolarising NMJ blockers. Suxamethonium is often used during caesarean sections, but accurate timing is necessary to prevent serious effects on the fetus. In addition, suxamethonium should be used with caution in individuals with fractures because the muscle contractions it causes might lead to additional trauma ; in individuals with narrow-angle glaucoma or penetrating eye injuries because intraocular pressure increases ; and in individuals with paraplegia or spinal cord injuries, which could cause loss of potassium from the overstimulated cells and hyperkalaemia. hyperpyrexia, thyrotoxicosis, collagen diseases and exposure to neurotoxic insecticides. If plasma cholinesterase levels are low, the serum levels of suxamethonium remain elevated and the paralysis can last much longer than anticipated.These people need support and ventilation for long periods after surgery. There is also a genetic predisposition to low plasma cholinesterase levels. People should be asked whether they or any family member has a history of either low plasma cholinesterase levels or prolonged recovery from anaesthetics. Alaskan Eskimos belong to such a genetic group, and they are especially likely to suffer prolonged paralysis and inability to breathe for several hours after suxamethonium has been used for surgery. If there is no other drug of choice for these people, special care must be taken to monitor their response and ensure their breathing for an extended postoperative period. Suxamethonium and paralysis Suxamethonium is broken down in the body by cholinesterase, an enzyme found in the plasma. Several conditions may cause the body to produce less of this enzyme, including cirrhosis, metabolic disorders, carcinoma, burns, dehydration, malnutrition,