Chapter 13: Central Nervous System Depressants and Muscle Relaxants
Lilley’s Pharmacology for Canadian Health Care Practice, 4th Canadian Edition
Chapter Summary
ANATOMY, PHYSIOLOGY, AND PATHOPHYSIOLOGY OVERVIEW
Sedatives and hypnotics are drugs that have a calming effect or that depress the central nervous system (CNS).
A drug is classified as either a sedative or a hypnotic drug depending on the degree to which it inhibits the transmission of nerve impulses to the CNS.
Sedatives reduce nervousness, excitability, and irritability without causing sleep, but a sedative can become a hypnotic if it is given in large enough doses.
Hypnotics cause sleep and have a much more potent effect on the CNS than do sedatives.
Many drugs can act as either a sedative or a hypnotic, depending on dose and patient responsiveness, and for this reason are called sedative–hypnotics. Sedative–hypnotics can be classified chemically into three main groups: (1) barbiturates, (2) benzodiazepines, and (3) miscellaneous drugs.
Physiology of Sleep
Sleep is defined as a transient, reversible, and periodic state of rest in which there is a decrease in physical activity and consciousness. Normal sleep is cyclic and repetitive, and a person’s responses to sensory stimuli are markedly reduced during sleep.
Sleep research involves study of the patterns of sleep, or what is sometimes referred to as sleep architecture, consisting of two basic elements that occur cyclically: (1) rapid eye movement (REM) sleep and (2) non-REM sleep.
Prolonged sedative–hypnotic use may reduce the cumulative amount of REM sleep; this is known as REM interference. This can result in daytime fatigue because REM sleep provides a certain component of the “restfulness” of sleep.
Upon discontinuance of a sedative–hypnotic drug, REM rebound can occur in which the patient has an abnormally large amount of REM sleep, often leading to frequent and vivid dreams.
Misuse of sedative–hypnotic drugs is common.
PHARMACOLOGY OVERVIEW
Benzodiazepines and Miscellaneous Hypnotic Drugs
Benzodiazepines were once the most commonly prescribed sedative–hypnotic drugs; however, the nonbenzodiazepine drugs are now more frequently prescribed.
The sedative and hypnotic action of benzodiazepines is related to their ability to depress activity in the CNS.
Benzodiazepines are commonly used for sedation, relief of agitation or anxiety, treatment of anxiety-related depression, sleep induction, skeletal muscle relaxation, and treatment of acute seizure disorders.
Contraindications to the use of benzodiazepines include known drug allergy, narrow-angle glaucoma, and pregnancy.
Benzodiazepines have a relatively favourable adverse effect profile; however, they can be harmful if given in excessive doses or when mixed with alcohol. Adverse effects associated with their use usually involve the CNS. Commonly reported undesirable effects are headache, drowsiness, paradoxical excitement or nervousness, dizziness or vertigo, cognitive impairment, and lethargy.
Most sedative–hypnotic drugs suppress REM sleep and should be used only for the recommended period of time. This time frame varies, depending on the specific drug used.
An overdose of benzodiazepines may result in one or all of the following symptoms: somnolence, confusion, coma, and diminished reflexes.
Benzodiazepines and miscellaneous sedative–hypnotic drugs are prescription-only drugs, and they are designated as Schedule IV controlled substances.
Barbiturates
Barbiturates were the standard drugs for treating insomnia and producing sedation. Only a few are in use today (and rarely prescribed) due to the favourable safety profile and efficacy of the benzodiazepines.
Barbiturates are CNS depressants that act primarily on the brainstem in an area called the reticular formation.
Short-acting barbiturates include pentobarbital sodium and secobarbital. Intermediate-acting barbiturates include amobarbital, aprobarbital, and butabarbital.
The pharmacokinetics of each group of barbiturates lends specific characteristics to the drugs. Health care providers administering them must understand their life-threatening potential because too rapid an infusion may precipitate respiratory or cardiac arrest.
Nursing interventions for barbiturates include careful consideration of parenteral administration. Knowledge about incompatibilities with other drugs in solution as well as dilutional fluid incompatibilities is important for safe administration.
Barbiturates can produce many unwanted adverse effects. They are physiologically habit forming and have a low therapeutic index.
The sedative and hypnotic effects of barbiturates are dose related, and they act by reducing the nerve impulses travelling to the cerebral cortex.
All barbiturates have the same sedative–hypnotic effects but differ in their potency, time to onset of action, and duration of action. They can be used as hypnotics, sedatives, and anticonvulsants and for anaesthesia during surgical procedures. It is important to note that the use of barbiturates is no longer recommended for sleep induction.
Contraindications to barbiturate use include known drug allergy, pregnancy, significant respiratory difficulties, and severe kidney or liver disease.
Barbiturates stimulate the action of enzymes in the liver responsible for the metabolism or breakdown of many drugs. By stimulating the action of these enzymes, they cause many drugs to be metabolized more quickly, which usually shortens their duration of action.
Adverse effects of barbiturates relate to the CNS and include drowsiness, lethargy, dizziness, hangover, and paradoxical restlessness or excitement.
Phenobarbital is also used to treat status epilepticus (prolonged uncontrolled seizures). In extreme cases, patients may be intentionally overdosed to the extent of causing therapeutic phenobarbital or pentobarbital coma.
An overdose of barbiturates produces CNS depression ranging from sleep to profound coma and death.
Muscle Relaxants
A variety of conditions, such as trauma, inflammation, anxiety, and pain, can be associated with acute muscle spasms; muscle relaxant drugs are capable of providing some relief.
Muscle relaxants are a group of compounds that act predominantly within the CNS to relieve pain associated with skeletal muscle spasms. Most muscle relaxants are known as centrally acting skeletal muscle relaxants because their site of action is the CNS.
Muscle relaxants are most effective when they are used in conjunction with rest and physical therapy. When taken with alcohol, other CNS depressants, or opioid analgesics, enhanced CNS depressant effects are seen.
Muscle relaxants are often used for the treatment of muscle spasms, spasticity, and rigidity. They result in varying levels of decreased sensorium and CNS depression, depending on the drug, dosage, and route of administration. They are not used as sedative–hypnotic drugs.
The primary adverse effects of muscle relaxants are an extension of their effects on the CNS and skeletal muscles. Euphoria, lightheadedness, dizziness, drowsiness, fatigue, confusion, and muscle weakness are often experienced early in treatment.
The toxicities and consequences of an overdose of muscle relaxants primarily involve the CNS. There is no specific antidote (or reversal drug) for muscle relaxant overdoses.
NURSING PROCESS
Assessment
Before administering any CNS depressant drug, perform an assessment focusing on insomnia; vital signs; results of a head-to-toe physical examination for baseline comparisons; neurological findings; miscellaneous information such as medical history; allergies; history of smoking and alcohol use; caffeine intake; past and current medication profile, including use of any prescription drugs, over-the-counter drugs, and herbals; alternative or folk practices; and any changes in health status, weight, nutrition, exercise, life stressors, or lifestyle.
For muscle relaxants, always note drug allergies before use, and perform a complete head-to-toe assessment with focus on the neurological system. In older adults, there is increased risk of CNS toxicity, with possible hallucinations, confusion, and excessive sedation.
Implementation
The miscellaneous drug ramelteon is a newer medication that is used for insomnia but is not associated with CNS depression, does not carry the potential for misuse or dependence, and does not lead to withdrawal symptoms when treatment stops. Therefore, this drug can be used for patients who are likely to misuse CNS depressants.
Patients taking benzodiazepines and other CNS depressants experience sedation and possible ataxia, hence the need for patient safety measures.
Before giving any CNS depressant, it is always important to try nonpharmacological measures to induce sleep.
Evaluation
If medication therapy is indicated, preventing respiratory depression and other problems associated with CNS depression is of prime importance, as is maintaining patient safety and preventing injury.
Some of the criteria by which to confirm a patient’s therapeutic response to a CNS depressant include the following: an increased ability to sleep at night, fewer awakenings, shorter sleep induction time, few adverse effects such as hangover effects, and an improved sense of well-being because of improved sleep.
Therapeutic effects related to muscle relaxants include decreased spasticity, reduction of choreiform movements in Huntington’s chorea, decreased rigidity in parkinsonian syndrome, and relief of pain from trigeminal neuralgia.