CNS Depressants - Pharmacology

Central nervous system depressants are medications that suppress the transmission of information throughout the central nervous system. There are seven broad classifications of central nervous system depressants. These are sedative-hypnotics, general and local anesthetics (discussed later in this chapter), analgesics, narcotic analgesics (Chapter (Narcotic Agonists) ), anticonvulsants, antipsychotics, and antidepressants (discussed later in this chapter)

SEDATIVE-HYPNOTICS

Sedative-hypnotics are commonly referred to as sedatives and are the mildest form of central nervous system depressant. Sedative-hypnotics are given in low doses to diminish the patient’s physical and mental responses without affecting the patient’s consciousness.

With increased doses, the patient experiences a hypnotic effect causing the patient to fall asleep. Even higher doses of sedative-hypnotics anesthetize the patient. Such is the case of the ultra-short-acting barbiturate thiopental sodium (Pentothal) that produces anesthesia.

Sedative-hypnotics and barbituates were first used to reduce tension and anxiety. However, other medications have been developed for this use. Chronic use of any sedative-hypnotic should be avoided.

It is important to understand that sedative-hypnotics are not the same as sleep medications purchased over-the-counter such as Nytol, Sominex, Sleep-eze, and Tylenol PM. Over-the-counter sleep medications such as diphenhydramine contain an antihistamine not barbiturates to achieve sedation.

Short-acting sedative-hypnotics are ideal for patients who need assistance falling asleep but who must awaken early without experiencing a lingering aftereffect from the medication. Intermediate-acting sedative-hypnotics are useful to sustain sleep. Patients may experience residual drowsiness (hangover) after awakening.

The use of sedative-hypnotics for sleep (hypnotic) should be short term or there is a chance that the patient could become dependent on the medication or develop a tolerance. Patients who take high doses of sedative-hypnotics over long periods must gradually discontinue the medication rather than abruptly stopping the drug which can cause withdrawal symptoms. Sedative-hypnotics should not be administered to patients who have severe respiratory disorders or who are pregnant.

Before a patient is prescribed a sedative-hypnotic to aid with sleep, the patient should try non-pharmacological methods that promote sleep such as:

  • Arise at a specific hour in the morning.
  • Take few or no daytime naps.
  • Avoid heavy meals or strenuous exercise before bedtime.
  • Take a warm bath, read, or listen to music before bedtime.
  • Decrease exposure to loud noises.
  • Avoid watching disturbing television before sleep.
  • Avoid drinking a lot of fluids before sleep.
  • Drink warm milk before sleep.

BARBITURATES

Barbiturates are a type of sedative-hypnotic that is used to induce sleep, as an anesthetic, and in high doses to control epileptic seizures. Barbiturates are classified by duration of action referred to as ultrashort-acting, short-acting, intermediate acting, and long-acting.

Ultrashort-acting barbiturates such as thiopental sodium (Pentothal) is a commonly used anesthetic. Secobarbital (Seconal) and pentobarbital (Nembutal) are short-acting barbiturates that induce sleep. For longer periods of sleep, patients are prescribed intermediate acting such as amobarbital (Amytal), aprobarbital (Alurate) and bubatabarbital (Butisol). Phenobarbital and mephobarbital are long-acting barbiturates used for controlling epileptic seizures.

Barbiturates are Class II Controlled Substances and should be prescribed for no more than two weeks because of the adverse side effect. Barbiturates increase CNS depression in the elderly and should not be used for sleep.

Other-Barbiturates

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ANESTHETIC AGENTS

Anesthetic agents depress the central nervous system causing a loss of con-sciousness. They are classified as general and local.

Anesthetic agents were introduced in surgery in the early 1800s in the form of nitrous oxide (laughing gas), which continues to be used today for dental pro-cedures. Other anesthetic agents became widely used by the mid-1800s. These included ether and chloroform. Ether is a highly flammable liquid with a pungent odor that causes nausea and vomiting and is seldom used today. Chloroform is toxic to the liver and is no longer used.

General anesthetics are used for general surgery, cardiac surgery, neurosurgery, and pediatric surgery. They are administered by an anesthesiologist or a nurse anesthetist. They are inhaled through a mask or breathing tube.

A general anesthetic can consist of one medication or a combination of med-ications called balanced anesthesia depending on the patient’s age, weight, medical history, general health, and allergies.

This balanced approach is used when administering general anesthetics to patients in phases to minimize cardiovascular problems, decrease the amount of general anesthetic needed, reduce possible post-anesthetic nausea and vomiting, minimize the disturbance of organ function, and increase recovery from anes-thesia with fewer adverse reactions.

The night before the surgery, the patient is given a hypnotic to assist with a good night’s sleep. On the day of the surgery, premedication may be given to the patient about one hour before surgery. Premedication typically consists of two medications. One is a benzodiazepine such as lorazepan (Ativan). This medication sedates and decreases anxiety. The other is an anticholinergic such as atropine to decrease secretions. A short-acting barbiturate such as thiopental sodium (Pentothal) is then administered in the operating room to induce anesthesia. The patient is then given inhaled gas and oxygen to maintain anesthesia. Sometimes the anesthetic is administered IV. Depending on the nature of the operation, the patient may also receive a muscle relaxant.

The patient experiences four stages of anesthesia, some of which are not observable because they occur rapidly. These stages are:

Stage one: analgesia

The patient experiences analgesia (a loss of pain sensation) but remains conscious and can carry on a conversation.

Stage two: excitement

The patient may experience delirium or become violent. Blood pressure rises and becomes irregular, and breathing rate increases. This stage is typically bypassed by administering a barbiturate such as sodium pentothal before the anesthesia.

Stage three: surgical anesthesia

Skeletal muscles relax. Breathing becomes regular. Eye movement slows then stops. It is at this point when surgery begins.

Stage four: medullary paralysis

Breathing and other vital functions cease to function because the respiratory center (medulla oblongata) is paralyzed. Death results if the patient is not revived quickly. Careful administration of the anesthesia prevents reaching this stage.

Commonly-administered-intravenous-anesthetic-agents

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TOPICAL ANESTHETIC AGENTS

Topical anesthetic agents (see chart) are solutions, liquid sprays, ointments, creams, and gels that are applied to mucous membranes, broken or unbroken skin surfaces, and burns to decrease the sensitivity of nerve endings in the affected area.

The first topical anesthetic agent was TAC, which is a combination of tetracaine, adrenaline (epinephrine), and cocaine, and was used for face and scalp lacerations. A version of TAC called LET is used today. LET is a combination of lidocaine, epinephrine, and tetracaine. Lidocaine replaced cocaine. LET gel isgenerally preferred over TAC as analgesia for skin that is not intact because LET has a superior safety record and is more cost-effective than TAC.

Perhaps the most well known topical anesthetic for intact skin is EMLA. EMLAis commonly used to anesthetize skin before IM injections, venipuncture, and simple skin procedures such as curettage or biopsy. EMLA is most effective if administered 90 minutes before the procedure.

ELA-Max (4% Liposomal Lidocaine) is another topical anesthetic for intact skin that works faster than EMLA. EL-Max is an over-the-counter medication that uses the liposomal delivery system. Liposomes are tiny lipid (fat) balls that deliver moisture to the skin. Because of their small molecular size, they are able to penetrate the cell wall reasonably well and can be used to deliver medications.

Other topical anesthetics can be delivered using iontophoresisa therapy that uses a local electric current to introduce the ions of a medicine into the tissues and anesthetic patch.

Topical-Anesthetic-Agents

Other-Local-Anesthetic-Agents

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LOCAL ANESTHESIA

A local anesthetic (see chart) blocks pain at the site where the medication is administered without affecting the patient’s consciousness. It is commonly used for dental procedures, suturing of skin lacerations, short-term surgery at a localized area, spinal anesthesia by blocking nerve impulses (nerve block) below the insertion of the anesthetic, and diagnostic procedures such as lumbar punctures.

Local anesthetics are divided into two groups according to their basic chemical structure. These are esters and amides. An ester is a chemical compound formed from the reaction between an acid and an alcohol. Amides are an organic chemical compound formed by reaction of an acid chloride, acid anhydride, or ester with an amine. Amides have a lower incidence of causing an allergic reaction than esters.

SPINAL ANESTHESIA

Spinal anesthesia is a local anesthetic injected into the spinal column in the third or fourth lumbar space to produce a regional neural block. If it is given too high, the respiratory muscles could be affected and respiratory distress or failure could result. There are 4 types of spinal anesthesia: subarachnoid block, epidural block, the saddle block, and a caudal block.

A subarachnoid block is the injection into the subarachnoid space in the third or fourth lumbar space to produce anesthesia.

The epidural block occurs when the anesthetic is injected into the outer covering (dura mater) of the spinal cord near the sacrum.

The saddle block is given at the lower end of the spinal column to block the perineal area for procedures such as childbirth.

The caudal block is placed near the sacrum.

The patient may experience headaches and hypotension as a result of these procedures because of a change in cerebrospinal fluid pressure when the needle is inserted into the spine. The patient should remain in the supine position following the procedure and increase fluid intake.


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