General Anesthetics

evolve.elsevier.com/Haveles/pharmacology

 

Reversible loss of consciousness and insensibility to painful stimuli.

Contemporary general anesthetic techniques use balanced anesthesia, which employs a combination of drugs to produce a reversible loss of consciousness and insensibility to painful stimuli while minimizing adverse reactions, taking into account the patient’s physical status and preanesthetic and postanesthetic needs.

 

Hospital: General anesthesia

Dental office: Conscious sedation

In oral health care, oral and maxillofacial surgeons use general anesthetic drugs in their offices. General dentists use nitrous oxide to provide conscious sedation and pain control. Although dental hygienists are not generally involved with the administration of general anesthesia, many states allow them to monitor and/or administer nitrous oxide for conscious sedation. In today’s practice, the dental hygienist should have an understanding of the principles of general anesthesia because it is an indispensable tool used to meet the needs of special patients and for extensive oral and maxillofacial surgery.

History

The original methods of producing general anesthesia involved either strangulation or cerebral concussion. Later, opium, belladonna, hemp, and alcohol were used to render patients unconscious. During this time, the operations were “quick and dirty.” Nitrous oxide was discovered in 1776, and 20 years later Sir Humphrey Davy suggested that the administration of nitrous oxide might be useful in surgery. About the middle of the 1800s, true general anesthetics were discovered in the United States. In 1846 surgically related mortality dropped dramatically with the introduction of ether. Today, anesthesiologists use a combination of inhaled and intravenous general anesthetics and adjuvant drugs to produce a generalized, reversible depression of the central nervous system (CNS) that results in a loss of consciousness, amnesia, and immobility but not necessarily complete analgesia.

Mechanism of action

Overview

Many theories have been proposed to explain the mechanism of action of the various general anesthetic agents, but unfortunately no theory does so completely. It may seem relatively simple to say that these drugs are CNS depressants. However, the way in which they depress the normal functions of the CNS is complicated by the lack of knowledge of the physiologic and biochemical events of arousal and unconsciousness. Proposed mechanisms for the action of different general anesthetics involve an increase in the threshold for firing, facilitation of inhibitory γ-aminobutyric acid (GABA), and a decrease in the duration of opening of nicotinic receptor-activated cation channels. The increase in the threshold or hyperpolarization is a result of the activation of the potassium channels.

Stages and Planes of Anesthesia

 

Guedel described four stages and planes of anesthesia.

The extent of CNS depression produced by general anesthetics must be carefully titrated to avoid excessive cardiorespiratory depression. In 1920, Guedel described a system of stages and planes to describe the effects of anesthetics (Table 10-1). Although Guedel’s classification applied to the effects produced by ether during the open drop method of administration, modern anesthetic techniques seldom show these exact stages. However, the four stages are briefly described here because Guedel’s terminology is still used to describe the depth of anesthesia.

Table 10-1

Stages and Planes of Anesthesia

Stage and Plane Patient Response
Stage I: Analgesia Patient is unresponsive
Reduced sensation to pain
Can still respond to commands
Reflexes are present
Regular respiration
Some amnesia
Loss of consciousness (end of stage)
Stage II: Delirium or excitement Unconsciousness
Amnesia
Involuntary movement and excitement
Irregular respiration
Increased muscle tone
Sympathetic stimulation: tachycardia, mydriasis, hypertension
Stage III: Surgical anesthesia Return to regular respiration, muscle relaxation, and normal heart and pulse rates
Divided into four planes
Stage IV: Respiratory or medullary paralysis Cessation of respiration
Subsequent circulatory failure
Respiration must be artificially maintained

Copyright 1996 Elena Bablenis Haveles.

Induction is the term used to refer to the quick change in the patient’s state of consciousness from stage I to stage III, as follows:

Stage I analgesia is characterized by the development of analgesia or reduced sensation to pain. The patient is conscious and can still respond to command. Reflexes are present, and respiration remains regular. Some amnesia may also be evident. Nitrous oxide, as used in the dental office, keeps the patient in stage I. The end of this stage is marked by the loss of consciousness.
Stage II delirium or excitement begins with unconsciousness and is associated with involuntary movement and excitement. Respiration becomes irregular, and muscle tone increases. Sympathetic stimulation produces tachycardia, mydriasis, and hypertension. This can be an uncomfortable time for the patient because emesis and incontinence can occur. As the depth of anesthesia increases, the patient begins to relax and proceeds to stage III. To ensure the patient’s comfort and safety, it is important to have a smooth and rapid induction. The ultrashort-acting barbiturates accomplish this readily. When balanced anesthesia is used, the patient does not pass through each stage as listed. Adjunct drugs reduce the side effects of each of the drugs used during surgery.
Stage III surgical anesthesia is the stage in which most major surgery is performed. This stage is further divided into four planes that are differentiated on the basis of eye movements, depth of respiration, and muscle relaxation. The onset of stage III (planes I and II) is typically characterized by the return of regular respiratory movements, muscle relaxation, and normal heart and pulse rates. Reflexes associated with the eye disappear during planes I and II. Vomiting reflex stops during stage II, but swallowing reflex is maintained until stage III, plane I. Plane III is associated with decreased skeletal muscle tone, dilated pupils, tachycardia, and hypotension. Beginning in plane III and progressing to plane IV, stage III is characterized by intercostal muscle paralysis (diaphragmatic breathing remains), absence of all reflexes, and extreme muscle flaccidity. If the depth of anesthesia is allowed to increase, the patient will rapidly progress to the last stage, with cessation of all respiration.
Stage IV respiratory or medullary paralysis is characterized by complete cessation of all respiration (diaphragmatic respiration is the last to go) and subsequent circulatory failure. At this point, pupils are maximally dilated and blood pressure falls rapidly. If this stage is not reversed immediately, the patient will die. Respiration must be artificially maintained.

Modern anesthetic techniques now use more rapidly acting agents than those associated with the four stages described by Guedel. Flagg’s approach, used to describe the levels of anesthesia (Figure 10-1), uses the following categories:

f10-01-9780323171113
Figure 10-1 Levels of anesthesia: induction, maintenance, and recovery.
Induction: The induction phase encompasses all the preparation and medication necessary for a patient up to the time the operation begins, including preoperative medications, adjunctive drugs for anesthesia, and anesthetics required for induction.
Maintenance: The maintenance phase begins with the patient at a depth of anesthesia sufficient to allow surgical manipulation and continues until completion of the procedure.
Recovery: The recovery phase begins with the termination of the surgical procedure and continues through the postoperative period until the patient is fully responsive to the environment.

Adverse reactions

 

Risk of general anesthesia must always be compared with the benefit of surgery.

The goals of surgical anesthesia are good patient control, adequate muscle relaxation, and pain relief. To produce anesthesia, potent CNS depressants are given in relatively high doses, and many combinations of drugs are used in balanced anesthesia. The hazards encountered with the administration of general anesthetics are summarized in Table 10-2.

Table 10-2

Adverse Reactions to General Anesthetics

System Effect/Comment*
Cardiovascular system Cardiovascular collapse
Cardiac arrest
Arrhythmias Ventricular fibrillation with halogenated hydrocarbons
Blood pressure Hypertension (stage II)
Hypotension
Respiration Depressed respiration (stage III)
Respiratory arrest (stage IV)
Laryngospasm with ultrashort-acting barbiturates
“Boardlike” chest with neuroleptanalgesia
Explosions/flammability Cyclopropane
Ether
Teratogenicity (either male or female exposure) Chronic exposure fetal abnormalities
Spontaneous abortions
Hepatotoxicity (repeated exposure) Operating room personnel
Halogenated hydrocarbons
Other Headache, fatigue, irritability, addicting

* Stage refers to Guedel’s stage of anesthesia.

General anesthetics

Classification of Anesthetic Agents

The general anesthetic agents can be classified according to their chemical structures or routes of administration. Table 10-3 categorizes the agents according to their routes of administration.

Table 10-3

Classification of General Anesthetics by Route of Administration

t0020

* Of historic interest only.

Induction Anesthesia

Intravenous Anesthetics

The intravenously administered general anesthetics are a diverse group of CNS depressants that include the opioids, the ultrashort-acting barbiturates, and the benzodiazepines. Although most injectable general anesthetics are administered intravenously, one agent, ketamine, can also be given intramuscularly. These drugs find their greatest utility in induction of general anesthesia but may occasionally be used as single agents for short procedures. Although they offer the advantage of convenience, the depth and duration of anesthesia are less easily controlled with them than with the inhalation agents.

Ultrashort-Acting Barbiturates

The ultrashort-acting barbiturates used include methohexital (meth-oh-HEX-i-tal) sodium (Brevital), thiopental (thye-oh-PEN-tal) sodium (Pentothal), and thiamylal (thye-AM-i-lal) sodium (Surital). These ultrashort-acting agents have a rapid onset of action (about 30-40 seconds) when given intravenously. They are highly lipid soluble, and repeated dosing during anesthesia can result in a prolonged recovery period. However, blood and brain levels of these agents quickly decrease as the drugs redistribute to lean tissues (skeletal muscle).

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Apr 12, 2015 | Posted by in Dental Hygiene | Comments Off on General Anesthetics
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