copyright © 2013, 2010 by saunders, an imprint of elsevier inc. chapter 27 general anesthetics

Copyright © 2013, 2010 by Saunders, an imprint of Elsevier Inc.

Chapter 27

General Anesthetics

2Copyright © 2013, 2010 by Saunders, an imprint of Elsevier Inc.

General Anesthetics

General anesthetics are drugs that produce unconsciousness and lack of responsiveness to all painful stimuli

Local anesthetics do not reduce consciousness, and they blunt pain only in a limited area


Pain

Analgesia Loss of sensibility to pain

Anesthesia Loss of pain and loss of all other sensations


General Anesthetics

Two main groups Inhalation anesthetics Intravenous anesthetics

Before 1846 Surgery: brutal and excruciatingly painful Strong people and straps used to restrain patient Survival based on surgeon’s speed


General Anesthetics

Given only by licensed anesthesiologists (physicians) and CRNAs (nurses)

Used to facilitate certain procedures Endoscopy, urologic procedures, radiation therapy,

electroconvulsive therapy, transbronchial biopsy, various cardiologic procedures


Basic pharmacology of inhalation anesthetics Properties of individual inhalation anesthetics Intravenous anesthetics

General Anesthetics


Basic Pharmacology of Inhalation Anesthetics

Properties of an ideal inhalation anesthetic Balance anesthesia Molecular mechanism of action Minimum alveolar concentration


Pharmacokinetics

Uptake and distribution Uptake

• From the lungs• The greater the concentration, the more rapid the uptake

Distribution• To CNS and other tissues• Determined largely by regional blood flow

Elimination Exported in the expired breath

• Inhalation anesthetics are almost entirely eliminated by the lungs

Metabolism• Hepatic metabolism is minimal


Pharmacokinetics

Adverse effects Respiratory and cardiac depression Sensitization of the heart to catecholamines Malignant hyperthermia Aspiration of gastric contents Toxicity to operating room personnel Hepatotoxicity


Drug Interactions

Analgesics, CNS depressants, and CNS stimulants can influence the amount of anesthetic required to produce anesthesia

Opioids allow for a reduction CNS depressants add to the depressant

effects of anesthesia


Adjuncts to Inhalation Anesthesia

Preanesthetic medications Given to reduce anxiety, produce perioperative

amnesia, and relieve preoperative and postoperative pain• Benzodiazepines• Opioids• Clonidine, an alpha2-adrenergic agonist• Anticholinergic drugs


Neuromuscular Blocking Agents

Surgical procedures require skeletal muscle relaxation

NBAs reduce amount of anesthesia needed NBAs prevent contraction of all skeletal

muscles, including diaphragm and muscles of respiration (flaccid paralysis)

21 million patients undergo surgery 20,000–40,000 wake up during the procedure


Postanesthetic Medications

Analgesics Mild pain: aspirin-like drugs Severe pain: opioids

Antiemetics Ondansetron (Zofran)

Muscarinic antagonists Abdominal distention and urinary retention Bethanechol


Properties of Individual Inhalation Anesthetics

Halothane (no longer available in United States)

Isoflurane Enflurane Desflurane Sevoflurane Nitrous oxide Obsolete inhalation anesthetics


Halothane

Prototype for volatile inhalation anesthetics No longer available in United States

Anesthetic properties High-potency anesthetic Time course: smooth and relatively rapid Weak analgesia Muscle relaxation generally inadequate for surgery


Halothane Adverse effects

Hypotension Respiratory depression

• Requires support with oxygen-rich gas mixture Promotion of dysrhythmias

• Epinephrine and catecholamines should be used with caution. Malignant hyperthermia

• Genetically predisposed Hepatotoxicity

• Rare Other adverse effects

Elimination 60%–80% by lungs and 20% hepatic


Isoflurane

Most widely used inhalation anesthetic Properties much like those of halothane Better muscle relaxant, but still requires NMB Not associated with renal or hepatic toxicity


Enflurane

Has pharmacologic properties very similar to those of halothane

Newer agents now preferred


Desflurane (Suprane)

Nearly identical in structure to isoflurane Induction occurs more rapidly than with any

other volatile anesthetics


Sevoflurane (Ultane)

Similar to desflurane Approved for induction and maintenance


Nitrous Oxide

Also known as “laughing gas” Very low anesthetic potency Very high analgesic potency Never used as primary anesthetic Frequently combined with other inhalation

agents to enhance analgesia 20% nitrous oxide = Pain relief of morphine No serious side effects (nausea and vomiting)


Obsolete Inhalation Anesthetics

Ethylene Cyclopropane Diethyl ether (ether) Vinyl ether Ethyl chloride Abandoned because they are explosive and

offer no advantage over newer anesthetics


Intravenous Anesthetics

Short-acting barbiturates (thiobarbiturates) Benzodiazepines Propofol Etomidate Ketamine Neuroleptic-opioid combination: droperidol

plus fentanyl


Short-Acting Barbiturates (Thiobarbiturates)

Administered intravenously Used for induction of anesthesia Two agents

Thiopental sodium (Pentothal) Methohexital sodium (Brevital)


Benzodiazepines

Diazepam Induction with intravenous diazepam (Valium) Unconsciousness within a minute Very little muscle relaxation

Midazolam Unconsciousness within 80 seconds Can cause dangerous cardiorespiratory effects


Propofol

Most widely used anesthetic Actions and uses

Unconsciousness develops within 60 seconds and lasts 3–5 minutes

Sedative-hypnotic for induction and maintenance of analgesia• Mechanical ventilation and procedures

Adverse effects Can cause profound respiratory depression Can cause hypotension Risk of bacterial infection


Propofol

Risks for abuse Not a controlled substance Supplies are not closely monitored Widely available in operating rooms, etc. No “high” Instantaneous but brief sleep period Patients awaken “refreshed” and talkative, and

report feeling elated and even euphoric


Fospropofol

Fospropofol (Lusedra) Intravenous prodrug Conversion to propofol in the lever Effects similar to propofol Slower onset of sedation (4 min vs. 1 min) Lower risk of bacteremia Schedule IV drug


Etomidate

Potent hypnotic agent Used for induction Repeated administration can cause

hypotension, oliguria, electrolyte disturbances, and high incidence of postoperative nausea and vomiting


Ketamine

Anesthetic effects Dissociative anesthesia Sedation, immobility, analgesia, and amnesia

Adverse psychologic reactions Hallucinations, disturbing dreams, and delirium Soothing environment

Therapeutic uses Anesthesia for young children with minor

procedures


Neuroleptic-Opioid Combination: Droperidol Plus Fentanyl

Quiescence Indifference to surroundings Patient appears to be asleep but is not in a state

of complete loss of consciousness. Used for diagnostic and minor procedures Adverse effects

Prolongs the QT interval, hypotension, and respiratory depression

copyright © 2013, 2010 by saunders, an imprint of elsevier inc. chapter 27 general anesthetics

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