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Chapter 27

Chapter 27. General Anesthetics. 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

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Chapter 27

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  1. Chapter 27 General Anesthetics

  2. 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

  3. Pain • Analgesia • Loss of sensibility to pain • Anesthesia • Loss of pain and loss of all other sensations

  4. 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

  5. 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

  6. General Anesthetics • Basic pharmacology of inhalation anesthetics • Properties of individual inhalation anesthetics • Intravenous anesthetics

  7. Basic Pharmacology of Inhalation Anesthetics • Properties of an ideal inhalation anesthetic • Balance anesthesia • Molecular mechanism of action • Minimum alveolar concentration

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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

  13. Postanesthetic Medications • Analgesics • Mild pain: aspirin-like drugs • Severe pain: opioids • Antiemetics • Ondansetron (Zofran) • Muscarinic antagonists • Abdominal distention and urinary retention • Bethanechol

  14. Properties of Individual Inhalation Anesthetics • Halothane (no longer available in United States) • Isoflurane • Enflurane • Desflurane • Sevoflurane • Nitrous oxide • Obsolete inhalation anesthetics

  15. 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

  16. 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

  17. 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

  18. Enflurane • Has pharmacologic properties very similar to those of halothane • Newer agents now preferred

  19. Desflurane (Suprane) • Nearly identical in structure to isoflurane • Induction occurs more rapidly than with any other volatile anesthetics

  20. Sevoflurane (Ultane) • Similar to desflurane • Approved for induction and maintenance

  21. 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)

  22. Obsolete Inhalation Anesthetics • Ethylene • Cyclopropane • Diethyl ether (ether) • Vinyl ether • Ethyl chloride • Abandoned because they are explosive and offer no advantage over newer anesthetics

  23. Intravenous Anesthetics • Short-acting barbiturates (thiobarbiturates) • Benzodiazepines • Propofol • Etomidate • Ketamine • Neuroleptic-opioid combination: droperidol plus fentanyl

  24. Short-Acting Barbiturates (Thiobarbiturates) • Administered intravenously • Used for induction of anesthesia • Two agents • Thiopental sodium (Pentothal) • Methohexital sodium (Brevital)

  25. 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

  26. 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

  27. 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

  28. 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

  29. Etomidate • Potent hypnotic agent • Used for induction • Repeated administration can cause hypotension, oliguria, electrolyte disturbances, and high incidence of postoperative nausea and vomiting

  30. 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

  31. 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

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