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Inhalation Anesthetics

Inhalation Anesthetics. Chapter 3 Isoflurane and sevoflurane ( halogenated compounds) Nitrous oxide and desflurane Enflurane Halothane Methoxyflurane Diethyl ether. Diethyl Ether. 1 st inhaled anesthetic No longer used as an anesthetic agent

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Inhalation Anesthetics

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  1. Inhalation Anesthetics Chapter 3 Isofluraneand sevoflurane (halogenated compounds) Nitrous oxide and desflurane Enflurane Halothane Methoxyflurane Diethyl ether

  2. Diethyl Ether • 1st inhaled anesthetic • No longer used as an anesthetic agent • Classic stages and planes of anesthesia described using ether • Desirable characteristics • Stable cardiac output, rhythm, and blood pressure • Stable respirations • Good muscle relaxation

  3. Diethyl Ether (Cont’d) • Undesirable characteristics • Tracheal and bronchial mucosal irritation • Prolonged induction and recovery • Postoperative nausea and vomiting • Flammable and explosive

  4. Halogenated Organic Compounds • Isoflurane and sevoflurane are the most commonly used agents in this class • Others include Desflurane, Halothane, Methoxyflurane, and Enflurane, but these are not commonly used • Liquid at room temperature • Stored in a vaporizer on an anesthetic machine • Vaporized in oxygen that flows through the vaporizer • Exception being Desflurane- has a special injection type vaporizer

  5. Uptake and Distribution ofHalogenated Organic Compounds • Liquid anesthetic is vaporized and mixed with oxygen • Mixture is delivered to the patient via a mask or endotracheal tube (ET tube) • Mixture travels to lungs (alveoli) and diffuses into the bloodstream • Diffusion rate is dependent on concentration gradient (alveoli/capillary) and lipid solubility of the anesthetic gas • Concentration gradient is greatest during initial induction

  6. Uptake and Distribution of Halogenated Organic Compounds • Distribution to tissues is dependent on blood supply • Tissues with greater blood flow (brain, heart, kidney) are more quickly saturated with anesthetic gas • Lipid solubility of the gas determines entry into tissues through cell walls • Maintenance of anesthesia is dependent on sufficient quantities of anesthetic delivered to the lungs

  7. Elimination of Halogenated Organic Compounds • Reducing amount of anesthetic administered reduces amount in the alveoli • Anesthetic will move from the brain into the blood and then into the alveoli where it is finally breathed out • Patient wakes up

  8. Inhalant Anesthetics EFFECTS: ADVERSE EFFECTS: EFFECTS: ADVERSE EFFECTS: EFFECTS: ADVERSE EFFECTS:

  9. Physical and Chemical Propertiesof Inhalant Anesthetics • Important properties to consider • Vapor pressure • Partition coefficient • Minimum alveolar concentration (MAC) • Rubber solubility

  10. Vapor Pressure • Is the amount of pressure exerted by the gaseous form of a substance when in equilbrium • i.e. – it’s ability to evaporate • Determines how readily an inhalation anesthetic will evaporate in the anesthetic machine vaporizer • Temperature and anesthetic agent dependent

  11. Vapor Pressure • Volatile agents • High vapor pressure- evaporates readily • Isoflurane, sevoflurane, desflurane, and halothane • Delivered from a precision vaporizer to control the delivery concentration • All precision vaporizers are made to deliver only one specific halogenated agent • Nonvolatile agents • Low vapor pressure- no need for precision vaporizer • Methoxyflurane *Vaporizers are specific to that gas, and is unacceptable to combine agents in the same vaporizer. Although it is safe to switch patient from one gas to another

  12. Blood:Gas Partition Coefficient • The measure of the solubility of an inhalation anesthetic in blood as compared to alveolar gas (air) • Indication of the speed of induction and recovery for an inhalation anesthetic agent • Low blood:gas partition coefficient • Agent is more soluble in alveolar gas than in blood at equilibrium • Agent is less soluble in blood • Faster expected induction and recovery

  13. Blood:Gas Partition Coefficient • High blood:gas partition coefficient • Agent is more soluble in blood than in alveolar gas at equilibrium • Agent is less soluble in alveolar gas • Agent is absorbed into blood and tissues (sponge effect) • Slower expected induction and recovery

  14. Blood:Gas Partition Coefficient • Blood: gas partition coefficient determines the clinical use of the anesthetic agent • Induction: Can a mask be used? • Maintenance: How fast will the anesthetic depth change in response to changes in the vaporizer setting? • Recovery: How long will the patient sleep after anesthesia?

  15. Minimum Alveolar Concentration (MAC) • The lowest concentration of which 50% of patients shows no response to a painful stimulus • The measure of the potency of a drug • Used to determine the average setting on the vaporizer that will produce surgical anesthesia • The lower the MAC, the more potent the anesthetic agent and the lower the vaporizer setting • MAC may be altered by age, metabolic activity, body temperature, disease, pregnancy, obesity, and other agents present • Every patient must be monitored as an individual • Age, disease, temperature, pregnancy, obesity, pre medications

  16. Isoflurane • Most commonly used inhalant agent in North America • Approved for use in dogs and horses; commonly used in other species

  17. Isoflurane • Properties • High vapor pressure: need a precision vaporizer • Low blood:gas partition coefficient: rapid induction and recovery • Good for induction with mask or chamber • MAC = 1.3% to 1.63%: helps determine initial vaporizer setting • Low rubber solubility • Stable at room temperature; no preservatives needed = no build up in the machine

  18. Effects and Adverse Effects • Maintains cardiac output, heart rate, and rhythm • Fewest adverse cardiovascular effects • Depresses the respiratory system • Maintains cerebral blood flow • Almost completely eliminated through the lungs- 0.2% metabolized by the liver • Induces adequate to good muscle relaxation • Provides little or no analgesia after anesthesia • Difficult to mask patient • Can produce carbon monoxide when exposed to a desiccated carbon dioxide absorbent

  19. Sevoflurane • High vapor pressure: need a precision vaporizer • Low Blood:gaspartition coefficient = rapid induction and recovery • Good for induction with a mask or chamber • High controllability of depth of anesthesia • MAC = 2.34% to 2.58% • Cost about 10x more than Isoflurane • Easier to mask a patient, more pleasant smelling

  20. Effects and Adverse Effects of Sevoflurane • Minimal cardiovascular depression • Depresses respiratory system • Eliminated by the lungs, minimal hepatic metabolism- 2-5% • Maintains cerebral blood flow • Induces adequate muscle relaxation • Some paddling and excitement during recovery • No post-op analgesia • Can react with potassium hydroxide (KOH) or sodium hydroxide (NaOH) in desiccated CO2 absorbent to produce a chemical (Compound A) that causes renal damage

  21. CNS and Respiratory Stimulants • Doxapram • Analeptic agent (CNS stimulant) • Stimulates respiration and speeds recovery • Acts at the carotid sinus and the aortic arch • Used in neonate puppies and kittens after C-section • IV administration or sublingual drops (neonates) Adverse effects • Wide margin of safety, but the following may be seen: • Hyperventilation and hypertension • Lowers seizure threshold • CNS damage

  22. Use of Doxapram • Repeat injections may be necessary • Reverses respiratory depression from inhalant agents and barbiturates • 1-5 ggt under the tongue of a puppy, or 1-2 ggt in a kitten if needed after C-section or dystocia

  23. UNCOMMONLY USED INHALANT ANESTHETICS

  24. Desflurane • Closely related to isoflurane • Expensive • Lowest blood:gas partition coefficient: very rapid induction and recovery • Used with a special heated electronic precision vaporizer • MAC = 7.2% and 9.8% • Least potent inhalant agent • Eliminated by the lungs- 0.02% metabolized in liver

  25. Effects and Adverse Effects of Desflurane • Strong vapors cause coughing and holding the breath= difficult to mask • Other effects are similar to isoflurane • Transient increase in heart rate and blood pressure (humans) • Produces carbon monoxide with spent soda lyme

  26. Other Halogenated Inhalation Agents • Halothane (Fluothane) • Not available anymore • replaced by isoflurane and sevoflurane • B:G -2.54 • 20-46% metabolized in the liver • MAC- 0.87-1.19 • Sensitizes heart to catecholamine and induces arrhythmias • Cardiac, respiratory depression • Increased cerebral blood flow • Increased temperature- malignant hyperthermia- Dantrolene is used for treatment

  27. Methoxyflurane • Methoxyflurane • No longer available in North America • B:G- 151! • Used in a non precision vaporizer- wick • 50-75% metabolized by the liver, excreted by the kidneys!! • Fluoride ions and other potentially toxic metabolites produced by the liver= renal damage • Enflurane • Used primarily in human medicine

  28. Nitrous Oxide • Nitrous oxide • Used primarily in human medicine; some veterinary use • A gas at room temperature; no vaporizer is required • Mixed with oxygen at 40-67%, then delivered to patient • Reduces MAC 20-30% • Used with Halothane and Methoxyflurane to reduce the adverse effects of these gases

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