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

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

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

Halogenated Organic Compounds Isoflurane and sevoflurane are the most commonly used agents in this class Also Desflurane, Halothane, Methoxyflurane, and Enflurane 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

Uptake and Distribution of Halogenated Organic Compounds Liquid anesthetic is vaporized and mixed with oxygen gas 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 Concentration gradient is greatest during initial induction

Uptake and Distribution of Halogenated Organic Compounds (Cont’d) Distribution to tissues is dependent on blood supply Lipid solubility determines entry into tissues through cell walls Depth of anesthesia is dependent on partial pressure of anesthetic in the brain Partial pressure in the brain is dependent on partial pressure of the anesthetic in blood and alveoli Maintenance of anesthesia is dependent on sufficient quantities of anesthetic delivered to the lungs

Elimination of Halogenated Organic Compounds Reducing amount of anesthetic administered reduces amount delivered to the alveoli Blood level is initially higher than alveolar level Concentration gradient now favors anesthetic diffusion from blood into the alveoli Blood levels drop quickly as patient breathes out anesthetic from the alveoli Brain levels drop as less anesthetic is delivered by blood Patient wakes up

Effects of Halogenated Organic Compounds CNS Dose-related reversible CNS depression Hypothermia Cardiovascular system Depress cardiovascular function Effects on HR variable Respiratory system Dose-dependent ventilation depression

Adverse Effects of Halogenated Organic Compounds CNS (Cont’d) Increased intracranial pressure in patients with head trauma or brain tumors Considered safe for epileptic animals Cardiovascular system Decrease blood pressure and may decrease renal blood flow Respiratory system Hypoventilation Carbon dioxide retention and respiratory acidosis

Physical and Chemical Properties of Inhalant Anesthetics Important properties to consider Vapor pressure Partition coefficient Minimum alveolar concentration (MAC) Rubber solubility

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

Vapor Pressure Volatile agents Nonvolatile 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

Blood:Gas Partition Coefficient Solubility is a ratio of concentration of an agent in 2 substances 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

Blood:Gas Partition Coefficient (Cont’d) 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

Blood:Gas Partition Coefficient (Cont’d) 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?

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

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

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

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

Sevoflurane High vapor pressure: need a precision vaporizer Blood:gas partition 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 the Isoflurane Easier to mask a patient, more pleasant smelling

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 of 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

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

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

Other Halogenated Inhalation Agents Halothane (Fluothane) Not commonly used anymore Being 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

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 toxid metabolites produced by the liver= renal damage Enflurane Used primarily in human medicine

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

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 Hyperventilation and hypertension Lowers seizure threshold CNS damage

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