Inhalation General Anesthetics – Flashcards
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General Anethesia
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-can be achieved with either injectable anesthetics or inhalational (gas) anesthetics
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Why Inhalational Anesthesia is Considered Safer than Injectable Anesthesia
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1. The depth of anesthesia can be more readily altered using gas anesthetic -Can lighten up the depth of anesthesia faster -Can only be made deeper with injectables. Cannot make them lighter until the drug is metabolized 2. Since inhalant (inhalation) agents are excreted through the respiratory system, they are often safer for animals with liver or kidney problems 3. Animals under inhalation anesthesia are breathing 100% oxygen -Animals anesthetized with injectables are breathing room air (20-21% O2) 4. Most patients under inhalation anesthetic are intubated and we know the advantages of that!
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Disadvantages of Inhalational Anesthesia
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1. Inhalation anesthesia requires an anesthetic machine -Cost and skills to use it 2. Induction is much slower with gas anesthetics than injectables 3. You have to deal with the waste anesthetic gas produced -Safety issues to humans
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Conclusion
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•Both inhalational and injectable general anesthetics have their place in veterinary medicine •Regardless of which one is used, there is an overall depressant effect on the cardiovascular, respiratory, and thermoregulatory systems > monitoring is crucial!
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When was Nitrous Oxide Introduced
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-"laughing gas" -1844
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When Was Chloroform Introduced
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-Diethyl Ether -1847
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When Was Halothane Introduced
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-1956
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When was Isoflurane Introduced
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-1981
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When was Desflurane Introduced
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-1992
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When was Sevoflurane Introduced
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-1994
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Most Common Inhalation Drugs Used
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-Isoflurane -Sevoflurane -Desflurane -Nitrous oxide -Halothane is no longer available to buy
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Characteristics of an Ideal Inhalation Agent
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1. Minimal Toxicity -Especially to the cardiovascular, respiratory, renal, hepatic, and nervous systems 2. No unwanted side effects -Such as seizures, vomiting, nausea 3. Safe for personnel -Minimal toxicity of waste gas vapors 4. User-friendly -Pleasant smell and non-irritating vapor so patients fight the 'smell' less 5. Rapid and gentle induction and recovery 6. Depth of anesthesia easily controlled and quickly altered when necessary 7. Not dependent on liver or kidney function for metabolism or excretion -Metabolized and excreted through the respiratory system 8. Good muscle relaxation 9. Adequate analgesia 10. Low cost 11. Adequate potency to achieve surgical anesthesia 12. Handling ease and safety -Nonflammable, non-explosive, chemically stable 13. Doesn't require specialized or expensive equipment 14. Doesn't react with anesthetic machine equipment -Rubber, metal, or carbon dioxide absorber
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Halogenated Compounds
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a) Isoflurane b) Sevoflurane c) Desflurane d) Enflurane -The most commonly used inhalation anesthetics in veterinary medicine -Come in a liquid form and must be vaporized with the use of oxygen and a vaporizer -Includes isoflurane, sevoflurane, enflurane, and desflurane
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Mechanism of Action of Inhalation Anesthetics
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-Not well understood -Anesthetic molecules may actually dissolve in nerve cell membranes and change the way the nerves conduct impulses = CNS depression
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Inhalation Agents Affect on Vital Signs
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1. Depress respiration in a dose-dependent manner 2. Depress cardiovascular function 3. Some sensitize the heart to epinephrine 4. Inhalation agents may decrease BP 5. All cause a dose-dependent, reversible depression of the CNS
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Depressed Respiration in a Dose-Dependent Manner
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-Hypoventilation due to decreased tidal volume and decreased respiratory rate -Therefore, we can see decreased O2 levels and increased CO2 levels
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Depressed Cardiovascular Function
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-Variable effect on heart rate -All cause vasodilation and decreased cardiac function -This could decrease BP and tissue perfusion, especially to the kidney and brain
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Sensitized Heart to Epinephrine
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-Can result in potentially serious (and potentially fatal arrhythmias) -Halothane was the worst one but is not used anymore
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Decreased BP
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-This could also decrease blood flow to the kidneys -Most significant if giving with other drugs that can also be hard on the kidneys (many of our analgesics), or if the animal has kidney disease
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Dose-Dependent, Reversible Depression of the CNS
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-Use with caution in animals with trauma -Can increase intracranial pressure. Safe to use in animals with epilepsy -Depresses the temperature regulating center causing hypothermia
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Safety of Inhalation Anesthesia
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-Considered safe for most patients -However, safety depends on the care with which these agents are administered and the person monitoring!
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What Happens When We Administer Gas Anesthetics?
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-Liquid anesthetics are vaporized, mixed with oxygen, and delivered via mask or ET tube -The anesthetic goes to the lungs and is absorbed into the bloodstream through the alveoli -Rate of diffusion from the lungs to the blood depends on the -Concentration gradient and -Lipid solubility of the anesthetic -During induction, the concentration gradient is high, (high in lung and low in blood) so diffusion is rapid into the blood. -Tissues with greater blood flow are quickly saturated with anesthetic -ie: Brain, heart, kidney. -Slower saturation of skeletal muscle and fat. -Because most gas anesthetic agents have high lipid solubility, they pass readily from the circulation into the brain, inducing anesthesia -As long as sufficient quantities of anesthetic are delivered to the alveoli so that the blood, alveolar and brain concentrations are maintained, anesthesia is maintained and the animal stays anesthetized
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What Happens When We Stop?
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-The concentration of the anesthetic in the alveoli decreases, so now anesthetic starts moving out of the blood into the alveoli and leaves the body when the animal breathes out. -Bagging the animal with 100% oxygen further increases the concentration gradient and wakes the animal up faster -As the concentration in the blood falls, the concentration gradient favors movement of the anesthetic from the brain into the bloodstream and the animal wakes up
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Does it Always Work This Way?
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-Some inhalational anesthetics may accumulate in body fat stores and are not "breathed out" -These agents rely on liver metabolism and renal excretion -As a result, we get a slower recovery and a prolonged anesthetic hangover -The newer inhalation anesthetics are mostly cleared from the body now via the lungs -Halothane depended more on metabolism and excretion but not used anymore so this is really not much of a concern anymore
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Physical Properties of Inhalation Agents
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1. Vapor pressure 2. Solubility coefficient = Partition coefficient 3. Minimum alveolar concentration (MAC)
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Vapor Pressure
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-A measure of the tendency of a molecule to move from the liquid phase to the vapor (gas) phase (evaporate) -Depends on the anesthetic agent and the temperature -Usually given at 20-22 degrees C (room temp) -The higher the vapor pressure, the more quickly it will evaporate -Agents with high vapor pressures could reach concentrations of 30% easily - this is way too high and could quickly OD a patient -We use precision vaporizers to limit the evaporation (maximum we can give is 5%) -Agents include isoflurane, sevoflurane, desflurane, halothane -A different vaporizer must be used for each inhalational anesthetic
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Low Vapor Pressure
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-Agents with low vapor pressures will not evaporate as quickly (may only reach 4% instead of 30%), so a precision vaporizer isn't needed to "control" the evaporation. IE: Nitrous Oxide
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Partition Coefficient
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Blood:Gas Partition Coefficient = Solubility Coefficient -A ratio of the concentration of an inhalation agent in the blood and in the alveoli -Measures the solubility of an inhalant anesthetic in the blood as compared to the alveoli -Important because it indicates the speed of induction and recovery for an anesthetic agent -Also called Solubility Coefficient
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Low Partition Coefficient
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-A low blood:gas partition coefficient indicates that an anesthetic is less soluble in the blood than it is in the alveolar gas -Example: Partition Coefficient = 0.5 -Means the anesthetic agent is half as soluble in the blood as in the alveolar gas -So 1/3rd of the anesthetic will be in the blood and 2/3rd in the alveolar gas
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High Partition Coefficient
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-A high blood-gas partition coefficient means the anesthetic is more soluble in the blood as in the alveolar gas -Example: Partition Coefficient = 2 -Means the anesthetic is twice as soluble in the blood as in the alveolar gas -So 2/3rd of the anesthetic will be in the blood and 1/3rd will be in the alveolar gas
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Low blood:gas partition solubility
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-Low solubility -less soluble in the blood -Tends to remain in gas phase in the alveoli so there is a high concentration gradient between alveoli and blood -Enters blood and brain quickly and leaves quickly -Results in fast inductions and fast recoveries
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High blood:gas partition solubility
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-High solubility -more soluble in the blood -Highly soluble in blood and tissue so low concentration gradient between alveoli and blood because it does not build up in alveolar gas -Therefore we get a slower induction and recovery
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Conclusion for Solubility
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-The lower the blood:gas partition coefficient for an inhalational anesthetic, the faster the expected induction and recovery -Wants to get back to the lungs fast! -The higher the blood:gas partition coefficient for an inhalational anesthetic, the slower the expected induction and recovery -Wants to stay in the blood and the tissues!
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Induction
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-Low solubility drugs are good choices for mask or chamber induction. Eg. Isoflurane or sevoflurane -High solubility drugs are not good choices for mask induction
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Maintenance
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-If an animal is too light or too deep and you turn the anesthetic level up or down, you'll see a response to low solubility drugs quickly (in a minute or so). -Higher solubility drugs take longer
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Recovery
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-Much quicker recovery times when agents with low solubility coefficients are used -Low solubility - patients usually awake within minutes -High solubility - 10-15 minutes to wake up -Higher solubility - 30-60 minutes to wake up
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Mean Alveolar Concentration (MAC)
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-MAC of an anesthetic agent is defined as the lowest concentration that produces no response in 50% of the patients exposed to a painful stimulus -Indicates the "effective concentration", but also the potency/strength of an agent -You need less to do the same job! **An agent with a low MAC is a more potent anesthetic than an agent with a high MAC.**
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Example of MAC
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-Halothane has a lower MAC than isoflurane and is therefore more potent > a higher concentration of isoflurane will be necessary to maintain a similar anesthetic depth -This means if you need to keep a patient at 1.5% on halothane, you may need to keep the same patient at 2.5% on isoflurane to get the same depth of anesthesia -Want to Maintain a 2%
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Rule of Thumb for Any Inhalation Agent
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-1 x MAC will give you light anesthesia -1.5 x MAC will give you surgical anesthesia -2 x MAC will give you deep anesthesia -These numbers will vary with age, metabolic activity, body temperature, etc. as well as any premed given
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Individual Inhalational Anesthetics
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1. Isoflurane 2. Sevoflurane 3. Desflurane 4. Enflurane 5. Nitrous Oxide
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Isoflurane
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Most commonly used inhalant anesthetic in veterinary medicine in North America
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Why is Isoflurane so Great?
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1. wide margin of safety 2. doesn't sensitize the heart to epinephrine 3. quick inductions and recoveries
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Physical and Chemical Properties
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1. High vapor pressure -Need a precision vaporizer to administer 2. Extremely low blood:gas solubility 3. MAC is a 1.3 to 1.6 in most species -Most animals maintain on 1.5-2.5% (ave 2%) depending on the premed and the animal's condition 4. Isoflurane is stable at room temperature and no preservative is required -Therefore, there's no residue buildup in the machine
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Extremely Low Blood:Gas Solubility Means...
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a) Extremely rapid induction -Well suited to mask induction but irritating to the mucous membranes and many animals resent the smell and will fight. -Allows us to change the depth of anesthesia rapidly during surgery b) Extremely rapid recovery -Don't turn anesthetic off until surgery is over - will wake up in a few minutes
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Effects of ISO on the Cardiovascular System
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-Isoflurane is considered to have the fewest adverse effects on the heart of all the inhalation anesthetics we use in veterinary medicine -Considered to be the safest inhalation agent in patients with cardiac disease -Only a very small decrease in cardiac function -Close to preanesthetic levels -Little effect on heart rate -Does not sensitize heart to epinephrine -Can still see hypotension especially at deeper planes of anesthesia
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Effects of ISO on the Respiratory System
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-Significant respiratory depression -Can see apnea at concentrations as low as 3%
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Effects of ISO on the CNS
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-Maintains cerebral blood flow so good choice for patients with head trauma or brain swellings if concentration is kept low
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Effects of ISO on the Kidneys
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-Nearly all the isoflurane is exhaled quickly once the vaporizer is turned off due to low lipid solubility (<2%) -Very little stays in the fat/muscle -Therefore, very easy on the liver and kidneys so safe for animals with liver and kidney disease -Safe for neonates and geriatrics
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Effects of ISO on the Muscle
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-Good muscle relaxation
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Analgesia Properties of ISO
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-Little to none -Analgesics in the premed or post op period are important -May see pain and excitment on recovery from painful procedures if analgesia isn't provided
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Sevoflurane
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-Gained popularity in veterinary medicine, especially in horses -More money than isoflurane! -Closely related to isoflurane - many similarities -Some practices (primarily feline practices) have switched to sevoflurane, but it will likely be awhile before it becomes commonplace, due to the high cost
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Physical and Chemical Properties of Sevoflurane
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1. Vapor pressure is high = a precision vaporizer specially made for sevoflurane is required 2. Very low solubility coefficient (blood-gas partition coefficient) induction and recovery even faster than isoflurane 3. MAC is much higher than ISO 4. Reacts with the potassium hydroxide or sodium hydroxide in the CO2 absorber to produce a chemical that has been shown to damage the kidneys in rats -Hasn't been reported in dogs or cats, but it may be best to only use in a Bain system (Cat practices) 5. Animals struggle less with mask induction than when using isoflurane because it is less irritating and smells better -Sevoflurane is therefore better than isoflurane for mask and chamber inductions
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MAC for Sevoflurane
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-Higher than ISO -Much less potent -Higher concentrations required -2.5-4% for maintenance -6-8% for induction
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Effects of Sevoflurane on Cardiovascular System
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-Similar effect on the heart as isoflurane. -hypotension common -Does not sensitize heart to epinephrine
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Effects of the Sevoflurane on the Respiratory System
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-May depress respiration slightly more than isoflurane -Apnea lasting at least 30 seconds and tachypnea have both been reported
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Effect of Sevoflurane on the Kidneys
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-Primarily excreted via the lungs with minimal metabolism in the liver
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Effect of Sevoflurane on the CNS
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-Safe in patients with head trauma and tumors
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Effect of Sevoflurane on Muscles
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-Gives adequate muscle relaxation -Paddling, excitement, and muscle tremors have been reported, particularly during the recovery period
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Analgesic Effects of Sevoflurane
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-NONE
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Desflurane
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-Relatively new - occasionally used in veterinary medicine. Still very expensive -Effects on the CNS, cardiovascular and respiratory systems similar to ISO
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Advantages of Desflurane
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-Undergoes very little metabolism in the body -Lowest partition (solubility) coefficient of any of the inhalants -twice as fast as isoflurane for induction and recovery
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Disadvantages of Desflurane
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-Extremely high vapor pressure - requires a special vaporizer which is very expensive -MAC much greater than any other halogenated agent - therefore much less potent than iso or sevo - needs to be maintained at 7-10% - Unpredictable: a lot of variability between patients so depth must be monitored closely -Desflurane vapors are very pungent and will induce coughing and breath-holding with mask induction
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Enflurane
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-Widely used in human medicine but very little in veterinary medicine because of the adverse side effects
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Advantages of Enflurane
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-Relatively rapid and smooth recovery -Minimal effects on heart rate and minimal epinephrine sensitization
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Disadvantages of Enflurane
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-Profound respiratory depression - often have to breathe for them (ventilate them) -Poor analgesia and muscle relaxation -May also see seizure-like muscle spasms in dogs
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Nitrous Oxide
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•Been around for 150 yrs •Still used in human medicine •Is a gas at room temperature •Comes in blue compressed gas cylinders •Administered using a flowmeter -Does not require a vaporizer •Mix with oxygen before being delivered to patient
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MAC in Nitrous Oxide
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-Used with other inhalation agents, not alone because -it has an extremely high MAC (100-250%) -because of its low potency, it's impossible to achieve a surgical plane of anesthesia with nitrous oxide alone
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Physical and Chemical Effects of Nitrous Oxide
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-When used with another inhalation agent, it reduces the MAC (and therefore the vaporizer setting) by 20-30% for the halogenated agent it is being used with -Reduces adverse effects on CV, Respiratory and other systems
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Pharmacological Effects of Nitrous Oxide
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-Wide safety margin -Has little effect on the cardiovascular, respiratory, hepatic or renal systems -Offers good analgesia, but is not a good muscle relaxant
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Risk of Hypoxia Using N2O
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•The use of nitrous oxide in an anesthetic machine limits the amount of oxygen delivered to the patient because nitrous oxide is "taking up space" in the circuit •Any patient receiving N2O is at an increased risk of hypoxia and should be monitored closely
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Special Precautions Using N2O
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-Diffusion into air pockets -Attracted to "pockets of gas" anywhere in the body because of its low solubility coefficient -Can diffuse into air in the chest or gut -Contraindicated in animals with intestinal obstruction, gastric torsion, pneumothorax or diaphragmatic hernia -Fire hazard -Also called "laughing gas"
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Doxapram
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•Injectable agent used to stimulate respiration •Stimulates the respiration centre in the brain stem •Used mostly in emergency situations •Within 2 minutes of IV injection will increase respiration and depth •Also used by placing a couple drops under the tongue of neonates after C-sections
