Inhalant Anesthesia

Inhalant Agents

  • commonly used inhalation anesthetics

    • isoflurane

    • sevoflurane

    • desflurane

  • less commonly used inhalant anesthetics

    • methoxyflurane

    • nitrous oxide

    • halothane

  • new inhalant anesthetics

    • xenon

Clinical considerations for selecting an inhalation agent

  • metabolic consideration

    • how quickly we can wake up from anesthesia

  • anesthetic potency consideration

  • rate of induction, rate of change in anesthetic depth, and rate of recovery

  • cardiopulmonary consideration

  • cost

Metabolic consideration

  • major elimination route of inhalation anesthetics = through the respiratory tract

  • hepatic dysfunction patient = choice of inhalant = isoflurane

    • isoflurane: 0.17% is metabolized by the liver and the kidneys

    • sevoflurane: 3.0% is metabolized by the liver and the kidneys

  • nitrous oxide = not commonly used in veterinary anesthesia

    • not spare other inhalants much

    • requires higher fresh gas (o2) flow

    • diffusion hypoxia

Anesthetic potency

  • MAC: minimum alveolar concentration

    • the concentration of a vapor in the alveoli of the lungs that is needed to prevent movement in 50% of subjects in response to surgical stimulus

  • used to compare inhalation anesthetic potency

    • similar mg/kg for those of injectable anesthetics

  • clinically surgical plane of anesthesia = 1.5~2 times of MAC

  • The less the MAC, the more potent the anesthetic agent

    • sevoflurane is less potent than isoflurane

  • an anesthetic dose increased, mean arterial pressure decreased = most potent vasodilator

Factors affecting MAC

  • Decreasing MAC

    • Hypotension (ex) hemorrhage intra-op

    • metabolic acidosis

    • extreme hypoxia

      • PaO2 <38 mmHg

    • hypothermia

    • age

    • premedication

      • opioids, sedatives, tranquilizer

    • pregnancy

  • increasing MAC

    • hyperthermia: increase cerebral metabolic rate of brain

    • hyperthyroidism

    • hypernatremia

    • drugs increase CNS catecholamines

      • cocaine, tricyclic antidepressants

  • Not affecting MAC

    • type of stimulation

    • duration of anesthesia

    • species

    • sex

    • PaCO2 or PaO2

Rate of induction, rate of change in anesthetic depth and rate of recovery

  • rate of induction, change in anesthetic depth, and rate of recover = BLOOD / GAS SOLUBILITY of each inhalation anesthetic

  • the higher the blood/gas solubility, the slower the induction and recovery rates

  • the higher the blood/gas solubility, the slower rate of change in depth of anesthesia

Cardiopulmonary consideration

  • all inhalant anesthetics = cardiopulmonary depression (dose-dependent)

    • CO decrease

    • BP decrease

    • RR decrease

  • myocardial depression

    • halothane >or = methoxyflurane > isoflurane = sevoflurane = desflurane > N2O

  • reduction in cardiac output:

    • halothane > or = methoxyflurane > isoflurane = sevoflurane = desflurane > N2O

  • Reduction in systemic vascular resistance (vasodilation):

    • isoflurane = sevoflurane= desflurane > methoxyflurane > or = halothane > N2O

  • respiratory depression:

    • isoflurane = sevoflurane = desflurane > methoxyflurane > halothane > N2O

Clinical use of inhalant anesthetics

  • Induction

    • Advantages

      • accurately controlling anesthetic depth during induction with the safety of being able to discontinue the administration of the inhalant anesthetic immediately if problems arise

      • can be eliminated quickly via ventilation

      • provide high-inspired oxygen

    • disadvantages

      • slow speed of induction- excitement stage

      • hold breaths due to the pungent smell

      • polluting the working environment

        • waste inhalant gas, pregnant worker

  • Maintenance of general anesthesia

    • protection of the airway

Methods of inhalant anesthetic induction

  • Face Mask Induction:

    • select a tight-fitted face mask and place it on the face of the animal

    • suitable in birds, neonate ruminants or foals, or in healthy dogs or cats with premedication

    • debilitating patients

      • reduced inhalant anesthetic %

  • chamber induction

    • suitable in small viscous animals

      • cats, reptiles, other small exotics

    • provides “hand free” induction with minimal physical restraint to the patient

    • start highest concentration of inhalant and high oxygen flow

    • once patient becomes unconscious, take the patient out from the chamber and place the patient on face mask to deepen anesthetic and intubate

  • Nasotracheal intubation

    • suitable in foals and calves

    • most commonly done in foals up to 200 lbs in body weight

    • calves are more difficult it be nasotracheal intubated than foals

    • use of premedication will be helpful!

      • butorphanol & diazepam

Patient waking up intra-op

  1. increase vaporizer dial setting the the highest %

  2. increase oxygen flow

  3. increase ventilation rate and volume

  4. once your patient become deepened and relaxed, reduce vaporizer dial setting

Cost

  • Desflurane > sevoflurane > isoflurane> halothane

  • methoxyflurane is relatively expensive due to lack of protection

  • sevoflurane cost about four times more expensive than isoflurane when compared in mL basis

Maintenance

  • 1.5-2 X MAC = surgical maintenance

  • use of premedication = decrease maintenance concentration of inhalation anesthetics

  • Anesthetic maintenance:

    • Methoxyflurane: 0.5-1.5%

    • Isoflurane: 1-2.5%

    • sevoflurane: 2.4-4.0%

Recovery

  • recovering from inhalant can cause dysphoria

    • tx. apply sedation drug during recovery and provide high concentration of oxygen

      • once you turn off the inhalant vaporizer, keep the patient on oxygen for at least 5 minutes

    • equine: during recovery, after extubation and the patient starts spontaneous breathing, we usually give some sedation to make sure the patient is laying down and blowing off inhalant anesthetic from the lungs