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Inhalational agents

OBJECTIVES FOR INHALATION AGENTS. Recognize the historic impact of the development of inhalation anesthesia on the care delivery and in relief of human suffering.Review the possible mechanisms of action of inhaled anesthetics.Review the pharmacodynamic principles related to inhaled anesthetics, in

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Inhalational agents

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    1. Inhalational agents Vicente Gonzalez MS, CRNA/ Linda Wunder MSN, CRNA Florida International University Anesthesia Nursing Program

    2. OBJECTIVES FOR INHALATION AGENTS Recognize the historic impact of the development of inhalation anesthesia on the care delivery and in relief of human suffering. Review the possible mechanisms of action of inhaled anesthetics. Review the pharmacodynamic principles related to inhaled anesthetics, including potency, minimal alveolar concentration, and system-by-system effects. Review the pharmacodynamic principles related to inhaled anesthetics, including solubility, equilibration, uptake, distribution, concentration effect, second gas effect, metabolism, and elimination. Identify other issues related to use of inhaled anesthetics, including differences by system, rare toxicity events, and environmental concerns.

    3. 1840’S Nitrous oxide Diethyl Ether Chloroform 1920’s – 1940’s Ethylene Cyclopropane Divinyl Ether 1950’s Discoveries in fluorine chemistry

    4. Inhalational agents Nitrous Oxide (N20) Halothane (Fluothane) (not in common use) Enflurane (Ethrane) (not in use) Isoflurane (Forane) Desflurane (Suprane) Sevoflurane (Ultane)

    5. Chemical structures

    6. Chemical structures

    7. Mechanisms of action General anesthesia is an altered physiologic state characterized by reversible loss of consciousness, analgesia of the entire body, amnesia, and to some degree muscle relaxation Meyer-Overton Rule (Critical Volume Hypothesis) Protein Receptor Hypothesis (Agent Specific Theory) Microcrystaline Theory Alteration in Neurotransmitter Availability

    8. MAC Definition: Minimum Alveolar Concentration. It mirrors brain concentration and therefore allows for comparison of different agents. Defined as the concentration at which 50% of subjects do not respond to a standard stimulus. Can be thought of as the ED50 of the agent. It is fairly uniform among species.

    9. MAC Some other terms associated with MAC MAC BAR- block adrenergic response. MAC awake- 50% of subjects will follow a standard command at this level. MAC-BAR= MAC*1.3-1.4 MAC awake= MAC*0.3-0.4 MAC decreases 6% per decade of age. Tends to be highest at 6months.

    10. MAC MAC is additive, that is roughly ½ MAC of N2O and ½ MAC of Isoflurane is the same as 1 MAC of Isoflurane alone but the physiologic effect may not be the same. Other factors also affect MAC. It is also patient specific. Calculated from measurements done on healthy 33 to 55 year olds. Table 8-2 Miller.

    11. N2O First discovered in the 1840’s MAC 105% (requires hyperbaric conditions to achieve MAC. Only inorganic agent in use. Clear and odorless. Non- flammable but it will support combustion.

    12. N2O Systemic effects. Cardiovascular: Sympathetic stimulation. It depresses the myocardium but it stimulates catecholamine release. Increases pulmonary vascular resistance. Can cause myocardial ischemia in hypovolemic patients and in patients with CAD.

    13. N2O Respiratory: Causes tachypnea (as do all agents). It increases respiratory rate but it decreases tidal volume the net effect is no change in minute ventilation. Response to CO2 is depressed with nitrous as well as with the other agents. Increases the apneic threshold. (need a higher CO2 to initiate spontaneous ventilation)

    14. N2O Neuro: N2O probably increases CBF and CMRO2. Uncoupling still occurs. The use of N2O in neurosurgery is not contraindicated but the practioner needs to be aware that adjustments may need to be made to the respiratory rate to compensate. Renal: No appreciable effect. GI: in prolonged cases > 3 hours bowel distention may occur.

    15. N2O Contraindications: any procedure that involves working on a non- distensible space i.e.; craniotomy, pneumothorax, eye globe etc. It diffuses 34 times faster than N2 N2O interferes with folic acid metabolism, it inhibits any enzyme that is vitamin B12 dependent. Prolonged exposure can lead to bone marrow depression.

    16. Desflurane Halogenated anesthetic agent. MAC 6.0% Very pungent and irritating to the airway, cannot be used for inhalation induction. Non- flammable. Poor solubility in blood = rapid rise of FA/Fi

    17. Desflurane Systemic Effects Cardiovascular: Increasing the concentration quickly can cause tachycardia, may be attenuated by fentanyl 2-5 mcg/kg prior to induction.

    18. Desflurane Respiratory : Tachypnea, with a reduction in TV so minute ventilation is minimally decreased. Irritating to the airway. Depresses CO2 response. Increases apneic threshold.

    19. Desflurane Neuro: Decreases CMRO2 and increases CBF (uncoupling). Need to hyperventilate patient to counteract. Potentiates muscle relaxants. Renal: no change.

    21. Ideal Anesthetic Agent Quick onset and recovery Easy to administer Clear indication of anesthetic depth Inexpensive Wide margin of safety Minimal effect on organ systems

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