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General anesthesia

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Anesthetics - Pharmacology
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General anesthesia

  1. 1. Inhalational Anesthetics PGI Alm eñana
  2. 2. General vs regional anesthesia RA – aim to block a certain part of the body - e.g Appendectomy GA – brain is the target organ - block CNS causing patient to fall asleep
  3. 3. Course of general anesthesia 1.Induction – giving medications prior to intubation of patient (Sedative, Muscle Relaxants) 2.Maintenance – Muscle Relaxants, Gases (now running on ET tube) 3.Emergence – post op medications given
  4. 4. Stages 1.Amnesia 2.Delirum/Excitement/Uninh ibited Response 3.Surgical Anesthesia 4.Impending Death/Overdosage
  5. 5. 1. AMNESIA • From beginning of induction to loss of consciousness (loss of eyelid reflex)
  6. 6. 2. Delirum/Excitement/Uninhibited Response From loss of consciousness to onset of automatic breathing   Agitation, delirium, irregular respiration, breath holding   Dilated pupils and eyes are divergent   Vomiting, laryngospasm, hypertension
  7. 7. 2. Delirum/Excitement/Uninhibited Response   DO NOT INTUBATE Make sure you have given sedatives and muscle relaxants Propofol: onset of 1-2 minutes Atravirium: onset of 2-3 minutesso do intubation after 2-3 minutes (if done earlier, patient may be at risk of having bronchospasm episodes)
  8. 8. 3. Surgical Anesthesia   From onset of automatic respiration to respiratory paralysis   Central gaze   Stage for INTUBATION
  9. 9. 4. Impending death/overdosage   From onset of diaphragmatic paralysis to apnea and death   Dilated and non-reactive pupils, hypotension   Circulatory failure
  10. 10. PHARMACOKINETICS
  11. 11. “The higher the FGF rate, the smaller the breathing circuit volume, and the lower circuit absorption, the closer the inspired gas concentration will be to the fresh gas concentration.”
  12. 12. Partition Coefficients • The relative solubility of an anesthetic in air, blood and tissues. • Insoluble agents are taken up by the blood less avidly than soluble agents. As a consequence, alveolar concentration rises faster and induction is faster.
  13. 13. Different Tissue Groups based on Perfusion Highly perfused Brain, heart, liver, kidney and endocrine glands Muscle group Skin and muscle Fat group Minimal/vessel poor groups Bones, ligaments, teeth, hair, cartilage
  14. 14. pharmacodynamics
  15. 15. General Anesthesia • Reversible loss of consciousness • Analgesia of the entire body (pain reliever) • Amnesia • Muscle relaxation
  16. 16. Brain Areas affected by Inhalational Anesthetics • Reticulating Activating System (RAS) • Cerebral cortex • Cuneate nucleus • Olfactory cortex • Hippocampus • Spinal cord –dorsal horn interneurons
  17. 17. Inhalational Anesthesia • Depress spontaneous and evoked activity of neurons in many regions of the brain • GABA-receptor chloride channel
  18. 18. Pharmacology of Inhaled Anesthetics
  19. 19. Theories of Anesthetic Action Agent-specific theory Various agents produce anesthesia by different methods Unitary hypothesis All inhalational agents share a common mechanism of action at the molecular level Meyer-Overton rule The anesthetic potency of inhalational agents correlates directly with their lipid solubility
  20. 20. Depth of Anesthesia Minimum Alveolar Concentration (MAC) The alveolar concentration of an inhaled anesthetic that prevents movement in 50% of patients in response to standard stimulus (surgical incision)
  21. 21. Depth of Anesthesia MAC-Beta-adrenergic response (MAC-BAR: 1.5MAC) Concentration of inhaled anesthetic required to block the autonomic responses to nociceptive stimuli 1.7 to 2 times the MAC
  22. 22. Depth of Anesthesia MAC-awake (0.5MAC) The dose of inhaled anesthetics necessary to produce unconsciousness
  23. 23. Depth of Anesthesia MAC Amnesia (0.25MAC) You can use this if you just want to patient to not remember to procedure.
  24. 24. Factors that modify mac Increase MAC • Young age • Hyperthermia • CNS Hypo-osmolality • Habituation to alcohol • CNS Stimulus (Dextroamphetamine, Cocaine) • Physostigmine
  25. 25. Factors that modify mac Decrease MAC • Old age • Hypothermia • CNS hyperosmolality • Acute effects of alcohol (occasional drinker) • CNS depressants (Benzodiazepines, Barbiturates, Propofol) • Tranquilizers (Chlorpromazine) • CNS effects of local anesthetics • Narcotics • Pregnancy • Alpha-2-adrenergic agonist (Clonidine, Dexmedetomidine)
  26. 26. Agents Clinically Used in Anesthesiology • Nitrous Oxide • Methoxyflurane • Enflurane • Halothane • Isoflurane • Desflurane • Sevoflurane
  27. 27. 1. Nitrous Oxide • Laughing gas • Only inorganic anesthetic gas • Colorless and odourless • Gas at room temperature and ambient pressure; liquid under pressure • Can cause bone marrow depression (megalobalastic anemia) and even neurologic deficiencies (peripheral neuropathy and pernicious anemia) upon prolonged exposure
  28. 28. 1. Nitrous Oxide • Has possible teratogenic effect hence avoided in pregnant patients Tends to diffuse into air containing cavities • Nitrous oxide is 35X more soluble than nitrogen so usually it goes inside the air containing cavities
  29. 29. 1. Nitrous Oxide Contraindicated if: Pneumothorax Air embolism Acute intestinal obstruction Intracranial air Pulmonary air cyst Intraocular air bubbles Tympanic membrane grafting Pulmonary hypertension
  30. 30. 2. Methoxyflurane • Most potent of all (because it has the lowest MAC, only .16) • Sweet, fruity odor • High solubility and low vapour pressure • 50% is metabolized by cytP450 enzymes • Associated with vasopressin-resistant high- output renal failure Due to free fluoride metabolites (inhibits tubular function)
  31. 31. 3. Enuflurane • Non-pungent, non-flammable • Depresses myocardial contractility and sensitizes the myocardium to epinephrine • Increases CSF and the resistance to CSF flow • Causes tonic-clonic seizures • Don’t use this in patients with history of seizures
  32. 32. 4. Halothane • Halogenated alkane • Carbon-fluoride bonds are responsible for its non-flammable and non-explosive nature • Least expensive volatile anesthetics • Halothane hepatitis Increased risk to patients exposed to multiple halothane anesthetics, middle-aged obese women, persons with familial predisposition to halothane toxicity (centrilobular necrosis)
  33. 33. 4. Halothane Contraindications Liver dysfunction Intracranial mass lesions Because of possibility of intracranial hypertension
  34. 34. 5. Isoflurane • Non-flammable, pungent, ethereal odor • Coronary steal syndrome  Dilation of normal coronary arteries could divert blood away from fixed stenotic lesions  Causes regional myocardial ischemia during tachycardia or drops of perfusion pressure • Good bronchodilator like Halothane • The difference between isoflurane and sevoflurane is isoflurane is pungent; that’s why sevoflurane is preferred but it is expensive
  35. 35. 6. Desflurane Similar to Isoflurane Substitution of Fluorine atom for Isoflurane’s chlorine atom High vapour pressure (681 vapor pressure) Boils at room temperature at high altitude that’s why it has special vaporizer Low solubility in blood and body tissues Causes rapid wash in and wash out meaning its blood coefficient is very low, 0.4-1
  36. 36. 7. Sevoflurane • Most commonly used because of its sweet odor • Halogenated with fluorine • Non-pungency and rapid increase in alveolar concentration  Excellent choice for pediatric and adult patients • Low blood solubility  Rapid emergence • Compound A  Fluoromethy-2,2-difluoro-1-vinyl ether)  Nephrotoxic end product of soda lime + Sevoflurane • Avoid Sevoflurane in patients with pre-existing renal dysfunction
  37. 37. FIN.

Editor's Notes

  • e.g. Sevoflurane has MAC of 2.0. we usually increase our MAC since according to our definition, 50% will respond to stimulus, 50% will not. if our patient belongs to the 50% that has negative response, chances are, the patient may wake up... we usually increase our concentration, sometimes we double it or 1.5 times.
  • e.g. Sevoflurane has MAC of 2.0. we usually increase our MAC since according to our definition, 50% will respond to stimulus, 50% will not. if our patient belongs to the 50% that has negative response, chances are, the patient may wake up... we usually increase our concentration, sometimes we double it or 1.5 times.
  • If the patient is anxious and she is to undergo breast mass excision, if she wanted to sleep you can use the MAC-awake. It does not produce much problem.
  • If the patient is anxious and she is to undergo breast mass excision, if she wanted to sleep you can use the MAC-awake. It does not produce much problem.
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