Inhalational anaesthetic agents

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Inhalational anaesthetic agents

  1. 1. INHALATIONAL ANAESTHETIC AGENTS History & Minimum alveolar concentration Presenter: Dr.Anoop Kumar Moderator:Dr.Anand Kulkarni
  2. 2. History <ul><li>Jean Baptiste Van Helmont(1577-1644) </li></ul><ul><li>invented the word Gas </li></ul><ul><li>Boyle, Dalton & Charles out lined the basic properties of gases </li></ul><ul><li>Joseph priestley & Carl scheele (1742-1786) performed first investigation on gas inhalation </li></ul>
  3. 3. History Contd .. <ul><li>On Dec 10 th ,1844 Prof Colton presented his exhibit featuring inhalation of nitrous oxide at union hall in Hartford, Connecticut. </li></ul><ul><li>On Oct 16 th 1846 Morton demonstrated Ether anaesthesia at Massachusetts general hospital. </li></ul>
  4. 5. History contd… <ul><li>Wells, Morton and Jackson all claimed to have discovered anaesthesia. </li></ul><ul><li>In 1847 David Waldie at Edinburgh Medical school England suggested Chloroform as an alternative agent. </li></ul>
  5. 6. History contd… <ul><li>In 1951 Halothane was synthesised by Suckling and introduced into clinical practice in 1956 </li></ul><ul><li>Between 1959 & 1966 Terrell and colleagues at Ohio medical products synthesised more than 700 products </li></ul>
  6. 7. History contd… <ul><li>347 th and 469 th compounds were Enflurane & Isoflurane </li></ul><ul><li>Wallin and colleagues at Travenol laboratories described other new compounds in 1970-Sevoflurane </li></ul><ul><li>Desflurane was introduced in 1993 </li></ul>
  7. 8. Minimum Alveolar concentration (MAC) <ul><li>Eger and colleagues defined this concept. </li></ul><ul><li>“ The alveolar concentration of an inhaled anaesthetic at 1 atm pressure in 100%Oxygen at equilibrium, that produces immoblity in 50% of those subjects exposed to a standardized noxious stimuli. ” </li></ul>
  8. 9. Determination of MAC <ul><li>For humans the noxious stimulus is a surgical incision of the abdomen. </li></ul><ul><li>In animals it is usually produced by clamping of tail or passing electric current through subcutaneous electrodes. </li></ul>
  9. 10. MAC Contd… <ul><li>It mirrors brain partial pressure after a period of equilibration. </li></ul><ul><li>Best estimate of anaesthetic potency of an inhalational anaesthetic. </li></ul><ul><li>Allows comparison between agents and provides a standard for experimental evaluation. </li></ul>
  10. 11. MAC Contd… <ul><li>MAC values are roughly additive. </li></ul><ul><li>MAC for other response parameter are not additive. </li></ul><ul><li>Relatively constant within species and between species. </li></ul>
  11. 12. MAC contd.. <ul><li>Analogous to ED 50 expressed for intravenous drugs. </li></ul><ul><li>1.3 MAC would prevent 95 % subjects from moving and is equivalent to ED 95. </li></ul>
  12. 13. MAC Awake <ul><li>MAC of anaesthetic that would allow opening of eyes on verbal commands during emergence from anaesthesia </li></ul><ul><li>=0.3-0.4 MAC </li></ul><ul><li>MAC Intubation </li></ul><ul><li>MAC that would inhibit movement and coughing during endotracheal intubation.=1.3 MAC </li></ul>
  13. 14. MAC Bar <ul><li>MAC of anaesthetic necessary to prevent adrenergic response to skin incision, as measured by conc. of catecholamine in venous blood =1.5 MAC </li></ul><ul><li>When different agents are compared the ratio of MAC skin incision to MAC intubation or MAC awake is relatively constant </li></ul>
  14. 15. MAC Rationale Why this measures anaesthetic potency? <ul><li>Alveolar conc. can be easily measured </li></ul><ul><li>Near equilibrium alveolar and brain tension are virtually equal </li></ul><ul><li>High cerebral blood flow produces rapid equilibration </li></ul>
  15. 16. Factors supporting use of MAC <ul><li>MAC is invariant with variety of noxious stimuli. </li></ul><ul><li>Individual variability is small. </li></ul><ul><li>Doses of anaesthetic in MAC are additive. </li></ul>
  16. 17. <ul><li>Hypothermia </li></ul><ul><li>Hyperthermia(If >42 0 C) </li></ul><ul><li>Hyponatremia & Hypocalcemia </li></ul><ul><li>Hypoxia(PaO 2 <38mm of Hg) </li></ul><ul><li>Induced hypotension (MAP<50mmHg) </li></ul>Factors affecting MAC MAC
  17. 18. Factors sing MAC contd… <ul><li>Hypercarbia(PaCO 2 >95mm of Hg) </li></ul><ul><li>Anemia(Hematocrit<10%) </li></ul><ul><li>Pregnancy </li></ul><ul><li>Acute ethanol administration </li></ul><ul><li>Hypo osmolality </li></ul><ul><li>Increasing age(max. at 6months) </li></ul>
  18. 19. <ul><li>Local anaesthetics(except cocaine) </li></ul><ul><li>Opioids. </li></ul><ul><li>Ketamine,Barbiturates,Benzodiazapines. </li></ul><ul><li>Sympatholytics. </li></ul><ul><li>Verapamil, Lithium. </li></ul><ul><li>c/c administration of amphetamine. </li></ul>Drugs MAC
  19. 20. Factors MAC <ul><li>Chronic ethanol use. </li></ul><ul><li>Hypernatremia. </li></ul><ul><li>a/c admin. of amphetamine. </li></ul><ul><li>Cocaine. </li></ul><ul><li>Ephedrine. </li></ul><ul><li>MAO inhibitors. </li></ul><ul><li>Levodopa. </li></ul>
  20. 21. No changes in MAC <ul><li>Gender. </li></ul><ul><li>Hypo/Hyper thyroidism. </li></ul><ul><li>Duration of inhaled anaesthetic Administration. </li></ul><ul><li>Hypercalcemia. </li></ul><ul><li>Metabolic alkalosis. </li></ul>
  21. 22. Clinical implications of MAC <ul><li>Understanding of MAC helps in prevention of awareness.(will not occur at end tidal conc. of 1MAC). </li></ul><ul><li>Tool to measure anaesthetic potency. </li></ul><ul><li>Helps to study side effects of inhalational anaesthetics at equipotent doses. </li></ul><ul><li>Combinations are additive. </li></ul>
  22. 23. MAC of common inhalational anaesthetics 2 Sevoflurane 1.2 Isoflurane 0.75 Halothane 105 N2O 1.6 Enflurane
  23. 24. Thank you

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