General Anesthesia and Sedation

1. General Anesthesia and Sedation Josh Smith, M.D. Assistant Professor Department of Anesthesiology

2. Objectives: • Review of pharmacology • Inhalational agents • Induction agents • Muscle relaxants • Narcotics • Benzodiazepines • Local anesthetics • General anesthesia • Sedation • Crisis management

3. Ideal Inhaled Anesthetic • Maintains cardiovascular function • Maintains respiratory function • Allows adequate oxygenation • Chemically stable • Nontoxic • Easy to administer • Inexpensive

4. Minimum AlveolarConcentration (MAC) The Alveolar concentration in volumes % at which 50% of subjects will hold still for a painful stimulus, usually a skin incision. 1.3 (MAC) prevents a response to noxious stimuli in 95% of subjects. Method of comparing anesthetic potency.

5. The MAC on any inhalational anesthetic is reflective of the Oil:Gas partition coefficient of the gas itself. The higher the Oil:Gas partition coefficient the more potent the inhalational anesthetic and the lower the MAC value.

6. Minimum Alveolar Concentration Inhaled Anesthetic ( Vols % ) Nitrous oxide 104 Cyclopropane 9.2 Desflurane (Suprane) 6.0 Diethyl ether 1.92 Sevoflurane (Ultane) 1.71 Enflurane (Ethrane) 1.70 Isoflurane (Forane) 1.15 Halothane (Fluothane) 0.77 Methoxyflurane (Penthrane) 0.16

8. Factors that Increase MAC • Increased central neurotransmitter levels • Acute amphetamine use • Cocaine use • Hyperthermia • Young age

9. Anesthetic Partial PressureGradients May Exist Between • Vaporizer • Inflow • Circle System ( inspired partial pressure ) • Alveoli ( alveolar partial pressure ) • Blood • Brain

10. Gases In Solution • Any gas dissolved in a liquid exerts a force to drive molecules out of solution and into the gas phase that, at equilibrium, is counteracted by molecules in the gas phase exerting a force that drives them into the liquid phase (Henry’s Law).

11. At equilibrium, the PARTIAL PRESSURE in all phases of a closed system (alveolar gas, blood, and tissues ) is equal. However, the CONCENTRATIONS within those solutions or tissues may vary.

14. Anesthetic Solubility in Blood • Anesthetics with low B / G partition coefficients are insoluble in blood. Examples: N2O, desflurane, sevoflurane • Anesthetics with high B / G partition coefficients are highly soluble in blood. Examples: Diethyl ether, methoxyflurane

16. Blood / GasInhaled Anesthetic (O/G) Partition Coefficient Cyclopropane (9.2) 0.41 Desfluane (6.0) 0.42 Nitrous Oxide (104) 0.47 Sevoflurane (1.7) 0.60 Isoflurane (1.15) 1.4 Enflurane (1.7) 1.91 Halothane (0.77) 2.3 Diethyl ether (1.92) 12.1 Methoxyflurane (0.16) 12.0

17. Induction agents • Propofol • Thiopental (barbituate) • Etomidate

18. Propofol • Don’t use with egg allergy • Support bacterial growth • Frequent changing of IV sets in ICU patients • Rapid awakening • extrahepatic metabolism • Neuro - like barbituates but will reduce cerebral perfusion pressure • Pronounced cardiovascular effects-age/dose • Respiratory depression – beware with sedation • Antiemetic

19. Barbituates • Can induce porphyria • Cerebral (most similar to volatile) - ICP/CMR • Dilate capacitance vessels -volume status key • Respiratory depression - as are all nonvolatile -synergistic effects

20. Etomidate • Neuro - similar to barbs; myoclonus • Minimal cardio-respiratory effects • Postoperative nausea/vomiting • Adrenocortical suppression – studies showing increased mortality in critical patients

21. Muscle relaxants • Depolarizing • Succinylcholine • Nondepolarizing • Steriod based • Rocuronium, vecuronium, pancuronium • Benzylisoquinolones • Cisatricurim, atracuruim

24. Succinylcholine • 2 Ach molecules • Rapid onset, short duration • Pseudocholinesterase metabolism • Low levels in pregnancy, resulting in prolonged block • Pseudocholinesterase deficiency • Heterozygous atypical – 1/50; slight increase in block duration • Homozygous atypical – 1/3000; 6-8 hour block • Dibucaine number • 80% metabolism – normal • 20% metabolism – seen in homozygous atypical

25. Succinylcholine • Bradycardia after 2nd dose • quicker release from muscarinic receptors • Fasciculations – defasciculating dose • Hyperkalmia - burn, trauma, denervation injury • Myalgias • MH trigger • Not used routinely in pediatrics due to possibility of undiagnosed myopathy, which can cause hyperkalemic arrest in these patients.

26. Nondepolarizing agents • Slower onset, varying durations (> with ESRD) • Mivacurium(s) • Metabolized by pseudocholinesterase • Cisatracurium (m) – isomer of atracurium • Not dependent on organ elimination • Vecuronium(m) -devoid of CV effects • Rocuronium(m) -alternate for RSI • Pancuronium(l) -vagolytic

27. Narcotics • Morphine, merperidine, fentanyl derivatives • Bind mu, kappa, delta, sigma receptors • Morphine is less lipid soluble, and has a slower onset. • Meperidine has metabolite which can cause seizure activity • Minimal effect on cerebral blood flow • Bradycardia occurs, except with meperidine • Can be reversed with naloxone

28. Narcotics • Histamine release caused by these agents causing puritis • Respiratory depression due to mu receptors • Chest wall rigidity • Stimulate chemoreceptor triggering zone – postoperative nausea/vomiting • Biliary spasm/colic • Meperidine and MAOI – combination to be avoided

29. Benzodiazepines • Diazepam, Lorazepam, Midazolam • Acts on GABA receptors • IV and PO preparations • Oral midazolam for pediatric premedication • Moderate preservation of respiration • Produces antegrade amnesia, anxiolysis, muscle relaxation and will control seizures • Minimal cardiovascular effects • Intermediate neuro/respiratory changes

30. Ketamine • Analogue of phencyclidine • Both analgesic and amnestic effects • Dissociates thalamus/limbic cortex • Excellent preservation of respiration • Bronchodilator • Sialagogue • Sympathetic effects – elevates BP, HR • Only IV agent that increases CBF • Not reversible

31. Local anesthetics • Esters • Cocaine, prilocaine, benzocaine, tetracaine • Amides • Lidocaine, bupivicaine, ropivicaine • Length of action: • Ropivicaine, bupivicaine>lidocaine

32. Local anesthetics • Bind Na channels in inactivated state and prevent influx/action potential • Bezene ring and tertiary amine separated by ester or amide • Potency – dependent on lipid solubility • Onset – dependent on pKa • Shorter if close to 7.4 • Acidic conditions delayed onset • Duration of action – dependent on protein binding

33. Local anesthetics – absorption and metabolism • Site of injection determines absorption – iv > trachael > intercostal > epidural > brachial plexus > subq • Vascocontrictors - better for short acting drugs • Metabolism -Esters: pseudocholinesterase -Amides: hepatic clearance

34. Complications of local anesthetics: • Neuro: • Circumoral parasthesias, tinnitus • Seizures • Rx: ABC’s, induction, +/- muscle relaxation • Cardiac: • Circulatory arrest • Rx: ABC’s, CPR, ACLS, Intralipid

35. Local anesthetics - Neuro • Cocaine stimulates the central nervous system • Blocks norepinephrine reuptake • Cauda equina syndrome • Subarachnoid injection via microcatheters • Transient neurologic symptoms • Subarachnoid injection of lidocaine

36. Local anesthetics • CV - depress automaticity, eventually causing circulatory arrest in some cases • Respiratory - blocks hypoxic drive and relaxes bronchial smooth muscle • Immunologic – esters more likely than amides to cause allergic reaction due to preparation in para-aminobenzoic acid (PABA)

37. General Amnesia Analgesia Unconsciousness Muscle relaxation Sedation Amnesia Analgesia General Anesthesia vs. Sedation

38. Sedation • Primary goal in sedation is to preserve spontaneous respirations. • Common medications include benzodiazepines, narcotics, and oxygen • Monitoring hemodynamic parameters is of obvious importance

39. Crisis managment • First goal is to avoid the crisis. • Preoperative evaluation is of extreme importance. • A complete review of preoperative evaluation is not possible here. • High points to hit:

40. Preoperative evaluation • Airway • Ability to intubate? Less important. • Ability to ventilate? Absolutely important. • French factors – predictors of difficult mask ventilation • BMI >26 • Age > 55 • Edentulous • Beard • History of sleep apnea/snoring

42. Ventilation • Mask ventilation • One hand • Two hand

43. Prior to doing either of these… • Call for help.

46. Oral airway, nasal airway

48. Laryngeal mask airway

49. Endotracheal intubation

50. Grade 1 view of vocal cords