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General Anesthesia and Sedation. Josh Smith, M.D. Assistant Professor Department of Anesthesiology. Objectives:. Review of pharmacology Inhalational agents Induction agents Muscle relaxants Narcotics Benzodiazepines Local anesthetics General anesthesia Sedation Crisis management.

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general anesthesia and sedation

General Anesthesia and Sedation

Josh Smith, M.D.

Assistant Professor

Department of Anesthesiology

objectives
Objectives:
  • Review of pharmacology
    • Inhalational agents
    • Induction agents
    • Muscle relaxants
    • Narcotics
    • Benzodiazepines
    • Local anesthetics
  • General anesthesia
  • Sedation
  • Crisis management
ideal inhaled anesthetic
Ideal Inhaled Anesthetic
  • Maintains cardiovascular function
  • Maintains respiratory function
  • Allows adequate oxygenation
  • Chemically stable
  • Nontoxic
  • Easy to administer
  • Inexpensive
minimum alveolar concentration mac
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.

slide5
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.
slide6
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

factors that increase mac
Factors that Increase MAC
  • Increased central neurotransmitter levels
    • Acute amphetamine use
    • Cocaine use
  • Hyperthermia
  • Young age
anesthetic partial pressure gradients may exist between
Anesthetic Partial PressureGradients May Exist Between
  • Vaporizer
  • Inflow
  • Circle System ( inspired partial pressure )
  • Alveoli ( alveolar partial pressure )
  • Blood
  • Brain
gases in solution
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).
slide11
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.
anesthetic solubility in blood
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

blood gas inhaled anesthetic o g partition coefficient
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

induction agents
Induction agents
  • Propofol
  • Thiopental (barbituate)
  • Etomidate
propofol
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
barbituates
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

etomidate
Etomidate
  • Neuro - similar to barbs; myoclonus
  • Minimal cardio-respiratory effects
  • Postoperative nausea/vomiting
  • Adrenocortical suppression – studies showing increased mortality in critical patients
muscle relaxants
Muscle relaxants
  • Depolarizing
    • Succinylcholine
  • Nondepolarizing
    • Steriod based
      • Rocuronium, vecuronium, pancuronium
    • Benzylisoquinolones
      • Cisatricurim, atracuruim
succinylcholine
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
succinylcholine1
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.
nondepolarizing agents
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

narcotics
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
narcotics1
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
benzodiazepines
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
ketamine
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
local anesthetics
Local anesthetics
  • Esters
    • Cocaine, prilocaine, benzocaine, tetracaine
  • Amides
    • Lidocaine, bupivicaine, ropivicaine
  • Length of action:
    • Ropivicaine, bupivicaine>lidocaine
local anesthetics1
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
local anesthetics absorption and metabolism
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

complications of local anesthetics
Complications of local anesthetics:
  • Neuro:
    • Circumoral parasthesias, tinnitus
    • Seizures
    • Rx: ABC’s, induction, +/- muscle relaxation
  • Cardiac:
    • Circulatory arrest
    • Rx: ABC’s, CPR, ACLS, Intralipid
local anesthetics neuro
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
local anesthetics2
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)
general anesthesia vs sedation
General

Amnesia

Analgesia

Unconsciousness

Muscle relaxation

Sedation

Amnesia

Analgesia

General Anesthesia vs. Sedation
sedation
Sedation
  • Primary goal in sedation is to preserve spontaneous respirations.
  • Common medications include benzodiazepines, narcotics, and oxygen
  • Monitoring hemodynamic parameters is of obvious importance
crisis managment
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:
preoperative evaluation
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
ventilation
Ventilation
  • Mask ventilation
    • One hand
    • Two hand
malignant hyperthermia
Malignant Hyperthermia
  • 1/15k ped; 1/40k adults
  • Trigger: succinylcholine or halogenated agents
  • Abnormal Ca release
  • Hypermetabolic state: oxygen consumption

and CO2,lactic acidosis, hyperthermia,

hyperkalemia, rigidity

  • Vfib, renal failure, DIC; mortality 5-30%
  • Tx: dantrolene, cooling, diuresis, correct H/K
malignant hyperthermia1
Malignant Hyperthermia
  • Muscular dystrophy, MMR
  • Non-triggering anesthetic
  • NMS, pheo, thyroid storm, sepsis