General Anesthesia and Sedation

Report
General Anesthesia and
Sedation
Josh Smith, M.D.
Assistant Professor
Department of Anesthesiology
Objectives:
• Review of pharmacology
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Inhalational agents
Induction agents
Muscle relaxants
Narcotics
Benzodiazepines
Local anesthetics
• General anesthesia
• Sedation
• Crisis management
Ideal Inhaled Anesthetic
• Maintains cardiovascular function
• Maintains respiratory function
• Allows adequate oxygenation
• Chemically stable
• Nontoxic
• Easy to administer
• Inexpensive
Minimum Alveolar
Concentration (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.
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.
Inhaled Anesthetic
Minimum
Alveolar
Concentration
( Vols % )
Nitrous oxide
Cyclopropane
Desflurane (Suprane)
104
9.2
6.0
Diethyl ether
Sevoflurane (Ultane)
Enflurane (Ethrane)
1.92
1.71
1.70
Isoflurane (Forane)
Halothane (Fluothane)
Methoxyflurane (Penthrane)
1.15
0.77
0.16
Factors that Increase MAC
• Increased central neurotransmitter
levels
• Acute amphetamine use
• Cocaine use
• Hyperthermia
• Young age
Anesthetic Partial Pressure
Gradients May Exist Between
• Vaporizer
• Inflow
• Circle System ( inspired partial pressure )
• Alveoli ( alveolar partial pressure )
• Blood
• Brain
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).
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
• 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
Inhaled Anesthetic (O/G)
Blood / Gas
Partition Coefficient
Cyclopropane (9.2)
Desfluane (6.0)
Nitrous Oxide (104)
Sevoflurane (1.7)
0.41
0.42
0.47
0.60
Isoflurane (1.15)
Enflurane (1.7)
Halothane (0.77)
1.4
1.91
2.3
Diethyl ether (1.92)
Methoxyflurane (0.16)
12.1
12.0
Induction agents
• Propofol
• Thiopental (barbituate)
• Etomidate
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
• 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
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Neuro - similar to barbs; myoclonus
Minimal cardio-respiratory effects
Postoperative nausea/vomiting
Adrenocortical suppression – studies
showing increased mortality in critical
patients
Muscle relaxants
• Depolarizing
– Succinylcholine
• Nondepolarizing
– Steriod based
• Rocuronium, vecuronium, pancuronium
– Benzylisoquinolones
• Cisatricurim, atracuruim
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
Succinylcholine
• Bradycardia after 2nd dose
– quicker release from muscarinic receptors
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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
• 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
• 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
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
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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
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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
• Esters
– Cocaine, prilocaine, benzocaine, tetracaine
• Amides
– Lidocaine, bupivicaine, ropivicaine
• Length of action:
– Ropivicaine, bupivicaine>lidocaine
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
• 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:
• Neuro:
– Circumoral parasthesias, tinnitus
– Seizures
– Rx: ABC’s, induction, +/- muscle relaxation
• Cardiac:
– Circulatory arrest
– Rx: ABC’s, CPR, ACLS, Intralipid
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 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
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Amnesia
Analgesia
Unconsciousness
Muscle relaxation
Sedation
• Amnesia
• Analgesia
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
• 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
• Airway
– Ability to intubate? Less important.
– Ability to ventilate? Absolutely important.
• French factors – predictors of difficult mask
ventilation
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BMI >26
Age > 55
Edentulous
Beard
History of sleep apnea/snoring
Ventilation
• Mask ventilation
– One hand
– Two hand
Prior to doing either of these…
• Call for help.
Oral airway, nasal airway
Laryngeal mask airway
Endotracheal intubation
Grade 1 view of vocal cords
Grade 3 view of cords - bougie
Malignant Hyperthermia
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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 Hyperthermia
• Muscular dystrophy, MMR
• Non-triggering anesthetic
• NMS, pheo, thyroid storm, sepsis

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