MS3 General Anesthesia Overview

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MS3 General Anesthesia Overview
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General Anesthesia

• A controlled reversible state of
• Amnesia (with loss of consciousness)
• Analgesia
• Akinesia (skeletal muscle relaxation)
• Autonomic and sensory reflex blockade
• Called the 4 As of General Anesthesia

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Anesthesia Terms
Balanced Anesthesia- GA with several
Agents Regional Anesthesia- using LAs to
anesthetize a body region Combined Technique-
regional plus light GA Conscious Sedation- IV
agents for analgesia/ anxiolysis
maintaining consciousness
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Minimum Alveolar Concentration (MAC)

• The steady state minimum alveolar concentration
  (percent) of an inhalational agent that is
  required for immobility of 50 of the subjects
  exposed to a noxious stimulus (e.g., surgical
  incision)

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Minimum Alveolar Concentration (MAC)

• Provides a means to compare the potency of the
  various inhalational agents
• Serves as a guide to determining dose
• Similar to an ED50 value for an intravenous agent
• MAC values are additive

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Dose Response(Multiples of MAC)

• DOSE RESPONSE ALVEOLAR CONCENTRATION at
  which
• .5 X MAC MAC Awake 50 of patients can be
  awakened
• 1.0X MAC MAC 50 of patients will not move
  at surgical incision
• 1.3X MAC ED 95 95 of patients will not
  move at surgical incision
• 1.5-2.0X MAC-BAR 50 of patients have
  blocked autonomic response

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Stages of AnesthesiaGuedel 1930s

• Stage of Analgesia
• Analgesia without amnesia, impaired judgement,
  vertigo/ataxia, increased respiration, blood
  pressure, heart rate
• Stage of Excitement
• Delirious, excited, amnestic. Irregular
  respirations, struggling, retching and vomiting

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Stages of Anesthesia- cont.

• Stage of Surgical Anesthesia
• Recurrence of regular respiration --gt cessation,
  Loss of corneal, swallowing, eyelid reflexes
  Skeletal muscle relaxation Decreased blood
  pressure
• Stage of Medullary Depression
• Begins at cessation of spontaneous respiration
  --gt severe depression of vasomotor and
  respiratory centers--gtwithout support Death

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Inhalational Anesthetics

• Drugs administered as gases
• Via facemask or endotracheal tube
• Volatile liquids vaporized in a carrier gas
• Potent Agents widely used in USA halothane,
  isoflurane, desflurane and sevoflurane
• Nitrous Oxide (N2O), a gas at ambient
  temp/pressure, is a low potency adjunct

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Modern Vaporizers allow Fine Control of
Inhalational Agent Delivery
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Big Picture
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Partial Pressure

• The concentration of a gas in a mixture is
  proportional to its Partial Pressure. The terms
  partial pressure (torr) and concentration (vol
  per cent) are used interchangeably to describe
  the dosage of inhaled anesthetics PPA
  x 100 concentration (vol ) Total P

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Inhalational AnestheticsPharmacokinetics

• Agents achieve increased effect as the steady
  state concentration (partial pressure) increases
  in the brain
• Rate of partial pressure increase in the brain
  depends on multiple factors Solubility,
  Inspired anesthetic concentration Pulmonary
  ventilation Arteriovenous concentration
  gradients Pulmonary and cerebral blood flow

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Solubility

• Defined in terms of a partition coefficient
• BloodGas partition coefficient describes an
  anesthetics relative affinity for the blood
  compared to air
• molecules in blood/ molecules in gas at SS
• Agents with low solubility require relatively few
  molecules to dissolve into the blood to raise
  partial pressure to equilibrium (opposite for
  highly soluble agents)

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Inspired Anesthetic Concentration

• The rate of induction of anesthesia by an
  inhalational agent can be increased by increasing
  the inspired concentration of the agent
• Several multiples of the MAC value for an agent
  are often used initially to try to rapidly
  achieve an adequate brain level of an agent

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Pulmonary Ventilation

• The rate of rise of anesthetic in the blood is
  also determined by the minute ventilation of the
  patient
• Increasing ventilation generally increases the
  speed of induction, i.e. carrying agent into the
  alveoli faster means gas taken up is replenished
  more quickly and a higher concentration is
  maintained in alveoli

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Pulmonary Blood Flow

• Increased pulmonary blood flow (increased cardiac
  output) decreases the rate of rise of the
  arterial anesthetic gas tension
• Agent has a greater volume of blood to saturate
  and the partial pressure will increase more
  slowly
• Patients with low cardiac output would therefore
  have a relatively quick induction

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Arteriovenous Concentration Gradient

• Dependent on the uptake of the anesthetic by the
  tissues
• TissueBlood partition coefficients
• Rates of blood flow to the tissues
• Concentration gradients
• The larger the partial pressure gradient between
  arterial and venous blood, the more time it takes
  to achieve equilibrium

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Gas Movement on Induction
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Elimination

• Recovery processes are similar (reversed) to
  those of induction
• Major route of elimination is via the lungs and
  the agents follow a gradient back to the alveoli
• Agents with low solubility are eliminated quickly
• Minimal metabolism, primarily in the liver

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Gas flow Brain --gt Lung
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Cardiovascular Effectsof Inhalational Agents

• Dose-dependent decrease in MAP
  (N2O sympathomimetic effects may obscure effect
  when used with potent agents)
• Newer agents predominately decrease SVR,
  Halothane depresses the myocardium
• Potent agents reduce myocardial O2 consumption
• Halothane sensitizes myocardium to
  catechol-amines, predisposing to ventricular
  arrhythmias

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Respiratory Effectsof Inhalational Agents

• Decreased tidal volume with an increased
  respiratory rate --gt overall decrease in minute
  ventilation (except N2O)
• All inhalational agents decrease the ventilatory
  response to increases in PaCO2
• Decreased ventilatory response to hypoxia

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Malignant Hyperthermia

• Rare, inherited, potentially lethal syndrome
• Characterized by hypermetabolic activity, marked
  CO2 production, altered skeletal muscle tone and
  metabolic acidosis
• Triggers potent inhaled agents and SUX
• Probably altered Calcium metabolism
• Treatment Dantrolene (CA blocker)
• DX requires muscle biopsy/ in vitro testing

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Halothane

• Halogenated hydrocarbon
• __________________
• 1 agent worldwide
• Venerable, breathable
• __________________
• Myocardial Depressant
• Myocardial Sensitization
• Metabolized 15-20

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Isoflurane (Forane)

• Halogenated Ether
• _______________
• 1 in USA
• Decreases SVR
• Little myocardial depression
• Minimal metabolism

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Nitrous Oxide

• Low molecular weight gas
• ______________________
• Low solubility (fast on/off)
• Good adjunct
• Minimal Myocardial effects
• _______________________
• Low Potency

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Intravenous Anesthetics

• Sedative-hypnotics
• Barbiturates
• Etomidate
• Propofol
• Benzodiazepines
• Opioids
• Dissociative anesthetics
• Ketamine

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Barbiturates

• Thiopental ultra short acting
• Crosses BBB rapidly
• Short effect due to redistribution (t1/2ª)
• Dose dependent decreases in SV,
  MAP, CO
• Potent respiratory depressant i.e.-gtApnea
• Reduces CMRO2 and CBF

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Etomidate

• Carboxylated imidazole
• Minimal CV effects
• Potential adrenocortical suppression
• Myoclonic movements

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Propofol

• Phenol derivative
• Effects similar to barbiturates
• Amnestic and anti-emetic effects
• Useful as sedation agent
• Most popular ambulatory surgery induction agent
  --gt least residual sedation

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Benzodiazepines

• Primarily for anxiolytic/amnestic effects
• Preop/intraop sedation
• Uncommonly used as induction agents
• Midazolam most popular (short T1/2 and water
  soluble)
• Specific antagonist Flumazenil

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Opioids

• Main analgesic agents
• CV stability allows high dose techniques
• Dose dependent respiratory depression i.e.
  decreased RR, decreased minute ventilation
• Chest wall rigidity

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Opioids

• Prototype Agent Morphine
• Common synthetic agents Fentanyl, Sufentanil,
  Alfentanil, Remifentanil
• Specific Antagonist Naloxone

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Ketamine

• PCP cousin
• Dissociative action
• Sympathomimetic effects
• Analgesic
• Nightmares

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