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General anesthesia

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General anesthesia General anesthesia was not known until the mid-1800 s Diethylether was the first general anesthetic used for surgery General Anesthetics are ... – PowerPoint PPT presentation

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Title: General anesthesia


1
General anesthesia
  • General anesthesia was not known until the
    mid-1800s
  • Diethylether was the first general anesthetic
    used for surgery
  • General Anesthetics are divided into two
    classes
  • Inhaled anesthetics (usually halogenated
    compounds)
  • Intravenous anesthetics or induction agents

2
Modern Anesthesia
  • It combines the following
  • Analgesia
  • Sleep (loss of consciousness)
  • Skeletal Muscle relaxation
  • amnesia
  • Abolition sensory autonomic reflexes
  • No single drug can produce all these effects

3
Ideal anesthesia is
  • Induce loss of consciousness smoothly and rapidly
  • Allow for prompt recovery of cognitive function
    after its administration is discontinued
  • Possess wide margin of safety
  • Have no side effects
  • No single drug can produce all these effects

4
Stages of anesthesia
  • Stage 1analgesia
  • Decreased pain awareness, sometimes with amnesia
    ,conscious may be impaired but not lost
  • Stage 2disinhibition
  • Delirium, excitation, amnesia, enhanced reflexes,
    irregular respiration and incontinence
  • Stage 3surgical anesthesia
  • Unconsciousness ,no pain reflex, regular
    respiration and maintained blood pressure
  • Stage 4medullary depression
  • Severe CVS and respiratory depression and the
    patient require pharmacological and ventilatory
    support

5
Anesthesia protocols
  • For minor procedure, conscious sedation conscious
    sedation techniques that combine IV agent with
    local anesthetics are often used these can
    provide profound analgesia, with retention of the
    patient ability to maintain a patent airway and
    response to verbal commands
  • For extensive surgical procedure protocol
    commonly includes IV drug for induction, inhaled
    agent(with or without IV)for maintenance and
    neuromuscular junction blockers to cause muscle
    relaxation

6
General Anesthetics
  • Absence of sensation associated with a reversible
    loss of consciousness, skeletal muscle
    relaxation, and loss of reflexes.
  • Drugs used for anesthesia are CNS depressants
    with action that can be induced and terminated
    more rabidly than conventional sedative and
    hypnotics
  • Most sensitive site of action for general
    anesthetics is the reticular activating system of
    the brainstem (RAS)
  • Anesthetic dose does not cause depression of
    cardiac, vasomotor or respiratory centers
  • Has a small margin of safety

7
Inhaled Anesthetics
  • Include
  • Nitrous oxide
  • Halothane
  • Enflurane
  • Isoflurane
  • Desflurane

8
Intravenous Anesthetics
  • Include
  • Barbiturates
  • Thiopental Methohexital
  • Opioids
  • Alfentanil, Meperidine, Fentanyl, Sufentanil
    (agonists)
  • Naloxone (antagonist)
  • Benzodiazepines
  • Diazepam, Midazolam
  • Flumazenil (antagonist)

9
Intravenous Anesthetics
  • Miscellaneous Agents
  • Etomidate non-barbiturate hypnotic agent
    without analgesic properties
  • Droperidol - Neuroleptic (similar to Haloperidol)
    - combined with Fentanyl and is used for
    neuroleptanalgesia (state of analgesia and
    amnesia)
  • Ketamine - dissociative anesthetic
  • Propofol

10
General Uses of IV Anesthetics
  • Primary Use induction of general anesthesia
  • Supplement general anesthesia
  • maintain general anesthesia
  • provide sedation
  • control Blood Pressure

11
Intravenous agents
  • Mechanism of action
  • Act at cell surface receptors
  • Barbiturates and benzodiazepine act at GABA-A
    receptors to increase Cl- influx
  • Opioids act on m and other subtypes
  • Ketamine antagonizes PCP site on NMDA receptors
    (prevent excitation)
  • Pharmacokinetics
  • Rapid induction shorter acting
  • Duration of effect proportional to redistribution
    from brain to other tissue

12
Barbiturates Thiopentone
  • Ultra-short acting hypnotic with no analgesic
    action
  • High lipid solubility promotes rapid entry to the
    brain
  • Eliminated by the liver
  • Has rapid onset of action and recovery
  • M.O.A. potentiates GABA, decrease glutamate
    activity, increase chloride ion conductance
  • Adverse reactions decreased myocardial and
    respiratory activity

13
Etomidate
  • Imidazole derivative that provide induction with
    minimal change in cardiac function and
    respiratory rate and has short duration of action
  • It is not analgesic , and its primary advantage
    is in anesthesia for patient with limited
    respiratory and cardiac reserve
  • Activates GABA receptors
  • Uses
  • Induction of anesthesia
  • Side effects
  • Myoclonus
  • Post-operative nausea and vomiting

14
Ketamine
  • This drug produce dissociative state in which the
    patient is patient remains conscious but has
    marked catatonia, analgesia, and amnesia
  • It is a chemical congener of the psychotomimetic
    agent, phencyclidine (PCP)
  • It is a cardiovascular stimulant drug and this
    action may cause increase ICP
  • Emergency reactions include disorientation
    ,excitation and hallucination which can be
    reduced by preoperative administration of
    benzodiazepines

15
  • Uses- Induction of anesthesia
  • in children
  • in severely hypovolemic patients
  • Contraindications
  • Increased intracranial pressure
  • Ischemic heart disease
  • Psychological disorders
  • Effects
  • Analgesic with dissociative anesth. properties
  • Dreaming in children

16
Propofol
  • Uses
  • Induction and maintenance of anesthesia
  • As anesthetic agent at outpatient surgery
  • Also effective in producing prolog sedation in
    patient in critical care setting
  • Contraindications
  • Cardiovascular instability due to marked
    reduction in the peripheral resistance
  • Effects
  • Hypnosis ,Antiemetic
  • Fast acting, short duration. Fewer peripheral
    side effects compared to barbiturates

17
Opiates
  • Potent analgesics
  • Fentanyl -Potency 50-100X gtMorph
  • Alfentanil -Potency 25-30X gt Morph
  • Sufentanil -Potency 5-10X gtFentanyl
  • Meperidine
  • Uses
  • Supplementation of general anesthesia or
    analgesia
  • Effects
  • respiratory depression
  • nausea and vomiting
  • muscle rigidity

18
  • INHALATION ANESTHETICS

19
MAC(minimal alveolar concentration)
  • MAC of anaesthetic measures potency of
    anaesthetic vapour. High MAC means low potency
  • Defined as the concentration of anesthetic that
    prevents movement induced by a painful stimulus
    in 50 of subjects.

20
Mechanism of Action
  • Potency is correlated with lipid solubility
  • Olive oilgas partition coefficient
  • The greater the number, the more potent the
    anesthetic Methoxyfluranegthalothanegtisoflurane
    etc.

21
Theories for Mechanism of Action
  • Theory 1
  • Gas movement into lipid membrane disrupting ion
    channels and action potential propagation
  • Increased Atmospheric pressure will reverse
    effects
  • Theory 2
  • Binding theory anesthetics bind to hydrophobic
    portion of the ion channel
  • Theory 3
  • Neuromodulator theory anesthetics bind to
    cell-surface receptors.
  • increased Cl- flux (possible GABA mediation)

22
Pharmacokinetics of Inhaled Anesthetics
  • Factors influencing the effects of inhaled
    anesthetics
  • Amount that reaches the brain
  • Indicated by oilgas ratio (lipid solubility)
  • Partial pressure of anesthetic
  • 5 anesthetic 38 mmHg (10 76 mmHg)
  • Solubility of gas into blood
  • The lower the bloodgas ratio, the more
    anesthetic will arrive at the brain
  • Cardiac Output
  • Increased CO greater Induction time

23
Rate of Entry into the Brain Influence of Blood
and Lipid Solubility
24
General Actions of Inhaled Anesthetics
  • Respiration
  • Depressed respiration
  • Kidney
  • Depression of renal blood flow and urine output
  • Muscle
  • High enough concentrations will relax skeletal
    muscle

25
General Actions of Inhaled Anesthetics
  • Cardiovascular System
  • Generalized reduction in arterial pressure and
    peripheral vascular resistance. Isoflurane
    maintains CO and coronary function better than
    other agents
  • Central Nervous System
  • Increased cerebral blood flow and decreased
    cerebral metabolism

26
Toxicity and Side Effects
  • Depression of respiratory drive
  • Depressed cardiovascular drive
  • Fluoride-ion toxicity from methoxyflurane
  • Metabolized in liver release of Fluoride ions
  • Decreased renal function allows fluoride to
    accumulate nephrotoxicity
  • Malignant hyperthermia
  • To treat this, rapidly cool the individual and
    administer Dantrolene to block release of Calcium
    from muscle sarcoplasmic reticulum

27
Advantages and Disadvantages of Selected Inhaled
Anesthetics
  • Isoflurane
  • Cardiac output is maintained
  • Arrhythmias are uncommon
  • Potentiates the actions of muscle relaxants
  • Minimally metabolized and no reports of heptato-
    or nephrotoxicity
  • most widely used agent
  • MAY CAUSE MALIGNANT HYPERTHERMIA

28
Advantages and Disadvantages of Selected Inhaled
Anesthetics
  • Desflurane
  • More irritating to airways than other agents
  • Rapid recovery
  • No reports of malignant hyperthermia

29
Preanaesthetic Medication
30
Focus Points
  • Induction of anesthesia is through use of any of
    the IV agents (Barbiturates Thiopental, Opiate
    Fentanyl, Benzodiazepines Midazolam,
    Dissociative Ketamine, Others Propofol,
    Etomidate and Droperidol)
  • Majntenance of anesthesia is through use of any
    of the ihalation agents
  • -N2O (70 in oxygen) is not suitable alone
  • - N2O is usually combined with another
    inhalation agent or with opioids e.g. fentanyl

31
A comparison
  • halothane
    N2O
  • Speed of induction intermediate fast
  • Potency v.potent
    weak
  • MAC2
    MAC 80
  • Muscle relaxation some
    none
  • Cardiac arrhythmia yes
    no
  • Liver damage yes
    no
  • Recovery slow
    rapid

32
  • NOTES
  • Enflurane releases flouride ions which may cause
    renal failure
  • All inhalation anesthetics can cause resp.
    depression, myocardial depression, cardiac
    arrhythmias, hypotension and PONV
  • A mixture of N2O(50-70) and haothane 1 is
    usually used in anesthesia.

33
Nitrous Oxide
  • Characterized by inert nature with minimal
    metabolism
  • Colorless, odorless, tasteless, and does not burn

34
  • Simple linear compound
  • Only anesthetic agent that is inorganic
  • Major difference is low potency
  • MAC value is 80 - 105
  • Weak anesthetic, powerful analgesic
  • Needs other agents for surgical anesthesia
  • Low blood solubility (quick recovery)

35
  • GOOD LUCK
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