General%20anesthesia%20Outline%20of%20lecture

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General anesthesiaOutline of lecture

• Components and phases of general anesthesia
• Indications for GA
• Induction of GA
• Standard
• Rapid sequence induction
• Inhalation
• Monitors employed

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Basic components of general anesthesia

• Definition of Anesthesia state of being unaware
  and unresponsive to painful stimuli
• Several aspects are involved
• a) lack of conscious awareness unconsciousness
• b) lack of perception of pain analgesia
• c) lack of movement muscle relaxation
• d) modification of autonomic responses (HR,BP) to
  painful stimuli.

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Components of general anesthesia 2

• Definition of Anesthesia state of being unaware
  and unresponsive to painful stimuli
• a) lack of conscious awareness unconsciousness
• Regional anesthesia (spinal, epidural, plexus
  block) is perhaps more correctly termed regional
  analgesia.
• Analgesia is an altered sensation of painful
  stimuli. The stimulus is felt as movement,
  pressure.
• Patient is usually partly aware of surroundings
• pregnant woman having C. Section under spinal
  analgesia

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Components of general anesthesia 3

• Two aspects of conscious awareness being
  awake and the formation of a memory of being
  awake.
• Goal of providing a level of sedation adequate to
  prevent patient being awake.
• Amount of required sedation depends on intensity
  of stimulation.
• If also give analgesia, one can prevent a patient
  being awake and in pain.
• If give relaxants and no analgesia, a patient can
  be awake and paralyzed and in pain.

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Awareness during GA

• Sedation with midazolam also causes amnesia
  (failure to form a memory of event even when
  awake)
• Volatile anesthesia at a depth greater than 0.7
  MAC is thought to prevent awareness.
• Titration of level of anesthesia to a BIS level
  less than 60 is claimed to prevent awareness.

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Components of general anesthesia 4

• Definition of Anesthesia state of being unaware
  and unresponsive to painful stimuli
• b) lack of perception of pain analgesia
• c) lack of movement in response to painful
  stimuli
• This will occur at MAC level of anesthesia or sub
  MAC levels and use of muscle relaxants
• d) modification of autonomic responses (HR,BP) to
  painful stimuli. This usually requires a dose of
  more than 1.0 MAC and is easier to achieve with
  specific drugs (beta blockers, potent narcotics)

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Components of general anesthesia 5

• Practically impossible to create state of general
  anesthesia with a single drug
• A combination of various drugs of specific types
  is commonly used.
• Result of the combination satisfies all the
  desired categories and often has a synergistic
  effect.
• A sedative narcotic is more potent than bigger
  dose of either alone

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Components of general anesthesia 6

• Volatile agent e.g. sevoflurane has large amount
  of sedation, some muscle relaxation, but no
  analgesia.
• (Nitrous oxide has above features and analgesia)
• Propofol has sedation, some relaxation, some
  amnesia and no analgesia.
• Fentanyl has mild sedation, no relaxation, no
  amnesia and large amount of analgesia
• Muscle relaxants have no sedation, amnesia or
  analgesia

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5 phases of general anesthesia

• (Preparation)
• Induction
• Maintenance
• Emergence
• Recovery

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Preparation for GA

• Patient assessment
• NPO status
• Airway
• Functional reserve of major organ systems
• CVS, respiratory, renal, hepatic
• Medications used regularly
• Allergies and previous experience with GA
• Type of planned procedure
• Urgency
• Position of patient during surgery
• Area of body involved

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Phases of general anesthesia

• Induction phase transition from awake state to
  full affect of anesthesia on CNS, CVS,
  respiratory and muscle system
• Changes in CNS function are always accompanied by
  those of other systems
• Magnitude of changes in various systems reflect
  physiological state of patient
• age, stress level, physiological reserve, fluid
  balance, drug therapy

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Induction of anesthesia

• Drug effect on CNS is primarily depression of
  usual response
• There may be contrary effects related to loss of
  inhibitory actions of CNS (excitement)
• Examples movements of limbs, hiccough, cough

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Induction of anesthesia

• Addition of supports is required to ensure
  adequate function of respiratory and CVS systems
• Airway control with oral airway, LMA, or ETT
• Ventilatory support
• Protection of the airway
• Blood pressure support with medication or IV
  fluids
• Further adjustment of anesthesia levels based on
• Patient response
• Stage of surgery
• Trends of monitored variables

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Maintenance of anesthesia

• Further adjustment of anesthesia levels based on
• Patient response
• Stage of surgery
• Trends of monitored variables
• Maintenance phase usually a stable period unless
• Changing level of surgical stress
• Impaired state of patient fitness
• Anesthesia gases form the major component with
  some IV narcotics or relaxants as background

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Emergence from anesthesia

• Slower version of induction phase in a reverse
  order
• CNS wakes up in stages or by regions
• Brainstem or lower functions first (breathing,
  cough, shivering)
• Cerebral cortex later (purposeful movements,
  response to commands)
• Removal of supports at appropriate time intervals
  
• Excitement aspects are common limb movement,
  restlessness, coughing.
• Potential for vomiting, laryngospasm, upper
  airway obstruction

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Indications for general anesthesia

• Defined by surgical procedure
• Requires profound muscle relaxation
• Incision location above umbilicus
• Inability to provide comfort with local/regional
  anesthesia
• Duration of surgery more than 3 hours
• Defined by patient
• Airway protection
• Respiratory failure
• Unstable clinical state
• Inability to cooperate/ understand regional

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Complications of general anesthesia

• Respiratory failure
• Atelectasis
• Aspiration
• Hypotension
• Injury to peripheral nerves, cornea
• Injury to respiratory tract

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Intravenous induction

• Indications
• Usual or default method of starting general
  anesthesia
• Risk of aspiration (see rapid sequence)
• Standard method involves drug combination
• Sedative in large dose (propofol) usually with
  narcotic and/or anxiolytic (midazolam)
• Muscle relaxant if doing intubation
• Mask 100 O2 during process (before, during,
  after)
• Drug doses are initially based on weight and age
  of patient. Extra doses as directed by response
  of patient

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Intravenous induction

• Contraindications
• Lack of proper equipment for resuscitation (IPPV,
  oxygen, airway devices, suction)
• Uncertainty about ability to ventilate or
  intubate patient if they become apneic
• Patient with partial airway obstruction (avoid
  apnea)

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Intravenous induction

• Precautions
• Patient with limited or uncertain CVS reserve
  (hypovolemia, CHF, valvular stenosis, sepsis)
• Patients with poorly controlled CVS disease (high
  BP, angina, disturbed heart rhythm)
• Patients with risk of aspiration
• Patients with respiratory failure

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Intravenous induction

• Standard form vs slow form
• Standard form indicates use of standard doses
  given on basis of body weight.
• Slow form indicates careful titration of strong
  sedative drugs (propofol) or narcotics. Possible
  substitution with or addition of other
  medications (ketamine)
• Goal is the use of minimal but sufficient doses
  of anesthesia to reduce intensity of CVS and
  respiratory effects and allow time for
  compensation

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Rapid Sequence Induction
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Rapid sequence induction

• Indications
• Patient at risk for regurgitation and aspiration
  who require GA
• History of recent vomiting or recent meal
• Pregnancy
• Increased intra-abdominal pressure
• Abdominal distension
• Poorly controlled GE reflux
• Decreased level of consciousness

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Rapid sequence induction

• Contraindications
• Potential difficult intubation
• Potential airway obstruction
• Laryngeal injury
• Cervical spine injury
• Poorly controlled BP

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Rapid sequence induction

• Precautions
• Potential for loss of airway control
• Potential for severe BP change (high or low)
• Potential for cardiac dysrhythmias, including
  arrest, in predisposed patient.
• Potential for marked increase in ICP

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Rapid sequence induction

• Method
• Preoxygenation is critical best method unclear.
• Suction and airway alternatives available
• Use adjuvant drugs to control BP, HR response
  midazolam, narcotics, lidocaine, ketamine, etc
• Explain and rehearse use of cricoid pressure with
  the patient. Optimize position of upper airway.
• Dose of potent sedative (propofol) as per body
  weight or titrate depending on reserve of CVS

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Rapid sequence induction

• After patient is asleep, apply cricoid pressure
  and give relaxant in large dose.
• Two choices
• no active ventilation, proceed with laryngoscopy
  as relaxant has peak effect
• Gentle IPPV (Paw 10-15 cm H2O) with 100 O2 until
  relaxant has peak effect.
• Place ETT, and inflate cuff and confirm correct
  position of ETT before removing cricoid pressure

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Inhalation induction
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Inhalation induction

• Indications
• Difficult IV access
• Potential airway obstruction e.g. epiglottitis
• Thoracic diseases which preclude use of IPPV
• Mediastinal mass, foreign body in airway,
  broncho-pleural fistula
• Patients unable to cooperate with awake airway
  endoscopy

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Inhalation induction 2

• Contraindications
• Aspiration risk (unless overruled by airway
  concerns)
• Active bleeding in airway (risk of cough,
  laryngospasm)
• Note profound changes in BP are unusual with this
  as compared to rapid sequence with IV drugs
• No controlled studies in this area of right way
  to do induction in this type of patient

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Inhalation induction 3

• Precautions
• Lack of patient cooperation or comprehension
• Preexisting respiratory failure
• Patients may become restless before falling
  deeply asleep. This is a temporary phenomenon
  excitement phase. Use gentle assisted
  ventilation and wait.
• After several minutes of anesthesia, expect
  improved conditions for starting an IV, if not
  already done.

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Inhalation induction 4

• Describe steps briefly to patient. Emphasis on
  deep breaths with maximal breath holding
  interval.
• Best agents are sevoflurane, enflurane,
  halothane.
• Desflurane and isoflurane are irritating to
  airway.
• Avoid narcotics give sedation with midazolam.
• Coach patient with calm, reassuring voice
• Choices of technique
• Several deep breaths from a primed circuit
• Slow incremental doses with normal ventilation

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Inhalation induction 5

• Single / several deep breath technique
• Prime circuit with anesthesia agent from
  vapourizer at maximum setting, FGF at 8L/min, pop
  off valve open and patient end of circuit
  occluded.
• Have patient exhale maximally, then apply face
  mask to patient and inhale maximally from primed
  circuit.
• Expect prompt onset of sleep (60 seconds)
  followed by transient apnea, then pattern of
  rapid shallow respirations.

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Inhalation induction 6

• Slow incremental doses with normal ventilation
• Prime circuit with N2O 70, FGF at 8L/min, pop
  off valve open and patient end of circuit
  occluded.
• When patient is comfortable with situation, begin
  volatile agent increasing vapourizer setting by
  0.5 every 3 or 4 breaths. Reassure patient with
  calm voice encouraging a regular smooth breathing
  pattern.
• Use of a deep breathing pattern here may lead to
  premature onset of apnea with prolonged phase.
• Expect several minutes to fall asleep. Assist
  ventilation

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Inhalation induction 7

• Time to safe airway insertion Use eye signs and
  elapsed time, not ET concentration as guide.
• Consider response to oral airway as trial
• With single deep breath technique, authors
  suggest possible insertion of LMA after at least
  2 minutes, ETT at least 5 minutes following onset
  of sleep.
• Laryngospasm, coughing, inadequate view of larynx
  is possible. Do not rush.
• Place patient on 50-100 O2 shortly before
  attempted insertion of LMA / ETT

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Monitors used during Induction of Anesthesia
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Monitors during inductionof anesthesia

• Pulse oximetry and end tidal CO2 are critical
• Eyes and ears of the anesthesia person
• Experienced assistant is very important
• Stethescope, BP, EKG
• Prepare with plan B