Transcript Basic components of general anesthesia

General anesthesia
Outline of lecture
 Components and phases of general anesthesia
 Indications for GA
 Induction of GA
 Standard
 Rapid sequence induction
 Inhalation
 Monitors employed
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.
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
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.
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.
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)
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
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
5 phases of general anesthesia
 (Preparation)
 Induction
 Maintenance
 Emergence
 Recovery
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
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
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
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
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
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
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
Complications of general anesthesia
 Respiratory failure
 Atelectasis
 Aspiration
 Hypotension
 Injury to peripheral nerves, cornea
 Injury to respiratory tract
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
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)
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
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
Rapid
Sequence
Induction
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
Rapid sequence induction
 Contraindications:
 Potential difficult intubation
 Potential airway obstruction
 Laryngeal injury
 Cervical spine injury
 Poorly controlled BP
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
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
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
Inhalation induction
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
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”
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.
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
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.
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
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
Monitors
used
during
Induction
of
Anesthesia
Monitors during induction
of 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