general anesthesia outline of lecture components and phases of general anesthesia indications for ga...
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General anesthesiaOutline of lecture
Components and phases of general anesthesia
Indications for GAInduction of GA
StandardRapid sequence inductionInhalation
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 involveda) lack of conscious awareness =
unconsciousnessb) lack of perception of pain = analgesiac) 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 = unconsciousnessRegional 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 GASedation 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 = analgesiac) 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 5Practically impossible to create state of
general anesthesia with a single drugA 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 6Volatile 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 MaintenanceEmergenceRecovery
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Preparation for GAPatient assessment
NPO statusAirway Functional reserve of major organ systems
CVS, respiratory, renal, hepaticMedications used regularlyAllergies and previous experience with GA
Type of planned procedureUrgencyPosition of patient during surgeryArea of body involved
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Phases of general anesthesiaInduction 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 anesthesiaDrug 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 anesthesiaAddition of supports is required to ensure
adequate function of respiratory and CVS systemsAirway control with oral airway, LMA, or ETT Ventilatory supportProtection of the airwayBlood pressure support with medication or IV fluids
Further adjustment of anesthesia levels based on Patient responseStage of surgeryTrends of monitored variables
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Maintenance of anesthesiaFurther adjustment of anesthesia levels based on
Patient responseStage of surgeryTrends of monitored variables
Maintenance phase usually a stable period unlessChanging level of surgical stressImpaired state of patient fitness
Anesthesia gases form the major component with some IV narcotics or relaxants as background
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Emergence from anesthesiaSlower version of induction phase in a reverse
orderCNS 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 anesthesiaDefined by surgical procedure
Requires profound muscle relaxationIncision location above umbilicus Inability to provide comfort with local/regional
anesthesiaDuration of surgery more than 3 hours
Defined by patientAirway protectionRespiratory failureUnstable clinical stateInability to cooperate/ understand regional
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Complications of general anesthesiaRespiratory failureAtelectasisAspirationHypotensionInjury to peripheral nerves, corneaInjury 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 intubationMask 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 aspirationPatients with respiratory failure
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Intravenous induction Standard form vs slow formStandard 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 SequenceInduction
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Rapid sequence induction Indications: Patient at risk for regurgitation and
aspiration who require GAHistory of recent vomiting or recent mealPregnancyIncreased intra-abdominal pressureAbdominal distensionPoorly controlled GE refluxDecreased level of consciousness
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Rapid sequence induction Contraindications:
Potential difficult intubationPotential airway obstructionLaryngeal injuryCervical spine injuryPoorly controlled BP
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Rapid sequence induction Precautions:
Potential for loss of airway controlPotential 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 availableUse adjuvant drugs to control BP, HR response:
midazolam, narcotics, lidocaine, ketamine, etcExplain 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 Indications:
Difficult IV accessPotential airway obstruction e.g. epiglottitisThoracic 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 comprehensionPreexisting 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 voiceChoices of technique:
Several deep breaths from a primed circuitSlow incremental doses with normal ventilation
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Inhalation induction 5Single / 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 6Slow incremental doses with normal ventilationPrime 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 7Time to safe airway insertion: Use eye signs and
elapsed time, not ET concentration as guide.Consider response to oral airway as trialWith 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 criticalEyes and ears of the anesthesia personExperienced assistant is very importantStethescope, BP, EKGPrepare with plan B