General anesthesiaOutline of lecture

Components and phases of general anesthesia

Indications for GAInduction of GA

StandardRapid sequence inductionInhalation

Monitors employed

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.

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

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 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.

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)

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

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

5 phases of general anesthesia(Preparation)Induction MaintenanceEmergenceRecovery

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

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

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

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

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

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

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

Complications of general anesthesiaRespiratory failureAtelectasisAspirationHypotensionInjury to peripheral nerves, corneaInjury 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 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

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 aspirationPatients with respiratory failure

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

Rapid SequenceInduction

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

Rapid sequence induction Contraindications:

Potential difficult intubationPotential airway obstructionLaryngeal injuryCervical spine injuryPoorly controlled BP

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

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

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 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

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 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.

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

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.

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

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

Monitors used

during Induction

of Anesthesia

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