Induction, Maintenance, Emergence

Question 1

Define induction

Answer 1

Administration of a drug or combination of drugs at the beginning of an anaesthetic that results in a state of general anaesthesia

Question 2

What are the 2 broad categories of induction

Answer 2

Inhalational vs Intravenous

Question 3

When is a rapid sequence induction indicated

Answer 3

All patients that are non-fasted, or at high risk of aspiration of gastric contents
(hiatus hernia, pregnancy, recent trauma, high opiate requirement, intoxicated, abdo pathology, bowel obstruction, peritonitis, obtunded, metabolic disease - diabetes, other cause of delayed gastric emptying)

Question 4

What are the indications for an inhalational induction

Answer 4

2 main indications

• Avoid IV induction due to requirement for an IVC (children, severe needle phobia, difficult IV access)
• Need to maintain airway patency and spontaneous ventilation during induction (anticipated difficult airway or ventilation, inhaled FB, bronchopleural fistula)

Question 5

Outline the process of an inhalational induction

Answer 5

Explain process to patient/parents
Apply routine monitoring
Close fitting facemask
Have a backup plan (esp if anticipated DA)
Have a 2nd person ready to cannulate
Pre-oxygenate as able
Consider addition of N2O - speeds onset (avoid in DA)
Sevoflurane and Halothane best tolerated gases
Halothane introduced in stepwise fashion from 1-1.5%, Sevo sometimes started immediately at 8%, sometimes stepped from 2%

Question 6

Which are the commonly used IV induction agents?

What are the associated dosages for inducation

Answer 6

Propofol 2mg/kg
Thiopentone 3-5mg/kg
Ketamine 1-2mg/kg
(Midazolam)

Question 7

What are the advantages to use of Propofol

Answer 7

Short duration of action due to rapid distribution
High clearance - less accumulation cf Thiopentone
Marked reduction in airway and pharyngeal reflexes - ideal for LMAs
Reduces CBF, CMRO2 and ICP
Can be used for TIVA
Safe in porphyria

Question 8

What are the disadvantages of Propofol as an induction agent

Answer 8

Produces the most marked fall in blood pressure of all induction drugs, greatest in hypovolaemic and elderly
Causes respiratory depression and apnoea
Can cause epileptiform movts (not true seizure)
Painful injection
PRIS has been described after a single dose

Question 9

What are the advantages of Thiopentone

Answer 9

Rapid and smooth onset of hypnosis - fastest induction
Reduces CBF, CMRO2 and ICP
Potent anticonvulsant
More haemodynamically stable than Propofol (still a CVS depressant though)
Painless injection

Question 10

What are the disadvantages of Thiopentone

Answer 10

Slow metabolism can lead to accumulation and prolonged emergence especially in large doses (emergence is usually due to distribution)
Negative inotrope, and decreases venous tone - effect more pronounced in hypotension and hypovolaemia
Can precipitate porphyria
Unsuitable for use with LMAs - no effect on airway reflexes
Can cause histamine release, bronchospasm and laryngospasm

Question 11

What are the advantages of Ketamine

Answer 11

Has hypnotic, ANALGESIC and anaesthetic properties
Multiple routes of administration (IV, IM, PO, PN, PR)
Short duration of action
Preserves CVS function - may inc BP, and CO (best drug for shocked, unwell pt)
Preserves resp function and acts as bronchodilator (excellent in asthma)
Painless injection
Likely safe in porphyria

Question 12

What are the disadvantages of Ketamine

Answer 12

Slowest induction agent (1-2mins) without definitive endpoint (pt appears awake)
Occasional purposeful movt
Emergence delirium
Hypersalivation
Possible inc CBF and ICP (now disputed) - avoid in head injuries

Question 13

Outline the process of a TIVA induction

Answer 13

Select a concentration less than anticipated (4-6mcg/mL is the requirement for most patients)
Allow time for the effect site concentration to increase towards target concentration
Administer O2 during induction phase
Increase target concentration to achieve desired level of anaesthesia for procedure, individual pt, and balance of other agents (analgesia)

Question 14

What are the steps involved in induction of anaesthesia

Answer 14

Pre-operative assessment and preparation
IV access (can be post induction if needle phobic)
Positioning
Pre-oxygenation
Drug administration
Assess airway control, oxygenation, ventilation
NMB
Establish airway once safe
Monitoring and maintenance of anaesthesia

Question 15

Outline the role of co-induction agents - Midazolam and Opioids

Answer 15

Midazolam - Anxiolysis

Opioids - provide analgesia, suppress laryngeal reflexes, obtund sympathetic response to airway manipulation

Question 16

Who needs an ETT

Answer 16

Any patient requiring paralysis for surgery (laparotomy, thoracic surgery)
Long surgery (>1hr)
Lack of access to airway (shared airway procedures, position other than supine - includes head down)
Obese patients (>100kg)
Need for IPPV with high airway pressures (asthma, laparoscopy)
Failure of oxygenation by other means
Compromised airway - aspiration risk, obstruction risk, trauma, obtunded

Question 17

What are some potential hazards during induction

Answer 17

Difficult or failed intubation
Laryngospasm
Haemodynamic instability
Adverse drug reaction
Awareness

Question 18

What is maintenance of anaesthesia

Answer 18

Maintaining a state of anaesthesia involves the balance of reducing factors that normally maintain wakefulness, minimising adverse effects while preventing awareness

Question 19

What is involved in maintaining anaesthesia

Answer 19

Care of the unconscious patient (eg preventing pressure injury, preventing heat loss, infection control etc)
Appropriate positioning
Maintaining and protecting the airway
Preventing awareness
Analgesia
Ventilation
Monitoring of vital signs
Fluid inputs and losses
Record keeping

Question 20

What are the 2 main options for maintaining anaesthesia

Answer 20

TIVA and Inhalational

Question 21

Outline inhalational anaesthesia

Answer 21

Use of volatile liquids +/- N2O

Target a specific MAC that will provide adequate anaesthesia for the specific procedure

Question 22

What are the MACs for Sevoflurane, Desflurane and Isoflurane

Answer 22

Sevoflurane 1.8
Desflurane 6.6
Isoflurane 1.17

Question 23

What are the disadvantages of Sevoflurane

Answer 23

Recognised precipitant of MH
Reacts with dry soda lime to produce toxic compounds A-E
Myocardial depression and reduction in SVR
Blunts response to hypoxia and hypercarbia
Inhibits pulmonary hypoxic vasoconstriction

Question 24

What are the advantages of Sevoflurane

Answer 24

Reasonably pleasant smell - can be used for induction
Does not sensitise myocardium to catecholamines
Non-irritant to upper airways
Bronchodilator
Low B:G coefficient - rapid onset and offset
Minimal metabolism

Question 25

What are the disadvantages of Desflurane

Answer 25

Trigger for MH
Respiratory tract irritant
Myocardial depressant and reduction in SVR
Tachycardia
Blunts response to hypoxia and hypercarbia
Dose dependent depression of respiration

Question 26

What are the advantages of Desflurane

Answer 26

Improved preservation of SVR compared to sevoflurane
Lowest B:G coefficient of all volatiles - fastest onset and offset
Only 0.02% is metabolised

Question 27

What are the steps involved in TIVA

Answer 27

Programme pt details into pump - ht, wt, age
Choose appropriate concentration (Propofol 3-6mcg/mL, Remi 5ng/mL or 0.02mcg/kg/min)
Pre-oxygenate
Start infusions - consider bolus or premed with Midaz
Monitor BIS and pt
Once anaesthetised secure airway
Adjust concentration depending on depth as given by BIS (aim 40-60)
If pt breathing spontaneously - RR and EtCO2 are good markers of adequate anaesthesia (Aim RR ~12)

Question 28

What other drugs complement maintenance of anaesthesia

Answer 28

Analgesia - opioids, paracetamol, NSAIDs
Anti-emetics
Muscle relaxants
Sedatives - a2 agonists
Local and regional anaesthetics

Question 29

Define emergence

Answer 29

Return of spontaneous respiration, airway reflexes and consciousness after cessation of administration of anaesthetic agents

Question 30

Outline a process for emergence

Answer 30

Check TOF and reverse if necessary
Cease/slow IV infusion or cease inhalational agent
Change method of ventilation to a spontaneous mode
Can increase FGFs to ‘wash-out’ agent
Can allow EtCO2 to rise to stimulate respiration
Monitor for return of spont respiration - if prolonged assess for evidence of opioid overdose (Naloxone)
Watch for return of airway reflexes (return at Stage III plane I-II)
Once purposeful movt returns - extubate, ensure appropriate analgesia