Induction, Maintenance, Emergence Flashcards
Define induction
Administration of a drug or combination of drugs at the beginning of an anaesthetic that results in a state of general anaesthesia
What are the 2 broad categories of induction
Inhalational vs Intravenous
When is a rapid sequence induction indicated
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)
What are the indications for an inhalational induction
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)
Outline the process of an inhalational induction
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%
Which are the commonly used IV induction agents?
What are the associated dosages for inducation
Propofol 2mg/kg
Thiopentone 3-5mg/kg
Ketamine 1-2mg/kg
(Midazolam)
What are the advantages to use of Propofol
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
What are the disadvantages of Propofol as an induction agent
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
What are the advantages of Thiopentone
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
What are the disadvantages of Thiopentone
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
What are the advantages of Ketamine
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
What are the disadvantages of Ketamine
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
Outline the process of a TIVA induction
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)
What are the steps involved in induction of anaesthesia
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
Outline the role of co-induction agents - Midazolam and Opioids
Midazolam - Anxiolysis
Opioids - provide analgesia, suppress laryngeal reflexes, obtund sympathetic response to airway manipulation
Who needs an ETT
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
What are some potential hazards during induction
Difficult or failed intubation Laryngospasm Haemodynamic instability Adverse drug reaction Awareness
What is maintenance of anaesthesia
Maintaining a state of anaesthesia involves the balance of reducing factors that normally maintain wakefulness, minimising adverse effects while preventing awareness
What is involved in maintaining anaesthesia
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
What are the 2 main options for maintaining anaesthesia
TIVA and Inhalational
Outline inhalational anaesthesia
Use of volatile liquids +/- N2O
Target a specific MAC that will provide adequate anaesthesia for the specific procedure
What are the MACs for Sevoflurane, Desflurane and Isoflurane
Sevoflurane 1.8
Desflurane 6.6
Isoflurane 1.17
What are the disadvantages of Sevoflurane
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
What are the advantages of Sevoflurane
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
What are the disadvantages of Desflurane
Trigger for MH
Respiratory tract irritant
Myocardial depressant and reduction in SVR
Tachycardia
Blunts response to hypoxia and hypercarbia
Dose dependent depression of respiration
What are the advantages of Desflurane
Improved preservation of SVR compared to sevoflurane
Lowest B:G coefficient of all volatiles - fastest onset and offset
Only 0.02% is metabolised
What are the steps involved in TIVA
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)
What other drugs complement maintenance of anaesthesia
Analgesia - opioids, paracetamol, NSAIDs Anti-emetics Muscle relaxants Sedatives - a2 agonists Local and regional anaesthetics
Define emergence
Return of spontaneous respiration, airway reflexes and consciousness after cessation of administration of anaesthetic agents
Outline a process for emergence
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
