Anaesthetics

Question 1

Describe the stages of Guedel’s signs

Answer 1

• Stage 1 - analgesia and consciousness
  
  • Conscious sedation - use of small amounts of anaesthetic or benzodiazepines to produce a sleepy-like state
  • Maintain verbal contact but feel comfortable
• Stage 2 - unconscious, breathing erratic but delirium could occur, leading to an excitement phase
• Stage 3 - surgical anaesthesia, with 4 levels describing increasing depth until breathing weak
• Stage 4 - respiratory paralysis and death

Question 2

Describe the different types of anaesthesia used in modern practice

Answer 2

• Premedication - given hour before surgery (hypnotic-benzodiazepine)
• Induction - usually IV but may be inhalational
• Intraoperative analgesia - given at time of surgery (opioid)
• Muscle paralysis - facilitate intubation/ventilation/stillness
  
  • Relax airways
• Maintenance - IV or commonly inhalational
  
  • Keep asleep during surgery
• Reversal of muscle paralysis and recovery which includes postoperative analgesia (opioid, NSAID, paracetamol)
  
  • Opioids are short acting, so another may need to be given when waking up to relieve pain
• Provision for post operative nausea and vomiting (PONV)

Question 3

State examples of inhalational and IV anaesthetics

Answer 3

• Inhalational
  - N2O, diethyl-ether, xenon, chloroform, cyclopropane
• IV
  - Propofol, barbiturates, etomidate, ketamine

Question 4

Define minimum alveolar concentration

Answer 4

• Concentration of anaesthetic in alveoli at equilibrium at which 50% of subjects fail to move to surgical stimulus
  
  • At equilibrium, alveolar concentration = spinal cord concentration
• Patients are given more than MAC to ensure anaethesia works
• Lower MAC means drug has higher potency

Question 5

What are factors affecting MAC

Answer 5

• Age - high in infants, low in elderly
• Hyperthermia (increase), hypothermia (decrease)
• Pregnancy (increase)
• Alcoholism (increase)
• Central stimulants (increase)
• Other anaesthetics and sedatives (decrease)
  
  • Eg Nitrous oxide (N2O) can be given alongside normal anaesthetic to reduce dosing needed to sedate patients - given for patients likely to have side effects to anaesthetics
• Opioids (decrease)

Question 6

Describe the role of GABA receptors in anaesthesia

Answer 6

• GABA is the major inhibitory transmitter
• Ligand gated ion channel allowing chlorine to enter the cell and thus hyperpolarising it
  
  • Increases difficulty to stimulate action potential
• Apart from Xe, N2O, and ketamine, all anaesthetics potentiate GABAA mediated Cl- conductance to depress CNS activity
  - These 3 block NMDA receptors, which is an excitatory receptor

Question 7

Define local anaesthetics and the PK of short and long acting anesthetics

Answer 7

• Local anaesthetics anaesthetize a small area of tissue for a minor procedure
• Ester link are short acting and amide link are long acting anaesthetics
• Lipid soluble - higher greater potency
  
  • Amide have greater lipid solubility
• Dissociation constant (pKa) - lower pKa = faster onset
• Protein binding - more binding = longer lasting
  
  • Amide have greater protein binding

Question 8

Define regional anaesthetics

Answer 8

• Regional anaesthetics anaesthetize a part of the body

- Block off a nerve and hence the patient remains awake

Question 9

Describe the mechanism of action for wound analgesia

Answer 9

• Local anaesthetics not charged so freely pass through plasma membrane into the cytoplasm
• Once inside, becomes charged and blocks Na channels (anaesthetic effect)
• Prevents Na from entering cell, therefore prevents action potential from happening
• Targets and blocks small myelinated (afferent) nerves in preference
• Use-dependent block - degree of block proportional to the rate of nerve stimulation

Question 10

State the main ADRs of anaesthetics

Answer 10

• General anaesthesia
  
  • PONV - post operative nausea and vomiting
  • CVS - hypotension
  • POCD - post operative cognitive dysfunction
    
    • Increases with age
  • Chest infection
• Local and regional anaesthesia
  
  • CVS - cardiovascular toxicity due to blocking of Na channels
• Allergic reactions / anaphylaxis