S7) Introduction to Anaesthetics

Question 1

Outline the different types of anaesthetic techniques

Answer 1

Question 2

What is conscious sedation?

Answer 2

Conscious sedation is the use of small amounts of anaesthetic or benzodiazepines to produce a ‘sleepy-like’ state

Question 3

In 7 steps, describe the process of administering anaesthesia

Answer 3

⇒ Premedication e.g. benzodiazepine

⇒ Induction (IV/ inhalation)

⇒ Intraoperative analgesia (opioid, usually)

⇒ Muscle paralysis (facilitate intubation, ventilation, etc)

⇒ Maintenance (IV/ inhalation)

⇒ Reversal of muscle paralysis (postoperative analgesia)

⇒ Provision for PONV (post-op nausea and vomiting)

Question 4

Provide four examples of intravenous anaesthetics

Answer 4

• Propofol (rapid)
• Barbiturates (rapid)
• Ketamine (slower)

Question 5

Provide four examples of inhalational anaesthetics

Answer 5

• Nitrous oxide
• Xenon
• Chloroform
• Cyclopropane

Question 6

What is the term given to the range of effects on the CNS produced during general anaesthesia?

Answer 6

Guedel’s signs

Question 7

Identify and describe the four stages in Guedel’s signs

Answer 7

• Stage 1: analgesia and consciousness
• Stage 2: unconscious, breathing erratic but delirium could occur, leading to an excitement phase
• Stage 3: surgical anaesthesia, with four levels describing increasing depth until breathing weak
• Stage 4: respiratory paralysis and death

Question 8

General anaesthesia is a combination of which four events?

Answer 8

• Analgesia
• Hypnosis (loss of consciousness)
• Depression of spinal reflexes
• Muscle relaxation (insensibility and immobility)

Question 9

How might one determine the volatile anaesthetic potency?

Answer 9

Volatile anaesthetic potency is described by the minimum alveolar concentration (MAC)

Question 10

What is the anatomical substrate for MAC?

Answer 10

The anatomical substrate for MAC is the spinal cord

Question 11

What is MAC?

Answer 11

• MAC is the [alveolar] at 1 atm, at which 50% of subjects fail to move to surgical stimulus (unpremedicated breathing air)
• At equilibrium [alveolar] = [spinal cord]

Question 12

Explain how partition coefficients (solubility) affect the induction and recovery of general anaesthesia

Answer 12

• Blood:Gas partition (in the blood) – low value fast induction and recovery e.g. desflurane
• Oil:Gas partition (in fat) – determines potency and slow accumulation due to partition into fat e.g. halothane

Question 13

Identify five factors which affect MAC by increasing it

Answer 13

• Age (high in infants, low in elderly)
• Hyperthermia
• Pregnancy
• Alcoholism
• Central stimulants

Question 14

Identify three factors which affect MAC by decreasing it

Answer 14

• Age (high in infants, low in elderly)
• Other anaesthetics and sedatives
• Opioids

Question 15

Describe the relationship between anaesthetic potency, lipid solubility and GABA_A activity

Answer 15

Anaesthesia potency correlates with lipid solubility and GABA_A activity

Question 16

Describe the role of the GABA receptors in general anaesthesia

Answer 16

• GABA_A receptors are a critical target and act as major inhibitory transmitter for Cl^- conductance
• All anaesthetics (except for 3) potentiate GABA_A mediated Cl^- conductance to depress CNS activity

Question 17

What are the three effects which arise from potentiating GABA activity?

Answer 17

• Anxiolysis
• Sedation
• Anaesthesia

Question 18

Which three general anaesthetics do not potentiate GABA_A activity?

Answer 18

• Xe
• N₂O
• Ketamine

Question 19

Briefly, explain how general anaesthetics modulate consciousness in the brain

Answer 19

• In the brain consciousness is a balance between excitation (Glutamate) and inhibition (GABA)
• Anaesthetics modulate this balance

Question 20

In four steps, describe the effects of general anaesthesia on the brain circuity

Answer 20

⇒ Reticular formation (hindbrain, midbrain and thalamus) depressed and connectivity lost

⇒ Hippocampus depressed (memory)

⇒ Brainstem depressed (respiratory and some CVS)

⇒ Spinal cord – depressed dorsal horn (analgesia) and motor neuronal activity (MAC)

Question 21

What is involved in regional anaesthesia?

Answer 21

- Local anaesthesia involves selectively anaesthetising a part of the body

• It is the ‘block’ of a nerve and hence the patient remains awake and uses local anaesthetic / an opioid

Question 22

Provide four examples of nerves blocked in the regional anesthesia in the upper extremity

Answer 22

• Interscalene nerve
• Supraclavicular nerve
• Infraclavicular nerve
• Axillary nerve

Question 23

Provide four examples of nerves blocked in the regional anesthesia in the lower extremity

Answer 23

• Femoral nerve
• Sciatic nerve
• Popliteal nerve
• Saphenous nerve

Question 24

Identify five circumstances where local and regional anaesthesia are used

Answer 24

• Dentistry
• Obstetrics
• Regional surgery (patient awake)
• Post-op (wound pain)
• Chronic pain management (PHN)

Question 25

Provide four examples of local anaesthestics

Answer 25

• Lidocaine
• Bupivacaine
• Ropivacaine

Question 26

Describe the characteristics of local anaesthestics in terms of the following:

• Lipid solubility-potency
• Dissociation constant
• Chemical link
• Protein binding duration

Answer 26

• Lipid solubility-potency – higher greater potency
• Dissociation constant (pK_a) – lower pK_a, faster onset
• Chemical link – metabolism
• Protein binding – higher for longer duration)

Question 27

Describe the mechanism of action of wound analgesia

Answer 27

Bupivacaine infiltration for wound analgesia:

• Block small myelinated (afferent) nerves in preferance hence nociceptive and symp block
• Adrenaline ↑ duration

Question 28

What are the side effects of general anaesthesia?

Answer 28

• PONV (opioids)
• CVS
• Hypotension
• POCD (post-op cognitive dysfunction)
• Chest infection

Question 29

What are the side effects of local anaesthesia?

Answer 29

Local and regional – depends on the agent used, locals are Na⁺ channel blockers so cardiovascular toxicity