Peri-op Care: Anaesthetics

Question 1

Name the 2 main types of general anaesthetics.

Answer 1

1. volatile (inhaled)
2. IV

(Usually use combinations - enhanced effects allowing smaller doses)

Question 2

Describe the mode of action of volatile (inhaled) anaesthetics.

Answer 2

All volatiles except Xe and NO2 potentiate GABAa receptors, increasing Cl- entry on GABA binding

Causes membrane hyperpolarisation, central inhibition and thus anxiolysis, sedation and anaesthesia

NMDA receptor blocking probably other site of action

Question 3

What is Minimum Alveolar Concentration (MAC)?

Answer 3

Alveolar drug concentration at 1 atm at which 50% Pts fail to respond to surgical stimulus.

Describes volatile agent potency.

Question 4

Suggest factors that increase or decrease a volatile agent’s MAC.

Answer 4

MAC is increased by (so less potent in):

• young age
• pregnancy
• alcoholism
• hyperthermia
• central stimulants

MAC in decreased by (so more potent in):

• hypothermia
• opiates
• other anaesthetics and sedatives (NO2 often added to decrease dosing)

Question 5

Name the 2 partition coefficients of a volatile agent that affect Pt induction and recovery.

Answer 5

1. Blood-gas partition (= solubility in blood): low value indicates faster induction and recovery
2. Oil-gas partition (= solubility in fat): high value indicates higher potency and slow accumulation due to partition into fat (obese Pts have longer recovery)

Question 6

Name 3 examples of IV anaesthetics and describe their mode of action.

Answer 6

• Propofol (rapid) SE = cardiac depression
• Barbiturates (rapid)
• Ketamine (slower)

With exception of ketamine (NMDA) all potentiate GABAa.

Question 7

What are the respective advantages of bupivacaine and lidocaine as local anaesthetics?

Answer 7

Bupivacaine: longer acting (4-8hrs) but longer onset of action (5-10min). Prefered for spinal and epidural injections.

Lidocaine: rapid onset of action (2-5min) but intermediate duration action (2hrs without and 3hrs with adrenaline).

Question 8

Which agent is typically injected alongside lidocaine? Why?

Answer 8

ADRENALINE: vasoconstricts arteries, reducing bleeding and delaying resorption of local (almost doubles duration of anaesthesia)

Question 9

Describe the mode of action of bupivacaine and lidocaine.

Answer 9

Bind to intracellular portion of voltage-gated Na+ channels and block Na+ influx into nerve cells, preventing depolarisation and AP generation.

Use-dependent action as have higher affinity for open Na+ channels in inactivated state (hydrophilic pathway).

Question 10

Give examples of a muscle relaxants and possible SE?

Answer 10

Suxamethonium
Actracurium
Rocuronium

Suxamethonium apnoea = pt does not have the enzymes to metabolise it, they remain paralysed for an increased length of time and cannot breathe adequately at the end of an anaesthetic

Question 11

What are the 3 elements to anaesthesia?

Answer 11

Hypnosis - IV or inhaled

Analgesia

Muscle relaxation