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 anaesthetics.

Answer 2

• All volatiles exc. 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

Which IV anaesthetic agent is usually used for induction and maintenance of GA? For rapid-sequence induction? What are their MOA and characteristic features?

Answer 6

Induction and maintenance of GA: IV PROPOFOL

• moderate cardiac depression
• pain on IV injection
• little accumulation of metabolites and anti-emetic properties

Rapid-sequence induction: IV SODIUM THIOPENTONE (barbituate)

• v. rapid onset of action
• marked myocardial depression can occur and metabolites build up quickly (unsuitable for maintenance infusion)

Act by potentiating GABAa Rs… increased Cl- entry… membrane hyperpolarisation.

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).