HNNS L17 - General Anaesthetics

Question 1

What are included in Modern Balanced Anaesthesia?

Answer 1

1. Unconsciousness (anaesthetics)
2. Analgesia (e.g. Opioids)
3. Muscle relaxation (NMJ blocker)

Question 2

What is the antidote for anaesthetics?

Answer 2

There is NO antidote for anaesthesia! (Unlike opioids)

Question 3

List some of the risks of general anaesthesia

Answer 3

1. Extremely narrow therapeutic window (*cardiorespiratory depression)
2. Airway obstruction
3. Aspiration pneumonia (*Nausea, vomiting caused by anaesthetics)

Question 4

Examples of IV general anaesthetics

Answer 4

1. Thiopental
2. Propofol
3. Ketamine
4. Etomidate

Question 5

Thiopental (Thiopentone)

Answer 5

Barbiturate
Can be used as an IV general anaesthetic
(Not so commonly used now)

Dissolve the powder in bicarbonate water
–> make it more lipid soluble

Long half life but short duration of action
(due to redistribution effect)
- repeated dosage would increase half-time

Cardiopulmonary depression effect

• decrease systemic vascular resistance and mean arterial pressure
• decrease respiration

Question 6

How to prepare thiopental for general anaesthesia?

Answer 6

Dissolve in bicarbonate water

make it more lipid soluble –> cross BBB

Question 7

Thiopental for general anesthesia: long or short

(1) half life
(2) duration of action?

Answer 7

Long half life but short duration of action

• redistribution effect
• drug level increases in vessel-rich group first, but decreases quickly
• then to muscles and fat
• *repeated dose may saturate muscles and fat –> slower drop in plasma level

Question 8

Propofol

Answer 8

Most commonly used IV general anaesthetic
- dissolved in lipid solvent (‘milk’)

???(Not sure)???
Less redistribution effect…?
Still gradually reduce dose to maintain the concentration (with the use of target controlled infusion)

Not for hypotensive patients due to its cardiopulmonary suppression effect
- Drop systolic pressure by 75-80 mmHg (most, exc hypotensive, can tolerate)
(Use ketamine instead)

Question 9

IV general anaesthetic contraindicated for hypotensive patients?

Answer 9

Propofol
cardiopulmonary suppression effect
- Drop systolic pressure by 75-80 mmHg (most, exc hypotensive, can tolerate)
(Use ketamine instead)

Question 10

IV general anaesthetic for hypotensive patients?

Answer 10

Ketamine
- but with more hallucinations (nightmare during operation)
For patients who cannot tolerate propofol due to its suppression in BP

Question 11

Ketamine

Answer 11

IV general anaesthtic

• Esp for very painful operations
• Also for hypotensive patients who cannot use propofol (ketamine increases systemic vascular resistance and mean arterial pressure)
• Lots of hallucination (nightmare during surgery)

Question 12

Etomidate

Answer 12

IV general anaesthetic
No change in BP and HR –> for cardiovascular surgery
- BUT lots of vomiting
Cardiopulmonary effect between propofol and ketamine

Question 13

Which IV general anaesthetic is good for a cardiovascular surgery?

Answer 13

Etomidate

No change in BP or HR

Question 14

Examples of inhaled general anaesthetics

Answer 14

Desflurane
Isoflurane
Sevoflurane

Question 15

What does 1% vapour mean for inhaled general anaesthetics?

Answer 15

1 mL general anaesthetic vapour in 100mL air

Question 16

What is minimum alveolar concentration?

Answer 16

Alveolar concentration of the inhaled agent which prevents movement in response to a standard pain stimulus in 50% of subjects.

• Blood concentration as a convenient indicator
• Mouth concentration at the end of expiration same as alveolar concentration!

Question 17

Highest minimum alveolar concentration (e.g. for inhaled general anaesthetics) in which age group?

Answer 17

1-6 months.

Question 18

Inhaled general anaesthetics:

Effect of oil:gas partition coefficient on minimum alveolar concentration?

Answer 18

They are inversely related. (Meyer-Overton correlation)
Higher oil:gas partition coefficient
- lower minimum alveolar concentration (higher efficacy)

Question 19

Inhaled general anaesthetics:

Effect of blood:gas partition coefficient on wash-in (FA/FI)?

Answer 19

Inverse relationship
Lower blood:gas partition coefficient
- Slower to get into blood stream
- More drugs accumulate within alveoli
- Higher FA/FI (alveolar fraction / inhaled fraction) and wash-in

Question 20

Two things that affect the wash-in of inhaled general anaesthetics

Answer 20

1. Blood:gas partition coefficient (inversely)

2. Cardiac output (inversely)

Question 21

Effect of isoflurane on CO, systemic vascular resistance, mean arterial pressure and HR?

Answer 21

Decrease CO, systemic vascular resistance, mean arterial pressure
Increase HR

Question 22

Effect of desflurane on CO, systemic vascular resistance, mean arterial pressure and HR?

Answer 22

Not much effect on CO
Decrease systemic vascular resistance, mean arterial pressure
Increase HR

Question 23

Effect of sevoflurane on CO, systemic vascular resistance, mean arterial pressure and HR?

Answer 23

Decrease systemic vascular resistance and mean arterial pressure
No much change in CO and HR

Question 24

Among isoflurane, desflurane and sevoflurane, which would decrease cardiac output most significantly?

Answer 24

Isoflurane

Question 25

Among isoflurane, desflurane and sevoflurane, which would decrease mean arterial blood pressure?

Answer 25

All 3

Question 26

Among isoflurane, desflurane and sevoflurane, which would increase heart rate?

Answer 26

Isoflurane, desflurane.